[evlatests] PDif Issues at L-band

[Ken Sowinski]

On Mon, 9 Jul 2012, Rick Perley wrote:

>    A test to try clarify the origin of the PDif compression problem
> (8-bit style) was done Saturday evening.  The test is rather
> complicated, and analysis is ongoing.   The array was in a bit of an odd
> state due to the power issues of that day.
>
>    The SY table data are in general quite good.  But some clear
> failures occurred and are noted below for completeness.  Note that we
> have only L-band data in this test.
>
>    ea08:  In IF 'A' only, both PSum and PDif alternate between two
> different states on a rapid (1 second or less) timescale.
>    ea10:  In IF 'A' only, PSum has two different states, similar to
> ea08.  PDif has many discrepant values, which look to me to be due to
> the rapid change of states seen in 'PSum'.

This is also seen at X band.  Resetting the sampler and formatter
did not help.

>    ea11:  The PDif values are all zero (with noise), while PSum looks
> fine.  Look like there is no switched power on any IF.
>    ea16:  The switched power in IFs 'A' and 'C' is about a factor of
> 100 too low.
>    ea22:  The PSum data look perfectly normal, but the PDif values are
> bizarre, like nothing I've ever seen before!  There are equal numbers of
> positive and negative PDif values, but these are not due to noise.
> These are real signals, but just plain crazy, on all IFs.

This, to, was seen at X band.  The L305 MIB was not responding.
When repaired the switched power looked more normal.

>    ea28:  IFs C and D (LCP polarization) have zero-mean, noisy PDifs.
> The PSum data for these IFs are pretty good, but closer inspection shows
> they are much noisier than they should be.  RCP polarization (IFs A and
> C) are much better, but still not as good as they should be.  (This
> antenna has always shown this behavior at L-band).
>
>    Note that antennas 1, 2, 13, 15, 19, 23, 24 and 26 were out of the
> array.
>
>
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