[evlatests] Array Status

[Rick Perley]

    The usual stress test, in continuum, was run.  I've looked (so far) 
only at L, C, and X bands.  Here are the basic results:

    1) Antenna 17 retains its fast square-wave amplitude pattern -- all 
four IFs at all bands.  The period is exactly 1.666 seconds, and the 
signal shows three 'up' integrations, followed by one 'down'.  But all 
are weaker than they should be, and the antenna's basic sensitivity (as 
judged by statistical weights) is low by a factor of four.  The 1.67 
second period is that of the Walsh function switching, so there's likely 
something wrong there.  We should turn off phase switching to see if 
that removes the problem.   This problem has been around for at least a 
week, maybe two. 

    2) Visibility amplitudes from antenna 13 and 25 are very, very low 
at all bands.  The statistical weights show that both antennas have 
their sensitivities low by a factor of 200 or more. 

    3) VLA antenna 4 remains essentially non-functional at L-band (with 
amplitudes showing hundreds of short dropouts over a few minutes, on all 
four IFs) , but is o.k. at C and X bands, other than a curious 
modulation of the gain on IF 'A' -- like a square wave, with about a 20% 
modulation, and 15 second period. 

    4) There is all manner of odd amplitude instabilities, on timescales 
of minutes, at L-band almost exclusively.  As badly behaving antennas at 
L-band seem to work fine at C-band, I suspect external RFI which is 
messing with the Tsys, or maybe the correlation coefficient.  No time to 
check, yet. 

    5) There were EVLA to VLA phase jumps at L-band, on IFs A and C 
only, upon change of scan with no change in source, frequency, or 
bandwidth.  The characteristics are identical to what I have reported 
last week.  Barry will be looking at this problem, when time permits.  
As always, we cannot tell which array is responsible. 

    6)  Other than the 'global' phase jumps, all phases look relatively 
normal. 

    7) Flagging was generally reasonable, except for the initial L-band 
observation (which was the first in the  test), where all EVLA antennas, 
except 11, had bad data which were not flagged for one minute before 
good data began.  VLA antenna 8 also had some early bad, unflagged, 
data.  Weird. 

    8) The last 1 second of all three scans (L, C, X) band had low 
amplitudes.  Each of these events preceded a change in band. 

    9) The 'short shallow drop' problem remains with us.  This is 
uniquely identified with EVLA antennas, and the higher numbered ones 
seem to be more prone to this behavior.  The characteristics are always 
the same:
       - the amplitude drops, for a single 0.418 second integration, by 
~15%.
       - the effect always occurs together on the two polarizations 
within any given IFpair.  That is, A and C always drop together, as to B 
and D.  But the drop behavior on the two different pairs is entirely 
uncorrelated.
       - There is no phase effect whatever. 
       - - It's not a common problem -- at most about one drop in 30 
seconds (about 1% of the data, at most).  The overall effect on 
astronomers' maps is negligible. 

     10)  EVLA antenna 14 has erratic, unstable amplitudes.  There is no 
regularity to the problem, and there are varying timescales, probably 
indicating varying causes.  I'll have to make some plots for viewing.