[evlatests] C-Band VLA-EVLA Incoherence

[Rick Perley]

    The C-band scan taken yesterday showed that the EVLA and VLA were 
incoherent, in IFs A and C only, throughout the scan.  Intra-array 
baselines gave good fringe power. 

    The problem is likely to be on the VLA side.  Because there were no 
'cross' baselines, gain solutions were done separately for the two arrays. 
    For the VLA, I find that IF 'A' (only!) has a uniform phase wind 
w.r.t the reference antenna (10, at E8).  The rates are large, and 
clearly proportional to baseline component 'u':  for the maximum 
distance of 21 km, the phase rate is about 8 turns per minute -- about 
48 degrees per second.  IFs B, C, and D are not affected by this, and 
show normal phase behavior. 
    This has to be a Fluke issue.  This scan was at 50 MHz BW, and 
Flukes were set to 100, 200 MHz.  The previous observation was at 
P-band, so it would appear that Fluke 'A' was set to the wrong 
frequency.  (Perhaps Ken or Barry can explain why 'C' appeared to track 
phase correctly, but still gave no fringes to EVLA antennas). 

    All is fine on the EVLA side. 

    Other C-band issues:

    1) There were two visibilities, clearly belonging to the previous 
scan at the beginning of this observation, and were not flagged as bad.  
(Same issue as noted for P-band, following L-band). 
    2) The last 8 seconds of VLA antennas were good, but flagged bad.  
This is a known problem, and Hichem says he'll have it licked within a 
few days. 
    3) Antenna 21 is showing large, erratic jumps in amplitude, on all 
IFs.  Ken says this is due to a non-functional synchronous power, so the 
'Tsys' correction is essentially just noise.  *** This is an important 
problem to correct ***
    4) This scan followed a P-band scan.  It took about 35 seconds for 
the system to stabilize, with a ten-second spread (max to min) amongst 
the antennas.   Is this a reasonable duration?  (The flagger got all 
this correctly).