[evlatests] 128-MHz spectral artifacts

[Paul Demorest]

Rob,

The "slight" shift was 20 MHz so I think it's likely that only the L302/T304 tuning changed as you suggest (Ken could probably confirm).  So far we have only seen this at L-band but we have not looked very hard at other bands.  We may do more tests next week, will send updates as we find out more.

thanks,
Paul

________________________________________
From: Rob Long <rlong at nrao.edu>
Sent: Friday, August 30, 2024 2:36 PM
To: Paul Demorest; evlatests
Subject: Re: [evlatests] 128-MHz spectral artifacts

When "LO tuning is shifted slightly" is mentioned, this is strictly the
L302 tuning to the T304 converter? If this is an L302 artifact, we
should be able to see this effect at (theoretically) all bands. If we
only see this at lower bands, possibly a different problem.

Just thoughts,

Rob

On 8/30/2024 1:02 PM, 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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