[evlatests] X-Band Beats

[Vivek Dhawan]

Correction:

I had searched for some of the n*512 +_ m*128 lines from the comb
generators. only 1408 and 222272 were detected, the second appeared
to be a IF line, not at K-band. (in January- february)

At X-band I saw very weak lines at 8448, and not elsewhere - but
due to glitches and other happenings, 8192 data were missed.
So the lines may be there after all. Time to do this again.

V.

Rick Perley wrote:
>     I reported yesterday (at the bottom of a message about drop-outs) on 
> a strong
> beat phenomenon seen on the EVLA antennas at two frequencies:  8185, and 
> 8785 MHz.
> 
>     I took a much closer look at the data today.  The results:
> 
>     A) This is indeed a EVLA-only phenomenon.  All EVLA to VLA baselines 
> are clean.
> 
>     B) The 8185 MHz beat is between the astronomical signal and a signal 
> whose phase
> is unchanging between the two antennas.   We know this because the beat 
> phenomenon
> has a period *exactly* equal to the natural fringe rate.   Noting that twice
> the 4096 MHz LO is 8192 -- which lies within the passband at 8185 MHz, 
> it is suggested
> that there is a significant leakage of LO power.  If it is a CW, it must 
> be quite strong,
> as the modulation of the beat signal is near 100% (i.e., we get as much 
> correlation in
> the LO as we do in the astronomical broad-band signal). 
>     I checked with Vivek, who has long ago searched for such LO signals 
> in the
> bandpass, but he reports that he found nothing at 8192 MHz.    Perhaps 
> we should
> use Mike's spectral snaphot program to look for spikes?  (The correlator 
> is out of
> commission for most of the day, so single-antenna tests are all we can 
> get). 
> 
>     C)  The 8785 MHz beat must have a different origin.  Although it 
> clearly has a
> periodicity, the period has no obvious relation to the period of the 
> natural fringe.  For
> example, on baseline 13 x 14, whose U-coordinate is 3.5 Klambda, we 
> would expect a
> 5-second beat.  The apparent period is 0.8 seconds (or shorter -- hard 
> to tell with 0.4
> second integration).  Baseline 13 x 18 has a U-coordinate of -12 
> KLambda, which would
> predict a 1.5 second period.  What we see is a beautiful cyclic beat of 
> 7 second period. 
> Baseline 14 x 18 has a U-coordinate of -15 KLambda , so a natural period 
> of 1.2 seconds
> would be expected.  We see no obvious periodicity at all, but instead a 
> ratty amplitude
> and phase, which would be expected if the actual beat has a period very 
> much less than
> the 0.4 second integration.    Is this some sort of aliased broadband 
> signal? 
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