Lbw cal values measured 06aug04

12sep04

Links to PLOTS:
hcorcal value versus freq from sky and absorber
spectra of hcorcal on/off-1 on sky
power levels during hcorcal measurements

Ratio of other cals to hcorcal tracking blank sky.

Compare Tsys using the old and new cal values.

Links to SECTIONS:
Measuring the high correlated cal using blank sky and absorber
Measuring the other cals on blank sky relative to the high correlated cal.
Has the cal value stopped drifting??

Measuring the high correlated cal using sky and absorber: (top)

    The high correlated cal value (diode 1 going to polA and polB) for lband wide was measured 06aug04 on the telescope using the sky/absorber technique. The telescope tracked blank sky and used 3 second cal on/offs. The data was taken with the radar blanker on for both the sky and absorber.
    Tsys for polB started to drift on 24apr04. It continued to drift until the first of jun04. This drifting was in the polB measurement. It did not matter whether diode1 or diode2 was feeding polB. The sefd was measured for polA and polB using an unpolarized source and they remained constant. So the drift was in the cal measurement. It was coming from after the diodes. After waiting for polB to stop drifting, we remeasured the calvalues using sky and absorber.
    The Trcvr measured on the antenna test range in feb03 were used to remove the Trcvr.

The temperatures used in the computation were:

Tabsorber 301 K

Tsky 5 K

Treceiver from test shack

Tscattered 15 K

Each frequency band was measured 9 separate times on the absorber and 9 separate times on the sky. For each measurement the calon/off-1 spectrum was computed. A 3rd order polynomial was fit to this data to generate the cal values (throwing out any data points greater than 2 sigma on the first iteration of the fit). The cal Value versus frequency plots show the results.

Fig 1 is the measured cal value in deg K versus frequency. The colors are: Blue is measured using the sky and absorber ratio. Black is using just the absorber and red is using just the sky. The Top plot is polA and the bottom plot is polB. The dashed red line is the cal value measured at the antenna test range.. The dash green line is the receiver temperature used (from the antenna test range measurement). The cal value from the sky does not agree very well with the values from the absorber and absorber, sky ratio.
Fig 2 is a 3rd order polynomial fit to the calAbsorber and calRatio measured values (it will be used to generate the cal value table). The green crosses were used for the fit. The dashed red line is the cal values measured on the antenna test range.
Fig 3 and 4 show the cal values for each measurement (polA, polB). The top plot has the 9 measurements on the absorber. The center plot shows the 9 measurements on the sky. The bottom plot is the absorber, sky ratio. The jumps in the cal value spectra are from rfi.

The spectra of calOn/Caloff-1 for the 9 sky measurements shows the spectra for the on sky measurements. 100 Mhz junks at a time were taken. Pol A is clean in the 1300 to 1400 Mhz band while pol B has some gain jumps in this region. The radar blanker was on during this measurement. This same behavior was seen in polB in mar03. Maybe polB is saturating somewhere and not recovering??

The final plot has the power levels during the measurements.

Fig 1 top is the power at the upstairs fiber optic transmitter for all of the measurements. There were 324 records on sky followed by 324 records on the absorber. The large dips are at the edges is coming from the filters cutting off the unused part of the band. The spectral density is probably still ok (since -40dm is for 200 Mhz bw).
Fig 1 bottom is the power at the downstairs if power meter.
Fig 2 plots the 50 Mhz band (via the power counters) that is input to the correlator 8 bit a/d converter. Top to bottom is correlator board 1 (first 25 MHz of 100 MHz section) through board 4 (last 25 MHz of the 100 Mhz section). The level has been divided by the median value (the scale is linear in power). The jumps at the beginning are larger since the cal is a larger fraction of Tsys when on the sky (the first 324 records).
Fig 3 plots the 25 Mhz 9 level total power measured via the 0 lag of the correlator. A 25 MHz digital filter sits between the power counter of figure 2 and this measurement.

processing: x101/040806/hcorcal/lbwinp.pro,lbwcal.pro,lbwspec.pro,lbwdiag.pro

Measuring the other cals using sky and the high correlated cal from above. (top)

The high correlated cal was measured above using sky and absorber as the hot and cold load. The other cals were then measured relative to the high correlated cal. Blank sky was tracked and the following cal sequence was run:

hcorcal(on,off)
hcal(on,off),hxcal(on,off),h90cal(on,off)
hcorcal(on,off)
lcorcal(on,off),lcal(on,off),lxcal(on,off),l90cal(on,off)
hcorcal(on,off)

100 Mhz at a time was measured (4 by 25Mhz) going from 1120 to 1720 Mhz. The cal was cycled on/off for 3 secs at each step. The entire frequency range was repeated 6 times.
The calOn/calOff -1 spectra was computed for each measurement (giving cal in units of calOff or Tsys). Each of these 25 Mhz band passes was cumfiltered. This was done on the 6 loops of the same freq band (256 channels *6 numbers). The total power was computed and then the ratio of each cal value relative to the hcorcal was computed (via interpolation of the 3 measurements). These values were used to generate the cals table (after multiplying by the hcorcal in kelvins). No polynomial fit to the data was done. The figures show the other cals relative to Tsys and the hcorcal:

Fig 1. This is the cal value as a fraction of Tsys for each of the cal types. The top figure has the high cals while the bottom figure shows the low cals. The lines with * are polB. The cal types where the same diode feeds the same polarization (polA horcal,hcal.. polB hxcal,hcorcal, etc) all track extremely well even thought they were different measurements.
Fig 2. The calType/hcorcal. The top has the high cals while the lower plot is the lowcals. The frequency ripple in the high cal ratios is probably from the 90 deg hybrid (diode2 ->). The low cal diode1 ratios have a large sine wave. This is probably coming from the directional coupler used to generate the low cal from the high cal.
Fig 3. The hcorcal was measured 3 times in each set of 10 measurements. This plot shows how well the cal on and cal off values repeated. The places with jumps is coming from rfi.

processing: x101/040806/othercals/lbwcal.pro,newlbwcal.pro

Has the cal value stopped drifting?? (top)

The daily Tsys measurements showed the initial problem with the cal values. I took these values and recomputed the value of Tsys using the new cal values for the time period 4jun04 thru 12sep04. The plots show Tsys versus day number of year for this period.

Fig 1 Top. Tsys versus day number for 09apr04 to 12sep04 using the old cal values. Black is polA, red is PolB. The + are the uncorrelated cal with diode 1 feeding polA and diode 2 feeding polB. The * are the high correlated cal with diode 1 feeding polA and polB. You can see the initial jump around day number 115 (24apr04)
Fig 2 bottom. Tsys versus day number using the new cal values for 04jun04 (156) thru 12sep04. The blue dashed line is the start of the new cal values. The green dashed line is when the cal values were measured (06aug04). On the day of the measurement (219) the polA and polB tsys values are the same. Before this date there was some difference (you need to ignore the big jumps that are probably rfi or some source in the beam during the measurement). After day 219 it looks like the red and black measurements are starting to diverge a little. The problem with polB cal drifting has probably not gone away. We will have to monitor it for awhile longer to see how much it changes.

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