[evlatests] Zero PDifs

[Rick Perley]

    There are some antennas which provide zero mean PDif data -- 
meaning, the PDif values are just random noise with a zero average.  
This is a significant problem, since we cannot then use the monitor data 
to determine and correct antenna electronics gain changes, or monitor 
system temperatures. 

    The aborted test run from last weekend provides a 'map' of PDif 
problems from X, Ku, K, Ka, and Q bands.  Here is a summary of antennas 
with zero-mean PDifs:

    X-Band: 
               ea02 -- all IFs
               ea12 on IF 'D'
               ea27 -- on all IFs

    Ku-Band:
               ea27 -- on all IFs

    K-Band:
              ea23 -- on all IFs
              ea27 -- on all IFs

    Ka-Band:
              all are switching

    Q-Band:
             ea08 -- on all IFs.