[evlatests] antenna 23/ phase jumps

[Ken Sowinski]

On Mon, 27 Nov 2006, Ken Sowinski wrote:

> I started a long track on 3C345 beginning at 2356 IAT to try to
> assess the stability of antenna 23.  All the EVLA antennas were
> included and claim to be working.  There was a burble at IAT
> midnight which is surely due to the Modcmops.  I cursorily
> watched the data go by sampling the baselin of antenna 6 with 23
> and so only one interesting event at 00:06:20 IAT which changed
> the phases for both IFs A and B.  Someone should examine the
> data with more care to see if it was all EVLA antennas or just
> antenna 23.  I also saw variations in amplitude of about five
> percent on a time of minutes.  Again a more careful examination
> is needed.

Vivek has summarized what he saw.  All IFs of all EVLA antennas
experience a phase flip of close to 180 degrees at the same time.
It happened twice about 35 minutes apart.  This was during a
single scan with no change in setup.  The data was taken with
ten second integration time.  At each flip there is one record
with a bad amplitude, but there are no records with an intermediate
phase.

It is hard not to think that this is caused by a problem with
phase switching, however it is equally hard to think of a plausible
mechanism.  A simple timing error that comes and goes would
scramble the data and make a big mess.  The way to get what we
see is to suddenly complement all the bits in the Walsh sequence,
something that does not seem easy to do especially for all L302s
at the same time.

I wondered if the system controller could explain this but it cannot
distinguish VLA from EVLA antennas.

All this speculation hinges on the statement that all IFs flipped
by exactly 180 degrees.  A careful look at the data can confirm this.