[evlatests] EVLA-VLA incoherency statistics

[Jim Jackson]

There is no EVLA hardware fix for this.  If the hardware in the two 
systems gets out of sync (or phase lock) with each other, manual 
(human) intervention on the EVLA side is required to fix it - it 
would not self correct on the next scan.

Is it possible for VLA hardware to be causing this?  If it's only 
happening on the AC side, this implies that something is simply broke 
in the VLA system and needs to be repaired - presumably something 
associated with controlling the Flukes.  If VLA hardware is causing 
it and can't be fixed by a simple repair (ie - it's a design 
problem), then we should live with it until we turn the VLA system off.

Jim



At 08:28 AM 3/19/2008, Rick Perley wrote:
>     As has often been reported, we occasionally see the VLA and EVLA
>being mutually incoherent -- EVLA-EVLA baselines are fine, VLA-VLA
>baselines appear fine (but in fact, are not), and cross-baselines give
>no fringes at all.  This phenomenon *always* occurs for whole scans, and
>*only* occurs in IFpair 1 (AC side).     Presumably, the VLA antennas
>have been slightly mis-tuned.
>
>     One of my polarization tests, done early this month, involved
>numerous changes of mode and frequency settings -- alternating between
>50 MHz continuum, and 12.5 MHz mode PA, at X-band.  I find that:
>
>     7 of 199 continuum observations showed the incoherency (only on the
>AC IFs),
>     12 of 216 spectral line observations showed the incoherency.
>
>     About 5% of the scans are affected.  This is uncomfortably high,
>given that these data are not caught by the flagger.  The problem is
>easy to catch in observations of strong sources, but cannot be seen in
>observations of weak sources.
>
>     Is there a fix in the works?
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