Measuring the cband cal  values.

jan,2001 (last updated 02mar02)

 Links to sections:

Measuring the hcorcal
Measuring the ratio of the "other" cals to the hcorcal.

Links to plots:

The hcorcal measured values:
The ratio  of the other cals to the hcorcal:
Comparing Tsys measurements 2002 before, after cals updated.

Intro:

    A 2 db pad was removed from Pol A of the cband system (after the cal switches). The cal value was measured on 19jan01 using absorber in front of the horn. On 22jan01 the cal value was measured while tracking blank sky. The correlator was configured as 25 Mhz by 1024 lags by 4 sbc.  3950 to 6050 MHz  was covered in 100 Mhz chunks (22 steps). At each step a 5 second integration with the cal on was followed by a 5 second integration with the cal off. The frequency range 3950 to 6050 was covered 3 times (each pass separated by about 6 minutes). On 20dec01 blank sky was tracked and the ratio of the "other" cals was measured relative to the hcorcal. see measuring cals on the telescope using sky and absorber for how the cal value was computed.
 

Measuring the hcorcal: (..top)

The temperatures used for cband were:
 
Temperatures Used
Trcvr from amplifiers 8 K
Tomt orthmode transducer 4 K
Tabs absorber temperature 301 K
Tsky sky + scattered ground radiation 3 + 15=18K
Trcvr Tamp+Tomt 8 + 4 = 12K

The hcorcal measured values show the results of these measurements. Figure 1 plots the cal Values measured the 3 different ways:

  • Red shows cal polA, polB measured relative to the absorber.
  • Green shows cal polA,polB measured relative to the sky.
  • Light Blue is cal polA, polb measured using the ratios of the cals with absorber and sky.
  • The black line is the cal values measured in 1999 prior to the 2db pad being removed from polA.
    • All three methods give consistent values for 4300 through 5000 Mhz (of course I twiddled the value of Tsky for this to happen!). Outside this range the Cal values measured from Tsky do not agree with those from Tabs or the ratios. For each line and  frequency there are 3 separate measurements. They lie on top of each other so there was no problem with time variations of gain or Tsky changing with zenith angle. Between 4400 and 6000 Mhz the old value of Calb (which shouldn't have changed varies from calAbs,calRatio by about 2 degK.

        Below 4200 Mhz the cal measured from the sky and the absorber diverge in opposite directions. calAbs climbs rapidly (especially polB) while calSky drops off. This is most likely explained by the OMT being lossy in this region. Losses would cause the Tabs reaching the amps to decrease (TempOmt < Tabs) hence i used a Tabs too large. Tsky reaching the receiver would increase (tempOmt > Tsky) and i used a value of Tsky too small.

        The cal values from 4300 Mhz to 5000 Mhz is probably ok. The values outside this range are suspect. The data also make you wonder how well the receiver itself is going to work outside this limited range.

    Figure 2 shows the narrow band resonances found while looking at the absorber. It shows 

    (CalOn - CalOff)/(CalOff)  * Tabs+Trcvr  versus frequency for 3 bands.
    Since Tabs is greater then omtTemp, the CalOff bandpass goes negative in this region. When you do the division you get a positive going bump. Looking at Calon - CalOff with no division, the difference is constant which shows that the problem comes before the cal injection. The lowest resonance is close to where the measure cal values start to diverge. 
    processing: x101/010122/cal.pro , x101/010119/doit.pro for resonances.

    Measuring the ratio of the "other" cals to the hcorcal.   (..top)

        On 20dec01 blank sky was tracked with the cb receiver. A sequence of 10 cal on/off measurements were done using the correlator configured as 25 MHz x 4 boards. The sequence was (hcorcal, hcal, hxcal, h90cal, hcorcal, lcorcal, lcal, lxcal, l90cal, hcorcal). This sequence was repeated covering 3950 to 6050 Mhz. The hcorcal was measured using the sky and absorber (see above). The ratio of the "other" cals was then  computed relative to the hcorcal.(see measuring cals on the telescope using sky and absorber ).
        For each cal in the sequence, the cal on/off spectra was computed and then cumfiltered to remove any "obvious" rfi. The ratio with the hcorcal was then computed by interpolating between the 3 hcorcal measurements. The interpolation was used since  each cal is measured relative to the calOff (or tsys) and tsys varies as we track blank sky.
    The ratio  of the other cals to the hcorcal is shown in the plots:
    1. Fig 1 top shows the high cals measured relative to tsys versus frequency. The * are pol B. The vertical lines show the 100 Mhz steps. There were 4 25 Mhz measurements at each frequency setting. Some of the cal sizes track each other since they come from the same diode (e.g.. polA hcorcal:hcal,  hxcal:h90cal, polB hcorcal,h90cal). The bottom plot has the low cals. The bumps at 5750 and 6000 Mhz are rfi.
    2. Fig 2 top plots the ratio of the cal/hcorcal for the high cals. The horizontal red, black line have diode1->polA and diode 1->polB. This is the same configuration for the hcorcal so the ratio is close to 1. The horizontal pink and bright red lines have diode2->polA and diode2->polB. The ratio of about .88 is the ratio of diode2/diode1. The brownish,green diagonal line has diode2->polA through the 0deg leg of the 90 deg hybrid after diode2. This normally gives the 90 deg phase shift cals.  The slope in frequency is the response of the hybrid.
    3. Fig 2 bottom is the ratio of the low cals to the hcorcal. The added ripple (relative to the hcals) is probably the response of the 10 db coupler used to generate the lowcal. The cal values were computed by averaging the cal values over each 100 Mhz step. The black lines with the * show these values.
    4. Fig 3 show how the hcorcal measurement varied for the 3 samples taken at each frequency. The 2nd (red) and 3rd (green) hcorcal measurement were divided by the first hcorcal measurement (and then subtracted 1). Some of the slope is the change in Tsys with za (decreases at the start then increases at the end). 5800 and 6000 Mhz had rfi.
    To check the cals, the Tsys for jan,feb 2002 using the old and new cal values were plotted.
    1. The top figure plots Tsys using the old cal values. The solid lines are polA and the dashed lines are polB. The system temperature is all over the place.
    2. The bottom figure plots Tsys using the new cal values.
        The scatter in polB is < .5K (1.5%). The Tsys for polA are close except for hcal which differs by over 1K from the others.  The same cal value is used for polA hcorcal and hcal since it is diode 1 going to polA. The software assumes that the cal value for diode 1->polA is independent of which diode is feeding polB. From looking at the Tsys measurements it appears that this assumption is not always true. The "other" cal values above 5800 Mhz were not computed because of rfi  (the software will use the value at 5800 Mhz).
    processing: x101/Y01/011220/cbcal/cbcal.pro,chkcbcal.pro

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