[evlatests] phase noise on outer antennas {External}

[Paul Demorest]

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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