Intro
The xband receiver on the p12m telescope
was monitored for 19 hours on 04may21. The setup was:
- 14:54:50 04may21 to 10:31 05may21 AST
- telescope sitting at Az=0 el=75.
- xband receiver using 8050 to 9200 MHz
- Mock spectrometer used to record the data.
- 7 172 MHz bands centered at:
- 8132,8296,8460,8624,8788,8952,9116
- Each band spaced by 164 MHz
- 2 pols, 4096 channels/pol, 42 KHz channel width
- spectra dumped once a second
- When computing the total power, frequency channels with
rfi were rejected:
- compute rms/mean for each freq channels.
- do a linear fit to the rms vs freq and reject any
channels with fit residuals > 3 sigma.
- the rfi rejection was not very helpful. We had 19
hours of data with the mean value varying with temp. The
rms/chan was large so clipping at 3 sigma left a lot of
rfi.
- the total power for each 1 second spectra was then
computed over the good channels (scaling by the channels
used).
- The first and last bands were only partially filled with
rf power. They were cutoff by the p12m rf bandpass filter.
In these cases only the part of the band with power was
used for the computation.
- The total power vs time for each frequency band was
normalized by the median value (so the units become Tsys).
Total power vs time, temp, and ra
The total power vs time for the
entire 19 hour session was plotted:
The
total power vs hour of day with temp (.ps)
(.pdf)
- The x axis is hours since the start of 04may21 (so hr 30
is 6am 05may21)
- Top
- over plot total power vs hour of day for the 7
polA bands
- Middle:
- over plot total power vs hour of day for the 7
polB bands.
- The higher freq polB bands are unstable until close to
midnight.
- Bottom: pedestal temperature
- The pedestal temperature (behind the postamp chassis)
was recorded once a minute.
- there is lots of rfi in the 8788 and 8952 band
starting around 23:00 hrs.
Total
power vs temperature (.ps) (.pdf)
- Top: median (over the 7 freq bands) total power vs
hour of day . black polA, red polB
- The green curve is the temperature over plotted on
the total power (scaled to ft)
- Bottom: linear fit total power vs temp.
- Solid line: tp vs temp.
- dashed line: linear fit
- Black polA, red polB
- The coef:
- tpA=2.99 - .0216*tempF
- tpB=2.20 -.0128*TempF
- The total power units are Tsys.
- So the electronic gain must be changing by 2.2 and 1.3
% per deg F
Total
power vs Right Ascension (.ps) (.pdf)
- Top: PolA total power vs ra
- Bottom: polB total power vs ra
- the dashed red line is the galactic plane.
blowup total power vs time
total power vs time blowup
(.pdf)
- The total power vs time is plotted in 1 hour blocks.
- polA 1st and 3rd frames
- polB 2nd and 4th frames.
- The 7 bands for each pol are over plotted.
- Each 1 hour plot is normalized to unity.
- some jumps
- I've excluded the polB drifting. this lasted for hours.
- 22.13:
- polA jumped by about .5 %. it came back about 3
minutes later.
rfi
Strong rfi was seen in the 8788 and 8952
MHz bands starting around 23.5 hours.
Plots of
the rfi (.ps) (.pdf)
- Page 1: spectral density function and rms/chan
- black is polA, red is polB
- Top: a 1 second spectra at 01:00:10
- You can see rfi in the 5th (8788) and 6th (8952)
band passes,
- 2nd frame: rms/mean by channel using 960 1 second
spectra.
- the rfi sticks out here.
- the green line is the expected rms/mean from the
radiometer equation.
- 3rd frame: blowup of spectra around the rfi
- looks like and interesting modulation is used.
- Bottom: blowup of rms/mean by channel around the rfi
channels
- You can clearly see the frequency modulation of the
signal.
- The rfi covers 8732 to 8970 MHz (about 240 MHz)
- Page 2: Time periodicity of the rfi
- Top total power vs time for polA,polB in the 2
bands
- 3240 1 second samples were used starting at 24.1
hours]
- There is a comb in the time domain/
- Middle: spectral magnitude of the total power
(abs(fft(tp-mean(tp)))
- This shows the corresponding comb in the
frequency domain
- the freq resolution is 1/3240= .3 millihz.
- Bottom: blowup of the tp spectral magnitude
- the first spike is at .0556 hz
- The rfi has a periodicity of 18 seconds.
What is the rfi?
- 8.65 to 9.0 GHz has primary allocation of
radiolocation
- The notes are title 47.87 .. aviation
- 87.173:frequencies: 8750 to 8850 is airborne doppler
radars
- Our rfi covers more than this range.
- title 47.90 private land mobile services.
- didn't see any specific frequencies listed in this
range.
Summary:
- The electronic gain dependence with temperature :
- -2.2%/degF polA
- -1.3%/degF polB
- Pol B drifted by up to 10% over 9 hours. Pol A did not
show this drift
- The amplitude of the drift was a function of the band
frequency. Higher freq drifted more.
- PolA Did have some sharp steps of about .5%
processing: x101/210504/xbmontp.pro