[evlatests] Ant. 13, IFs BD: ~60d phase jumps every 20 seconds at C, X, L bands

[Michael Rupen]

AR603 data taken between July 12th and July 15th show an odd behavior
for ant. 13, IFs BD (AIPS IF 2 RR & LL): frequent ~60 degree phase jumps 
within a scan.

* Seen with similar character in X, C, and L bands.  The one K band data
   set was too noisy and short to be sure.
* Data taken on July 12th, 13th, 14th, and 15th all show this effect.
   It's also consistently seen on a given day, over a few hours.
* IFs AC look fine.
* Data taken on or before July 7th don't show this behavior.
* Jumps generally occur every 20 seconds or so (using 3.3second averaging),
   though there are some exceptions (where the jumps occur more often).
* Although the usual short amp. dropouts are seen in these not-yet-fixed IFs
   (IFs AC do NOT show these, as expected given the fix), there is no
   correlation between amp. dropouts and phase jumps; nor is there any sign
   of any amp. effect, at the times of the phase jumps.
* These phase jumps show up on all baselines, both VLA-13 and EVLA-13.
   The closure phases show no sign of these jumps, i.e., this is a closing
   error.
* No sign of any similar phase jumps associated with any other antennas.

I'll send a fuller report on EVLA behavior when I get a chance to look more
carefully at the data.

         -- Michael