[evlatests] phase noise on outer antennas {External}

[Bryan Butler]

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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