# lband drifting birdies with 60 Mhz spacing

#### Sections:

11jun04: First look at drifting birdie. Harmonics, drift, az dependance, time dependance.
28oct04: measure the drifting birdies at lband and the 430Mhz.
04nov04: The drifting harmonics do not come from the dome airconditioners.
12nov04: switching between sky and load at 430Mhz.
18nov04: 430 data sampled at 1 millisec shows signal jumps very 2.67 seconds.
19nov04: The birdie is coming from the dewar monitoring system.
How the dewar monitoring system works.
18nov04: Summary of what we know
14dec04: birdie seen at 60 Mhz with spectrum analyzer in the dome
17jan05: The 60Mhz birdie, what it was and how we fixed it..

#### 11jun04 First look at drifting birdie. Harmonics, drift, az dependance, time dependance.  (top)

A birdie that drifted through lband appeared in the middle of may04 (project A1861). There were copies of this birdie spaced by about 60 Mhz. Its drift rate varied from stable to 20 Mhz in 5 minutes. The birdie was not always present.

#### Data taking setup:

On 11jun04 data was taken during the day to investigate this birdie. The dome was set to 18 degrees za and the azimuth was swung (at .04 deg/sec) from az=450 to az=90 and then back to az=450. This was repeated twice. Data was taken with the interim correlator. The setup was 4 sbc, 50 Mhz each with 2048 channels.  After hanning smoothing, the resolution was 49 Khz. The sbc were centered at:  1335, 1377.5, 1422.5 and 1465 Mhz. The data was dumped once a second. Two data sets were collected each lasting for 2000 seconds. Each of these covered the azimuth swinging from 450 to 90 and then back to 450 degrees.

#### Data processing:

The data processing consisted of :
1. input 2000 records
2. Compute a bandpass correction for 500 records at a time. Use the rms by channel on each set of 500 records to throw out data points that had variations in time. Divide by the bandpass correction and subtract 1 (so the units are now Tsys).
3. For each 50 Mhz, 2000 records display the dynamic spectra. With the cursor, trace each drifting birdie storing the time and channel number.
4. Given the above time, channel numbers, interpolate the position of the birdie for the entire 2000 records. Shift all spectra to line up the birdies in frequency. Keep 40 channels (+/- 1 Mhz) about the birdie. After shifting, pick a strong record and cross correlate the 2000 samples with this. Shift each of the samples by this amount.
5. After alignment, compute the average power of 26 channels about each 1 second sample. Remove a baseline that is the median value for the 2000 records of the (81-26) channels that do not include the birdie. Use this data set for the power vs time/azimuth plots.

6.

#### Spectral density for 11 seconds.

The spectral density for 11 seconds of data shows the drifting birdies. The y axis is in units of Tsys. The different strips have been offset for plotting purposes. The 3 windows are centered on different multiples of 60 Mhz. The birdie gets up to about 8% of Tsys and looks dual humped. You can see it drifting to lower frequency with time.

#### The frequency variation of the  birdies with time and azimuth

The plots: (Freq. vs time and azimuth Data set 1)  (Freq. vs time and azimuth data set 2) show how the frequency of the birdie varied for the two data sets. The data does not cover the entire 2000 seconds because the birdie was not  visible for the entire time period.
• Top fig: The curves are the measured time/frequency positions of the  harmonics (4 for data set 1, 3 for data set 2)  This data was used to interpolate the frequency positions to 1 second spacing.
• Middle fig: The difference in frequency between adjacent harmonics are plotted versus time. The difference if changing from 58.5 Mhz to 60.8 Mhz and then back to 59.6 for the first data set.
• Bottom Fig: The frequency of the birdie versus azimuth angle. For both data sets the frequency increased till we hit 90 Degrees. It then remained stable for awhile and then started to decrease.

#### The strength of the signal versus time and azimuth.

The plots (strength of signal Data set 1) (strength of signal Data Set 2) show the variation of the strength of the signal with time and azimuth. The data was shifted in frequency to align it and then averaged over 26 channels that contained signal power.
• Fig 1 Top: The average of the birdie over the 2000 records for each of the harmonics. This average varies because the birdies were not present all of the time. If the double peaks are caused by two tones at the fundamental, then you would expect the separation of the birdies to increase with increasing harmonic number. This is not the case for the 24th harmonic.
• Fig 1 Bottom: The strength of each harmonic versus time. Occasionally the harmonics change strength together (but not normally). The large spiky jumps for the 22nd harmonic (black trace) are being caused by the 1330/1350 faa radar (and not the drifting birdie).
• Fig 2 shows the variation of the signal strength versus azimuth angle. Each box is a different harmonic. The red trace is the counter clockwise spin while the black trace is the clockwise spin. The repeatability in azimuth is not very high.
• Fig 3. This is figure 2 with the vertical scale blown up.

#### Dynamic spectra of the aligned data sets.

The spectra for the aligned data sets are shown below. 81 channels (2 Mhz) is displayed. Birdies that sweep across this image were constant in frequency before the alignment of the drifting birdies. The contrast of the image improves if you are not using too many windows (or print it out on the xp0 laser printer).

 data set 1 data set 2 Harmonic 22 1338 Mhz Harmonic 22 1338 Mhz Harmonic 23 1398 Mhz Harmonic 23 1398 Mhz Harmonic 24 1450 Mhz Harmonic 24 1450 Mhz Harmonic 25 1520 Mhz
In data set 1 harmonic 22, the birdie remains on for about 5 minutes. The birdie look like it has a modulation envelope is weak, increases to a maximum and then decreases again. This is probably caused by the azimuth arm sweeping through a strong signal. In the other images this is not so obvious because the signal is turning on and then off before the sweep can complete.

#### Summary.

• There are drifting birdies at lband with spacing close to 60 Mhz. If they are harmonics, then the fundamental is drifting from 58.5 Mhz to 60.9 Mhz.
• The birdies have two peaks in freq. which are separate by about 250 Khz. The spacing between the peaks does not increase with harmonic number for harmonics 23,24.
• The birdie strength  gets up to about 10% of Tsys.
• The birdie strength varies with time.
• The birdie strength may also vary with azimuth but is difficult to discern because of the time variation.
• The frequency of the birdie increases as we approach 90 degrees az. As we return to 450 it remains  constant for awhile and then decreases in frequency. This happened for both data sets.
processing: x101/040611/doit1.pro, doit2.pro

#### 28oct04: Measure the drifting birdies and lband and then 430 Mhz.  (top)

On 28oct04 the drifting birdies were measured using the lband wide receiver. The dome receiver was switched (within 1 minute) to the 430 Mhz system and the drifting birdies were seen at 430 Mhz. When we shifted back to lbw, the birdies and disappeared. The plots show the birdies at lbw and 430 Mhz (.ps)  (.pdf).
• Fig 1 top: The frequency versus time for the drifting birdies at lband. A image of the  dynamic spectrum (freq vs time) was used to interactively trace the position of the birdie versus time. The * are the marked points, the line is a linear interpolation thru the points. The black line is the 22nd harmonic, the red line is the 23rd harmonic.
• Fig 1 bot: The difference of the linear fits to the 22nd and 23rd harmonics was used to compute the fundamental. This was then multiplied by 7 to get the frequency at 430 Mhz. The black * are the computed values for the 430 Mhz for the time period 14.70 to 14.78 hours. During this time period the system was measuring the lband wide values. The green * show the frequency and time when the drifting birdie was measured at 430. The predicted frequency and drift rate line up with what the lband values predicted.
• Fig 2 Top: This is the strength of the drifting birdie measured at lband. I divided by Tsys and used Tsys=26K to scale the data. 300 seconds of data is overplotted. The peaks range from 1 to 7.5 K.
• Fig 2 Bot: This is the strength of the drifting birdies at 430Mhz. I divided by tsys and used Tsys=60 K to scale the data to kelvins. 300 seconds of data is overplotted. The peaks range from 100 to 500 Kelvins.
Using the difference between the lband harmonics, you can predict the frequency of the drifting signal at 430Mhz.  The signal is about 20db stronger at 430 Mhz than lband. The problem is that the 430Mhz receiver has a filter of about 20Mhz. Occasionally you will see the birdie in the lbw system and not in the 430Mhz system. That is because the birdie at 430 can fall outside the 20Mhz bandwidth of the receiver system. When searching for the drifting birdie with a spectrum analyzer/antenna you should use 430 Mhz since it is so much stronger. You will still probably need to use the lbw system to identify the drifting nature of the birdie.

processing: x101/040128/lbwand430_drift_28oct04.pro

#### 04nov04 The drifting birdie is not from the dome AC units.   (top)

While the lband 60 Mhz harmonic birdie was drifting, the dome air conditioning units were both turned off. The birdie remained. So the air conditioners can be ruled out as a source of the birdie. The image shows a plot of the drifting birdie while the dome AC's were off (.gif).
• The image is frequency (horizontal) versus time (vertical). The vertical axis labels records are sampled once a second. The dome AC's were off for about the first 180 seconds (records) of the image. The drifting birdie was still present.
processing: x101/041104/acchk60Mhz_04nov04.pro

#### 12nov04: look at birdies with 430 rcvr on and off load.  (top)

On 12nov04 the birdies where seen in the 430Mhz receiver. While monitoring the birdies, the receiver was switched from sky to load and then back to the sky. Tsys on the sky is about 60 K while Tsys on the load is probably 330K.
The image shows the dynamic spectra when switching between sky and load (.gif):
• The drifting birdie is close to 427 Mhz.
• The system was switched to load at 26 seconds and switched back to sky at 65 seconds.
• Each of the 3 sections were normalized to their median bandpass. This gives the same deltaT/Tsys for each plot (since the time*bandwidth product is the same).
The birdie went away when switched to load. This could happen if the birdie was coming in through horn, or the system temperature on load was too high to see the birdie.

The plot shows the spectra as second number 20 (.ps)  (.pdf) while on the sky. The signal is 25 Kelvins. The 1 second integration and 25Mhz over 1024 channels gives deltaT/T=.0064 . For a Tsys of 330 K, deltaT is .0064*330=2.1 Kelvins, so a 25 Kelvin signal is easily detectable.

Since the birdie went away while on load, the birdie is coming in through the horn. This is also supported by the harmonics at 430 and lbw. If the birdies were in the IF then you couldn't use the harmonic numbers to predict the 430 frequency using the lb spacing.

processing:041112/rfi.pro

#### 18nov04:  430 data sampled at 1 millisec shows signal jumps very 2.67 seconds.  (top)

On 18nov04 the birdie was visible in the correlator using 1 second dumps. The setup was switched to run the wapps in pulsar mode with 25 Mhz, 512 lags, and 1.024 milliseconds per dump. 300 seconds of data was taken. The images show the dynamic spectra for different smoothing of the integration times:
• Image 1: data is smoothed to 1.024 seconds per spectra and then plotted time vs frequency. A median bandpass has been removed from the data. The top plot is polA and the bottom plot is polB. The main birdie is at 435 Mhz. A weaker birdie is at 439 Mhz. The 439 Mhz version looks like it turns on and off.
• An unrelated comment:.the 423/427 Mhz birdies in polB are coming from the tv station intermods. PolA does not have them because it has the new notched filter in place.
• Image 2: data is smoothed to .05 seconds per spectra and then plotted. You can now see that the signal jumps from 434 to 439 Mhz every 2.67 seconds.
• Image 3: data is plotted at the 1 millisecond time resolution. The signal jumps to 439 Mhz and stays there for about 140 milliseconds and then jumps back to 434 Mhz. It looks stronger at 439 Mhz while it is there. The plots of the 1 second data show 439 Mhz to be stronger since it has the higher duty cycle.  The jumping is pretty fast. It is begin done within the 1 millisecond sample.
The 2.67 second spacing of the jumps was the key to figuring out where the birdie was coming from. The dewar monitoring system (the system that reads the dewar temperatures, voltages, currents... see below for a description.) takes 2.67 seconds to cycle through a single dewar. After 2.67 seconds, the same "address (temp,current, etc)" is being reread (but now from a different dewar).
The frequency jump is 4.39 Mhz at 430. This would be a jump of .628 Mhz at the 60Mhz fundamental. This jump is not seen in the lband data because it is too weak. The duty cycle of this "upper freq" is only 5%. If fast dump data was taken at lband, it would be seen as a .628*24=15.1 Mhz jump.
(note: the 19nov04 data showed that the amount of this jump varies).
processing: x101/041118/rfiwapp.pro

#### 19nov04: The birdie is coming from the dewar monitoring system.  (top)

The fast dump data from 18nov04 showed that the birdie was jumping every 2.67 seconds. This is the same time period that the dewar monitoring system needs to cycle through 1 receiver. On 19nov04 data was taken with the lband receiver. When the birdie appeared, the dewar monitoring program was stopped. The birdie was still there!!
I then moved to the 430 Mhz receiver. The signal is 20db stronger than lband and you can see the frequency jumping every 2.67 seconds. Data was taken with 1 seconds dumps, 25 Mhz bandwidth and 1024 channels.  The dewar monitoring program was stopped for about 50 seconds. The image shows the dynamic spectra when the dewar monitoring program was stopped.
• From 0 to 125 seconds, the dewar monitoring program was running. The birdie was jumping between 428.5 and 436.5 .
• The dewar monitoring program was stopped at second 125 and restarted at second 175. During this time the birdie did not jump in frequency.
• At 175 seconds the dewar monitor program was restarted and the birdie started jumping again.
The plot shows that the birdie is coming from the dewar monitoring system. During 140 milliseconds of the 2.67 dewar monitor cycle, something is being done that causes the birdie to jump up in frequency. This could be a particular mux address being accessed or maybe switching between dewars when the match of some circuit is changing.
Turning off the dewar monitor program does not make the birdie go away. It just stops it from jumping.  When stopped, the dewar monitor program is no longer cycling through the event that causes the jump. At lband you see no change when the program is stoppped because the jumped to higher frequency is too weak to ever see (because lband is 20db weaker than 430, and the higher frequency has a 5% duty cycle).
processing: x101/041119/rfi.pro.

#### The dewar monitoring system.  (top)

The dewar monitoring system measures the dewar temperatures, bias voltages, currents, etc... It is installed on 9 of the receivers. For each dewar there are about 28 analog readings to be made. The system conists of
•  an hp34970 that has an a/d converter and digital i/o to control the mux addresses.
• A muxbox that receives the addresses from the hp34970 and passes it on to the 9 receivers. It also takes the analog signal from each dewar and passes back only the receiver selected to the a/d converter in the hp34970. This is done via a 10 way analog multiplexor.
• The bias box of each receiver. It contains an analog multiplexor to select between the 28 values to be read for each dewar.
• The postamp chassis of each receiver. This contains the lakeshore temperature sensor that is used to read the dewar temperatures locally.
The digital i/o lines (mux/rcv addresses) go from the hp34970 to the muxbox. From the muxbox they fan out to the 9 receviers. The two bits that are used for the temperature diode select (16K,70K,omt) go first to the postamp chassis and then to the bias box. The other 3 bits go straight to the bias box. These i/o lines are common to all recievers. They are not buffered in the muxbox.
To read a receiver, the hp34920 first selects the address of a receiver. This controls the muliplexor in the muxbox. It then cycles through the 28 addresses for the data to read. All 9 receivers switch together and send their analog signal to the muxbox. The muxbox then passes the analog signal from the selected receiver back to the hp34970 a/d converter.

#### Summary of what we know:  (top)

From 11jun04 lband measurements.
• There are drifting birdies at lband with spacing close to 60 Mhz. These birdies were first seen in mid may 2004.
• If they are harmonics, then the fundamental is drifting from 58.5 Mhz to 60.9 Mhz.
• At lband the birdies have two peaks in freq. which are separate by about 250 Khz.
• The birdie strength  gets up to about 10% of Tsys.
• The birdie strength varies with time.
• The birdie strength may also vary with azimuth but is difficult to discern because of the time variation.

• The frequency of the birdie increases as we approach 90 degrees az. As we return to 450 it remains  constant for awhile and then decreases in frequency. This happened for both data sets.
From 28oct04 measurements.
• The birdies are seen at lband and at 430 Mhz.
• The frequency difference of the harmonics at lband can be used to predict the frequency at 430Mhz. During this day, the harmonic difference gave a fundamental that drifted
• The birdies at 430Mhz are about 20db stronger than at lband. They can be 400 to 500 Kelvins.
• The narrow (20Mhz) bandwidth of the 430Mhz system can cause the birdie at 430Mhz to fall outside of the 430Mhz rcvr system bandwidth.
• The linear fits to the two harmonics gave a fundamental the drifted from 61.17 to 61.84 Mhz. The drift rate was .047 Mhz/sec. This a drift of .00078 parts per second. When looking at the lband and 430 1 second integrations,   only 1 peak in frequency was seen. When we looked at the spectrum analyzer with the 430 Mhz system we could see two peaks and lots of jumping between them. The correlator resolution at 430 Mhz was 25Mhz/1024=25 Khz. Since the drift rate was 47 Khz/sec our integration time of 1 second was smearing the channel resolution to 47 khz. The spectrum analyzer sweep time of 4 milliseconds did not have as much smearing.
General:
• The biridies have been seen in the alfa receiver and the lband wide system.
• The birdies have been seen in the 430 dome receiver.
• The birdies have been seen during the day, thru the night, and in the early morning hours.
• If the birdies are harmonics of 60 Mhz, the measured differences in harmonics would have the fundamental drifting between 58 Mhz and about 63 Mhz.
• 04nov04: The birdies are not coming from the turret room air conditioners.
• 05nov04: The 430Mhz transmitter was switched to load while the birdies were seen with lbw. This switches the 327, 430dome,and 610 receivers to a load. The birdies remained in lbw. This means that the birdies are not coming from an amplifier oscillation in one of these receivers and then shooting back out the mouth of the horn. The amplifiers of the other receivers can not transmit 60 Mhz out of their horns since this frequency is below the waveguide cutoff (and the other receivers all have waveguide). See file  corfile.05nov04.x101.1 scan 431000003.
• 05nov04: The shutters for the receivers with shutters (xb,cbh,cb,sbw,sbh,sbn) were closed. The birdies did not go away. (see file corfile.05nov04.x101.1 scan 431000006).
• 05nov04: When the birdie was strong in the lbw receiver i looked at the output of the 430 ch receiver and could not identify the birdie (this should be repeated when we look at the birdie at 430 Mhz in the dome).
• 05nov04:When the birdie was strong at 430Mhz, we plugged the 260 Mhz IF into the control room radio receiver. We could hear a 120 Hz hum on the radio as well as the jumping between the 2 tones. The signal is not constant in time. It comes and goes.
• 05nov04: we looked at the output of the whip antenna on top of the 430 transmitter room and the wwv antenna by the rfi shack. We could not see any suspicious signals around 60 Mhz. This measurement was not very sensitive because of the loss in the cables and the type of antenna used (wwv).
• 12nov04: switching 430 between sky and load made the birdie disappear. So the birdies are coming in through the horn.
• 18nov04: The birdie jumps about 4 Mhz higher in frequency  every 2.67 seconds. It stays there for 2.67 seconds and then jumps back down. The jump takes less that 1 millisecond. This points to the dewar monitoring system as the culprit.
• 19nov04: stopping the dewar monitor program caused the birdie to stop jumping in frequency. The dewar monitoring is somehow causing the birdie.

# RESOLUTION:

19Nov04: The birdies are coming from the dewar monitoring system (writeup to come).

#### 14dec04: birdie seen with spectrum analyzer at 62.457 Mhz in the dome.  (top)

On 14dec04 the birdie was being monitored with lband. It appeared at 15:52 (same time as previous day!). The textronix ybt250 spectrum analyzer was used to search for the birdie in the dome with a small whip antenna. The birdie was seen at 62.457 Mhz using the spectrum analyzer. It was up to 30 db above the noise floor using a10 Mhz bandwidth.  The image shows a screen dump of a dynamic spectrum of the birdie.
• The dynamic spectra is 80 1 second integrations, with a 1 Mhz bandwidth. The rbw is probably (1Mhz/512=2 Khz).
• The birdie drifts from 62.447 to 62. 517 in the 80 seconds.  The absolute frequency is probably a little off since it did not use a reference signal.
• The color scale is 6 db per color. The biride is 20 to 30 db above the noise floor of the spectrum analyzer. For a 1 Mhz span the measured noise floor of the spectrum analyzer  is about -128 dbm. This gives a noise temperature of about 5000Kfor the device.
• The time stamp in the image is off by about 15 minutes, 51 seconds. I don't know whether it refers to the first or last spectra (probably the last).
• While this was happening, lband had a birdie at 1437.2 (62.487)  and drifting up .
• It may be hard to see this if the birdie falls near tv channel 3: video at 61.25 Mhz, color at 64.83, or audio at 65.75 Mhz.
• processing: x101/041214/rfi.pro

#### 17jan05: The 60Mhz birdie, what it was and how we fixed it.. (top)

The 60 Mhz biridie was rfi that produced harmonics close to 60 Mhz. They were seen in the lbw, alfa, and 430 Mhz receivers (the 327 receiver frequency range did not include 300 or 360 Mhz so it was not seen there).  These birdies were first noticed around may04. They were time variable. The interference would show up for awhile and then disappear only
to reappear later. Typical times would be 10 to 60 minutes when it would be always present. Another characteristic was that the birdies would drift in frequency. They could drift by 20 Mhz in 5 minutes and then settle down and remain constant. The  birdie drift  and the 60 Mhz spacing were used to identify the birdie as belonging to the 60 Mhz harmonics.

High time resolution data taken while the birdies was present showed that the birdies  jumped by a few Mhz every 2.7 seconds. The jump would last for about 140 milliseconds and then it would return to the old frequency. 2.7 seconds is the time that it takes the dewar monitoring system to cycle through all the readings of a single receiver. Further tests showed that if the dewar monitor system was not cycling through the receivers, then the birdie remained but this jumping in frequency stopped. This pointed to the dewar monitoring system as the culprit.

The dewar monitoring system is built into the post amp chassis's and the bias boxes for each receiver. The cables that control the receivers all run in parallel from the control device out to the post amp chassis and bias boxes. Using a portable spectrum analyzer and a small antenna we were able to see the birdie in the dome at 60 Mhz (although it was still variable).  After awhile we noticed that changing the cal configuration (correlated cal, uncorrelated cal, crossed cal..) could make the birdies start or stop (although not all the time).  This cal configuration is also in the post amp chassis. Eventually we narrowed the problem down to the sband wide (sbw) post amp chassis.

The cal configuration control was using some high speed cmos chips. Some of the gates on the chips were not used and they had been left floating. Some of the configurations of the cals setups would also leave some high speed gates floating. It turns out that the floating gates were oscillating. The oscillations were many volts. Putting the antenna close to these chips showed oscillations 50+db above the noise floor ( the noise floor was probably a few 1000 K) and they continued through the 0 to 200 Mhz range.

The solution was to ground all of the unused gates on the high speed chips (a better solution would be to remove all the
high speed chips since they are not really needed). Resistors were also added so that none of the cal configurations allowed any of the high speed gates to float. These changes on sbw got rid of the 60 Mhz birdie.

When the chips were oscillating, they would couple to all of the lines leaving the sbw post amp chassis. The dewar monitoring cables would then fan out to all receivers and act as radiating antennas. The 140 millisecond long frequency jump every 2.7 seconds was probably caused when one of the addresses in the dewar monitoring read back was selected. This address must have been affecting the oscillating chips and caused the oscillation to jump in frequently

Sbw was causing the oscillations, but all of the receivers have the same hardware circuitry (although the wiring for sbw looks  a bit of a mess!!). We are in the process of making the changes to all of the high speed chips in all of the post amp chassis so that this problem does not crop up in another receiver.