[evlatests] 4-band weirdness

[Paul Demorest]

hi all,

Earlier this week we ran a test project to try correlating VLA 4-band 
with the LWA.  We recorded baseband (VDIF) data from 4 MJP antennas 
(ea10, 12, 14, 19) in at 16 MHz band centered at 72 MHz.  The 
correlation will be done offline in software by the LWA group at UNM.

I've been checking out the VLA data in various ways, including making 
power vs time/freq plots from the individual antenna data.  This showed 
some strange features, not sure whether these are already known or not, 
but maybe some of you will be interested.  Most notably:

1. There is a huge 120 Hz modulation of the total power, on all 
antennas, both IFs.  The total power varies by about a factor of two.  I 
know some amount of this is to be expected but this seems kind of 
excessive to me.  See first attached plot showing 0.5 s of data with 
~1ms time resolution.

2. The bandpass shape is not stable.  It seems to jump back and forth 
between a few different shapes on timescales of 10's to 100's of 
seconds.  See second attached plot showing a hour-long observation (with 
~130ms time averaging).

In the plots, both of these are data from ea12, "polarization 0" (which 
I think is "YY").  I normalized by the median bandpass to make the 
variations easier to see.  The times with the brief power dropouts seem 
to be compression from strong transient narrowband interference.

-Paul
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