[evlatests] 'Global' Phase Jumps

[Rick Perley]

Vivek reported, last Monday:

Global phase jumps are back - There is a reference pointing scan
(X band) every hour, and bad things happen almost every time:

Antenna 4 has short phase glitches (but returns to the same phase)
at those times. At those same times ALL VLA antennas show phase
jumps between IF 1 and 2 (i.e. the A-B phase and C-D phase jumps)
- compared to any EVLA antenna. This alone cannot indicate which
array the jumps are on. BUT, in addition, VA 12 and 22 show (IF1-IF2)
phase jumps at those same times, when compared to any other antenna
(whether VLA or EVLA). This to me indicates that the jumps are on the
VLA and that 4, 12 and 22 are jumpier than the rest.


I reviewed my data from yesterday, for these antennas.  Recall
that I have four sequential L-band scans, with no change of source,
frequency, of BW.  I formed the IF1-IF2 phase difference, both both
polarizations, and find:

1)  Antenna 4 had only one working IFpair, so no differences are possible.
2) Antenna 12 shows a ~30 degree phase perturbation, last about two minutes.  
The perturbations are due to IF#1 (equal in both A and C), and are not visible
in IFs B and D.  
3) Antenna 22 is completely normal, and stable.  

My interpretation is that some VLA antennas are 'behaving badly' 
in terms of phase stability, and that there remains only one 
'global' phenomenon between the arrays, as I reported yesterday.  

More tests will be needed to see if the 'global' effect is restricted 
to L-band.