[evlatests] 128-MHz spectral artifacts

[Rob Long]

Dave Parker pointed out that this issue was documented well before JIRA 
and is now on Long Term under JIRA WO-131. The ticket indicates strong 
internal RFI as a (possible) L301 issue.

Rob

On 9/2/2024 9:14 AM, Ken Sowinski via evlatests wrote:
> A reminder that so far this has only been seen at L band so if LO
> spurs are the explanation, it could be the L301 rather than the L302.
> Band dependence might be investigaed by observing 'mega-masers', or by
> setting up a beacon at any convenient frequency.
>
>
>
> On Fri, 30 Aug 2024, Paul Demorest via evlatests wrote:
>
>> Hi all,
>>
>> Thanks to some careful investigation of L-band RFI by Lilia and 
>> Pedro, we have found a new (at least to us) type of spectral artifact 
>> in EVLA data.  We see low-level, frequency-shifted copies of strong 
>> RFI lines at frequencies +/- 128 MHz (and likely +/- 256 MHz as well) 
>> away from the real signal.  These signals are about -45 dB in power 
>> relative to the real signal, though this ratio is still somewhat 
>> uncertain due to the large signal strength and potential 
>> non-linearity.  We can see these most easily around 1425 MHz in the 
>> clean/protected part of L-band when pointing in specific directions, 
>> due to strong satellite RFI around 1550 MHz.
>>
>> We currently do not think this is a correlator artifact because:
>>  - The copies appear at the same sky frequency and amplitude 
>> independent of WIDAR subband BW (for 64 and 128 MHz subbands).
>>  - They also appear the same when the LO tuning is shifted slightly, 
>> thus moving the signal position with the subbands.
>>  - There is no change in their amplitude when the WIDAR "F-shift" is 
>> disabled.  (Other different artifacts do appear, as expected, when 
>> F-shift is turned off)
>>
>> This behavior appears on all four IFs, and on all baselines.  We 
>> might want to quantify the antenna/baseline dependence a bit better, 
>> but it does not appear to be limited to one or a few specific 
>> antennas.  We don't know if this is a new thing or has always been 
>> there.  It does require a very strong input signal to become noticeable.
>>
>> Some plots are attached to show how this looks.  These show 
>> "scalar-averaged" visibility data versus frequency from some of our 
>> recent experiments, for a variety of correlator setups and tunings.
>>
>> One explanation that we think fits the data is if an LO has low-level 
>> spurs at N*128 MHz away from the main tone.  This seems vaguely 
>> plausible to me since the L302's use a 128-MHz comb as part of their 
>> signal generation.  Of course others on this list know much more 
>> about the internals than I do.  It would be interesting to know if 
>> this sort of thing has been measured for the L302 output.  If not 
>> maybe someone could run some spectrum analyzer tests?  I'll put in a 
>> Jira ticket shortly for those who would rather track this there.
>>
>> Other suggestions are welcome of course!
>>
>> Cheers,
>> Paul
>
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