[evlatests] 128-MHz spectral artifacts

[Jim Jackson]

It probably wouldn't hurt us to take a closer look at a few synthesizers from offending antennas to make sure we're not seeing any performance degradations with age.  Some of these things are near 20 years old now.

Additionally, various folks have been visually noticing that the RFI gasketing in the vertex room racks is definitely showing its age. Maybe we need to take a closer look at that as well.  

Cheers,
Jim

-----Original Message-----
From: evlatests <evlatests-bounces at listmgr.nrao.edu> On Behalf Of Paul Demorest via evlatests
Sent: Tuesday, September 3, 2024 11:37 AM
To: Rob Long <rlong at nrao.edu>; Ken Sowinski <ksowinsk at nrao.edu>
Cc: evlatests <evlatests at nrao.edu>
Subject: Re: [evlatests] 128-MHz spectral artifacts

Thanks Rob and Dave.  I was aware of various reports of "N*128 MHz birdies" in the past.  I don't think this is the same issue, though I suppose it may have a related underlying cause in the LO system.  Rather than signals leaking out of the LO modules and getting into the RF signal path (which is what WO-131 seems to describe), our recent data suggests spurs in the LO outputs.  These would be near the LO output frequencies of ~14 GHz.  I do agree that I should not have been so quick to blame L302, and it could also/instead be L301.  I still think a (hopefully easy?) test of the LO output on a spectrum analyzer would be interesting, to measure the level of any spurs at +/- 128 MHz from the selected LO tuning.  If those are not seen we'll need to think harder about what might be causing these results.

-Paul

________________________________________
From: Rob Long <rlong at nrao.edu>
Sent: Tuesday, September 3, 2024 10:14 AM
To: Ken Sowinski; Paul Demorest
Cc: evlatests
Subject: Re: [evlatests] 128-MHz spectral artifacts

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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