[evlatests] The big 'resonance' in ea07 at Ku-band

[Rick Perley]

    The Friday 3-bit tests revealed an extraordinarily strong RFI-like 
'line' at 13894 MHz in the 3-bit data paths.  Unfortunately, this 
frequency was not covered by the 8-bit path tuning.

    Ken ran a short test yesterday morning, wherein he tuned the 8-bit 
path to include this frequency.  The 'line' was immediately obvious.  He 
turned off the 3-bit samplers, and saw the line was still there in the 
8-bit path.  However, he could not be sure that the 3-bit samplers were 
actually off, so the conclusion that the 'line' is unrelated to the 
samplers was not secure.

    I have loaded the data, and can conclude that indeed the 3-bit 
samplers were off when Ken turned them off, so that we can state 
definitively that this 'line' is a feature from earlier up the signal 
chain.  Here is how I know this:

    Attached are 5 png files of the autocorrelation spectra, and an 
individual baseline (7X14) cross-correlation.  The key point is that the 
horizontal blanked area in the 3-bit file occur when Ken turned off the 
3-bit samplers (and I flagged the resulting data).  Note that within 
these spectra, the other lines present (not the dominant central line -- 
this is the 13894 birdie) are curved, meaning their frequency is 
changing in time.  This is 'the phenomenon' -- the sampler resonances -- 
changing in frequency as the 3-bit samplers warm up after having been 
turned off. 

    Now note the dominant central line -- it never changes frequency, it 
is always present (in the 8-bit spectrum) whether the 3-bit samplers are 
on and off, and (very important!), it is of exactly the same intensity 
in all three autocorrelation spectra. 
   
    I deduce from this that this 'birdie' (which is significantly 
resolved) originates from higher up the signal chain.  It is present 
*only* in ea07, and it is equally strong in both polarizations.  In 
addition, the visibility data, and the switched power data from ea07 at 
this band are 'not quite right' (although I admit that trying to specify 
what this means is a bit dodgy.  Just call it a feeling ...)

   
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