• the cal Deflection total power vs time (.pdf)
• the calRatio vs freq plotted for each scan (.pdf)
• a fit to the CalRatio measurements (.pdf)
• computed Tcal and tsys in K using the new cal values (.pdf)

Links to SECTIONS:
Why the cals were remeasured.
The model used to compute the cal values
The parameter values used in the computation
Taking the data
Processing the data
Results
    Total power vs time by freq band.
     calRatio (caldif/caloff) vs freq
     Fitting to calRatio vs freq
     Computed Tcal and  tsys vs freq
Summary

Why the cals were remeasured.

• Prior to 08Nov21 the cal value was estimated to be 45 K (by looking at diode output and component losses).

Parameters used for Tcal computation

    The values i ended up using for the computations (see below for explanations) were:
 

	value	Notes
Tabsorber	301.3K	83 F from temperature sensor in absorber
Tsky	15K	includes any scattered energy. see below
Treceiver	74,68K	polA,B  see below
Tomt	300k	omt temperature.. a guess.
g	.05	reflection coef for horn/omt see below
alpha	.07	omt absorption

 Taking the data:

• The hardware winking cal was used:
• 20 millisecond on
• 20 millisecond off
• the cycle started on a hardware 1 second tick.
• The mock spectrometer was set up to measure the spectra.
  
• 7 mock boxes were used each with the following setup:
• 172.032 MHz bandwidth
• 2048 spectral channels
• polA,B  measured
• data  sampled at 2 milliseconds (winking cal was 20 ms on , 20ms off).
• each scan (sky or absorber) took 2 minutes of data.
  
• The 7 band frequencies were:
• 8219.00 8363.00 8505.00 8647.00 8789.00 8931.00 9073.00 MHz (center freq 8646)
  
• The bands are spaced by 142 MHz
• the freq  limits are (8219-172/2)  and (9073 + 172/2.)=  [8133,9160] or 1027 MHz bw
  
• The freq range of the measured cals was limited to 8150 to 9150 since there is a 1-2Ghz IF filter.
• The measurement started at 9:09 and finished by 10:27 ast.
• We did 3 sky measurement scans and 2 absorber scans.
  
• The scan sequence was:

• Loc	ra , dec j2000	az,el	mockFile #	Notes
  sky	15:53:30 , 36:59:00	336.6,69.4	100	power dip last 10secs.excluded that data
  abs	--	--	200
  abs	--	--	300
  sky	17:06:36 , 36:57:48	338.8,69.8	400
  sky	17:06:36 , 36:57:48	337.3,69.5	500

Processing the data:

    For each scan the following is done:

• Input the winking cal spectra and compute the ratio of calDeflection(freq)/Caloff(freq)
  
• Average the 20 ms of calon, caloff, giving bCalOn[25*120], and bcalOff[25*120]
  
• exclude the 2 ms spectra before and after each cal transition. This gives 16ms of data after the average
• Use the median for this averaging
• Compute the rms by chan for the bCalon and bCalOff data (for debugging)
  
• Compute the  avg calOn, calOff: bonAvg,boffAvg using the median for averaging
• compute the caldeflection and then the calRatio (by Tsys)
• bcalDef= bcalOn - bcalOff
• bcalRatio=bcalDefl/ bcaloff
• this removes the IF bandpass.
  
• interpolate bcalRatio so that the data across the 7 bands is a single spectra with fixed freq step (84Khz )
• This is then the cal value in units of Tsys (with cal off).
• Average calRatio for the 3 sky scans and the 2 absorber scans.
• Fit a curve to the calRatio for sky and abs.
  
• Compute Tcal using:
• Just the RatioAbs, RatioSky, and the combined RatioSky,RatioAbs (to remove Trcv).
• play around adjusting gamma, alpha, and Trcv to try and get the 3 different Tcal measurements to agree.
  

The Results

Total power vs time by freq band

    Plotting the cal Deflection total power vs time (.pdf)

• The calRatio=(CalOn - calOff)/calOff for each 40ms cal cycle was computed
• for each 172MHz band the total power for the cal ratio  was then computed (giving 25*120=3000 points/2 minutes scan).
• This total power was then normalized to its median value (for each 120 sec scan) so we could see the gain variation for each scan.
• There are 4 frames per page
• Each of the 7 freq bands uses 2 frames (top polA, lower: polB).
• In a single frame there are 5 scans (horizontally):
• sky,abs,abs,sky,sky.
  
• blue lines separate the scans. (there is a time lag between each set of  scans).
• What the plots show:
• the end of the 1st scan shows a dropout (the power dip). this data was not used
• The value seems to vary by +/- .5%
  
• This occurs for the sky and the absorber so it is probably a gain variation (via temperature?).
• the 8363 MHz frames show variation when on sky. This is from rfi.
• The 2nd abs scan decreases and then goes back up. This is probably the absorber shifting a bit during the observation.
• The final result will average over this +/- .5% variation.

Calratio (calDif/calOff) vs frequency

    The 2nd set of plots shows the calRatio vs freq plotted for each scan (.pdf)

• The division by calOff removes the IF bandpass.
• Page 1: calRatio vs frequency
• Each color is a different scan:
• black,blue,pink: on sky
• red,green : on abs
• The dashed vertical lines (purple) show where the rfi lies in the 8363 MHz total power band.
• The cal is .4 to .3 * Tsys on sky
• the cal is .1  * Tsys on absorber
• You can see ripples in the cal values vs freq in the sky and the absorber measurements.
• the ripples are smaller for the absorber (but the absorber Tsys is much higher).
  
• Page 2: Normalize each calRatio scan to their median value
• top frame: polA
• bottom frame: polB
  
• the ripples are now the same size for both the sky and absorber.
• Since we divide by the calOff, these ripples are probably not coming from the IF bandpass.
• The ripples in the cal are probably reflections between the cal diode and the cal coupler.
• Page 3: ACF of calRatio spectra. Ripple spacing.
• the acf of the calRatio spectrum shows the spacing between the reflection points (if it is a standing wave).
• Top: polA, bottom: polB
• The lag peak is about .015 usecs.
  
• With  n=1 this is a distance of 2.26 meters.
• with n=.7  the distance is about 1.6 meters.
• This is probably the cable going from the diode pwr splitter to the cal coupler (i wonder if there is a pad in line?).
• Page 4:  rms/mean by channel for the calon, calOff  spectra
• black polA, red: polB
• top frame: on sky
• bottom frame on absorber
• The rms by channel is computed for the 3000 calOn and calOff cycles for a 2 minute scan.
• The 3 sky scan and 2 abs scans are over plotted.
• You can see the larger rms in the 8319-8350 mhz spectra for the on sky calratios.
• the lines going off scale are the edges of the band passes.
• The mean value is a bit higher than the expected value, probably because of the gain variation during the 2 minutes.
  

Fitting a curve to the calRatios.

The plots show a fit to the CalRatio measurements (.pdf)

• The 3 sky measurements and 2 absorber measurements were averaged to give a sky and absorber calratio vs freq.
• A robust harmonic fit was then fit to the calRatios vs freq
• the fit included:
• a linear term
• A 17th order harmonic fit (fitting ripples of 172MHz down to 172/17 = 10MHz spacing).
• I looked at harmonic fits of 10 to 22 order and settled on 17 since the fit residuals did not improve much after this.
• The fit was iterated throwing out outliers and then redoing the fit.
• Page 1: calratio vs freq and fit
  
• black is polA , red is polB, green is the fit
• the blue * points were excluded from the fit.
• Top: On sky calratio vs freq and fit
• The blue * excluded points around 8320 were from rfi.
• 2nd frame: on sky fit residuals
• The residual rms was around .0019*TsysSky.
• 3rd frame: fit to calRatio on absorber.
• the rfi was not present on absorber
• bottom: absorber fit residuals
• the residual rms was around .0008*"TsysAbs
  

Computed Tcal and Tsys using the cal values.

    The  plots show the computed Tcal and tsys in deg K using the new cal values (.pdf)

• Page 1 Tcal vs frequency
• Tcal was  computed using calRatio from the y factor as well as  just calRatioAbs and calRatioSky.
• the values of Trcv, alpha, and gamma were modified to get the closest agreement with the 3 methods.
• PolA
• black thin line: used y factor calRatio
  
• black * used calRatioAbs
• green * used calRatioSky
• PolB:
• red thin line: used yfactor calRatio
• *  red used calRatioAbs
• * blue used calRatioSky
• We ended up using:
• Tsky + Tscattered: 15K
• Trcv(a,b): 74,68K
• gamma=.05,alpha=.07
• Page 2: Tcal  vs freq and Tsys vs freq
• Top:  Tcal vs freq
• this is from the y factor measurement after adjusting gamma and alpha.
• TcalPolA is about 40K,
  
• TcalPolB  is about 33K
  
• The + are the cal values sampled every 10MHz.
  
• they are entered in the cal table that users will use for the cal value
• routines will then linearly interpolate the cal value between these 10MHz points.
• Bottom Tsys vs freq
• the cal off data was averaged, divided by the cal deflection and then multiplied by Tcal to get the system temperature on Sky.
• Tsys ranges from 100K to 125K with polA being a little higher than  polB

Summary:

• The single cal diode feeding polA ad polB of the 12meter was measured on 17jan22 in the morning.
• the tcl routine /share/megs/phil/svn/aosoft/p12m/x101/220117_meascal.proc was used to take the data.
  
• The hardware winking cal data was taken for 2 minutes on each scan.
• There were 3 scans on sky
• there were few clouds and the elevation was around 70deg
• There were 2 scans on absorber (the absorber was 83F during the measurement)
• Looking at the total power vs time showed up to a +/- .5% variation in the calratio during a 2 minute measurement
• the noise diode is  temperature stabilized. This change is problem temperature variations of the electronics.
• The calRatio (calDeflection/calOff) vs freq showed ripples vs freq.
• this was seen on sky and absorber.
• this rules out a reflection from the structure.
• dividing by CalOff would cancel most ripples in the if/lo.
  
• the  period corresponded to a length of 2.26 meters with n=1, or 1.6 meters if n=.7
• this could be the cable from the cal splitter to the cal couplers.
• Rfi was seen from 8319to 8350 MHz while on sky
• a 17th order harmonic function was fit to the calRatios vs freq and then  used to compute Tcal
• the fit residuals were: .002*TsysOnsky, and .0008*TsysOnAbs
• The Tcal was then computed using:
• the Y factor (abs hot load, sky cold load)
• this removes Trcv but needs gamma(reflection coef horn,omt) and alpha (absorption in omt).
• Using just calRatioAbs
• using just calRatioSky
• Trcv, alpha, gamma were adjusted to get the 3 measurements as close as possible.
• Tcal values
• Tcal polA about 40K
• Tcal polB about  33K
• a table of values spaced every 10MHz is stored in a file
• /share/megs/phil/svn/aosoft/idl/data/cal12m.datR11
• the idl routines:
• calinpdata.pro will input the entire table
• calget1.pro will input a value at  a single freq.
• Tsys vs freq.
• using the calOff measurements on sky Tsys was measured.
• It varied 100 to 125k over the entire freq band with polA being higher than polB.

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