[evlatests] Delay Phase Problem, ea09

[Rick Perley]

     Unusual behavior in ea09's delay phase was found in data taken 
early  Sunday morning.

     Normally, the phase as a function of frequency is (more or less) a 
straight line, whose slope gives the delay.  (One nanosecond of delay 
gives one turn of phase over a span of 1 GHz -- a useful rule to 
remember).  By some means unknown to me, it is arranged that the phases 
from sub-band to sub-band are continuous.  That is -- there are never 
any discontinuous jumps from one spectral window (sub-band) to the next.

     But not in ea09 last weekend.  Attached is a plot of the raw 
bandpasses for IFs 'A1C1'.  The upper pair of plots is RCP, the lower 
pair is LCP.  Each pair shows the phase above, and the amplitude below.
For this baseline (ea04 -- ea09), the LCP bandpass is as we expect -- 
the phase is continuous.  But for RCP, the phase is discontinuous, 
although the delay slope is easily seen within each individual sub-band.

     This discontinuous effect was seen in both bands observed (X and 
C), and is different across the four IF pairs:

     A1C1:  Jumps in RCP only
     A2C2:  Jumps in both polarizations
     B1D1:  Jumps in both polarizations
     B2D2:  No jumps.

     The pattern is the same at both bands.
     No other antenna behaved in this way.  Data taken in November, with 
the same frequency tunings, did not show this effect.

     The effect is certainly antenna-based -- it calibrates out 
perfectly *providing that delays are derived separately for each SPW*.  
(This is how I found this problem -- my calibration script solves for 
one delay over the entire IF (16 subbands) -- which causes 'very 
interesting results' for those IFs where the phase is jumping from 
subband to subband ...)

     I've never seen this effect before (since the earliest days of the 
EVLA).  What has happened?
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