# 09mar11: 327 Mhz birdies and their Azimuth dependence

09mar11

Dynamic spectra:

the average and peak hold spectra for the az swings (.ps) (.pdf):
the birdie power versus azimith for the 60 strongest birdies (.ps) (.pdf):
Table summarizing the birdies.

#### Introduction (top)

On 09mar11 2 azimuth swings were done with the 327 Mhz receiver to see if there was an azimuth dependence for the birdies that were seen in the band. The dome was at 15 degrees.
The setup was:
• The azimuth was moved between -90 and 270 degrees at .35 degrees/second. This was done 2 times (clockwise CW and then counterclockwise CCW). A single swing took 1100 seconds.
• The 327Mhz rcvr (cooled) with the 302 to 352 Mhz filter was used.
• The mock spectrometer sampled a 57 Mhz band centered at 327 Mhz with 8192 channels. The spectra were integrated for a  second.
• The alfa motor controller and the dome cameras were turned off.
• This is a repeat of the data taken 28oct05 (which used the interim correlator and the 30 Mhz filter)

#### Processing the data: (top)

• For each azimuth swing a bandpass was constructed from the 1029 spectra. A fit to a bandpass was needed rather than an average since some rfi are constant and would remain in the bandpass average. The bandpass was created by:
• Compute a median bandpass from the 1029 spectra.
• Fit a 20th order harmonic and a linear polynomial to this median bandpass using robfit_polyfft(). This iterates the fit throwing out outliers.
• Divide each of the 1029 spectra by this fitted bandpass. This should remove most of the IF bandpass.
• For each spectra compute the median total power and subtract it. This should remove the source contribution.
• PolA and polB were averaged.

#### Dynamic spectra of the azimuth swings:  (top)

Dynamic spectra were made for each of the  azimuth swings showing spectra density plotted against azimuth and frequency. The plots were aligned so that azimuth always increased bottom to top (so the swings from 270 to -90  were flipped). At the bottom of each image is a peak hold spectra (maximum in each frequency channel) for the entire spin.
• Clockwise swing:
• Counter clockwise swing:
• There are a number of frequencies that are time variable and come and go together. They all have modulation on them. The strongest of these are:
• 308.7, 309.9, 311.07, 313.1. 314.3, 317.5, 318.9.
• Some other signals that are modulated and tend to be there all the time are: 312.31, 320 , 321.9 Mhz being the most prominent.
• The radio was tuned into the IF signal and the following radio stations were heard (in the IF):
• 312.31 is the 3rd harmonic of 104.1 (FM).
• 321.9 is the 3rd harmonic of 107.3 (FM). This is the strongest fm station.
• 317.5 was heard to be 107.3 (FM). This may mean that the other birdies (308.7,309.9.. ) are also intermods of 107.3 since they turn on and off with 317.5 .
• The large narrow spikes (327 Mhz 30*Tsys and 317.4 20*Tsys) only lasted for a few seconds. The spikes that are broader (and show the modulation ) were there more often.
• You can see a number of weak combs in the image. They will appear stronger when the channel width is narrowed (since they tend to be narrow).

#### Average/Peak spectra for each az swing:  (top)

For each azimuth swing an average spectrum (over the 1029 spectra) was computed as well as a peak hold spectrum (the largest value in each frequency bin was kept). Colors were used to differentiate between azimuth swings. The plots show
the average and peak hold spectra for the az swings (.ps) (.pdf):
• Top avg.: The average spectra for each swing. The largest birdie is at 306.98 (22*tsys)
• 307.00 (22*Tsys)
• 313.51    (2.*Tsys)
• 338.30 (3.0 Tsys)
• 2nd avg.: This blows up the vertical scale on the top plot
• 3rd Pk hold: This has a peak hold for each az swing (take the largest value in each freq channel from the 1029 spectra). The largest values are:
• 307.00 (600*Tsys)
• 309.90 (19*Tsys)
• 338.30 (65*Tsys)
• 4th Pk hold blowup: The vertical scale on the 3rd plot is blown up.

#### The azimuth dependence of the birdies:  (top)

The frequencies of the 60 strongest birdies were found and then a mask (in frequency) was placed around each one. The largest value in each mask was taken as the value of the birdie at each time sample (this was needed since some birdies drifted in frequency). Birdie power versus azimuth was plotted for each of the 60 birdies (5 birdies per frame). The black trace is the CW azswing while the red trace is the CCW trace. The vertical scale is in Tsys units.
• many of the birdies are changing together with azimuth.
• Some birdies with little az dependence may be in the dome:
• 16: 313.51,
• 25:319.46
• 33:327.00
• 59:350.39
• ind 58 (350.00 Mhz) is coming from the interim correlator in the control room
• ind53:346.53 has similar az dependence as 350.00.. it may be coming from the same spot.

#### The the birdies that have a common azimuth dependence.:  (top)

Looking at the dynamic spectra, there are a large number of birdies that occur at the same azimuth for the CW and CCW azswings. These are coming from the same az location (and probably the same set of intermods).
The dynamic spectra were blown up to show (300 to 327Mhz, 130 to240 az) how the signals change together:

• Blowup: Clockwise swing (.gif)
• Blowup: Counter clockwise swing (.gif)
• The 3rd harmonics of the 4 strongest radio stations are flagged with a solid vertical line:
• 102.9*3=308.7,
• 104.1*3=312.3,
• 106.5*3=319.5,
• 107.3*3=321.9 Mhz
• many of the birdies show azimuth (time) variation that matches the 308.7 (the 3rd harmonic of 102.9).
• dashed vertical lines centered on 308.7 (102.9*) and spaced by 1.2 Mhz fall on many of the signals.
• 1.2 Mhz is:
• 1.2= 3*.4 (the FM station spacing.
• 1.2= 104.1 - 102.9 (radio Rendor - HQ fm stations)
• These 2 station transmitters  are located in utuado within a few hundred feet of one another.
• The birdies at 314.5 and 314.5+1.2 do not line up in frequency with the comb from 308.7 although they show the same azimuth (time) dependence.

The plots show the azimuth dependence:

• The 11 birdies that show a FM station dependence (.ps) (.pdf).
•  Page 1 Clockwise spin:
•  top: Each color is a separate rfi vs azimuth.
• Bottom:  Different rfi's vs azimuth has been plotted with offsets for dispay
• Page 2: same plots for the CCW azswing.
• Page 3-5: plot each of these frequencies separately.
• Group then so that rfi with similar azimuth dependence is close together.
•  The black trace is the CW swing, the red trace is the CCW swing.
• The table below summarizes the values.

#### Summary:  (top)

• The third harmonics of some FM stations are seen in the band (see table below)
• Back in 2005 a filter was put in front of the receiver and the 3rd harmonics were still seen. This implies that the harmonics are being created outside of the receiver.
• A large number of intermods (11 to 15) are also related to the fm stations (see table below).
• There are some narrow constant birdies that may be coming from the az,el encoders. Need to test turning encoders on and off while monitoring the band.
• The table below summarizes the info on various birdies.
 rfi freq fm station Notes: 308.7 102.9 3rd harmonic of HQ . located next to 104.1 radio redentro in utuado 312.3 104.1 3rd harmonic of radio redentor transmitter in utuado 321.9 107.3 3rd harmonic of delta107.3 302.7 303.9 305.1 307.1 309.9 311.1 313.1* 314.3* 317.5* 318.7* 102.9 104.1 These all show time/azimuth variation that match that of 308.7 (102.9*3) Many of these are spaced by 1.2 Mhz (104.1-102.9) * these are spaced by 1.2 Mhz but are offset from the other set. They may be related to 107.3 (see below) 317.5 313.1 314.3 315.5 318.73 107.3 317.5:  back in 2005 This was heard (with the radio in the control room ) to be 107.3. Looking at the dynamic spectra, the other frequencies look like they may line up in az,time with 317.5 which would also make them related to 107.3 321.9-317.5 =4.4  Mhz. This is the difference 107.3-102.9 .. other birdies: 350.00 346.53 350.00 comes from the interim correlator in the control building. 346.53 has similar az dependence so it may also come from the same place 313.51, 319.46 327.00 350.39 Little az dependence ..may be in the dome.
processing:  x101/110309/327azswing.pro