• 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.

Parameters used for Tcal computation

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

	value	Notes
Tabsorber	305K	Tabs 31C @ xbstart and 35.5 at sb finish. Used 32C for xb abs.
Tsky	10K	includes any scattered energy. see below
Treceiver	80,70	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 13:15 and finished by 13:49 ast.
• We did 3 absorber scans and 3 sky scans.
  
• The scan sequence was:

• Loc	ra , dec j2000	az,el	mockFile #
  abs	--	--	000
  abs	--	--	100
  abs	--	--	200
  sky	07:43:40/25:50:05	40,80	600
  sky			700
  sky			800

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
• 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 3 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.
• Felix measured the flange temperature of the system looking at blank sky prior to installing the receiver on the telescope. The values showed lots  of ripple but the average value was higher that what i had previously been using. So i increased Trcv which made the 3 separate measurements (Yfactor, sky,abs) come closer together.
  

The Results

Total power vs time by freq band

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

• The total power is computed for each 20ms calOn, calOff spectra.
• This total power sequence was then normalized to its median value
• The absorber scans were normalized to the median value of the 3 abs scans
• the sky scans were normalized to the median value of the 3 sky scans.
• Previous cal measurements normalized each scan separately.
  
• This shows the gain variation during the abs and sky scans.
• There are 4 frames per page: polA,B 1 freq band, followed by polA,B  of the next freq band.
  
• In a single frame there are 6 scans (horizontally):
• abs,abs,abs,sky,sky,sky
  
• blue lines separate the scans. (there is a short time lag between each set of  scans).
• What the plots show:
• most freq bands show similar variation in level.
• The absorber varies by about .5% (of Tabs)
  
• On the sky,polA varies by < .5% of Tsys.PolB varies by up to 2%
• The 8363 MHz band shows ripple in both pol.A and polB (this is not seen in the other bands).
  
• the 8789 and 8931 bands on sky show rfi (this is later removed with a robust average).
  

Calratio (calDif/calOff) vs frequency

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

• compute the median spectra for the calon, caloffs
• compute  (calOn-calOff)/calOff  spectra
• The division by calOff removes the IF bandpass.
• Page 1: calRatio vs frequency:  (Calon-CalOff)/CalOff spectra
  
• Each color is a different scan:
• black,red,green : on abs
  
• blue,pink, light red : on sky
  
• The dashed vertical lines (purple) show where the rfi lies in the 8363 MHz total power band.
• The cal is .35 to .2 * Tsys on sky
• the cal is .08 to .06  * Tsys on absorber
• The attenuators that were installed in the cal line are now (3,2) db.
  
• You can see ripples in the cal values vs freq in the sky measurements. They may also be present in the abs measurement.
  
• 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.
• If the ripples were in the cal cables, then you would expect the ripples  in the abs to be much smaller than the ripples in the sky (since the Tcal value is the same, but the Tsys is very different).
  
• 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
• There are two lag peaks
• polA: .011,.015 usecs = 1.62,2.26 meters (if n=1)
• polB: .010,.015 usecs = 1.51,2.25 meters (if n=1)
  
• 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 3 abs scans are over plotted.
• You can see the larger rms in the 8319-8350 MHz spectra for the on sky calratios.
• the negative going lines are the DC channel of each spectra.
  
• 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 3 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
• 2nd frame: on sky fit residuals
• The residual rms was around .0013*TsysSky.
• 3rd frame: fit to calRatio on absorber.
• bottom: absorber fit residuals
• the residual rms was around .0006*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: used y factor calRatio
  
• green: used calRatioAbs
• blue: used calRatioSky
• PolB:
• red: used yfactor calRatio
• lightBlue:used calRatioAbs
• purple: used calRatioSky
• We ended up using:
• Tsky + Tscattered: 10K
• Trcv(a,b): 80,70K
• 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 34K to 23K
  
• TcalPolB  is 33K to 19K
  
• 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 95K to 120K with polA being a little higher than  polB
• The elevation was close to 80 deg.
  
• elevations close to 10deg will see a Tsys increase of about 20%.
• These values are a few kelvin higher that what was measured on 220607 (previous cal value measurements)
  

Summary:

• The single cal diode feeding polA ad polB of the 12meter was measured on 24jun22 in the afternoon.
• the tcl routine /share/megs/phil/svn/aosoft/p12m/x101/220624_meascalsbxb.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 80deg
• There were 3 scans on absorber (the absorber was 32C during the measurement)
• Looking at the total power vs time
  
• on Abs varied < .5% for the 3 scans
• on Sky:
  
• polA had < .5% variation for the 3 scans.
• polB had < 2% variation for the 3 scans.
• the noise diode is  temperature stabilized.
  
• If the change is an electronic gain variation with temperature you might expect the abs and sky to have similar variations.
• The polB drift is rather large.
  
• 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.
• If the ripple was in the cal cable, then the amplitude in sky, abs should be different after normalizing each.
  
• dividing by CalOff would cancel most ripples in the if/lo after the cal injection point.
• So the ripple must be somewhere between the horn flange and the lna.
  
• the  periods corresponded to a lengths of 1.6 and 2.26 meters (n=1)
• 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: .001*TsysOnsky, and .0006*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
• TcalPolA is about 34K to 23K
• TcalPolB  is 33K to 19K
• 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.

12 meter page
home_~phil