By frequency band

Non radar related by date:

Radars:

Azimuth dependence of birdies.

Cameras in the dome:

Distomats:

Power meter,multimeter:

Rfi info in other pages.

TV Information.
Cell phones

---

lband

• 26dec00 lbn. Azimuth swings with dome at 19 degrees za.  (rfimtop)
A 3 Mhz band pass was used. Swings were done centered at 1420 Mhz (257 10 second dumps) and  1419.5 Mhz (284 10 second dumps) The plots show :
• Figure 1: The  (average - baseline ) for each of the swings (pol Apollo B separate).    The birdies are at: 1419.36,1419.5, 1419.93, 1420.00,1420.8.
• Figure 2: a blowup of figure 1.
• Figure 3: The 1420 Mhz birdie channel versus azimuth angle with adjacent off channels over plotted. The bottom plot shows the difference between the 1420 Mhz birdie channel and the adjacent channels. There does not appear to be any azimuth dependence so the birdie is probably generated inside the dome (although the tsys bumps at -60, 60, and 180 which are perpendicular to the triangle sides are very interesting!!)
processing: x101/001226/26dec00azswinrfi.pro
 
• 22jan01 distomat birdies at 1300,1400 Mhz.    (rfimtop)
summary:
birdies at: 1287.5, 1299.84,1300.,1399.83, 1400.,1411.52,1412.5.Every 2 minutes, azimuth dependent. clk:27Mhz. looks like +/- 12.5 Mhz above/below the 1300,1400Mhz birdies
The distomats used for the laser ranging were generating birdies around 1300 Mhz. To measure this the correlator was setup for 25 Mhz bw/1024 channels centered at 1295 Mhz using the lband wide system. The telescope was pointed at each distomat in turn (n*120 +/- 35 degrees az, za=18 deg).  The distomats were run on a 30 second cycle starting on the minute. The correlator integrated for 180 seconds looking at each distomat separately. The distomat program spends about 7 seconds reading the temperature and tilt sensors and then it starts reading the 6 distomats. The measurement is usually through in 22 seconds.
• the images  (1.2mb ps file) show 1282.5 to 1307.5 Mhz by the 180 second measurements while looking at each of the 6 distomats.  Distomat 4 (az=205) is the worse with birdies at 1300 Mhz and 1300-12.5=1287.5.  For each 30 second cycle there is a spike at 7 seconds and then a longer duration that last for 3 or 4 seconds. This corresponds to the number of seconds it took to read the distomat.

    There were two frequencies close to 1300 Mhz: 1299.84 and 1300.0 Mhz measured with a  0.024 Mhz resolution.  On 5feb01 measurements with a spectrum analyzer at the distomats showed that distomat 3 = 1299.84 and distomat 4 was generating the 1300. Mhz birdie. The birdie from distomat 4 was about 5 times stronger than that from distomat 3. On 5feb01 distomats 1,3,4 all had large windows (distomat 1 window was changed from small to large after 22jan01). Distomat 1 showed no birdies on 5feb01 so the reason is not just the larger window.
• the plot shows the time series for the power in the 2 channels around 1300 Mhz (averaged). The bottom trace is distomat 1 and the top trace is distomat 6 (offsets were added for display). The units are fractions of Tsys. From this plot it looks like all of the interference is coming from distomat 4. The junk while looking at the other distomats may just be leakage from distomat 4. The birdie could be larger than .3 Tsys if its duration is very much less than 1 second (the correlator integration time).

The manufacturer says the clock rate is 27 Mhz so 1300 is the 48 Th. harmonic. Using azimuths spins from other days (11apr01) birdies were seen at:1299.85,1399.83, and 1400.
processing: x101/010122/doit.pro
• 27apr01-11may01 1407 Mhz birdie from tv station. (rfimtop)
A 1407 Mhz birdie appeared in apr01. It was about 300 Khz wide and seemed to be on all the time. Some of the characteristics of the signal were measured using the lbw receiver in the dome:
11may01 - final results: birdie was from call sign: w20bx channel 20 arecibo. barrio factor. 507.25 Mhz video carrier. approximate coordinates are: 18:27:46.0,66:38:18.7 (gps receiver close by). This was about 60 degrees east of north from the platform. Tower FCC erg number: 1211904. Engineer: Juan G. padding.
1. It is located at 214.4 Deg az and 31.8 degrees az (180 degrees away) and is at high za (19 deg).
2. It is very narrow in azimuth. It drops to 10% of maximum in 2 degrees az.
3. The strongest it gets is about 5*Tsys(40K) = 200 Kelvins coming through the telescope.
4. The signal appears to be left circularly polarized coming in the telescope.
5. There is a 15.8 Khz comb and a 60 hz comb in the signal.
6. There is a side band +4.5 Mhz from the peak and a weaker one -4.5 Mhz from the peak (tv signal audio offset from video carrier). The audio was not strong enough to demodulate.
7. The signal wanders around by up to 1 Mhz over hours.
8. The signal is almost always on.  We did see it turn on at 9:22 am 11apr01.
9. Looking with binoculars along the azimuth direction 214.2 ,  you can see the 107.3 Fm station tower (one of the strongest sky signal around). The strongest tv station in this direction is channel 22 with a video of 519.25 Mhz.
10. We inspected the 107.3 Mhz tower. It had the fm antennas, a microwave link, and one other whip. We did not see any believable 1407 Mhz emission ( the signals were so strong that we had trouble with intermods in the antenna/amp we were using).
11. From the platform we could not reliably see the signal at 1407 Mhz. We had a hard time getting a sensitive enough measurement using a log periodic (6 db gain), amplifiers, and spectrum analyzer.
12. We looked at the power levels coming out of the dewar. The caw output levels don't appear to be strong enough to cause intermods in the dewar. We see the 1407 birdie at the output of the dewar on the spectrum analyzer at 60% Tsys (Tsys=40K, 40 db gain in dewar, 10 KHz raw).
13. There are no harmonics of tv stations that fall at this frequency.
14. Thinking that it might be the IF output of our satellite receivers (900-1450) we turned off the amps/block converters for the tv dishes but the signal remained.
15. The radar blanker (with an lo of 1340 Mhz) was turned off but the birdie remained.
16. The signal drift rate was up to 1 part in 10-4 over 5 minutes. This is a pretty lousy oscillator.
17. On 10may01 we setup a receiver on the platform: log Periodic, circulator, 10db gain amp, 25 MHz filter about 1407, 20 db gain amp, mix to 30 MHz, and then sent it down to the control via the radar blanker rg7 cable (< 10db loss). We then input the signal to the correlator and integrated. We saw the birdie and it came from 60 degrees east of north.
18. On 10may01 we went out to route 10 overlook.. no results. Next was the arecibo airport. We got a good signal coming from the east.
19. On 11may01 we went farther east till we got a signal from the north.  Then into barrio factor of arecibo where we located the tower... W20BX channel 20 in arecibo. It was still transmitting a test pattern and tone. After contacting the engineer (juan g padding), we did a test turning the transmitter off and the signal  disappeared. The tv transmitter had been used for channel 28 (555.25) but was now being used at 507.25. He had been transmitting with an 8 watt executor at 507.25 Mhz. The engineer claimed that the bandpass filter was not working correctly. There must have been an lo of 900 Mhz lurking somewhere in the system.
Some plots of the signal are:
• Figure 1 shows spectra through the lbw receiver (circular) and the correlator using 1 second dumps and 6 Khz spectral resolution. Pol A (left circular) is red. The az=214.4 and za=19. degrees. Figure 2 shows the power in the signal channels during the azimuth spins. It peaks sharply at 214.2 and 31.8 az.
• Base band sampling 1 MHz bw about the birdie. The upper plot has 1 Khz resolution and shows the  spike and comb. The middle and lower plots are blowups to show that the comb is 15.8 Khz and the spike is a 60 Hz comb. These are the horizontal retrace and vertical retrace frequencies of the tv signal.
• Side bands of the signal.   The audio side bands can be seen with the wider bandwidth. There is a spike awfully close to the suppressed color carrier.
• Image showing the variation of the frequency over time. The image plots 122 channels about the video carrier for 5200 seconds. The signal is wandering about by a few  100 Khz.It has moved by up to 1 Mhz or more.
• The power levels at the output of the lwide dewar. The dewar can output 0 dbm so we aren't near the Mx (see Measuring the power levels at the output of the lband wide dewar below). The last page shows the size of the 1407 birdie at the output of the dewar on the spectrum analyzer (linear scale, 10 Khz raw).
• Birdie direction from log periodic antenna on the platform. The plots show the strength of the signal using a 100 second integration. The directions are only approximate since the antenna was turned by hand.
Discussion:
• The 15.8 Khz and 60 Hz combs are very narrow. If the signal was mixing with the FM tv station then it would  probably get smeared out.
• I had thought that a harmonic would cause the audio carrier to move farther out. Jon hagen straightened me out.
Suppose you have a tone of  Amplitude A and frequency w and a side band of amplitude B  and frequency (w+delta) then:
(Aexp(iwt)+Bexp(i(w+delta)*t)^2= AAexp(i2wt)+ ABexp(i(2wt delta)) + BB*exp(i(2w+2delta)t)
If B << A then the AB term will be much larger than the BB term so the delta signal will be stronger than the 2*delta.
If the video is a 2nd harmonic, then the audio below the video harmonic could be 3*video-audio.
• The wave guide at the input to the dewar will not let a tv station fundamental get into the dewar. The power levels in the dewar are not high enough to cause intermods so the signal must be entering the dewar at 1407 Mhz.
• The inability to see the birdie from the platform on the spectrum analyzer is probably a signal to noise problem. The spectrum analyzer noise floor is -107 db/30Khz raw. Using -198 dbm/hz/K gives a noise temperature of 40,000K. The Mx signal level through the telescope is 5*40K=200K. The log periodic noise temperature is at least 150K since it is looking at the horizon. After adding the noise figure of any amplifiers/filters, we probably have a system temperature going into the spectrum analyzer of 250 -300K. Our setup was a filter (1.7 db loss), 20 db gain amplifier, and then  the spectrum analyzer. We saw no 1407 birdie. Assume the signal is only losing 10 db getting into the dome and we want a signal to noise equal to what we saw at the output of the dewar. We would then need: (40dbDewarGain - 10 db loss getting in - 6 db gain of log periodic) + 300K/40K (8db tsys difference). This comes to about 30 db gain (and the signal at the output of the dewar was not very strong...).
• The block down converters for satellite tv receivers at cband and ku band use (5150-transponder frequency)  and (transponder frequency - 11.3 Ghz)for the If. For cband we would want a transponder frequency of 5150-1407=3743. The closest transponder frequency if 3740 (although i don't know the video carrier offset). I think that the satellite transmissions put the audio frequency 6.5 Mhz above the video so it's probably not a satellite IF from a neighbor's busted receiver.
• It seems strange that the azimuth dependence is so sharp. The sidelobe response should not be so sensitive to angle. Maybe the signal is doing some type of spectral reflection (say scattering off of the side of the azimuth arm, down into the dish, and then back up into the receiver. The fm tower lining up with the peak of the signal is probably just a coincidence. Scattering off of the side azimuth would also explain why we see the signal at 214 and 180 degrees away from that. This would be a tv signal at 120 or 300 degrees.
processing:x101/010427/rfifigs.pro, x101/010425/doitri.pro,x101/010504/doit.pro
• 02may01 Measuring the power levels at  lband wide dewar output.  (rfimtop)
We measured the output power levels of the lband wide dewar using the 22 Ghz spectrum analyzer and a power meter. The telescope was parked at az=214.4 and the gregorian was at 19 degrees. A 12 foot long semi rigid cable was used to connect to Pol A's output.  The plots show:
• Figure 1 plots the spectrum 1400 Mhz +/- 1000 Mhz. The resolution bandwidth was set to 3 Mhz. The system temperature at this za is about 40 K. The black line averages 10 sweeps of the spectrum analyzer. The red line is a peak hold for greater than 20 seconds. The black line shows continuous signals while the red line includes the impulsive radars. The dear's response goes from 1 Ghz out to 1.9 Ghz and is 10 db about the spectrum analyzer's noise floor. The total power measured with the power meter was -28 dbm. Integrating the spectra gave -22.5 and -17.9 dbm. The -28 dbm and -22.5 dbm should be the same. The -28 dbm value of the power meter is probably more reliable.
• Figure 2 is a blowup of the peak hold spectrum The known birdies are:
• 880 Mhz. cellular phones.
• 932, 956 Mhz. cellular communications.
• 1250 Mhz. aerostat radar.
• 1330,1350 Mhz. airport radar.
• 1950 Mhz. cellular phones, digital communications.
• Figure 3 is the 1407 Mhz birdie at the output of the dewar plotted on a linear scale.

 
Doing a consistency check on the power levels:
-198 dbm/hz/K + alog10(3Mhz rbw)*10 + alog10(40K)*10 + 40 db gain -77 dbm.
The spectrum analyzer value is about 5 db higher than this (it was also 5 db higher than the power meter reading).
The highest  continuous signals are in the 880 to 950 Mhz range. These signals could be filtered out by decreasing the diameter of the wave guide at the input to the dewar without affecting the normal observing band (say > 1100 Mhz). If a radar causes the dewar to go into saturation then 3*(850-950) - 1350 will mix back into the Fr band.

The final stage lbw amp can output 0 dbm of power so the caw levels are not high enough to cause the dewar to saturate. There may be a position of the azimuth, dome where these levels are higher than those  measured. These measurements do not tell whether the radar pulses are causing the dewar to go into saturation for the duration of the radar pulse. Some of the pulse lengths are on the order of ascends. The power meter averages for a lot longer than this while the chances that the spectrum analyzer peak hold caught the radar pulse when it was pointed at us is small (the sweep time/number of sampled points < radar pip).

The  1407 Mhz birdie is seen at the output of the dewar so it is not being caused downstream in the post amps. The power levels in the dewar are low enough that it is probably not an intermod created in the dewar.
processing:x101/010502/doit.pro

• 13aug,20sep01 lbn. Rfi from color camera on the service platform. (rfimtop)
A color camera  was placed on the service platform for monitoring purposes. It had no "extra shielding" added by us. On 13aug01 the lband narrow receiver was used to measure the rfi from the camera. 1337 to 1437 in 4 subtends was used. Each with 25 Mhz over 1024 channels. 1 second integrations were done. 120 seconds of data with the cameras on were followed by 120 seconds with the cameras off. The camera was removed and shielded. The rfi was remeasured on the service platform on 20sep01 using the same setup but a 240 second integration.
• image 1 (2 mb) shows the 240 records versus frequency (polA). They have been divided by the average of the camera off so the units are Tsys (28 K lbn). Horizontal lines are placed at the transition.
• plot of the average (cameraon/cameraoff)-1. The top plot shows the average for the 120 seconds on 13aug01.  PolA is black and polB is red. The units are fractions of Tsys. Vertical lines have been drawn every 14.3341 Mhz. The bottom plot shows 4 minutes of data with the camera on taken on 20sep01 after shielding.

 

 
 
 
 
 
 
 
 
 

The image shows that the frequency is moving by up to 100 Khz/second. This will decrease the average intensity of the tone when averaged over time.
A fundamental of 14.3341 would give harmonics at the measured positions. The strength of the signal increases every 4 of these units so there are probably two frequencies involved.
The shielding removed the rfi for a 4 minute integration. To go farther in sensitivity we should do the experiment at night (no sun and hopefully no punt saloons radars).

processing: x101/010813/dorfi.pro, x101/010920/colcamera.pro

• 04oct01 Radar harmonics created in the 2nd IF. (rfimtop)
Spectral line observing at lband normally has the following IF processing steps:
• The lband wide front end has a frequency response (see link).
• A set of filters between the dewar and the post amps limits the bandwidth.
• A high side lo mixes the Fr center frequency to 750 Mhz. It is then band limited to 500-1000 Mhz.
• A set of up to 4 high side second Los mix down to 260 Mhz. Each of these is then low pass filtered to 500 Mhz.
• Each of the 260 Mhz bands go through a distribution amplifier and then to the correlator.
• The correlator first places a 50 Mhz filter about 260 Mhz. It then amplifies the signal and sends it to the digitizer that samples the entire 50 Mhz band.
• Digital filters then reduce the band by factors of two from 25 Mhz down to 195 Khz.

 

 
 
 
 
 
 
 
 
 

In step 5, the second harmonic of any signal 130 Mhz below the center of a subcorrelator will land centered on the subcorrelator (130*2= 260 MHz is the center of the band).  If the sub band center is  130+dx Mhz above a radar, then the radar 2nd harmonic  will come 260 - (260-(130+dx))*2  = -2 dx Mhz above the center of the band. If abs(dx) < Bandwidth/4 then it will appear in the spectrum.
The table below shows the center frequency 130 Mhz above various radars. It also lists the range of vulnerability assuming you use a 25 Mhz digital filter (25/4=6.25)
 

radar freq	130 Mhz Above(6 -/+Mhz)
1233 modeA puntaSalinas	1363  (1357-1369)
1242 puntaSalinas modeA	1372 (1366-1378)
1248 puntaSalinas modeA	1378 (1372-1384)
1257 puntaSalinas modeA	1387 (1381-1393)
1241.75 aerostat	1371.75 (1365-1378)
1244.6 aerostat	1374.6 (1368-1381)
1256.5 aerostat	1386.5 (1380-1393)
1261 aerostat	1391 (1385-1397)
1269 ramey	1399 (1393-1405)
1290 ramey	1420 (1414-1426)
1330 airport	1460 (1454-1466)
1350 airport	1480 (1474-1486)

This problem will only occur if the radar signal in the 2nd IF (260Mhz) is strong enough to create harmonics. These harmonics have been seen for the 1269 Mhz radar with sbc cfr =1398Mhz . In this case dx=-1 and the birdie comes -2dx= +2 Mhz above the center (at 1400Mhz).

image 1 shows the 1330,1350 Mhz faa radar as well as 130 Mhz above each of these frequencies : 1460 and 1480 Mhz.
image 2 shifts the center frequency of the last two sub bands to 1461 and 1481. The 1330 Mhz harmonic is not visible but the 1350 harmonic is now at 1479. Using the formula above:
 

The 1350 Radar is (1481-1350)= 261 Mhz below the band center so dx = 1 Mhz.
The birdie then falls at  (cfr-2*dx)=1481-2= 1479 Mhz.

The other birdies in the 1330,1350 bands are intermods caused in the 8 bit A/D converter for the correlator by the radar.
processing: x101/011005/ifharmonics.pro
• 05oct01. tone at 1350 Mhz. (rfimtop)
The FAA radar is at 1330 and 1350 Mhz. When the radar blanker was used, the radar pulses were blanked but a tone remained at 1350 Mhz. On 05oct01 an azimuth spin was done with lband narrow (native circular) with the dome at 19 degrees za to look for any azimuth dependence of the tone. The correlator was setup for 25 Khz resolution and was dumped at 1 second intervals. The figures shows the results. Pol A is black, pol B is red, and the azimuth location of the control room is flagged with a green vertical line.
• The top figure shows +/- 180 degrees azimuth (from north).
• The center figure smoothes the 1 second samples  (.2 degrees azimuth /sample) by 11.
• The lower figure blows up +/- 20 degrees azimuth about the control room position. Asterisks mark the 1 second samples.

The tone is between azimuth -100 and +10 degrees az. The peak value is close to the control room values but the center is not.  The peaks last for about 8 seconds (or 1.6 degrees in azimuth). At 25 Khz resolution the tone was not resolved.

If the control room is causing the problem, then the center offset may come from the asymmetry in the cliffs adjacent to the control room blocking the beam. The lidar lab and the rfi shack are also in this general direction. We need to go up to the azimuth with an antenna and look around (bringing the data down into the correlator and using the blanker).
processing: x101/011005/1350tone.pro
 

• 06nov01. 1381 gps L3 birdie on all the time. (rfimtop)
There has been a birdie at 1381 Mhz  for the last few weeks that is 1 Mhz wide. It looks like GPS L3 at 1381.05 Mhz but occurs more often than the scheduled usage of GPS L3. The people at the VLA do not see this unusual usage of GPS L3.
Experiment # A1517 has been running from 19:00 AST thru 1:00 AST the next morning. Below is a list of what it has  seen at 1381. The dates are the AST end dates (1am AST). The times are AST (UTC-4).
 

date (of end time)	astTimes (hours)
03nov01	19.0->0.8am 4 bursts of 1 to 2 minutes duration.
04nov01	19.0->0.8am 1  two minute burst
05nov01	19.0->20.8 ...1 to 2min on for each 4 minute scan 20.8->23.3 off 23.2-> 0.8  on .. 1 to 2 min of for each 4 minute scan.
06nov01	19.->.8 on most of the time. There were the 1 or  2 minute bursts that were strong. When the strong bursts stopped, a lower level emission remained on.
07nov01	3 bursts of: 2min, 1 min, 1min for the entire night (19. ->0.8 am)
08nov01	4 bursts of:1min,2min,1min,1min for the entire night (19->.8am)
09nov01	3 bursts for entire night (19->.8am)

06nov01 was different from all of the other days. On this day, the bursts would last for a minute or two but the signal would not turn off completely after the bursts. A low level interference would remain on.

The figure shows two plots from 06nov01 when the birdie was on all the time and 1 plot from 08nov01 when the bursts were normal. The data was sampled at 1 second intervals. The telescope was tracking a celestial source while this data was taken on 06nov01. On 08nov01 the telescope was stationary while the data was taken. (The links for fig1,2,3 below are the the individual pages from the "figure" link above. They were added for the braindead browser at aerospace!!)

• Figure 1 top is a linear plot of 1 Mhz total power about 1381.  At 150 seconds it is the strongest. An image of spectral density versus time showed that that the signal remained there at a lower level for the entire scan.
• Figure 1 middle plot is the same plot but with a db scale. The FWHM for the peak at 150 seconds is 16 seconds.   This is close to the sidereal rate of 13 seconds for the main beam.  The signal is  moving through one of our sidelobes because the telescope is tracking a celestial source and  the rfi source is probably also moving ( GPS satellites have 12 hour nearly circular orbits so they are move in their orbital plane at about twice sidereal rate).
• Figure 1 bottom is the spectral density for 100 contiguous seconds around the peak. An offset has been added for display.
• Figure 2 top  occurred later in the night. The signal was off and then turned on at 136 seconds ( 1:58:42 UTC 06nov01).  It then looked again like it was drifting through a sidelobe.  At 200 seconds it was turned off and then came back on at 230 seconds.
• Figure 2 bottom is the spectral density function for 100 seconds around the first turn on. An offset has been added for display purposes.
• Figure 3 was taken on 08nov01 during the day with the telescope stationary.

    The top plot is 1 Mhz power (linear scale) about 1381 Mhz sampled at 1 second intervals. PolA is the upper trace and pol B is the lower one (lbn was used with circular polarization). You can see the satellite move through the sidelobes taking 54 seconds to move through 4 sidelobes (13.5 secs/ sidelobe). This burst was a clean burst where it turned on and then off completely. The telescope was stationary so there is no mixing of the motion of the satellite and the telescope. The difference in polarizations is probably do to the unknown polarization properties of the far out sidelobes.
    The bottom plot is the spectral density versus time for this period (offsets were added for display). The spectral shape does not have the modulation that was seen in the spectra on the 06nov01.


GPS L3 is used for nuclear blast detection. There are on board sensors that monitor electromagnetic and x-ray events that could be related to nuclear blasts. Using multiple satellites, the positions of the blasts can be determined. This data is formatted and transmitted to ground stations on the L3 frequency. The GPS Nuclear Detection System operators were contacted and they admitted that one of their birds had a problem with its detector being stuck on. This could have caused the continual transmissions that we were seeing on 5/06nov01. A single satellite failure would also explain why we saw it and the vla didn't.

processing:x101/011106/doit.pro.
• 28feb02 1400 Mhz birdie. (rfimtop)
3 On/off scans of 5 minutes each were taken with lbn. The plots show a 1400 Mhz birdie. The resolution was 6 khz and it is not resolved (on 01mar02 I used 190 hz channel width and it did not drift and it was not resolved). The birdie divides out between the on and the off so it is very stable.
1. Fig 1 shows the spectra for a 6 second integration.
2. Fig 2 has the channel corresponding to the 1400 Mhz birdie plotted versus start time of the on or the off for the 3 scans. The on and the off are overplotted. Black is polA, red is polB. There is a 100 second cycle that repeats in the on and the off. Since the start times between the on and the off is not a multiple of 100 seconds, the periodicity is in space rather than in time.
3. Fig 3 plots the 1400 Mhz channel for the 3 scans versus az. The on and the off track each other. It is strongest in the first scan with az=240 (pointing toward T8). It gets weaker as we move away but there is still correlated power in the on an the off.This was done at low zenith angle.
The distomats were turned off during the off of the first scan. The times (seconds from midnite are:
 

scan1 on start	78469
scan1 off start	78830
scan1 off end	79130
laser ranging last pnt	78962
laser ranging off by	79082

    The laser ranging points come every 2 minutes so the 2 minutes after the last point (78962+120=79082) the distomats were off. This is 48 seconds before the end of scan1 off. The on and the off for scan 1 track identically so turning off the distomats had no affect.
    What it could be?? It's narrow, stable and exactly at 1400 Mhz so it's probably us. It varies as you move the az,za and repeats for the same az,za  so it is fixed to the ground. The strongest az points at T8 so is could be something at the tiedown but I don't know of any locked signal (other than the 1 second tick) that goes to the tiedowns. The az direction of 240 degrees should not be relied on since the actual scattering path into the dome varies a lot.
processing: usr/a1400/28feb02.pro
• 01mar02 azimuth swing with 1400 Mhz birdie (rfimtop)
    On 01mar02 I did an azimuth spin with the dome at 18 degrees za ]moving at .4 degrees/second. The azimuth spin plot shows that the peak strength is in the azimuth direction 340 degrees (toward the control room). This is the same direction found for the peak of the 1350 Mhz birdie.
processing:x101/020301/rfi1400.pro
• 09apr02 rfi from color cameras in the dome. (rfimtop)

    Rfi at 1417 Mhz appeared in early apr02. Turning off the color cameras in the dome made the birdie go away. On 09apr02 the 6 cameras in the dome were turned off 1 at a time (by disconnecting the power to the camera). The correlator was run with a 6 Mhz by 2048 channels setup with 1 second dumps on lbn receiver. The plots show how the rfi at 1417 changed as each camera went on off.
• image of spectral density as cameras turned on off. The cameras went onoff at:
• 30-65 camera 1 on, off (on the stairwell).
• 130-160 camera 2 on, off.
• 175-190 camera 3 on, off.
• 220-255 camera 4 on, off.
• 312-345 camera 5 on, off.
• 382-412 camera 6 on, off.
• 525-560 camera 1 on,off.
• 600-630 camera 2 on,off.
camera 1 is causing the problem. The off glitches at 100 and 460 were caused by camera 1 being turned off to get at the cord for camera 2.
• time series for single correlator channel at 1417.49 Mhz. This show the vertical line from the image for the single channel (width=6Mhz/2048). Black in polA and red is polB. The turning on/off of the cameras is in green/blue. The vertical scale units is tsys (approximately..). The record numbers here are 281 counts larger than those in the image. The birdie is up to 20% of Tsys.
• Camera 1 was turned off and then 4 min on,off position swithing was done on blank sky. The cameras (less camera 1) were turned on for the on position and off for the off position. 3 of these on/off pairs were done. The onoff position switch plots show the individual on/off -1  in  fig 1-3 while figure 4 is the average of the last 2. The first on/off position switch accidentally had the cameras turned on then off during the on position for a few seconds. This caused a noticable birdie. The vertical green lines are the 14.3341 Mhz comb measured 20sep01 before shielding. The baselines are lousy since this was done at 5pm when the standingwaves from the sun are bad. Even with the lousy baselines it looks like the cameras are still making birdies at about .2% of Tsys. This is .002*30K=30 milliKelvin. This was not seen on 20sep01 because the intergration wasn't long enough
The problem with camera 1 was not there on 20sep01 so something must have changed since then. Maybe the shield on the cable that goes out to the camera has a break in it somewhere. Camera 1 should be left off until it is fixed. We also need to do a better job of shielding the other cameras.
    Note: 30jul02 the gasket material used to make good contact for the doors of the camera box degrades with time. see 2 mhz comb at 327 Mhz: resolution.
• 28apr02 Distomat biride at 1399.83 (rfimtop)

    Experiment A1475 (using lbn) had interference at 1399.83 with a 1 minute period. It repeated  in the on position and the off position. This occurred in multiple scans that were close to azimuth 175 to 191 deg. At the time the distomats were being run on a 60 second cycle. The rfi window had been installed on the distomats months ago. The plot shows the power in a 12 Khz channel at 1399.83 Mhz (black) and two adjacent channels 24 Khz away (red,green). The rfi is strongest in polB (upper plots). The data was sampled with 6 second intregations.
processing: usr/a1475/distomat.pro
• 24may02 Color camera birdies still present.  (rfimtop)

   The dome color cameras were checked on 24may02. Camera #1 that caused the problem on 09apr02 was not installed (so there were only 5 cameras). The setup used a 390Khz setup with 190 Hz resolution. There was a band  at 1388.86 and a second band at 1417.5. lband narrow was used while tracking blank sky with the  radar blanker on. In the test all of the cameras were turned on/off using the downstairs switch.
    The first plot shows 200 seconds cameras on followed by 200 seconds cameras off. 10 20 second integrations were done with the camera on followed by 10 20 second integrations with the cameras off. The black trace has the cameras on while the red trace is the cameras off (both figures are polA). The birdie at 1388.858 went away when the cameras were turned off. The smaller birdie at 1388.86 was there all of the time. The lower figure shows the birdie at 1417.495. The smaller birdie at 1417.482 was only in one of the 20 records. The birdie was not resolved with the 190 hz resolution. The signal strength in the 190 Hz wide channels is about 23% of Tsys or (.23*28K)=6 kelvins.
    The second image shows 1 second dumps while the cameras were turned on and off. The horizontal axis is frequency while the vertical axis is time (in seconds). The dashed horizontal lines are where the cameras were turned on or off. The cameras had been on for awhile before the test started. You can see that when the cameras were turned on, the birdie drifted by about 2 Khz. After 100 seconds it had still not come back to the frequency we started with. This is probably a temperature related drift when the power gets applied.

       Two birdies were found: 1388.858 and 1417.495 with a spectral width < 190 Hz. This would be a comb frequency of 14.3185 Mhz (corrsponding to the comb measured before shielding..see 20sep09). The other members are probably also present. I had initially used 25Mhz/1024lags =  25 Khz resolution to look for the birdies. With this setup you couldn't see these birdies in 1 second dumps. The frequencies drift up to 2 Khz when there power if first applied.  When debugging this problem you will need to use the high frequency resolution.

processing: x101/020524/colorcam.pro
• 18sep02. Measuring the power levels at the output of the lbw dewar.   (rfimtop)

    The power was measured at the output of the lbw dewar on 18sep02. The azimuth was stationary. Peak holds were done at 1300 Mhz 1Ghz bw and 1400 Mhz 400 Mhz bw.

• The peaks 1,2,3,4 are related to the cellar phone bands. This rfi is the reason you must run with at least the 1100 -1800 Mhz filter between the dewar and the postamps.
• Peaks 5,6 are the aerostat radar balloon.

• Peaks 1,2 are the aerostat radar.
• Peaks 3,4 are the san juan airport radar. They are about -45 dbm. In the 1 Ghz scan they were -68 dBm. This illustrates the problem in using a swept oscillator system to measure a pulsed radar. The radar has a 1 usec pulse, ipp of about 2 millisecs, and a rotation period of 10 seconds. In the 1 Ghz plot the spectrum analyzer was not over the 1330 freq bin when the radar was pointed at us and pulsing.
• Peaks 6,7,8,9,10.. are the punta salinas radar.

 
Compare these plots with the data taken may01 from after the lbw dewar. This data was taken with a 30Khz Resolution bandwidth filter. The radar bandwidths are  .2 to 2 Mhz so this setting undersampled the radar power by up to 1.6Mhz/30Khz= 55 or 17 db low.
 
• 04apr03: Radar power levels in the downstairs IF (if2)   (rfimtop)

The system was being driven far  into saturation so that the gain was decreasing during the radar pulses. I wanted to measure the downstairs IF power levels  when the radars were present to see if it was happening downstairs or upstairs. I set the power levels into the square law detector so that the largest pulsed from the radar would not clip. This meant that you could not see the off pulse noise floor.

Setup:

Data was taken with the correlator and the radar interface during the day on 04apr03. The telescope was parked at za=8.48, az=270. The new lband wide receiver was used in linear polarization mode with the 1100 - 1800 Mhz filter in and the 750 Mhz IF.
 

Upstairs power level (IF1)	-40 dbm on meter  -20 dbm into FO xmter
Upstairs attenuators	rf: 6 5 db if: 0 0
Downstairs  power level (IF2)	-52 dbm meter -32 dbm before coupler
IF2 attnenuation	-7 ,-9  or 3db ,1db + 10db amp
output power IF2 FO RCVR	-32   at meter coupler -26 before 4 way splitter -34 before gain/attn -52 before 18db amp (output FO RCVR)
output power bottom IF2	IF2  FO rcvr out + 22 db
square law detector	2 usec time constant
op Amp after sqrLaw	gain 5
8 bit digitizer	0 to 2.5 volts sampled at 1Mhz

The 1st sampled pair of 500 MHZ bandpass was taken from the 750 Mhz front panel output. This had a 20 db pad so the power level was the same as that measured by the power meter. It was sent to a square law detector and detected with a 2 usec time constant. This voltage was multplied by 5 in an opamp and sent to the 8 bit A/D converter (0 to 2.5 volts full scale).  The 2 signals (polA,polB) were sampled at 1 Mhz.

The noise power levels into the square law detector were -52 dbm so we could only see when the radar was on and pointing at us. This insured that the a/d square law were not saturated.

A second set of signals were taken from the bottom of the IFLO buffer amps. The output power of the buffer amps is 14db + gainUsed greater than the input power. The 8db gain used set the output power 22 db above the input power. The data was taken out of the front panel input, passed through 23 db of attenuation (to put the levels close to the first pair of signals) , detected,  and then sent to the AtoD converters.

The correlator ran at 1 second dumps, 3 level interleaved sampling 1225 to 1425 Mhz. This let us identify which radars were active.

Calibration:

To calibrate the system, a -20 dbm and then -17dbm tone at 750 Mhz was injected into the IF/LO at the transfer switch input (right after the FO rcvr output). This data was recorded and used to map the a/d levels into dbm in the downstairs if/lo. The power at the output buffer amps of the IFLO were 22 db above the input. The -20 dbm level would be + 2db output at the buffer amps.

The radars seen:

Three radars were seen: 1290 Mhz remy radar, 1330/1350 FAA airport radar, and the punta salinas fps117 frequency agile radar (the aerostat was not broadcasting). Each of these radars has a 12 second rotation period.  Three distinct peaks could be seen in any 12 second section of data. They occured when the radar pointed in our direction. The radar peaks were identified in the total power data stream by their  radar ipps (since they were known ahead of time).

The plots show polB,polA from the upper section of the iflo rack and then polA polB from the buffer amplifiers. A gaussian is fit to each sweep of the radar to measure the beam width as it passes in front of the observatory. The second page shows a single sweep (rotation) of the radar, ipps, and a single pulse.

• 1290 Mhz Remy radar.

This is a single pulse (5 usec), single ipp (2781 usecs) radar with a rotation period of 12 seconds. The power level gets to -17 dbm at the input. This is 5 dbm output at the buffer amps. Ther is compression in some of the sweeps since the *'s  sit below the gauss fit.
The second page shows the largest sweep (polB top of iflo). The single pulse does not show any negative going gain.
• 1350/130 FAA airport radar.

This is a single pulse (1 usec), 5 ipps cycle, dual frequency (1350,1330) radar with a period of 12 seconds. The 1350 pulse is sent and then  8 usecs later the 1330 usec pulse is sent. This radar got close to the -17dbm power level (+5 dbm at output of buffer amps). The top of the stronger sweeps (black,yellow) seem to be flat topped a little so it has probably gone into compresssion. The 6 pulse plot  shows the 5 different ipps. The single pulse has the two frequencies smeared together with the multi path scattering. There is no negative gain.
• Punta salinas frequency agile radar.

This is a frequency hopping radar with 18 channels. Each channel has a long pulse (400 usecs two frequencies) and a short pulse (51.2 usecs two frequencies). It is hard to charaterize the ipps because of the frequency hopping. They supposedly blank the radar when it points in our direction.
• Fig 1 is a single sweep of the radar past the observatory. The power levels reach the -20 dbm iflo input level (buffer amps +2 dbm). The four strong pulses in the center are the 400 usec long pulse. The smaller closer spaced ipps I think are the 50 usec pulses. The 50 usec pulses look like they are sweeping out a beam, but then they disappear. I don't know if this is blanking or what. The two levels we see with the 50 usec pulses are probably different frequencies.
• Fig 2 shows the sweep for polA top of iflo. It then plots the 4 400 usecs pulses and then a single 400 usec pulse. The power definitely goes down during the pulse. I don't know whether this is a part of the transmission or we are seeing some negative gain here. The bottom two plots show the 50 usec pulse. It should be stronger than the 400 usec pulse (higher power for shorter period of time) but it isn't. The single pulse last longer than 50 usecs. I don't know if they are transmitting the two frequencies staggered in time or not.
• Fig 3. This is a single 51.2 usec pulse. It is 102 usecs long so there are probably two pulses here (the dual frequency). All four of the outputs show a dip in the center of each pulse. The power levels in the downstairs iflo are all less than -17 dbm input or +5dbm at the buffer amps. If this is saturation and negative gain, then it must be occurring upstairs.

Conclusions:

The power levels from these radars are not close to the 1db compression level of the system (+16dbm for the amplifiers at the end). There is some compression occuring so it must be upstairs. The punta salinas 51 usec pulse may be going  negative. If this is true, it must be happening upstairs since the downstairs levels have not arrived at saturation.
    On 23apr03 we placed a 20 Mhz IF filter centered at 1435Mhz before the fiber optic transmitter. We  saw saturation with the 1350 FAA radar. We placed a 1435 rf filter before the  mixer and saw no saturation. This shows that the compression is occurring in the 750 Mhz mixer chassis.

• 19sep03 iridium signal stability. (rfimtop)

    Experiment a1840 dumped 100 Mhz bands every 2 milliseconds with the bands centered at 1370,1470,1570, and 1670 Mhz while looking at interplanetary scintillation's. There were 256 channels per board giving 390 Khz resolution. 14 300 second integrations were done over two days. I used this data to see how stable the iridium signal was. The signal has a 90 millisecond period with 45 milliseconds for downlink ( more iridium info).
    The band of interest were 1621 to 1626 for iridium and 1610 to 1613 for the radio astronomy band. Unfortunately the observer setup the filters so they abutted at 1620 Mhz. The plots show the iridium signal properties..
• Fig1  For each 300 second integration the rms/median() was computed for each frequency channel in the 1600 to 1630 Mhz range. The top plot is the rms/median() by channel for the 14 scans (300 sec integrations). Each strip has been offset for display. The middle plot is a blowup with no offsets. The iridium signal is clearly seen in the 1620 to 1626 Mhz range. The bottom plot is the median bandpass for each of the 14 scans. You can see the filter falloff in the middle.
• Fig2 The spectral data was bandpass corrected by scan (using the median bandpass) and then the total power over the range 1621.35 to 1626.35 was computed for every 2 millisecond sample. The total power data from each scan was  folded with a 90 millisecond period. The starting time for each scan relative to the first scan was then used to phase correct each scan to the first scan. The data was then shifted so that the off period started at T=0. The top plot has the folded total power in the iridium band plotted versus the 90 millisecond period of iridium. Each scan has been offset for plotting purposes. You can see a drift of about 4 milliseconds from the start to the end of the data set. For each scan there is about 42 milliseconds with no iridium signal. The bottom plot is the same data but the y axis is now the hours since the start of the first scan. You can see how the signals drifted over time.
• Fig 3. The total power was also computed in the radio astronomy band (1610.6 to 1618.8) and processed the same as the iridium data. The top plot is the 14 scans of data. The dashed green lines show were the strongest iridium signal sat. The bottom plot has the 14 scans averaged together.  There is no obvious difference between the iridium on and iridium off period.

 
The satellite signals look pretty stable with time. We can probably compute the phase of the start of the 90 millisecond cycle and then use the sps to generate a blanking signal. The blanking duration will probably have to be close to 50 milliseconds. As a double check, i need to find someone else's data and see if the phase of the signal from this data set continues to predict the phase of the iridium on cycle.
processing:usr/a1840/iridium/iridium.pro,irplot.pro

 
• 14oct03 puntaSalinas? birdie during a1803 experiment.  (rfimtop)

    Experiment a1803 has been having problems with radars at 1289.7 and 1298.7 Mhz. I took some data during the day on 14oct03 using lbn, the 1280Mhz hipass filter in, 25Mhz bandwidth with 1024 channels, and 209 1 second dumps. I covered 1278.5 to 1322.5 Mhz.
• Fig 1 below is a dynamic spectra image of the 209 secs for the two 25 Mhz bands.

    You can see the 12 second rotation period of the radar. There are two sets spaced by 15 Mhz.
• Figure 2  Is a peak hold of the 209 second records with a bandpass normalization of the median bandpass for the 209 seconds. Black is polA and blue is polB. The dotted lines show the paired birdies that are 15 mhz apart (Red and green).

 
    The 15 Mhz spacing looks like the punta salinas radar.  It also has a 12 second period like the punta salinas radar. It could be punta salinas channels  9 and 10 if their reference frequency was off by 3 Mhz. The bandwidth is a bit narrow but we may only be seeing the long range pulses (which are stronger and narrower in freq). I did not get a chance to measure the pulse duration of the signal. It went off of the air around 15:30. The temporal duration of the pulse should tell us if it really is a fps117 radar. By the way, I've notices that the faa airport radar has not been transmitting simulataneously at 1330 and 1350 Mhz (just one or the other). I wonder if they have changed their radar???
processing: x101/031014/doit.pro
• 13oct03 Color camera #5 birdie at 1417  (rfimtop)
On 16sep03 the dome camera #5 (looking at the klystron tube) showed a birdie at 1417.5 Mhz. The camera was removed and worked on in the lab. On 13oct03 it was reinstalled and tested using the correlator. The setup was lband wide, 2048 channels over 195 Khz (95 hz resolution) , with 1 second dumps. The image of the dynamic spectra  show the results.
• The horizontal dashed lines show (approximately) when the power to camera 5 was switched on and off. The vertical axis is 1 second integrations. The image has been normalized by the mean bandpass. The strongest birdie is at 1417.495 Mhz. When it turns on there is a frequency drift as it warms up. The comb is spaced by 15.85Khz which is very close to the 15.75Khz line rate of standard ntsc video. From record 240 to 330 camera 5 was off and all the other cameras were on so it looks like this is the only bad camera.

 
The camera was removed and returned to the lab for more testing. The question is how is the rfi getting out of our rfi shielded enclosure for the camera?
processing: x101/031013/camera.pro
• 13oct03 1422.5 Mhz birdie from tv channel 54.
A test was done with tv channel 54 to see if it was still generating birdies. The video carrier is at 711.25 Mhz. The 2nd harmonic of this falls at 1422.5 Mhz. A guard at the transmitter site opened and closed the transmitter doors while we were taking data. He also turned the transmitter on and off.  The lband wide receiver (Tsys about 28 Kelvins) was used in circular polarization mode. Data was taken with the interim correlator running in two modes:  381 Hz  and 95 Hz resolution. Data was dumped once a second. The plots show the results of the measurements.
• Fig 1(polA)  and Fig 2(polB). The telescope was sitting at an azimuth of 192 degrees and a zenith angle of 18 degrees. Each plot is a spectral average over time. The y axis is a fraction of Tsys (about 28K). You can see the 1422.5 birdie. It was not resolved in frequency by the 380 Hz channels. The top plot has the transmitter doors open. The second plot has the transmitter doors closed. The 3rd plot has the transmitter off. The bottom plot has the transmitter turned back on. You can see the birdie at 1422.5 with the doors opened and closed. It goes away with the transmitter off. This az,za was not the position of maximum signal strength (see fig 3).
• Fig 3 shows the azimuth dependence of the 1422.5 birdie (with the za=18degrees). The transmitter doors were open for these measurements. The 95 Hz resolution was used for these measurements. The single channel at 1422.5 Mhz was plotted. The top 2 plots show the birdie strength versus azimuth (black and blue traces). The red trace is an average of the system temperature close to this frequency channel. The 3rd plot shows the fractional strength of the birdie (birdie/Tsys) - 1 versus azimuth. The bottom plot blows up the azimuth around 245 to 265 degrees.  There is a maximum around az=258 degrees. It gets up to about .6 Tsys.
• Fig 4 has the telescope parked at az=257.2 and za=18 using the 95 hz resolution channels. The data is plotted as a fraction of Tsys versus time. The data samples have been smoothed to 5 secs.  At the beginning, the transmitter doors were open. At second 340 the inner doors were closed. The outer door of the transmitter building were shut around 350 seconds. There is no noticeable drop in the birdie strength with the doors closed.
A 1422.5 Mhz birdie is seen from the channel 54 transmitter. It is less than 95 hz wide. It has a maximum strength close to az=258,za=18 deg. Closing the transmitter doors makes a difference when the telescope is not in the maximum strength position. When the telescope is sitting close to the maximum strength position , closing the transmitter doors make little difference.
processing: x101/031013/yiyi.pro
• 28may04 1400 Mhz birdie.

    Project a1861 had a birdie at 1400. Mhz. Lband wide was used in linear polarization's with the 1370 wave guide filter in. They did 5 minute on/off position switching. The correlator was setup for 2048 channels over 25 MHz and 1 second dumps. The birdie was less that 1 channel wide (12.2 Khz). The plots (ps)  (pdf) show the birdie during 2 on/off patterns (300*2*2=1200 samples).
• Fig 1 plots the 1200 samples. The top plot is polA and the bottom plot is polB. The black lines are the birdie at 1400 Mhz (1 channel of 12.2 khz). The red lines are an average of 6 adjacent channels. The step every 300 samples is because the source had some continuum flux. The birdie is stronger in pola.
• Fig 2 plots (1400 MhzChn - avg6adjacent_channels) versus azimuth. Since position switching was done, the telescope tracks the same part of the dish for the on position and the off position. The black lines are on source while the red lines are off source. The top plot is polA while the bottom plot is polB. The strength of the birdie repeats with the telescope position. This means that the birdie is not coming from within the dome. The strongest position was close to an azimuth ot 206.2 degrees azimuth (the za was 11.945 deg).
• Fig 3 plots the birdie strength versus time. The top plot has polA in black and polB in red. There are 6 minutes between the two peaks (5 minute on with a 1 minute move). The peaks in polB (red) are occurring at 50 or 100 second intervals. The bottom plot is a blowup of one of the peaks in polA. It has a gaussian shape and lasts for about 25 seconds.
There is a distomat located at an azimuth of 207 degrees (distomat 4). Normally the distomat birdies turn on for a few seconds every 120 seconds and then turn off. This birdie looks like it is on all the time.  The blowup of the polA birdie shows that it is the beam sweeping thru the birdie that makes it go up and down, and not the distomat turning on and off (that would not give a gaussian profile). We need to look at this with higher resolution.
processing: usr/a1861/biride1400_may04.pro

 
• 28may04,07jun04 1374.495 Mhz birdie. (rfimtop)

    Project a1861 had a birdie at 1374.495 Mhz. Lband wide was used in linear polarization's with the radar blanker enabled.. They did 5 minute on/off position switching. The correlator was setup for 2048 channels over 12.5 MHz and 1 second dumps. The birdie was less that 1 channel wide (6.1 Khz). The plots (ps)  (pdf) show the birdie during 4 on/off patterns (300*2*4=2400 samples).
• Fig 1 plots the 2400 samples. The top plot is polA and the bottom plot is polB. The black lines are the birdie at 1374.495 Mhz (1 channel of 6.1 khz). The red lines are an average of 6 adjacent channels. The step at 600 sample boundaries are the start of new on/off pairs at different za's (and possibly different correlator attenuations).
• Fig 2(polA) and Fig 3(polB)  plots (1374.495 MhzChn - avg6adjacent_channels) versus azimuth. Since position switching was done, the telescope tracks the same part of the dish for the on position and the off position. The black lines are on source while the red lines are off source. Each frame (4 per page) is a separate on/off pattern. The strength of the birdie repeats with the telescope position. This means that the birdie is not coming from within the dome. It also probably means that is ground based (unless it is a geostationary satellite). The average za's for the for on/off patterns are: 14.5, 12.4, 11., and 11.2.
• Fig 4 plots the birdie strength versus time. The top plot has 1000 seconds with   polA in black and polB in red.  The bottom plot is a blowup m(150 seconds)  of polA. The peaks look like sidelobes drifting through the interference. They last from 20 to 25 seconds. To move through the 5 peaks takes about 26 seconds. ian shape and lasts for about 25 seconds.

 
This birdie is coming from outside the dome. The birdie strength correlates with azimuth  position so it is probably ground based. It does not correlate as well as the 1400 Mhz birdie above and does not line up with the distomat locations.

On 07jun04 base band sampled data with 10 Khz bandwidth was taken to see the high resolution structure of this birdie. The image (.gif) shows dynamic spectra for 500 seconds. The bandwidth is +/- 245 Hz about 1374.494794 Mhz. The frequency resolution of the image is .6 hz. The  top plot is polA while the bottom plot is polB (lbw was set to linear polarization mode). The signal is drifting by about 5 hz over 500 seconds. The sidebands of the signal are spaced by 60 Hz. The instantaneous width of the line is less than 1 Hz wide.

processing: usr/a1861/biride1374_may04.pro, x101/040607/doit.pro

 
• 11jun04 drifting harmonics in lband with 60 Mhz spacing    (rfimtop)

 
• 30may04: distomat birdies still there after converting to fiber communications. (rfimtop)

The communications between the distomats and the control room were converted from copper to fiber in xxxx 04. On 30may04 I  checked to see if the distomat birdies were still present after this conversion.
    Lbw in linear polarization mode centered at 1400 Mhz with 48 Khz resolution (after hanning smoothing) was used. 180 one second records were taken while pointing at each of the 6 distomats (the dome was at 18 degrees za). The set of birdies used was at 1399.83 and 1400.00 Mhz. The distomats were running on a 60 second measurement cycle. About 10 seconds after the minute the distomats begin their distance measurements. The data was synchronized to the start of the 60 second cycle. The plots show the current state of the birdies:
•  (polA) (PolB) dynamic spectra  (time vs frequency of spectral density). The 6 images correspond to the 6 distomats. Horizontal dashed lines are drawn at the start of each 60 second cycle. For polA you can see the birdies at distomat 4 (1399.83) and distomat 6 (1399.83 and 1400). For polarization B you can see the birdies at distomat 4 (1399.83,1400) and distomat 6 (1399.83, 1400). This spectral resolution is a lot wider than the distomat birdie so it is probable that you could see the birdies in the other distomats if narrower channels were used.
• Time series for 48Khz channels at 1399.83 and 1400 Mhz. These plots shows the time series for a single frequency channel. The top plot is 1400 Mhz, the bottom plot is 1399.83 Mhz. The black color is polA while the red color is polB. For each plot the distomat values are number 1 (bottom) to 6 (top). The offsets have been added for display purposes. The vertical dashed blue lines are the start of each 60 second cycle. You can see the birdie increase a few seconds after the start of each cycle. The vertical scale in Tsys units. The strongest birdie is distomat 3 polB where it gets up to .15 Tsys  in the 48 Khz channel width.
The conversion of the distomat communications had little affect on the distomat birdies that occur during the measurement (see 28apr02 measurements).
processing: 040530/doit.pro

 
• 29jun04:  birdies around 1400 Mhz during azswing.  (rfimtop)

Birdies close to 1400 Mhz were looked at with high spectral resolution during an azimuth swing. The correlator was set to 190 hz resolution (after hanning smoothing). The telescope was swung from az=540 to az=180 and then back at .4 degrees per second. Data was dumped at 1 second intervals. Lbw was used in linear polarization mode with the 1370 hipass filter in. The two az swings took about 2000 seconds.
• The image of dynamic spectra shows the time variation during the swings. The x axis is KHz from 1400 MHz. There are strong birdies at 1399.987 and 1400.00 Mhz. A more intermittent birdie is at 1400.009 Mhz.
• The power in the birdie channels versus azimuth shows how the signal varies with azimuth. The three plots are for the frequencies 1399.983,1400.000 and 1400.009. The data covers 190 hz. The black plot is the counter clockwise spin while the red plot is the clockwise spin.
• Figure 1. This has the complete azimuth swings. The black and red lines overlay each other so the signals are not coming from inside the dome. The first birdie is strong at 0 and 180 degrees azimuth. The second birdie (1400) peaks at -20 degrees azimuth. The 3rd birdie peaks around an az of 145 degrees. The 1400 birdie is 30 times the system temperature (in a 190 Hz channel).
• Figure 2: This is a blowup of the horizontal scale about the strongest peak for each birdie. The first two birdies overlap exactly in azimuth (comparing the clockwise and counter clockwise spins). The 3rd birdie has an offset of a few degrees between the two spins.
• Figure 3: This plots the power in the birdie versus time (minutes from midnight). The green lines at the bottom of the plot mark the even minutes. 7 seconds after the even minutes, the distomats begin their measurements. You can see that the bottom birdie aligns with these even minutes so it is coming from the distomats. The az of 140 is distomat 3. There is a beating between the position of the azimuth and the distomat turning on/off that make us loose some of the distomat birdies from the other distomats (seedistomat az swing).
Conclusions:
• there are 3 distinct birdies around 1400 Mhz. Previous measurements with wider resolutions have missed this.
• All three birdies are coming from outside the dome.
• The strongest birdie is at 1400 Mhz. It is 30 times Tsys in a 190 hz channel and has maximum strength at an az of -20 degrees. It was not resolved in a 95 hz channel spacing. This birdie is coming from the 100 Mhz buffers in the controlcorrelator  room. Mainly the wapp's who are running at 100 Mhz.
• The second birdie at 1399.983 is about 2*tsys in 190 hz channel and peaks up at 0 and 180 degrees.
• The 3rd birdie at 1400.009 is from the distomats (in this case distomat 3).

 
• 02jul04: 1400 Mhz birdie coming from 100 Mhz distribution/wapps.  (rfimtop)

The 29jun04 measurements showed that the 1400 Mhz birdie was not coming from inside the dome. On 02jul04 eddie castro and myself looked in the control room with the new tektronix spectrum analyze (ybt250) with 100 hz resolution. The birdie was 20 to 30 db above the noise floor (100 hz resolution) in the receiver room. The signal got to 50 db above the noise floor in the wapp,correlator room. We did the following tests:
1. We unlocked the phase lock loop for the 100 mhz distribution. This caused the birdie to drift by many khz.
2. We pulled the power on the 100 Mhz distribution and the birdie in the receiver room went away.
3. There were 3 cables leaving the 100 Mhz distribution: correlator, receiver room, and wapps. We replaced the 100 Mhz phase locked reference with a 100 Mhz from an hp synthesizer for these 100 Mhz (one at a time). We monitored the birdie in the receiver room while we did this. The portion of the 1400 Mhz birdie that came from the "different" 100 Mhz reference would jump be a few 100 hz (since the hp synthesizer was not synched to the maser). Of the 3 signals, the wapps had the strongest signal.
It's not surprising that the wapps had the largest contribution to the 1400 Mhz birdie since all of the correlator chips are running at 100 Mhz.

We spent some time looking at the strength of the signal outside in front of the control room. The results were:

• The signal was barely visible (2-3 db above the noise floor) in front of the main windows. The shielding placed on the windows is really working.
• The signal is 10 db above the noise floor in the office to the left of the control room (hectors office).
• At the entrance to the pathway between building 1 and the cliff, the signal is 30 db above the noise floor. Walking down the pathway, the strongest signals were coming from the plywood around the air conditioners that were installed through the building walls. This hole is breaking the faraday cage that encloses the floor, ceiling, walls of the correlator/computer rooms. Some signals are probably also coming from the doors/windows that are also not shielded.
• The other 100 Mhz harmonics are also visible. The odd harmonics are stronger (1300,1500..)
• The azimuth spin looking at this birdie shows how a birdie from the control room should vary as a function of azimuth. Any other birdies with this azimuth dependence most likely come from the control room.

 
• 28sep04: azimuth dependence of faa radar signal using alfa receiver.  (rfimtop)

     The faa radar specs are:
•  5 usecond pulse
• 2.5 millisecond prf (pulse repetition frequency)
• 12 second rotation period.
• It transmits at 1330 and 1350 Mhz every prf (one pulse at 1330 followed by 1350).
     When the radar beam points in the direction of the observatory  there is a large signal at 1330 and 1350 Mhz. Frequencies outside the 1330/1350 range go into compression (they decrease in power). The depth of the compression can be used to measure the strength of the faa radar in the receiver system.
    On 28sep04 an azimuth swing was done from az=270 to az=630 degrees. The azimuth moved at .3 degrees per second. The dome was at 18 degrees za. A 100 Mhz band  centered at 1420 Mhz was sampled every 64 useconds using the alfa receiver and the wapps. The alfa rotator was set to 0 degrees. The processing steps were:
• Compute the total power for each time sample.
• Remove Tsys by normalizing to a 1 second median and then removing it.
• Plot the data and find all the points that are 10% below Tsys (100Mhz and 64 usecs gives a deltaTsys of .0125).
• Knowing the 5 periods of the FAA radar find the phase of the faa compressed signal in the data.
• compute the "best" rotation rate for the radar so the 12 second pulse remain remain stationary for the az spin time.

 
The first plots shows power versus time when the faa radar pointed at the observatory (.ps)  (.pdf) (using pixel 4 of the alfa receiver).
Each line has 120 milliseconds centered on when the radar pointed at AO.  Successive plots have been offset for plotting purposes.  While this data was being taken, the azimuth was moving at .3 degrees/second. You can see that sometimes the compression was large and sometimes it is hard to see.  The successive spikes are the radar pulses very 2.5-3 milliseconds. It takes about 80 milliseconds for the faa beam to sweep by the observatory.
The second plot shows the compression versus azimuth angle (.ps)  (.pdf) .
The 7 plots are for the 7 beams of alfa. The black color is polA and the red color is polB. The farther negative the plot, the stronger  the compression. This is a linear scale. When the value reaches -.4, the system temperature has compressed by 40% (Tsys is now 60% of the original value).  You can see large values of compression near az=10,40,80, 340 degrees azimuth in most of the beams.
processing:040928/inputtp.pro,doproc.pro,mkplot.pro
 
• 06oct04: faa,punta salinas, and aerostat Radars. total power versus time.  (rfimtop)

On 06oct04 data was taken with pixel 2a,2b of the alfa receiver. The band was centered at 1410 Mhz. The front panel outputs were passed thru attenuators (pix2a 15db, pix2b 0b) and then detected with a 20 usecond time constant. The signal was then sampled at 10 usecond sampling for 48 seconds. The telescope was parked at az=270, za=1.9 degrees.
    The attenuated version was to let us see an unclipped version (in the A/D's) of the radar signal when it pointed at the AO.  The unattenuated version was to see detail when the radar was not pointed directly at us. These plots can be used as a reference when someone is looking at  a square law detected signal on the oscilloscope in the control room.

The total power versus time for the 48 seconds (.ps)   (.pdf)
    A peak hold on each 10 milliseconds of data was done to decimate the data for these plots.

• Top: The un attenuated version of the data. Dashed colored lines have been drawn where the faa, punta salinas, and aerostat radars appear. The red dotted lines are the sidelobes of the aerostat as it spins around (when the aerostat points at AO, it blanks its transmitter).
• Bottom: This is the data with the 15 db of attenuation in.


The Faa radar power versus time.   (.pdf)
    The FAA radar at 1330 and 1350 Mhz was flagged with the green lines in the first plot. It has a 5 useconds pulse  at 1330 Mhz followed by a 5 usecond pulse at 1350. This gets repeated every ipp useconds. There is a sequence of 5 ipps that are used by the radar (2500 to 2500 useconds). The radar does not blank when it points at the observatory.

• Fig 1: This has 120 milliseconds centered on when the faa radar pointed at the observatory.  You see the beam sweep by the observatory. This repeats every 12 seconds (11.985 seconds seemed to give the best alignment of the 4 beams). The FWHM of the beam is about 50 milliseconds. This translates to a beam width of 1.5 degrees (.05/12 *360). This data was taken from pix2a with the 15 dbs of attenuation. The red and green dashed lines show the data that is used for figure 2.
• Fig 2: The top figure shows 6 ipps. The measured spacing (with 20 Usec time constant) is close to that measured with  higher time resolution (i'm calling it the exact version here). The bottom figure is single radar pulse. It has been smeared out by the 20 usecond time constant.

 
The Punta Salinas frequency agile radar.  (.pdf)
    The punta salinas radar was flagged with the blue lines in the first plot. It is a frequency agile radar (more info..). It transmits two frequencies simultaneously and can hop between up to 20 different pairs. On each transmission there is a narrow strong pulse and a wider weaker pulse for the short range and long range detection. The rotation rate of the radar is 12 seconds. They  supposedly blank in  the direction of the observatory.
• Fig 1: Shows the 4 times when the radar pointed at the observatory.  There looks like there are two separate beams in each plot. The central blanking last for 40 milliseconds. The beam width is about 100 milliseconds min to min (looking at the 3 sweep). The third picture also shows that there are two blanking periods. During each blanking period all of the pictures show two strong pulsed during the blanking period.
• Fig 2: The top plot is the first sweep of fig 1. The colored lines show that data used for the following plots. The second plot has the two strong pulses during the blanking period. The first pulse is 800 useconds wide while the second is 100 useconds. They are separated by 4.2 milliseconds. The 3rd plot shows 11 consecutive pulses. Each of them are spaced by 1420 useconds. Using the entire data set, ipps of [1490,1340,1670, 1420] useconds were found. The bottom figure is a single pulse which is about 100 useconds wide. There is supposed to be a 52 useconds narrow pulse and a 400 usecond long pulse. The measured widths may be from the time constant or two pulses following one another. The 800 usecond long pulse in the middle of the nulling period is probably two of the 400 usecond pulses.


The aerostat balloon radar.  (.pdf)
    The aerostat balloon radar flies above lajas (more info..)  . It has 4 frequencies it can transmit on (1241, 1244,1256, and 1261). It usually transmits on a pair of these. The pulse length is 160 useconds long with a band width of 1.6 Mhz (it is chirped).  The two frequency pulses are transmitted back to back giving a 320 usec long transmission. There are 7 ipps that are cycled through (2800 to 3800 useconds). The rotation rate for the radar is 12 seconds. It blanks for about 1.4 seconds (42 degrees of azimuth) while it swings by the observatory. There are strong back lobes when the radar is at +/- 110 degrees from the observatory direction.

• Fig 1: The first figure shows the entire 48 seconds. The red lines are where the aerostat points at the observatory (and is blanking). The green lines are 108 degrees before the observatory direction while the blue lines are 115 degrees after the AO direction.
• Fig 2: This shows the 4 times the aerostat pointed at the observatory. The blanking lasts for about 1.4 seconds.  The relative size of the signal on both sides of the blanking region shows how well the blanker is aligned with our direction (it looks pretty good here).
• Fig 3,Fig4: These are the signals from the aerostat when it pointing -108/+115 from the observatory direction. Since it is symmetric about the AO position, it is probably from a sidelobe/backlobe of the transmitter rather than a reflection from a tower.
• Fig 5: The top figure shows 8 contiguous ipps. It shows the 7 ipp sequence of the radar. The measured values are within the time constant of the values measured with higher time resolution. The bottom plot shows a single pulse. It is 367 useconds wide. This is close to the expected width of 320 useconds (given the time constant).  The two peaks come from the two different frequencies (160 useconds one frequency followed by 160 of the second).


Summary:

• The time variability of the 3 radars is shown.
• You can use the time variability to distinguish between the different radars:
• The rotation is the same for all 3
• The pulse widths are: 5 usecs faa, 100/400 punta salinas, 320 usecs aerostat.
• The aerostat blanks for 1.4 seconds when it points at the ao.
• The aerostat has back lobes +/- 100 degrees from the ao direction
• punta salinas blanks for 40 milliseconds when it points at the ao. It looks like it has a second blanking period.
• It would be nice to know what exactly the 2 punta salinas pulses during there blanking period are doing.
• The punta salinas pulses were the strongest of any of the other radars at this az,za position.
• 17nov04: 1367/1382 radar.. punta salinas chn 18 left running.

    On 17nov04 project A1961 had two radars at 1367 and 1382 Mhz. They observed during the early morning hours. A peak hold of a 162 second scan showed that this was punta salinas channel 18 which had been left on. The plots show the radar signature (.ps)   (.pdf).
• Top plot: Peak hold by channel for 162 seconds of the wapp data. The black and red lines are polA and polB. The dashed color lines are the frequencies for the long range (blue) and short range (green) pulses transmitted by the radar.
• Middle: A blowup of the 1367 Mhz pulse. You can see the wide frequency, lower power density of the short range pulse, and the narrower frequency higher density of the long range pulse.
• Bottom: A blowup of the 1382 Mhz pulses.
The hilltop monitoring showed that punta salinas was in mode A,B,C and channel 18 on from midnight 17nov04 till about 15:00 17nov04. This only occurred on 17nov04. 16nov04,18nov04 did not have this problem.
processing:  usr/a1961/17nov04_rfi.pro

 
• 26feb05: punta salinas transmitter left at wrong frequencies??  (rfimtop)

On 26feb05 project A2010 was doing 100 Mhz drift scans centered at 1385 using alfa starting at 21:15 ast. Interference that looked like punta salinas radar was seen in the band. Punta salinas was called at 21:25 ast and they claimed they were in mode A (channels 2,3). The plot shows the hilltop monitoring 1325 Mhz band for 26feb05 (gif). Lighter color is stronger signal.
    Each punta salinas channel has two frequencies separated by 15 Mhz. Each channel pair is flagged with a different color.  For most of the day there was power in :
• Chan 3,4 (mode A)
• Chan 6,7 (mode B)
• chan 10,11 (mode C)
• chan 18 (diagnostic channel)
• chan 5 (maybe).
It seems unlikely that punta salinas was in mode a and someone else was using most of their channels...
The birdies at 1330/1350 Mhz are the faa radar. The 1290 signal is the remy radar.
processing: 050226/puntasalinas_050226.pro
• 18nov04: distomat birdies still there after added shielding. (rfimtop)

The shielding for the distomats was worked on an then more measurements were taken. On 18nov04 lbw was used in linear polarization mode centered at 1395 Mhz with 24 Khz resolution. 180 one second records were taken while pointing at each of the 6 distomats (the dome was at 18 degrees za). The distomats were set to run on a 60 second measurement cycle so there were 3 complete cycles of turnon, turnoff while looking at each distomat. About 5 seconds after the minute the distomats begin their distance measurements. The data was synchronized to the start of the 60 second cycle. Birdies were seen at 1400 Mhz. The plots show the current state of the birdies:
•  (polA (.gif)) (PolB (.gif)) dynamic spectra  (time vs frequency of spectral density). The 6 images correspond to the 6 distomats. Horizontal dashed lines are drawn at the start of each 60 second cycle. You can see the 1400 Mhz birdie in distomats 3 and 4 (although 4 is weaker). The spectral resolution used did not resolve the birdie (so they may be in other distomats at higher resolution).
• Time series for 24 kHz channels at 1400 Mhz (.ps)  (.pdf). These plots shows the time series (180 seconds) for a single frequency channel at 1400 MHz. The 6 plots correspond to the 6 distomats. Black is polA, red is polB, and green shows where the distomats turned on. They remained on for about 13 seconds. The strongest signal in polA of distomat 3 where the strength is about 4% of Tsys.
The distomats with the birdies have changed since the may04 measurements.
    On 29mar05 we went down and looked at distomats 3,4,5,6 using the portable spectrum analyzer and a helical lban antenna. With a 50 Khz span we saw the 1400 Mhz birdie in all of these distomats. It was strongest in D3,D5, and D6.
processing: 041118/distomat.pro

 
• 28sep05: 1300,1400 Mhz birdies coming from the wapps.  (rfimtop)

    Narrow birdies have previously been seen at 1300 and 1400 Mhz (more info:  2004). Some of these birdies were found to come from the correlator/wapp room. During the summer of 2005 the shielding of the doors and the windows of the wapp/correlator room was worked on. Measurements were taken on 28sep05 to see any remaining signals.
•  An lband helix antenna and the Tektronix portable spectrum analyzer were used to measure the strength of the signal coming from the clock room (rbw 100 hz, Tsys about 700 K).
• The measurement was made at the entrance to the walkway between the control building and the cliff (next to the operators office).
• The strength of the birdie at 1400 Mhz varied between 20 and 25 db (above the noise floor). In sep04 the value was closer to 30 db above the noise floor (same setup).
• Walking down the walkway (between the cliff and the building) the signal got as high as 40 db above the noise floor.


    Data was also taken with the lbw receiver (in linear pol mode) using  the interim correlator. The correlator was set to 95 hz resolution (190 hz after hanning smoothing) . 2 bands were centered at 1300.001 and 1400.001 (the offset was used to get away from the spike in the center of the interim correlator). To verify that the signal was coming from the wapp/correlator room, the 100 mhz input reference for the wapps was taken from an hp synthesizer. Data was taken with the synthesizer locked to the station clock and also with the synthesizer free running. When in free running mode, the 100 Mhz frequency shifted down by 85.4 hz. This offset got multiplied by 13 and 14 to get to 1300 and 1400 Mhz. The plots show the results of the measurements:

• 1300 and 1400 Mhz birdies when the 100 Mhz reference was unlocked (.ps) (.pdf). Data was taken with the wapps unlocked (red line) , locked (black line) , and then unlocked (red line). Each section lasted for about 60 seconds. The dome was at 19 deg za and the az was at 327.439 deg (this position was a maximum for the 1300 Mhz polA birdie).  When the reference to the wapps was unlocked the birdies moved away from 1300/1400 Mhz. When the reference was relocked, they moved back. When shifting, a residual birdie remained at 1300 Mhz. This is probably because there are other sources of 100 Mhz that we were not drifting (the interim correlator, the 100 Mhz in the dome). Since 1300 is an odd harmonic of 100 Mhz you would expect it to be stronger than 1400 Mhz. The top plots are polA while the bottom plots are polB. The birdie strengths  (in units of Tsys) are:

freq	locked polA/polB	unlocked polA/polB
1300.	3.8/0	.5/.2
1299.999	0/0	2.3/.25
1400.	.2/.2	.05/.08
1399.999	0/0	.05/.0

• The strength of the birdie versus azimuth (.ps) (.pdf): 4 sets of azimuth spins were done over 1.5 hours (colors differentiate the different spins). The first (black) swing was done at .4 deg/sec. The last 3 used .1 deg/sec. The 3rd (green) and 4th (blue) swings went over the same azimuth twice. The za was set to 19 degrees. The vertical scale is linear in power (the baseline would be Tsys).
• Fig 1 top: The 1400 Mhz polA birdie versus azimuth. The max. value  is about 1*Tsys.
• Fig 1 2nd plot: 1400 Mhz polB. The max. value is about 3*Tsys.
• Fig 1 3rd plot: 1300 Mhz polA. The max. value is 4*Tsys.
• Fig 1 bottom: 1300 Mhz polB. The max. value is 4*Tsys
• Fig 2 top: A blowup in az  of the 1300 polA swings 2 and 4 shows that the birdies FWHM are about .8 degrees in azimuth.
• Fig 2 bottom: This shows the platform vertical motion during the measurements. The dashed lines show when each az swing was taken. During the 3rd (green) swing there were no distomat measurements (it was pouring rain).
Summary:
• A large fraction (over 50%) of the  1300/1400 Mhz birdies are coming from the wapps (the correlator chips are being clocked at 100 Mhz).
• The 1300/1400 Mhz birdies maximums do not come at the same azimuth position.
• Within say 1/2 hour the azimuth positions of the peaks are repeatable.
• The maximum 1400 Mhz birdie in sep04 was about 25*Tsys. It is now about 3*Tsys so the shielding has improved things.
• The signal strength at the entrance to the corridor between the cliff and the control room (measured using a lband helix and spectrum analyzer) decreased from 30db (sep04) to about 25db.
processing: x101/050928/doit.pro

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327

• 24jul02 2Mhz comb in 327 Mhz receiver (resolved). (rfimtop)
A 2 Mhz comb was found in the 327 Mhz receiver.  It is at the odd Mhz 327,329,331,.. Mhz. It is offset from the Mhz by +3.737Khz. Looking more carefully you can also see a birdie offset by the same amount at 326,328,330,.... The comb must be a 1 Mhz square wave with the even harmonics non-zero because of imperfections in the square wave.The birdie goes away when the receiver is put on load so it is coming in through the front end. The first lo was shifted and the comb remained at the same rf frequency so it is not coming in after the first mixer.
    Data was taken with the correlator in 195Khz/2048 channels (with hanning smoothing). This has a 190 hz resolution.  Bandpasses were centered at 327,329,331,and 333 Mhz. 1200 records were then taken while various devices were turned on/off and doors opened, closed.
    The 327Mhz birdie was then mixed down close to baseband and sampled with the radar interface at 10 Khz. This measured the width and drift rate of the signal.
The plots show the results of these measurements (note.. the plots call it a 2Mhz comb since the odd harmonics were so much stronger):
1.  Comb offsets from 327,329,331, and 333Mhz   The comb is offset 3.737 Khz above 327,329,331,333, etc. The spacing is 2 Mhz within the measurement resolution.
2. Drift rate of birdie at 327 Mhz.   The for the 157 seconds of the measurement the signal drifted at .041 hz/second. At 327 Mhz this is a stability of .041/327e6= 1.25e-10 fractional change/second. 3.2 seconds of data were taken for each spectra giving a resolution of .3 hz. It has still not been resolved in the last spectra.
3. Data was taken for 1200 seconds using the correlator. During this time the doors going out to the stairwell were opened and closed a few times. The doors to the iflo racks were also opened and closed. The plot shows the birdie at 331 Mhz -Tsys  versus time for the 1200 seconds. The only thing that correlates is the opening and closing of the outer doors. Opening and closing the iflo racks did not make a large difference.

Resolution: Conducting gasket in huffman box degrades with time.

    After turning off a number of equipment it was found that the birdie was coming from the power meter that is used to measure the power in the 430 Mhz waveguide. It is located on the ceiling of the turret room inside a huffman box. It has an rf cable going to it from the waveguide, a fiber optic cable sending the signal downstairs, and an ac power code that goes through an ac power filter at the input.  Inside the box is a pc card with a VtoF converter on it. This card uses a 1 Mhz crystal. There is another of these boxes in the carriage house measuring the  430 Mhz output power. At first the ac power filter was suspected as the culprit. The box in the carriage house was brought down and tested. The main problem was the conducting gasket around the inside top of the huffman box. It was supposed to improve the contact between the top and the box when the top was closed. The gasket was a rubber (neopreen?) material with a conductive coating. Where the gasket was in contact with the box (top or bottom) the gasket was still conducting. The edge of the gasket had a dark strip which was no longer conducting so the top of the gasket was no longer electrically in contact with the bottom. When the box was closed the gasket was acting as an insulator for the top. The guess is that the part of the gasket in contact with the air oxidized the conducting material. This is a serious problem since most of the huffman boxes we have use this gasket material. It is probably why the color camera birdie was not there when the cameras were installed in their new shielded boxes and then reappeared 6 months later.
processing:x101/020724/327xxx.pro

 
• 24nov03 comb at 327 Mhz.  (rfimtop)

A comb appeared at 327 Mhz sometime in oct or nov03. Data was taken on 24nov03 with the interim correlator using a 25 Mhz bw, 2048 channels, 1 second sampling, and centered at 327Mhz. An az swing was done from az=290 (za=10) clockwise to an az at 660 using a rate of .4 deg/sec. The az was then swung counter clockwise from 660 to about 540 (where is stopped because the cable car was in use). The plots show the comb at 327 Mhz:
• Fig 1 Top is the average of the clockwise spin. Black is polA, Red is polB. The units are Tsys (about)...Bottom: This is the average of the counter clockwise spin. The vertical scale is blown up to show the smaller birdies.
• Fig 2 This shows the spacing between the birdies. A baseline was fit to the non-comb values and then removed. All measurements above 1.5 Tsys were flagged as comb members. Combs in adjacent freq channels were collapsed to the channel with the largest value. The frequency difference was then computed between all of the adjacent comb members. The data was plotted with the comb intensity versus the comb frequency step. The points clustered at multiples of 372 Khz with the largest number occurring at 327*2=745 Khz. Black is polA, Red is polB, and green lines are multiples of 372Khz. The top plot is the clockwise spin, the bottom plot is the counter clockwise spin.
• Fig 3: This is the strength of each comb element vs azimuth position for all of the comb members found. The top plot is the clockwise spin, the bottom plot is the counter clockwise spin. Colors differentiate different comb members (note that the colors on the top plot do not necessarily correspond to the same frequency for that color on the bottom plot).
The comb spacing is 372Khz with the strongest values at  745 Khz (actually 371.97Khz generates the comb at 327Mhz). There is variation in the power for each element vs azimuth but it does not look like something that would be coming from a tower far away. The dropouts and increases do not repeat with azimuth implying that they are a time variation rather than an azimuth dependence. The birdie is probably on the platform or even in the dome. It would be interesting to take some data with the doors to the turret room open and then closed.
processing:x101/031124/doit.pro

01dec03 327 Mhz comb coming from sb Tx room in dome.
    On 01dec03 data was taken with the 327 receiver while tracking blank sky. The correlator was set for 2048 channels over 25Mhz (centered at 327Mhz) and 2048 channels over 195Khz (centered at 321.7 Mhz. The data was dumped at 1 second intervals and hanning smoothed to give resolutions of 24Khz and 190 Hz. While the data was being taken, the doors to the sband transmitter room was opened and closed as well as the door to the turret room. Data was taken for 2200 seconds.
    We knew that the signal was coming in through the horn (since it went away when put on load). The doors should give some shielding to birdies generated inside the rooms. If the birdie strength increased when the door was opened, then it was probably coming from inside the room. The plots shows the 327 comb data while the room doors where opened and closed.
• Fig 1 This is the average of the 2200 seconds of data. Black is polA and red is polB (polB has been offset in frequency by a few channels for display). The top plot is the 25Mhz band (24Khz resolution while the bottom plot is the 195Khz band (190 Hz resolution). The 190 hz resolotion data did not resolve the comb.
• Fig 2. This is the time series for the 321.75 Mhz, 190 Hz wide channel. The top plot is the entire 2200 seconds (it took me awhile to get up to the dome to open/close the doors). The bottom plot is a blowup of 300 seconds when the transmitter room doors were opened/closed twice. The dashed green line is when the transmitter door was opened. The blue line is when the door was closed. (these times could have been off by a few seconds. You can see that the power in the channel went up by almost a factor of 7. This shows that the comb is coming from inside the room.

    The dip at 1250 seconds is when i opened the door rapidly and then closed it to get into the room for the first time. When I closed the door, it must have made better contact than previously.
processing: x101/031201/doit.pro

03dec03 327 Mhz comb from sband Tx Rack in dome.
        The 327 Mhz comb (with 372Khz spacing) is coming from the sband transmitter cabinet in the dome. Rey velez noted that the fingers for making contact between the door and the cabinet were not working. Data was taken with the correlator while the sband rack was on and off. 1 second dumps were done using a 24Khz resolution and a 190 Hz resolution (after hanning smoothing). A bandpass correction was fit to an average of the bandpasses when the rack was powered off. The plots show the 327Mhz comb when the sband rack was power on and off. The door from the sband transmitter room to the stairwell was open during these test.
• Fig 1 . A 30 second average with the rack on. The top plot has 24Khz resolution while the bottom plot has 190 Hz. The vertical units are Tsys.
• Fig 2. The 30 second average with the rack on (black) followed by a 30 second integration when the rack was off (red). The top plot has 24Khz resolution while the bottom plot has 190 Hz. You can see that turning the rack on created many birdies and it raised the entire continuum level by about 10%.
processing: x101/031203/doit.pro
• 03jun05 16Khz comb in 327 receiver from alfa motor.
• 16oct05 1 Mhz comb in 327 reciever from both dome AC units

 
• 21oct05: 327 Mhz spectral density at output of 1st amplifier (before the rf filter).

    The output of the 327 Mhz receiver after the first amplifier was measured with the tektronix ybt250 portable spectrum analyzer. The signal  was taken after the first amp and before the rf filter (the filter limits the band to 30 Mhz about 325 Mhz). The telescope was at an az of 270 and a za of 8.46 degrees. Data was taken covering 0 to 1000 Mhz and then 0 to 500 Mhz. At each band a regular sweep and a peak hold of about 1 minute were recorded. This was done for polA and polB. The plots show that television stations are the largest signals in the receiver band (.ps) (.pdf):
• Top: this covers 30 Mhz to 1000 Mhz. The black and red are the single sweeps for polA and polB. The green and blue are the one minute peak holds for polA and polB. The largest birdie in the band is 471.25. This is the video carrier for tv channel 14. The next two largest birdies are 519.25 (tv channel 22) and 187.25 (tv channel 9).
• Bottom: the 30 Mhz to 500 Mhz measurements.

 
     On 20oct05 we saw intermittent intermods/harmonics in the 327 Mhz band that included the fm station 107.3  (3rd harmonic at 322) and intermods with a tv station and 107.3 (we heard the fm station in the IF signal).  The power levels measured above do not look like they should be driving the first amp into compression.
    The 1 minute peak hold has values that are up to 10 db stronger than the single sweep values so the levels are time variable. There is most likely an azimuth dependence to the birdies. It still looks like a good idea to put an rf filter before the first amplifier (since the receiver isn't cooled).
processing: x101/051021/327.pro
• 02nov05 cleaning up the 327 Mhz band.
• 28oct05: azimuth dependence of birdies in the 327 Mhz rcvr.

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430

•  aug97 430gr. Power meter, multi meter, camera system birdies (ps file).
• 22jan01 Using the dome 430 Mhz system to look at the vittara jeep parked by the control room. (rfimtop)
The vittara jeeps are not shielded and are often used in the parking lot next to the control room. We tried to see if any rfi from the jeeps was getting into the 430 dome receiver system. The dome was as az=350deg and za=19deg. The jeep was as close to the control room as possible looking toward the dome. The jeep was turned on at the minute and turned off at the minute + 30 seconds. This was repeated for 7 minutes. The correlator was set for 12.5Mhz bw, 1024 channels and readout at 1 second intervals. Images were made of the spectral density function (frequency versus time).  (The images below are about 800Kb ps files). The solid lines in the images are the start of a minute (jeep on) and the dash lines are the 30 seconds (jeep off). (some of these lines were lost in the conversion from postscript back to the screen).
• Figure 1 has the band pass normalized to the median band pass for the 420 seconds, 1 subtracted, and then clipped to +/- .02 Tsys. There is rfi and continuum sources but nothing obvious from the jeep.
• Figure 2 is also flatten by frequency channels 400 through 500. This gets rid of the continuum sources (as well as any continuum from the jeep). There does not appear to be any frequency dependent spikes correlated with the jeep going on/off.

Looking at the total power versus time also does not show any obvious correlation with the jeep. We need to redo the experiment with the jeep in the beam (maybe at the fork before you get to the rim road).
processing: x101/010122/vittara.pro

 
• 11apr01 430 dome. Multimeter inside new rfi enclosure.     (rfimtop)
An rfi enclosure for the multi meters, power meter was placed inside the right cabinet on the turret floor.  The multi meter used to read the receiver temperatures was put in the box and hooked up to the gpib bus. The correlator was setup for 195 Khz by 2048 channels (95 hz channel width). Integrations were done with the rfi box open  and closed (both times inside the right cabinet). An integration was also done with the second multi meter in the left cabinet on  to show the reference birdie. A linear baseline was fit to the interference free part of the spectra and then used to normalize the averages to Tsys. 60 deg Kelvin was used to convert from fractions of Tsys to Kelvins.
The plot shows:
• black - 1400 second integration with the multi meter in the rfi box. No birdie is seen.
• red     - 100 second integration with the 2nd multi meter on in the left cabinet to show the birdie at 429.73993.
• blue    - 400 second integration with the front  panel to the rfi box open. No birdie is seen.
• dark red - the baseline used to normalize the spectra.

It is interesting that we see no birdie from the multi meter in the right rack even when the front to the rfi box is open.
processing: x101/010411/multimeter.pro
• 23apr01 430 dome. Multimeter, power meter inside new rfi enclosure. (rfimtop)
The rfi enclosure (see 11apr01) was tested with the power meter and multimeter inside the enclosure. The 2nd multimeter in cabinet 1 was turned off. The receiver monitoring and power cables were hooked up to the the devices inside the box via feed through connectors. The correlator was setup for 2048 channels and 195 Khz bw (95 hz/channel). 5 second integrations were done and then the data was hanning smoothed. The acquisition sequence was:
•   45 records: cabinet back door open, rfi box open, meters on.
•   30 records: cabinet back door closed, rfi box open, meters on.
•   60 records: cabinet back door closed, rfi box closed, meters on.
•   60 records: cabinet back door closed, rfi box closed, meters off.
• 120 records: cabinet back door closed, rfi box closed, meters on.
•   60 records: cabinet back door closed, rfi box closed, meters off.
The 120 records with the meters off were used as  a band pass correction. 200 channels adjacent to the power meter birdie at 433.325 Mhz were used to correct for sky variations with time.
1. The first image shows the 195 Khz band with horizontal stripes where the changes were made. The birdie at 433.325 is from the power meter. It completely disappears only when the meters are turned off (4th section from the bottom). The birdie at 433.351 also goes off when the meters are turned off.
2. The second image is a blowup in frequency of the first. You can see that the 433.325 birdie is still visible in the 3rd section from the bottom where everything is closed up and the meters are on.
3. The plots show the power in the 433.325 Mhz channel versus time. The data was normalized to the average value for the 30 records when the back door was open (maximum signal), divided by the average of 10 adjacent channels (to remove sky variation), and then converted to db. It contains the power from the birdie + Tsys (about 60K). Closing the back door of the cabinet reduced the power by 18 db. Closing the rfi box reduced it another 10 db. The bottom plot shows there is about .1 to .4 db leakage with the rfi box all closed up. Averaging the spectra in the 3rd section gave a birdie of about 2 Kelvins.
We need to redo the measurement placing an rfi tight cap on the gpib bus cable to see if this is the remaining problem.
processing: x101/010423/doit.pro
• 23oct02 410 Mhz birdies.     (rfimtop)

A 408 to 412Mhz filter was put on the dome 430 system to do an experiment at 410.5 Mhz. A number of  narrow birdies were found close to this value. They were narrow < 3 Khz width. Data was taken on 23oct02 and 24oct02 to characterize the birdies. The plots show the 408 and 410 birdie behavior.
1. Fig 1 frequency  of the birdies. 101 second averages with dome at 18 deg za and az at 270 deg.
2. Fig 2 time variation of the 410.5249 birdie. 101 seconds with telescope stationary.  Red is polB, black is polA. The adjacent channels are plotted in blue and green (blown up by 300 and 100). The birdies are varying by up to 100 % during these 100 seconds.
3. Fig 3  PolA azimuth variation of the signals. 4 Azimuth swings were done with the dome at 18 degrees za and the azimuth moving at .4 degs/second. The data was sampled once a second.
4. Fig 4  PolB azimuth variation of the signals. 4 Azimuth swings were done with the dome at 18 degrees za and the azimuth moving at .4 degs/second. The data was sampled once a second.
On 25oct02 rey velez and I spent the day in the rfi van searching for the 410.5249 Mhz birdie. We finally located it coming from the navy transmitter tower in aguada. There is the tall tower (1000'??) and two smaller towers. We measured a strength of -48 dbm (using a 4 element yaggi and a 15 db amp).  Using the same setup from the platform we measured a maximum strength of ? -70dbm??.
    The location of the tower from our gps receiver is: 18 24 12.1, 67 09 32.7 (probably 1/2 mile east of the tower). The observatory is at: 18 20 39.44,  66 45 10 so the tower is west of the observatory. This is the bearing we got using the yaggi from the top of the platform (275 degrees). The azimuth swing gave a peak at 150 deg az and a smaller one at 300 deg az. These are far from the 275 position of the transmitter. The 150 az is  90 degrees from the cables that go to T8 (240 deg az). It may be that the main cables from T8 to the platform are acting as a radiator. The az=300 birdie is perpendicular to the T12,T8 beam of the platform.
    Since the signal is not constant in time, on/off processing will no do a very good job of removing it. I might be interesting to measure the signal in full polarization mode and then subtract the polarized component of the signal (since it is probably highly polarized).
    processing: x101/021023/doall.pro
• 19jun03 430 birides vs az, Tv station intermods.  (rfimtop)

    The 430 receivers were creating intermods from 2 tv stations:
• chn 14 F1Video=471.25 F1Audio=475.7
• chn 22 F2Video=519.25 F2Audio=523.75

 
The table below shows the possible intermods:
 

chan 14,22 intermods in 430 band

combination	freq
2*F1Video - F2Audio	418.75
2*F1Video - F2Video	423.50
2*F1Audio - F2Audio	427.75
2*F1Audio - F2Video	432.25

We see the birdies at 427.75 and 432.25 clearly in the data. It turns out that the 435.75 Mhz birdie is the 432.5 biridies aliased back into the band.
    The correlator was setup for 12.5 Mhz bandwidth centered at 430 Mhz. The edges of the band are at 430 -/+6.25Mhz = 423.75 and 436.25 Mhz. The 423.25 intermod is .5 Mhz below the bottom of the band. The digital filtering in the correlator is done on the complex i,q paths separately. This causes aliases on one side of the band to appear on the opposite. A birdie at .5 Mhz below the bottom of the band will alias into .5 Mhz below the top of the band. So 423.25 will show up at (436.25-.5= 435.75Mhz). We see this birdie in the correlator data.
    Azimuth swings were done to measure the directionality of the birdies. The dome was set at 19 deg za and the azimuth was swung from -90 to +270 degrees and then back to -70 deg moving at .4 degrees per second. The correlator was setup to dump 1024 channels over 12.5 Mhz once a second with the center freq set to 430 Mhz. The input data was then hanning smoothed.
    The average of each spin was computed and then a fit was made to the bandpass using harmonic functions up to order 10*az and a 3rd degree polynomial in az. Outliers were excluded from the fit (see corautobl()). Each 1 second integration was bandpass corrected by dividing by this fit and then subtracting 1 (so the units are Tsys).  The channel containing the peak of each birdie was plotted versus azimuth. None of the birdies drifted in frequency.

The first plot shows the average and peak spectra for the first az swing.

• Top: this is the average of the entire swing.
• middle: The peak value in each channel for the entire swing is plotted. The spike at 427.75 continues up to 975.
• bottom: a blowup of the peak spectra.
The following plots show the Birdie strength vs az:
• 426.25 Mhz birdie. This peaks at 300 degs az. It is not the tv station.
• 427.75 Mhz birdie. Tv intermod 2*F1Audio-F2Audio
• 432.0 Mhz birdie.    No az dependance. It must be inside the dome.
• 425,426.84, 432.25, 435.26, 435.75 Mhz birdies.
• 432.25 is 2F1Audo-F2Video.
• 435.75 is the 423.5 birdie aliased to the other side of the 12.5 Mhz filter.
• 426.84, 435.26 may be from other parts of the tv signal (color,etc..).
• 425 Mhz may not be from the tv station. It does not have the strong spike in polA at az=124 degrees.
• PolA lines up with az =124 for all the signals.
• PolB is a bit more scatterd.
• The 430 dome main beam is circularly polarized. These signal are coming in the sidelobes and the polarization properties are probably different. Since there are two separate transmitters at two different locations creating these intermods, it may be that the two polarizations are sensitive to different transmitters.
processing: x101/030619/corazspin.

 
    To further investigate the 462.25 Mhz birdie, an azimuth spin moving at .4 deg/sec was done with the dome at 19 deg za and the carriage house at 17.6 deg. A 20Khz bandwidth centered at 426.24994 was baseband sampled from the dome and the carriage house. The motion of the azimuth will cause a doppler shift in the signal that can be seen by multiplying the two complex signals together.
    The maximum doppler shift will be: Dfrq=vel/C * freq = .3014met/sec /C* 426.25e6.
or +/- .43 Hz. To see this shift, spectra were constructed from 12 seconds of data (giving 1/12.=.08hz resolution). The spectra were then plotted vs azimuth. The image shows the doppler shift versus az and the spectra vs time,az.
• Top: The cross spectra (fft(chVolt*grVolt) image shows +/- 5 hz on the horizontal axis and -90 to 270 azimuth position on the vertical axis. You can see a sine wave in the data with an amplitude close to .5 Hz. The minimum is close to az=120 degrees. This puts the zero doppler at az=30. This should be the location where the azimuth arm is pointing at the transmitter (doppler=0). The amplitude peak of the spectra was at -60 degrees. This is 90 degrees away from the birdie direction. It should also be noted that -60 degrees lines the azimuth arm perpendicular to the T12,T8 side of the triangle.
• Center: This shows the spectral density (+/- 200 hz) of the dome signal on the horizontal scale and time on the y axis. You can see the signal chirp from 35 hz up to 110 hz and then back down in about 5 seconds. The pattern repeats every 12 seconds. This sweep looks like a phase locked loop that has become unlocked (see 100 Mhz reference comes unlock).
• Bottom. This is the same image as the center plot with the vertical scale changed to azimuth direction. You can see the strong signal at an azimuth of 300 degrees.
processing: x101/030619/riazspin.

 
• 30jul03 422-442 Mhz filter in 430 Dome.  (rfimtop)

 
The 10 Mhz filter centered at 430 Mhz was replaced with a 422 to 442 Mhz filter on 30jul03. Data was taken with the interim correlator with the dome at 18 degrees za and the azimuth at 124 degrees (this was the maximum for the tv station intermods). The correlator used 1024 channels over 25 Mhz, hanning smoothing (to give a resolution of about 50 Khz),  and 1 second dumps. Data was taken for 100 seconds. The plots show the birdies in the 20 Mhz band.
• Fig 1: This is the average of the 100 seconds. PolA is black, polB is red. The top plot is full scale while the bottom plot has the vertical scale blown up. The units are approximated Tsys.  The birdes in polA are much stronger than polb.
• Fig 2: The peak value (over the 100 seconds) is plotted here.


You can see the strong tv intermod at 423.5 Mhz. This is 2*F1Video-F2video of channels 14 and 22 (see 19jun03 430 birdies vs az, tv station intermods for a description of the various birdies). This data was taken at the azimuth peak of the tv intermods. It should have the same azimuth dependence as measured above.

processing: x101/030730/doit.pro

 
• 08oct04: 430Mhz filter in front of dewar removes tv station intermods. (rfimtop)

Tv channel 14 (471.25Mhz) and channel 22 (519.25 Mhz) were causing intermods in the both the carriage house and the dome 430 systems (more info..). A low loss bandpass filter with notches at 475 and 520 Mhz was placed in front of the gregorian dewar for polA at the beginning of oct04. This knocks down the tv station signals to a level that they will not created intermods in the dewar. On 08oct04 data was taken with the interim correlator to see how well the filter worked.
    The correlator was set for 12.5 Mhz and 1024 channels. One sec dumps were taken and then hanning smoothed to give 25 Khz resolution. The telescope was spun from 110 degrees to 380 degrees while data was being taken so we could see the peak of the tv intermods. 785 spectra were taken and then a peak hold on each channels was made (largest value in each of the 785 records became the value for each channel). The plots show the peak hold spectra for polA and polB during the azimuth swing.
• Fig 1. The black trace is polA (with the filter) while the red trace is polB (without a filter). The green dashed lines are placed at the tv station intermod frequencies. The top plot is the full vertical scale, the middle plot is a vertical blowup, while the bottom plot has had the bandpass removed. The bottom plot has had offsets of +/-.02 Tsys added to polB/polA for plotting purposes.
• Fig 2. This blows up the data in the frequency direction to give a better view of each tv intermod.
The response of the filter can be found here.
 
Results: The filter has removed the intermods. The worst case had a signal at 125 times Tsys (427.75 Mhz). The channel with the filter saw nothing down to the noise floor of .006 Tsys. The filter in front of the dewar has an insertion loss of  .04 db adding about 3-4 degrees K to the system temperature. We need to buy 3 more of these filters so that both pols of the dome and the carriage house are protected.
processing: x101/041008/430.pro

 
• 02dec05: birdies at 430 Mhz with 1 minute periods.  (rfimtop)

    Project A2125 was doing mapping at 430 Mhz. The data showed some time dependent birdies. On 02dec05 i took some data with the interim correlator and the 430 dome receiver. The setup was:
• 25 Mhz bw, 2048 channels, 1 sec dumps, cfr of 430.
• Data was taken for about 2 hours while sitting at az=270, za=10.
The plots show the birdies seen during this session:
• Dynamic spectra for 300 seconds, polA (.gif): This image is spectral density (time vs freq). The small dots are the time variable rfi. The short lines at the bottom of the plot are added to show where these birdies occur (some of the weaker periodic birdies were not flagged).
• Time and frequency extent of birdies (.ps) (.pdf): The top plot shows the power in the 429.89 Mhz birdie vs time. It lasts for 3 samples so the time duration of the signal is about 2 seconds. The bottom plot shows a spectral blowup of this same birdie. The freq. resolution (after hanning smoothing) is 24 Khz. The fwhm is about 3.5 channels so the birdie is about 30 khz wide.
• Time periodicity of channels over 1500 seconds (.gif): The spectra from the first 1500 seconds of data was interpolated in time and then  fft'd along the time direction. The image shows periodicity vs Rf frequency. The vertical scale is in milliHz. The horizontal dashed lines are plotted ever 16.66 milliHz (1/60.  seconds). You can see strong periodic combs around 422-423 Mhz, 430 Mhz, and 432.5 Mhz. These channels have birdies that have a 1 minute period. There are also some channels that have a weaker 1 minute periodicity (but do not show up well in this image).
• Birdie power vs time for 140 minutes (.ps)   (.pdf): Thirteen freq. channels with 1 minute periodicity were selected. The power in the channels were averaged over 7 freq. channels and then plotted versus time:
• Fig 1: all 140 minutes. Each color is a different birdie. The top plot is polA while the bottom plot is polB. This data taking session started at about 9 am. You can see that the birdies were stronger at the beginning of the run. Looking at the data toward the end of the run, the periodic birdies were still seen at some of the frequencies. The gaps in the data are where no data was taken (between scans).
• Fig 2: This is a blowup in time of fig 1 showing the first 5 minutes of data. The dotted lines in polA (top) show how the birdies are aligned in time. You can see two groups. The first starts at 422.72 (lower black trace) while the second group starts at 429.89 (light blue middle trace). The nth birdie of each group occurs at the same time.
• Fig 3: (power in periodicity) vs period:  For each birdie, 8500 seconds of data was interpolated and then fft'd. The plot shows the power vs periodicity. There is a comb with 16.6 millihz spacing (1/60 seconds). The width of the comb was 1 channel (about .1 millihz) so the 60 second period is relatively stable.

 
Summary of birdies:
 

period	1 minute
freq. width	about 30 Khz
time Duration	about 2 seconds
birdie groups	4 birdies per group rf spacing: .45,.1 .1 .1 Mhz tm spacing in group: 13,7,26 seconds each group repeats every 60 seconds.
group rf  spacing	7.2 Mhz between groups.
birdie freq.  (strongest)	422.72, 423.18, 423.25, 423.35 grp1 429.89, 430.31, 430.39, 430.48 grp2 432.67

    On 30nov05 A2125 also saw a group of birdies at 432 Mhz. The 7.2 Mhz spacing looks close to the comb that is generated by the alfa motor controller. We need to check this out.
 

processing: x101/051202/430.pro
• 06dec05 Ac units in dome cause birdies at 430Mhz   (rfimtop)

    The Ac units in the dome have a digital control that caused rfi. This rfi was seen as a  1 Mhz comb in the 327 Mhz receiver. Turning the Ac off with the remote control does not make the birdies go away since the digital remote control in the ac unit is still on. You need to turn off the power to the ac unit for the birdies to stop.
    On 06dec05 data was taken with the interim correlator. A switch had been installed allowing you to turn off the power to the old AC unit in the dome. We took data with the AC on, AC off, and then AC on.  The results are:
• A dynamic spectra of the 1000 seconds of data  shows the birdies. The Old Ac was turned off around 360 seconds and turned back on at 748 seconds. The dashed horizontal lines bracket when the old Ac was off. You can see the rfi that stopped when the old AC was off. The strongest are spaced 2 Mhz apart (missing the even harmonics).
• Median spectra when the AC was on and Off (.ps)  (.pdf): The median spectra was computed for when the Ac was off (black lines) and when the Ac was on (red lines). The green lines are spaced every 1 Mhz anchored at 418.52 Mhz. The vertical scale is in units of Tsys (about 60K). The Ac off spectra is offset by -.002 Tsys for display purposes. The strongest birdies are at 418.5 and 424.53 Mhz.

 
    The Ac units are causing problems at 430 Mhz (as well as 327).  The plots/images show another 1 Mhz comb that is still present when the OldAc is turned off. This is coming from the newer Ac unit (that was left running when this test was done). The 327 tests showed that the frequencies of the comb will wander by up to 100 kHz so the frequencies reported can vary. Hopefully we will replace the digital units in the Ac's with an analog one.
processing: x101/051206/430.pro

We measured the output power levels of the sband wide dewar using the 22 Ghz spectrum analyzer. The telescope was parked at az=309. and the gregorian was at 3.35 degrees.  The measurement was from 1000 Mhz to 2950 Mhz. The dewar response goes beyond 3 Ghz but the spectrum analyzer didn't want to switch to the next band. The data was taken during close to noon with the sun about 15 degrees away (dec=01 deg).  The plots show:
• Figure 1 top plots the spectrum 1000 Mhz to 2950 Mhz for polarization A. The resolution bandwidth was set to 3 Mhz. The system temperature is about 40 K. The red line averages 100 sweeps of the spectrum analyzer. The black line is a peak hold for greater than 30 seconds (I don't believe that the black line is a peak hold, it looks like it was left on averaging mode). Integrating the spectra gave -23.5 dbm.
• Figure 1 bottom plots the spectrum 1000 Mhz to 2950 Mhz for polarization B.  The red line averages 100 sweeps of the spectrum analyzer. The black line is a peak hold for greater than 30 seconds  Integrating the spectra gave -11dbm (peak hold) and -24.3 dbm (100 sweeps). The peak hold values where there is no interference should be about 3 db larger than the averaged version (not 13 db). So something went wrong in the measurement.

 
Doing a consistency check on the power levels:
-198 dbm/hz/K + alog10(3Mhz rbw)*10 + alog10(40K)*10 + 40 db gain= -77 dbm.
The spectrum analyzer value in the middle of the band where there is no interference is about 17 db higher than this. These measurements need to be redone.

The large interference at 1940 Mhz are from cellular phones. The large birdie at 1350 Mhz is the san juan airport radar. I don't think that there is any filter between the dewar and the post amps. This needs to be corrected.  We also need to decide what to do about the birdies at 1940 Mhz. Do we cut them out with a bandpass filter or try and use a notched filter?  Our spectral line list does not have any lines between 1720 (OH) and 2661 (HC5N).

processing: x101/010920/sbwdewar.pro

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xband

• 23nov04: birdie at 9186.2 Mhz with 85 Khz sidebands.   (rfimtop)

    A strong birdie was reported on 20nov04 at 9186.24 Mhz in the xband system. It was about 5000 Tsys in polB. The width was lt 25 Khz and it had side bands spaced 85 Khz away. The fractional occurrence of rfi by month showed that this birdie has been around for a long time (more than  a year).
    On 23nov04  the xband receiver was tuned to  9186.0 Mhz. The 260 Mhz IF was displayed on the spectrum analyzer with a 1 Mhz span. The birdie was 20 to 30 db above the noise floor. The IF signal was plugged into the radio. Using fm demodulation you could easily hear the birdie. It sound just like the dewar refrigerators going through their cycle !. The time for the period was .8 seconds (which is equal to the refrigerator cycle). Closing the horn shutters made the birdie go away. We then cycled off each refrigerator one at a time. The birdie did not go away. A metal short was placed over the horn. This would block the incoming radiation and leave tsys low (the shutter has some absorber on it). This also made the birdie go away.
    (this is where we got lucky...)
    While ganesh was on the service platform the shutters were closed again.  The birdie went away immediately, even before the xband shutter had moved very far. There are two kinds of shutters in use. On is mechanical (slow) and the other is compressed air (fast). The sbw and cband shutters were closing immediately while the others took a long time. We then powered off the sbw bias box and the birdie disappeared. Ganesh looked at the output of the sbw dewar and saw an oscillation at 9186 Mhz with the 85 Khz side bands (only in one channel of sbw). By adjusting the bias voltages he was able to get rid of the birdie. So the xband birdie at 9186 Mhz is now gone..

---

Azimuth dependence of birdies.  (rfimtop)

• 05oct01 :  1350 Mhz tone. radar blanker on. az swing at .2 deg/sec. Dome at 19 deg za. center plot smoothed to 11*.2=2.2 degrees.
• 01mar02: 1400 Mhz birdie . .4 deg/sec az swing with dome at 18 degs za.
• 29jun04: 1400 Mhz birdie  .4deg/sec az swing. dome at 18 deg za. rbw:190 hz. The center plot shows the response of the 1400.000 Mhz  birdie that comes from the control room.
• 23oct02 : 410 Mhz birdie from the tower in aguada. See figures 3 and 4. Spin at .4 degs/sec with dome at 18 degrees za. The tower is at an azimuth of 275 deg (via gps receiver). We measured a 275 deg az with a yaggi from the dome. The 430 system via the dome has a peak at 150 degrees. This is 90 deg from tower8 cables.
• 19jun03: 427.75 Mhz Tv chn 14,chn22 intermods. .4 deg/sec az swing with dome at 19 degs za. Peaks at az=124
• 19jun03: 426.25 Mhz birdie. .4 deg/sec az swing with dome at 19deg za. Peaks at az=301 deg.
• 19jun05:  432.0 Mhz birdie.    No az dependance. It must be inside the dome.
• az,za  dependence of faa, aerostat, remy, punta salinas using  1 second sampling.
• 28sep04: az dependance of faa radar compression in alfa receiver.   Az swing 270 to 630 at .3 deg/sec. Use alfa and dump data every 64 useconds.
• 28oct05: azimuth dependence of birdies in the 327 Mhz rcvr.

Az spin datasets: