[evlatests] New PDif compression test

[Barry Clark]

The old style X band receivers have very low output levels; most
end up with the T304 input attenuators pegged at zero.

Rick Perley wrote:
>     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. 
> 
>    
>    
>    
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