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Alfa filter bank amps cause aerostat radar to compress system.

may05

What was added to alfa and when:

A filter bank before the first mixers was added to the alfa receiver for OH measurements. It has the standard full bandwidth option and a set of filters for OH observing. A 10 db (actually a mix of 10 and 14 db ) amplifiers were added before the filter bank to improve the system temperature. These amplifiers were in the general rf path and affected regular and OH observing. The relevant dates are:

mar05: filter bank installed but left in wide band mode.
05apr05: 10db amps placed in pixel 2a,2b path in front of the filter bank.
08apr05: 10db amps placed in front of filter bank for all alfa pixels.
12apr05: 6db attenuators placed before 10db amps.
25apr05: 10db amps are now only in the OH filter bath. Normal wide band observations do not have any amps.

The 10db amplifiers improved the system performance of the OH observations but caused the regular wide band observations to compress when the radar signals were strong.

Looking at wapp data feb05 thru apr05 to look for compression:

The wapp pulsar data was recently placed in an idl data archive. This allowed the search of the data taken by P2030 (100Mhz bandwidth, cfr 1420Mhz, 64usec samples) between 01feb05 and 01may05 for signals of the aerostat radar. The aerostat frequencies (1241 thru 1261 more info) are outside the 1370-1470 Mhz band of the experiment but the radar can compress the entire RF/IF chain. This can then be seen in the p2030 data.
The processing steps were:

Make a total power time series model of the aerostat radar for a complete 12 second rotation (more info).
Transform the model and keep the first 1 Khz of data (1/12 hz resolution).
From the model find all of the periods whose amplitude is at least 10% of the maximum amplitude. Normalize these periods to the maximum period found. Use these periods to check the data taken by p2030.
For every file (1087 files) taken by P2030 between 01feb05 and 01may05 do the following:

create a total power time series for the first 12 seconds of the file (187500 points) for all 7 pixels.
transform each time series and then truncate to 1 Khz (1/12 hz resolution).
Remove the median value computed of this spectra (over 100 to 1000 Hz) and then remove it.
multiply the model period amplitudes (a few hundred) by the corresponding measured amplitudes and then compute the mean value.
The mean value will be large for data sets that are corrupted with aerostat compression.

The following plots show the strength of the compression plotted versus various parameters. When plotted versus date, this is the julian date. The tick marks are centered on 12 noon utc for that day (8am AST).

All the data vs sample (or file) (.ps) (.pdf) :

The next three links plot the compression versus azimuth, zenith, and julian date.

All the data (.ps) (.pdf) : This has all the data from 01feb05 thru 30apr05. The dashed vertical lines in the bottom plot show when the amps were installed. The az,za dependence is a bit misleading. The gap between az=50-90 is because there was not data taken here while the amps were in.
When the amps where in (.ps) (.pdf): This data is for the three days when all the amps were in (and no attenuators were installed upstairs). It seems strongest for azimuths around 110 and 290. This is about 180 degrees apart. It may have something to do with the orientation of the azimuth arm and the aerostat direction ( an azimuth of about 240 deg).
When the amps where out (.ps) (.pdf): These plots are for when none of the amps were in. There is no clear azimuth dependence. It may be a little stronger at higher za.

Summary:

The amps caused alfa to go into compression a large fraction of the time when the aerostat was up.
Pixel 0 seems to compress more than the other pixels. This probably has nothing to do with the gain of pixel 0 since the aerostat signal is coming in through the sidelobes.
The attenuators (installed 12apr05) did not seem to affect the compression on the 13th (the aerostat was only on for the latter part of the run). On the 15th of april the aerostat was on for the run but there was little compression (maybe the attenuators were installed on the 13th??).

Previous reports of this problem:

This compression was previously reported in:

08apr04: TotPwr images of a2010 apr05 data showing radar with new amp. The 1 second sampling could not distinguish which radar was causing this.

11apr05: P2030 compression of the system with amps in caused by aerostat.

processing: usr/p2030/rfirdr/aero/pltaero.pro

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