[evlatests] Feb 14 tests and confusion

[Barry Clark]

The glitching has, I think, reached the state that it disqualifies
use of the antennas for science - one never knows whether the antennas
are performing or not.

I summarize properties and possibilities of the glitch observed at X
band.  If they differ at other bands, or if I have it wrong at X band,
please let us know.

1.  The earliest reference I can find for something that may be identified
with this is Rick's message of December 29.

2.  The effect seems to know about 10 second intervals - it seems to occur
on the IAT 10 seconds when viewed with short integration times.  Rick's
first suggestion, that it preferred the 10s ticks with a scan change, does
not seem compelling to me.

This is very important.  There are only four things that know about IAT
10s in the normal course of affairs:  the L302, the ACU, the L304, and 
the central LO rack.

3.  The dropout is sometimes complete, but sometimes only a reduction by
a large factor.  When fringes are present, the drop is accompanied by a
jump in phase (often of order 120 degrees), but the phase remains stable
while the glitched state persists.  The first of these tends to exonerate 
the L302 walsh function switching, the second tends to exonerate the L302 
DDS.  It is still possible that the L302 is doing something to cause this, 
but the simplest failure modes in that module will not.

4.  When a dropout occurs, it tends to occur for all three antennas, but 
does not always.  Sometimes, but not always, when the drop affects two
EVLA antennas, normal fringes are seen between them.  This exonerates the ACU.


While not definitive, my suspicion falls on the central LO rack.