[evlatests] No PDif Compression!

[Rick Perley]

    We ran another test last night to locate the origin of the 'PDif 
Compression' problem.  In this test, Ken arranged to turn on the 20 dB 
'solar' attenuators which are located in the LSC Converter (T302).  The 
rest of the script ran as before:

    - on cold sky, the T304 input and output attenuators were adjusted 
to provide the correct power to the module, and the samplers.
    - the output attenuators were then stepped from +12 dB through to 
-12 dB attenuation, in 3dB steps, on each of three sources:  cold sky, 
calibrator, and Cygnus A. 

    Result:

    No (or almost no) PDif compression when on Cygnus A!!!  This result 
was true for every antenna and IF. 

    So, we can confidently claim that the compression occurs between the 
20 dB attenuators in the T302, and the input attenuators of the T304.   
Between these two points are amplifiers and LOs within the T302, and the 
input amplifier of the T304. 

   Keith Morris has further information regarding the output power 
levels of the L-band receivers, which I believe is consistent with the 
clues given above.  He'll send around a  report on his investigations 
later.