Receiver crosshead modulation of the gain.
Total power data was taken with the xband receiver.
A 40 Mhz band at 8800 Mhz was square law detected with a 20 millisecond
time constant and then sampled at 100 hz with the 12 bit A/D converters.
About 40 seconds of data was taken. The plot
shows a 1.2 hz variation in the total power.
1.2 Hz is the period of the crosshead used to cool the amplifier. This
variation should be checked on the other receivers.
The top plot is a time series of the total power data data (PolA) smoothed
to 3 hz.
The bottom plot is a fourier transform of the time series showing the peaks
at 1.2 and 2.4 hz. The 1.2 Hz is about .001 of Tsys.