[evlatests] X-band results

[Rick Perley]

    A quick summary of X-band results from Friday night tests:

    Data was taken only in fast continuum mode, and 4-channel spectral
line. 
   
    1) No dropouts, and no phase spins were observed. 

    2) Antenna 16 had a warm dewer -- clearly reported by the backend Tsys,
and easily seen in the rms statistics. 

    3) Antenna 14's backend Tsys was at normal values while on 3C84, but
while observing the nearby cold sky location, rose from these values (inthe
30s) to values from 350 to 2100 K, in a single minute.  The rms noise
matched this behavior -- but most of the data were flagged bad at the time
when the Tsys got really ridiculous.  It was not possible to judge 
sensitivity
on the basis of these data.  From the stable values while on 3C84, 14A 
showed
near-normal sensitivity (i.e., like a VLA antenna). 

    4) Phase stability for both antennas was excellent on all IFs -- 
with the
residual fluctuations matching those on the neighbor VLA antennas.  That is,
the residuals are due to atmospheric variations. 

    5) Amplitude stability was `interesting':

       a) 14A and 14B were outstanding, with pk-pk gain variations of 
less than
1% in amplitude -- the same as the best VLA antennas. 
       b) 14C and 14D were the same, except for 'an event', wherein the 
gain
rose then dropped by 4% in amplitude, over a 10-second duration, after which
all was normal.  I'm not inclined to worry about this -- such `events' 
are seen
in VLA antennas from time to time (and indeed, 22A had a much more 
impressive
event a couple of minutes later). 
       c) 16A is doing its 'square wave thing', as it has for as long as 
I can remember. 
Note that the period of this behavior 50 seconds, and the sharpness of 
the transition
between states (< 1 second) argues against an origin before the AGC, as 
I would
expect a power fluctuation of this type to be removed by the AGC.  
Hence, might
this problem not be an EVLA issue? 
       d) 16B, C, and D showed much poorer amplitude stability overall 
than 14.  Might
this be ascribed to the warm dewer/Tsys?