[evlatests] PDif Compression Still Lives!

[Rick Perley]

As part of the test yesterday, I observed Cygnus A, with the 8-bit samplers, at all high frequency bands from X through to Q.

Cygnus A adds appreciable power to the system at X and Ku bands — multiplying the 'cold sky' temperature by 1.4 to 1.6 at X band (higher at the low frequency end), and by 1.2 to 1.4 at Ku bands.  The effect at K-band is much reduced (because Cygnus A's radio lobes are mostly outside the primary beam):  a 3 to 10% increase at K-band, and essentially negligible at Ka and Q bands.

Early in the EVLA construction period, we found that the apparent gain (as measured by the PDif values) dropped when on very strong sources.  This is known to be a false indication — the gain in fact does not drop, although the PDif values would have us believe it does.  Much activity during the EVLA construction and commissioning period failed to find a viable explanation.

The purpose of this note is only to remind everyone that the problem is still with us.  Fortunately, the effect is fairly weak:


  1.
At X-band, where the Tsys is increased by typically 50% when on Cyg A, the PDif values, for all antennas and IFs, are see to decline.  Typical declines are by 1 to 3 %.  (This means that, if applied, the flux density of Cygnus A will be overestimated by about this amount).
  2.
At Ku-band, where Tsys is increased by typically 25% when on Cygnus A, PDif values are seen to decline by typically 1%.

At the higher bands, no PDif compression is seen on Cygnus A.

Rick



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