[evlatests] New PDif compression test

[Rick Perley]

    Following yesterday's meeting, I put together a Taurus A (3C144) 
test file, which cycles amongst the four lowest frequency bands (L, S, 
C, X) on and off Taurus A.  The file is 45 min long (two cycles around 
-- could be made shorter for quick tests), and ran at noon today. 

    This source is actually better for testing PDif compression overall 
than Cygnus A, as it has a higher flux density than Cyg A above 3 GHz. 
   
    I've taken a quick look at the results.  Our compression problem is 
seen at all four bands, and on almost all antennas.  After I finish up 
my talk for the Director's review (this weekend), I'll make up a 
'compression table' for all bands.  The compression is higher at low 
frequencies, but this is not surprising, given that the source is 
stronger at low frequencies.

    Many have asked if a clear separation between 'old style' and 'new 
style' X-band receivers can be seen.  Cygnus wasn't strong enough to 
make permit this judgement.  Taurus  is.  The answer is:  Yes -- the 
'old style' receivers appear to have no compression.  The 'new style' do.