[evlatests] More on 'The Phenomenon'

[Rick Perley]

    This is Mike's term for the 3-bit sampler chip resonance.  It is 
easy to see on autocorrelation spectra. 
   
    The good news is that it is not very strong on most antennas -- 
typically a few times the system spectral noise power.  (Be careful with 
this description -- the power in these 'lines' is *not* a few times the 
total system power.  Rather, the power within the 2 MHz channel, on the 
line, is a few times the power in a typical 2 MHz channel, off the 
line.)  The typical width of the line is perhaps 5 MHz, so with a 
subband width of 128 MHz, the resonance line adds 5 to 10% to the power 
within that subband, and a (trivial) ~0.5% to the total power seen by 
the sampler. 
   
   I have attached to files showing plots of the autocorrelation spectra 
for the 6 antennas and 2 IFs for both the A1C1 and A2C2 paths, for the 
subbands where the lines lie.  (The frequencies are from ~14.2 GHz 
through 14.8 GHz, for the tuning employed in this test).  .  These are 
*linear* scales, and are 'raw' -- no attempt at bandpass correction has 
been made.  Hanning smoothing was done (but doesn't make much 
difference, since the lines are resolved).  Note that:
    1) ea07 is much worse than the others -- as I've stated earlier, 
something is really wrong with that receiver/IF path.
    2) The 'phenomenon' is at a different frequency for each antenna, 
polarization, and path. 
    3) One antenna, ea26 in the C1 path, doesn't show the phenomenon at all.
   
    The effect of these lines in the cross-power spectra will be small 
-- a small loss in sensitivity for the channels in which the resonance 
occurs.  Think of it as a narrow-band rise in system temperature.  The 
effect will be much more troublesome for people wanting high resolution 
spectral observations at frequencies 'covered' by these lines.   It's 
probable that the lines won't move much with frequency, so we should be 
able to alert people to where they are, so frequencies of interest can 
avoid these 'noisy' zones. 


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