[evlatests] More on VLA/EVLA phase behavior

[Bryan Butler]

is this (the VLA-VLA problem) related to the new correlator controller (i hope not)?

	-bryan


On 10/7/06 10:42, Rick Perley wrote:
> It was discovered yesterday that VLA-VLA phases were for many antennas 
> badly behaved, in the sense that many antenna-IFs did not return to the 
> prior phase after change of band.  Ken and I used the dynamic time last 
> evening to repeat the experiment which revealed this behavior, adding a 
> number of different modes to see if any particular combination caused 
> the problem. 
> 
> Observational setup:
> 
> Alternating between C and X-band, one minute at each band. 
> A single source -- 3C454.3 (10 Jy).
> Two different bandwidths:  12.5 and 50 MHz
> Two different correlator modes:  Continuum and spectral line mode '4' 
> (giving RR and LL correlations in both IFs). 
> 
> There were thus 8 different combinations (two bands, two BW, two 
> correlator modes), which were cycled thusly:
> C-cont-50MHz
> X-cont-50
> C-line-50
> X-line-50
> C-cont-12.5
> X-cont-12.5
> C-line-12.5
> X-lint-12.5
> 
> All six EVLA antennas were in the array, and give strong fringes at both 
> bands, except 26, which is outfitted only at X-band, IFs A and C. 
> 
> Some EVLA antenna flagging was required -- see below for details. 
> 
>         A)   Phase stability results:
> 
>     - Both VLA-VLA and EVLA-EVLA baselines showed significant problems, 
> as described next. 
>     - There appears to be no dependence of these problems on bandwidth, 
> band, or correlator mode. 
> 
> 1) VLA-VLA baselines.  Using a central VLA antenna as phase reference, 
> it is clear that many VLA antennas do not return to phase after a band 
> change.  The worst offender by far is antenna 12 on the A and C IFs 
> only, but there are many others which show the same problem although 
> less frequency.  I can find no pattern in these phase changes. 
> 
> 2) EVLA-EVLA baselines.  Using antenna 18 as a reference, I find that 
> antenna 13, 14AC, 24, and 26 show good return to correct phase after a 
> band change.  All four show strong long-term trends (as reported 
> yesterday).   On the bad side, we have 14BD, which is a jumper (changes 
> phase upon band change), and 16AC, which has two phase states seen 
> within all 1-minute scans -- separated by 80 degrees, with the phase 
> changing that much within that time interval.  Antenna 16BD also jumps, 
> but much less frequently than 14BD.   These characteristics are exactly 
> the same for all bands, modes, and BW. 
> 
> 3) VLA-EVLA baselines.  With a VLA antenna as reference, the EVLA phases 
> are terrible.  We have what appears to be multiple semi-stable phase 
> states -- each 1minute scan is stable, but after a band change, the 
> phase is somewhere else.  The two arrays have essentially no stable 
> phase between them. 
> 
>     B) EVLA-specific flagging issues. 
> 
> Some special flagging was needed to remove some data:
> 
> 1) The 10-seconds-at-the-end-of-scan problem occurs on about 10% of the 
> scans.  In all cases *except 1*, all EVLA antennas go dead 
> simultaneously.  In a single case, antenna 26 did not go dead, while the 
> others did.  This oddity occurred on the last scan of the file -- but I 
> can't see if there is any significance to this? 
> 
> 2) We occasionally get no fringes from an EVLA antenna for an entire 
> scan.  This is a rather common occurance -- perhaps 10% of the data from 
> some antennas is lost.  The problem usually, but not always, affects all 
> four IFs.  In a few cases, only one IF pair (e.g. A and C, or B and D) 
> are affected.  Antennas 13, 14, and 16 are by far the most likely to 
> have this happen.  Antenna 24 never lost any fringes.  Antenna 26 was 
> dead only once (but, remember that it works only at X-band, in IFs A and 
> C).   It seems clear that this is an LO setup problem. 
> 
> 
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