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Intro

   On 12apr21 12meter xband spectra were dumped at a 1millisecond rate to check on the dropouts we had seen on the oscilloscope (more info). We later found that  these dropouts were caused by the sband portion of the signal saturating the system (probably the  Fiber tx ?). On 26apr21 we  retook some fast dump spectra with the sband signal terminated (at the input to the postamp chassis in 50 ohms).
     The setup we used was:

• Telescope sitting at az=120, el=41.285 . It was a little after noon so the sun had transited and was not close to the beam.
  
• The sband signal was terminated at the postamp chassis input so the system would not go into compression
• take 7 172 MHz bands covering xband,.
• take 512 channel spectra across 172MHz every .238 milliseconds for 180 seconds.

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Processing the data

• For  each .238 millisecond spectra compute the total power (2pols by 7 freqbands)
• there were 756000  spectral/totalpower samples in the 180 seconds of data.
  
• When computing the spectra of the total power
• spcTp= abs(fft(totalPwrTmseries - meanvalue))

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Displaying the results

Total power time series for each freq band (.ps) (.pdf)

• Each frame is as separate 172 MHz freq band
• Black is polA, red is polB
• The .238 millisecond samples have been smoothed/decimated to 7.38 millisecs (so the file doesn't get too big)
• The timeseries has been divided by it's mean value (to put it in units of Tsys).
• offsets have been added for display: +.01 for polA, -.01 for polB
• The slow variation is similar for all 7 freq bands. It is a about 1% of Tsys..

Total power spectra: polA   (.ps)  (.pdf)

Total power spectra: polB   (.ps)  (.pdf)

• The frequency of each band is plotted in a different color with an offset for display.
• The spectra was smoothed/decimated to .056 Hz frequency resolution.
• You can see birdies at 60hz, 120hz, and around 1Khz.
• The 1Khz spike is seen in polA and polB

Blowup of the total power spectra (.ps)  (.pdf)

• These spectra have the full freq resolution: .0056 Hz
  
• Page 1: blowup 0 to 2 hz
• top is polA, bottom is polB
• each color is the spectra from a different 172 MHz band
• Nothing stands out.
• Page 2: blowup around 1020 Hz.
• top  polA
• The birdie is centered around 1018.71Hz
• Bottom: polB
• the birdie is centered around 1021.589
• it is a little stronger and wider than polA
• The 1KHz birdie is probably coming from the fiber optic transmitter.
• It is dithering the output at 1Khz.
• Each fiber optic transmitter has a slightly different freq (differs by about 3hz).

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Summary

• the total power  varied by about 1%.
• We  should probably try this at night to make sure the sun is not coming in through a sidelobe.
• There is a 1018.7 Hz birdie in polA
• there is a 1021.589 birdie in polB
• These may be coming from the dithering of the fiber optic transmitter.

processing: x101/210426/p12mfastsample.pro