[evlatests] Switched power at P-band

[Rick Perley]

     The 'lunacy' polarization experiment (run last December) requires 
accurate gain calibration.   Because the system is linearly polarized, 
it's important that we use unpolarized calibrators (to simplify the 
process, primarily).  But which sources are unpolarized?  At P-band, 
most are, but some (like 3C345) are polarized at the ~2% level.  Using 
this source as a gain calibrator has the undesirable effect of making 
the Stokes Q vanish.  (I did this as a test -- and Q vanished).

     So to enable tracking the gain variations (which are often very 
large in our P-band system), I thought of using the switched power system.

     The good news is that for most antennas, the required quantities 
(PDif) are present and (after suitable filtering) can be used.

     The bad news is that some antennas have non-operating, or partially 
operating switched power.  Here is the list of malfunctioning systems:

     ea12:  The PDif values are zero-mean noise on both polarizations.  
PSum is present, and good.  So I suppose the noise diode is not swtiching?

     ea13.  PDif were zero-mean noise for most of the run, but at about 
75% of the way through, they sprang to life.  The transition from noise 
to signal was instantaneous.  Following this time, all data were good.

     ea20X.  PDif zero-mean noise for about 70% of the run, then slowly 
(over a few minutes) the values rose to reasonable levels. On the Y 
polarization, there were recognizeable values, but they are very weak 
(about 10% of what they should be) in all SPWs.

     ea22Y  The PDifs were present, but the signal was very weak -- 
maybe 20% of normal levels.

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     One more item:  ea24 worked well, but had a slowly changing gain 
change of remarkable value -- nearly a doubling of the power -- over the 
12 hours of the run.  The power rose, maximized, then declined.  The 
maximum was bout 4AM in the morning -- perhaps an extreme temperature 
sensitivity?