lbw check the rf filterbank

11mar06

A diagnostic was written to test the lband filterbank.

The data taking consists of:

Set the cal to hcorcal and the cal port to the 1 second winking cal (external port 7 with the 1 second toggle plugged in.
For each of the 9 filters banks: select the filter and the then take 20 seconds of data (10 with cal on, 10 with cal off).
Do this for each of the 9 filters. Then loop back and repeat the sequence 9 times.

processing the data:

Use the following definitions:

Tsys - temperature from sky, omt, ground
Tcal - temperature from cal diode
Ttm - time dependent Temperature caused by the filter switches not closing properly or changes caused by sources drifting through the beam. The changes in Tsys between the calOn,calOff will probably be small (spaced by 1 second).
Gfilt - gain of the filter. This could change if the switches did not close properly
Gif - gain for the rest of the IF. This will change as we adjust the attenuators.

For each filter we have N sets of 10 measurements of calon,caloff (with 2 seconds between each measurement. We compute:

CalDeflection=(calOn-calOff) = (Tsys+Tcal+Ttm - (Tsys +Ttm))GfiltGif=Tcal*Gfilt*Gif
CalRatio = CalDeflection/Caloff = Tcal*Gilt*Gif/(Tsys+Ttm)*Gfilt*Gif=Tcal/(Tsys+Ttm)
For each of these we compute the median spectra over the 10 measurements.

The calDeflection will show changes caused by Gif or Gfilt. It will not be sensitive to changes in Ttm (at least those that change slower than 2 seconds).

The calRatio will show variations in Tsys caused by Ttm. Ttm can change if the filter relays do not close properly and increase Tsys. It will also change when sources drift through the beam increasing Tsys. When sources drift through, it should occur in both PolA and polB (since most sources are not highly polarized). The calratio will not be sensitive to changes in the gains.

The data

09mar06. 6 complete passes were made through the filters. The plots show:

CalRatio for the 9 filters (.ps) (.pdf): For each filter there is a separate plot for polA and polB. The median calRatio spectra (over the 10 calonoffs) is plotted. There are 6 in each plot since we made 6 complete passes through the 9 filters (each spectra is separated by about 9*22=200 seconds). The y axis has units of Tcal/Tsys. If a filter did not close properly then you would expect the entire 100 Mhz band to be affected. There are some jumps in individual 25 Mhz bands. These are probably caused by the digitizer for this band being hit hard by rfi.
CalRatio for the 6 filters measured at 1370-1470 Mhz (.ps) (.pdf): 6 filters were measured at 1370-1470 Mhz. They are over plotted here to show the calratio measured with different filters. There are 6filters*6 passes over plotted in each plot.
Filter 9 (1280 hipass) shows Tsys variation (.ps) (.pdf): The top figure shows the fractional change in Tsys for polA (black) and polB (red) for the 10 cal off measurements that were made the first time the 1280 hipass filter was measured. Pol A varied by about 10%. PolB was stable so this was not a continuum source passing through the beam. The bottom plot shows the cal deflection. This remained relatively stable. This shows that the temperature was changing rather than a gain variation (since the cal deflection would have seen a gain variation). This variation was not seen in the other 5 passes through the 1280 hipass filter. This variation started when the filter was selected and lasted for the 20 seconds of the measurement. It did not show up in the Calratio plots since the median value of the 10 measurements is close to the normal value. Looking at the spectra, the change was across the entire bandpass.
Conclusions:

The filters do not show any variation in Tsys except for the first measurement of the 1280 hipass filter. On 10mar06 this was repeated and no variation in filter 9 was seen.

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