[evlatests] EVLA behavior, 9 Oct 2006

[Rick Perley]

    From a closer look at this morning's quick test.   Data were taken 
in 50 MHz continuum, alternating between C and X bands, 5 minutes 
duration on each. 

    A)  EVLA antenna amplitudes:

    Numerous beginning-of-scan effects are seen, always within the first 
10 seconds.  The effect is always an asymptotic approach to a stable 
state, indicating the Tsys correction mechanism reacting to changes in 
power level -- likely due to changing bands.    Those with the strongest 
effects are:  13AC (only at C-band), 14C (X-band), 16BD (X-band), 18AC 
(C-band), 24A (C-band). 

    16AC is clearly broken.  The amplitudes show two states -- and 
'upper' and a 'lower', with the upper being much in the majority.  
Transitions between states are very fast -- even with 0.41 seconds 
integration, no amplitudes other than upper or lower were seen.  

    There is a slow drift in amplitude gain, over the 1.5 hours of this 
test.  Fortunately, this is entirely, and confidently, attributed to the 
resolution of the source. 

    Other than these effects, the quality of the data is excellent. 

    B)  EVLA antenna phases

    1) With EVLA antenna 18 as reference. 

       Antennas13, 14AC, 16BD, and 24 phases are entirely consistent 
with atmospheric path fluctuations.  There are no jumps, spins, or other 
worrisome phenomena. 
      Antenna 14BD has multiple phase states.  Jim J. has reminded us 
that this antenna IF-pair still has old software, so the phase 
connection problems of last spring remain.  (When will this change?) 
       Antenna 16AC is clearly broken (see above description of 
amplitudes).  there are multiple phase states -- at least four!  Since 
the effect is at X-band as well as C-band, I presume the problem is with 
the L302? 

    2) With VLA antenna 5 as reference (i.e. -- phase connectivity 
between the VLA and EVLA). 

    There were three global EVLA-VLA phase shifts, two of which occurred 
*in the middle of a scan*!    The transition is very quick -- much less 
than 1 second, the phase change at X-band is about 120 degrees, at 
C-band about 80 degrees -- plausibly the jump scales with the wavelength. 

    Otherwise, VLA-EVLA phases were as good as VLA-VLA and EVLA-EVLA.