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Intro

   Debugging the xband hub amps (20db)  showed that they were causing a problem. on 23jun24 they were brought down to the lab for testing. The xband chain was left connected with a bullet rather than the amps. This reduced the gain in the chain by 20db and increased Tsys by about 40 K (more info).

    Cal on,offs were then repeated on and off the sun, as well as a few driven scans across the sun. This was to see whether the system was still saturating while on the sun. This would give us an idea of how large an attenuator we'd need when looking at the sun.

    The cal on off setup was:

• A jpl horizons ephemeris was used to track on and then off the sun.
  
• 30 second cal on followed by 30 second cal off was performed on and then off the sun.
  
• the time was increased to 30 secs so we could average over the total power variations while on the sun.
  
• The sequence was; off sun calon/off, on sun calonoff, off sun calonoff, on sun calonoff.
• The cal diode is probably around 45K (need to measure in more accurately).
• When off the sun.
  
• 2 deg (great circle ) offset in azimuth was used to get off the sun.
• 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 were dumped once a second
• the power levels were adjusted off source so 1 sigma voltage rms was 20 samples (rather than the normal 30).
• This was to try and give us a little more dynamic range on,off sun.
  

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Processing the data

• The total power was computed for each 1 second sample of each 172 MHz band. no rfi excision was attempted.
• For each frequency band, the total power was normalized to the average of the cal Off data when off the sun.
  

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Plotting the calOn,off results:

    The plots show the results of the calon,offs (.ps) (.pdf):

• Page 1: calOn,offs all freq bands
• The x axis is the total power samples (1 per second). 30 calon, 30 caloff,, repeated for the 2 sets
• the y axis units are Tsys (caloff, off sun avg).
  
• 1st,2nd frames: polA
• 1st : on Sun: calOn,off on sun
  
• On the sun the  signal increased by 13 to 22 TsysOffSun (depending on freq band)
• When the hub amps were in the cal deflection on the sun was negative (we were well into compression).
  
• 2nd: Off sun, calonoff
• the cal off is .18 to .34 of Tsys (depending of freq band).
  
• removing the hub amps should have increased Tsys from 50 to 90 K
  
• The cal deflection  off sun with the amps in was about .4 to .6 Tsys, so these numbers are consistent.
  

• 3rd,4th frames: PolB
• 3rd: on sun
• the levels are similar to those measured with polA
  
• 4th: off the sun. we see a cal deflection similar to polA
  
• Page 2: same plots blown up showing a single freq band: 8624 MHz
• top: cal deflection  on sun polA,B 
  
• there is lots of total power variation from the sun.
• Black: polA cal deflection
• red: polB cal deflection
• green line, average calon value
• blue line: average caloff value.
• Using the average values:
• polA Caldeflection .20 [Tsys off sun]
• polB calDeflection .15 [Tsys off sun]
  


• Bottom: cal deflection off sun polA,B
• black: polA
• red: polB
• both deflections are now about .26 TsysOfSun
• With the hub amps in the calDeflections off the sun were .5,.3 TsysOfSun
• So it looks like the system temps for polA ,B are now a lot closer (assuming the cal is the same in polA,B).
• The ratio calDeflection on sun to cal deflection Offsun:
• polA  .20OnSun/.27OffSun = .74
• polB  .15OnSun/.25OffSun  = .6
• So the system looks to have compressed going on the sun.
• This compression is either in the fiber optics or maybe the mocks.
• This is a lot less compression that what we saw with the 20db hub amps installed (but remember the Tsys also probably doubled when the amps were removed.
• So a solar attenuator will need to be more that 23 db (may 30db?).

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Scanning across the sun

Processing the data:

    The azimuth offset was computed using the azimuth position in the header and the azimuth value coming from the ephemeris file (that tracked the center of the sun);.   

Plotting the strips    

The plots show 3 scans across the sun (.ps) (.pdf)

•  Page 1: scans using the 8624MHz band
• black is polA, red is polB
• Sampling rate: 10Hz.
• The scanning was started prior to the datataking.
• The first strip went -1.5 to +5 deg across the sun (great circle) and got well off the sun..
• The scaling for all 3 strips used the average value of offset 3 to 5 degrees of the first strip.
  
• Top: scanning at .1 deg/sec +/- 2deg (great circle)
• the sun is about 30 amin across, the beam is 10amin, so we had about 3 beams across the sun.
  
• scanning at .1 deg/sec= 6amin/sec
• sampling at 10 Hz gave .6 amin/sample.
  
• with a beam of 10 amin this gave 16 samples across the beam
• there were about 50 samples across the sun.
• Middle: scanning at .05 deg/sec +/- 1deg
• bottom: scanning at .05 deg/sec +/- .05 deg
• Page 2: over plot all 7 freq bands for the +/= 1 deg .05 deg/sec scanning
  
• top: polA
• middle : polB
• bottom: tpPolA/tpPolB
• each strip was first normalized to the off sun value
• Looks like the ratio is always less than one.
• if Tsys polB was < tsys polA you would see this difference.
• there may also be a bit of beam squint so they may not be pointing at exactly the same spot.

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Summary:

• With the hub amplifiers removed, the cal deflection (normalized to Tsys) is now withing 12% for  polA and polB.
  
• We thing the cal values to polA,B are close so this means that TsysA is close to TsysB
• When the hub Amps were in, the deflections were .5 and .3 Tsys. So the hub amps were affecting Tsys.
• With the hub amps out the gain went down by 20db and Tsys went up from 50 to 90 (computed, not measured).
• The cal deflection on the sun is now .74(a),.6(b) times that off the sun.
• So the system is still compressed, but no where near the compression with the amps in (where cal Off was greater than calOn)
• a solar attenuator for observing the sun needs to be at least 23 db (probably closer to 30db).

processing: x101/210624/onoffsun.pro,scansun.pro