[evlatests] Delay Phase Problem, ea09

[Vivek Dhawan]

Antenna 9 had various weirdness happen to it regarding delay,
between Tuesday April 7 evening (when the LO rack was reset)
and Monday Apr 13 afternoon (when the station boards for that
antenna were rebooted).

I urge you to forget you ever saw this - it is fixed now.

On Wed, April 15, 2015 13:47, Rick Perley wrote:
|      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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