[evlatests] Switched Power Compression?

[Jim Jackson]

Antenna 24 has a different T302 LSC converter than before.  It now 
has the modified one with all of the filters added to it so it will 
be interesting to see if it behaves any differently than the other antennas.

Unfortunately, the T302 that came out of 24 could now be anywhere 
since we didn't realize anything was wrong with it. If you suddenly 
see the same strong compression on RCP in another antenna, let us know.

Jim


At 03:31 PM 10/19/2011, Rick Perley wrote:
>     We have long noted that the values of 'PDif' (Cal On - Cal Off)
>changed when the antennas observed a strong source.  This, in spite of
>holding the antenna electronic gains constant.  In some cases, the
>change has been noted to be very large -- exceeding 25%, and generally
>in the 'compressive' sense -- the values of 'PDif' decreased when
>observing a strong source such as Cygnus A.
>
>     There was reported (by me), a few months ago, weak evidence that
>this 'compression' did not occur to the visibilities -- so that
>application of the switched power values would degrade the visibility
>data.    These data were taken on 3C273, which provides only ~2K of
>antenna temperature.
>
>     As we are now in D configuration, a much better test of whether the
>visibilities are not affected by the PDif compression issue can now be
>done using Cygnus A.  This source has a big effect on Tsys (up to 120K
>at L-band), and certainly causes the PDif values to change.
>
>     A short observation was taken last evening of 3C286 (calibrator) and
>Cygnus A.  Unfortunately, the observations of the former were taken at a
>low elevation, and four antennas were shadowed when on 3C286 and could
>not be used for calibration.  This is a problem only in that these
>represent many short spacings, which are the useful ones for this
>experiment.  Nevertheless, the major result seems to be clear:
>
>     *** The Visibility data *are* compressed in a manner similar to that
>of the PDif Values ***
>
>     Application of the PDif values provides  flux values for Cyg A much
>closer to the known total flux.  Calibration without applying the PDif
>values results in Cygnus A flux density significantly too low --
>especially at L and S bands, where the antenna temperatures due to Cyg A
>are high.
>
>     Almost every antenna shows a strong reduction in the 'PDif' values
>when observing Cygnus A at L-band.  In a following report, I will
>provide the compression factors.  In general, the amount of compression
>declines with increasing frequency.
>Curiously, the strong compression formerly seen on ea24 in RCP at C-band
>is now completely absent!!!
>
>     L-band also provides another problem -- some baselines, notably
>those connecting antennas 8, 20, 23, and 28 -- are giving *enormous*
>closure errors when on Cygnus A, but not on 3C286.  'Enormous' means
>tens to hundreds of percent!  Something is seriously amiss here.
>
>     There are other (unhappy) issues with the PDif system which I shall
>report on separately.
>
>
>
>
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