[evlatests] phase noise on outer antennas {External}

[Bryan Butler]

Correct.  Tropospheric phase (delay) noise would be the same in the two 
polarizations and have the normal linear increase with frequency, but it 
wouldn't be on such short timescales (10 ms).  The source being resolved 
out would just mean that there is no signal, all just noise, and it also 
wouldn't be on such short timescales (and unique to those four antennas).

I think you're probably right that it's electronic, and LO phase jitter 
is a likely culprit.

	-Bryan


Paul Demorest wrote on 10/28/24 16:55:
> There may be a small increase in noise on N64 and W64.  E64 looks fine.  All the numbers are in that table I sent if you want to take a look.
> 
> Resolved source structure (ie, decreased amplitude) should not cause ~100%-correlated phase fluctuations in the IFs (different polns, freqs).. right?
> 
> ________________________________________
> From: evlatests <evlatests-bounces at listmgr.nrao.edu> on behalf of Bryan Butler via evlatests <evlatests at listmgr.nrao.edu>
> Sent: Monday, October 28, 2024 4:34 PM
> To: Walter Brisken; evlatests at listmgr.nrao.edu
> Subject: Re: [evlatests] phase noise on outer antennas {External}
> 
> 
> Wouldn't that degrade more gracefully with distance?  Do the antennas at
> W64, E64, and N64 show worse noise than others?
> 
>          -Bryan
> 
> 
> Walter Brisken via evlatests wrote on 10/28/24 16:28:
>> Is the source you are looking at being resolved out, simply leading to
>> sensitivity-based noise?
>>
>> On 10/28/24 16:24, Paul Demorest via evlatests wrote:
>>> Well, I would have expected troposphere fluctuations to have a "red"
>>> spectrum, not look like white noise on 10ms scales..  but people who
>>> do more high-freq observing than I do are welcome to comment if this
>>> isn't right!  The "slow" (>1s) fluctuations in the plots are likely
>>> atmospheric.
>>>
>>> Also I don't think troposphere can explain why W32 looks persistently
>>> bad.. but there could be multiple things going on.
>>>
>>> ________________________________________
>>> From: Claire Chandler <cchandle at nrao.edu>
>>> Sent: Monday, October 28, 2024 4:12 PM
>>> To: evlatests; Paul Demorest
>>> Subject: Re: phase noise on outer antennas
>>>
>>> Hi Paul,
>>>
>>> Wouldn't delay fluctuations from the troposphere also look similar?
>>>
>>> Claire
>>>
>>> ________________________________
>>> From: evlatests <evlatests-bounces at listmgr.nrao.edu> on behalf of Paul
>>> Demorest via evlatests <evlatests at listmgr.nrao.edu>
>>> Sent: Monday, October 28, 2024 4:06 PM
>>> To: evlatests <evlatests at nrao.edu>
>>> Subject: [evlatests] phase noise on outer antennas
>>>
>>> hi all,
>>>
>>> Now that we're in A-config, recent testing at high time resolution
>>> (10ms) has shown excess short-timescale phase noise affecting the
>>> outer three antennas (ea21 at E72, ea24 at W72 and ea18 at N72), as well as
>>> ea26 at W32.  A few plots are attached so you can see what I mean.  These
>>> were done via 10ms-dump-time observations of a bright source at K-
>>> band, and show two of the bad antennas (ea18, ea21) plus a good
>>> antenna (ea13) for comparison.  I also took data at X and Ka bands, a
>>> summary of these results is in the attached text file.  The worst ones
>>> have short-term phase RMS of ~20 deg at Ka, which is >10x larger than
>>> the good antennas.  This is enough to cause decorrelation (sensitivity
>>> loss) at the ~5-10% level and may be at least partially responsible
>>> for reduced high-freq performance seen on these antennas in recent
>>> stress tests.
>>>
>>> This noise has the following properties which make me suspect
>>> something like LO phase jitter is the cause:
>>>     - It looks totally random vs time.
>>>     - All 4 IFs for a given antenna show exactly the same noise (see
>>> zoom-in plot for example).
>>>     - The amplitude of the noise scales in proportion to observing
>>> frequency, higher freqs are more affected.
>>>
>>> It's also notable that all 3 outer antennas looked fine in B-config,
>>> and only started showing this noise once they moved to their A-config
>>> locations.  I found some 10ms X-band data from last A-config (Aug
>>> 2023) and it shows a similar pattern:  the outer pads and W32 are
>>> noisier than the rest, even though some different antennas were
>>> involved at the time.
>>>
>>> Since this seems to be pad-related rather than antenna-related, my
>>> hand-wavy guess is maybe LO optical power is getting a bit too weak
>>> over the very long fibers?  And as for W32 maybe it just has some
>>> problem with its fiber connection causing a similar effect.  Even if
>>> that's not the right explanation, I do think this is worth
>>> understanding and (if possible) fixing.  Let me know if you have any
>>> questions/suggestions.
>>>
>>> Cheers,
>>> Paul
>>>
>>> ________________________________
>>> From: evlatests <evlatests-bounces at listmgr.nrao.edu> on behalf of Paul
>>> Demorest via evlatests <evlatests at listmgr.nrao.edu>
>>> Sent: Monday, October 28, 2024 4:06 PM
>>> To: evlatests <evlatests at nrao.edu>
>>> Subject: [evlatests] phase noise on outer antennas
>>>
>>> hi all,
>>>
>>> Now that we're in A-config, recent testing at high time resolution
>>> (10ms) has shown excess short-timescale phase noise affecting the
>>> outer three antennas (ea21 at E72, ea24 at W72 and ea18 at N72), as well as
>>> ea26 at W32.  A few plots are attached so you can see what I mean.  These
>>> were done via 10ms-dump-time observations of a bright source at K-
>>> band, and show two of the bad antennas (ea18, ea21) plus a good
>>> antenna (ea13) for comparison.  I also took data at X and Ka bands, a
>>> summary of these results is in the attached text file.  The worst ones
>>> have short-term phase RMS of ~20 deg at Ka, which is >10x larger than
>>> the good antennas.  This is enough to cause decorrelation (sensitivity
>>> loss) at the ~5-10% level and may be at least partially responsible
>>> for reduced high-freq performance seen on these antennas in recent
>>> stress tests.
>>>
>>> This noise has the following properties which make me suspect
>>> something like LO phase jitter is the cause:
>>>     - It looks totally random vs time.
>>>     - All 4 IFs for a given antenna show exactly the same noise (see
>>> zoom-in plot for example).
>>>     - The amplitude of the noise scales in proportion to observing
>>> frequency, higher freqs are more affected.
>>>
>>> It's also notable that all 3 outer antennas looked fine in B-config,
>>> and only started showing this noise once they moved to their A-config
>>> locations.  I found some 10ms X-band data from last A-config (Aug
>>> 2023) and it shows a similar pattern:  the outer pads and W32 are
>>> noisier than the rest, even though some different antennas were
>>> involved at the time.
>>>
>>> Since this seems to be pad-related rather than antenna-related, my
>>> hand-wavy guess is maybe LO optical power is getting a bit too weak
>>> over the very long fibers?  And as for W32 maybe it just has some
>>> problem with its fiber connection causing a similar effect.  Even if
>>> that's not the right explanation, I do think this is worth
>>> understanding and (if possible) fixing.  Let me know if you have any
>>> questions/suggestions.
>>>
>>> Cheers,
>>> Paul
>>>
>>> _______________________________________________
>>> evlatests mailing list
>>> evlatests at listmgr.nrao.edu
>>> https://listmgr.nrao.edu/mailman/listinfo/evlatests
>>
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