[evlatests] phase noise on outer antennas {External}

Rob Long rlong at nrao.edu
Mon Nov 18 10:26:37 EST 2024 

We can grab a fast readout of the optical power, but will have to go 
into the MIB and change the update rate to something quicker for better 
resolution.

The fiber is connected directly to the attenuator and a single fiber 
into the antenna. However, Rick brings up an interesting point. We are 
still running through the optical coupler in the L304. I've never seen 
this cause a problem (like the isolators Bill was using), but it is 
something else we could look into.

Rob

On 11/16/2024 9:28 AM, Barry Clark via evlatests wrote:
> Cab we get a fast readout of the optical power at the antenna? It 
> would be interesting to know if there is a carrier power variation on 
> the same timescale as the signal phase variation.
>
> Is the long fiber directly connected directly to the 2db attenuator at 
> the patch panel, or is there another device between?  I recall Bill 
> Shillue had trouble with a directional coupler.
>
> On 11/15/2024 10:42 AM, Rob Long via evlatests wrote:
>> I still think it would be interesting to set up a test on the round 
>> trip fiber comparing the the 512 MHz phase stability (central LO vs 
>> return) with the mixer/signal analyzer.
>>
>> Rob
>>
>> On 11/15/2024 9:36 AM, Jim Jackson via evlatests wrote:
>>> Hi Ken,
>>>
>>> I've been wondering this all along.  Are we just seeing an effect at 
>>> the extremes of the system that's maybe exacerbated by the fact that 
>>> the electronics and fiber infrastructure is approaching 20 years old.
>>>
>>> I guess I'll ask everyone the management question that will 
>>> inevitably come up.  While the engineer in me would love to 
>>> understand the cause and fix it, how serious is this effect and how 
>>> much time & money should we invest in tracking it down (beyond fine 
>>> tweaking on levels and cleaning the fiber)?
>>>
>>> Cheers,
>>> Jim
>>>
>>> -----Original Message-----
>>> From: Ken Sowinski <ksowinsk at aoc.nrao.edu>
>>> Sent: Friday, November 15, 2024 8:47 AM
>>> To: Jim Jackson <jjackson at nrao.edu>
>>> Cc: Paul Demorest <pdemores at nrao.edu>; Ken Sowinski 
>>> <ksowinsk at nrao.edu>; evlatests <evlatests at nrao.edu>; Jeffrey Yarnell 
>>> <jyarnell at nrao.edu>; Michael LeBlanc <mleblanc at nrao.edu>
>>> Subject: RE: [evlatests] phase noise on outer antennas {External}
>>>
>>> On Thu, 14 Nov 2024, Jim Jackson wrote:
>>>
>>>> Thanks Paul,
>>>>
>>>> That's interesting and hopefully a clue that it did seem to have 
>>>> some effect.
>>> That, along with the earlier changes, suggests that what is 
>>> happening is intrinsic to the fiber.  Jim, many years ago you and 
>>> Steve wrote a paper for SPIE which was also published as an EVLA 
>>> memo describing the issues for choice of fiber. Perhaps some 
>>> combination of the effects described there explains this?
>>>
>>> Ken
>>>
>>>
>>>
>>>> Cheers,
>>>> Jim
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: Paul Demorest <pdemores at nrao.edu>
>>>> Sent: Thursday, November 14, 2024 2:52 PM
>>>> To: Jim Jackson <jjackson at nrao.edu>; Ken Sowinski <ksowinsk at nrao.edu>;
>>>> evlatests <evlatests at nrao.edu>
>>>> Cc: Jeffrey Yarnell <jyarnell at nrao.edu>; Michael LeBlanc
>>>> <mleblanc at nrao.edu>
>>>> Subject: Re: [evlatests] phase noise on outer antennas {External}
>>>>
>>>> After yesterday's optical power adjustment the RMS phase noise 
>>>> seems to have decreased a bit on the outer antennas (by ~30% or so) 
>>>> but is still notably elevated relative to the rest of the array:
>>>>
>>>> At X-band:
>>>>   ea18 and ea21 previously had RMS about 6.5deg, now they are about 
>>>> 4.5deg.
>>>>   ea24 was 3.5deg, now about 2.4deg.
>>>>   The good antennas are about 0.7 to 1.0deg.
>>>>
>>>> -Paul
>>>>
>>>> ________________________________________
>>>> From: Jim Jackson <jjackson at nrao.edu>
>>>> Sent: Thursday, November 14, 2024 8:49 AM
>>>> To: Ken Sowinski; Paul Demorest; evlatests
>>>> Cc: Jeffrey Yarnell; Michael LeBlanc
>>>> Subject: RE: [evlatests] phase noise on outer antennas {External}
>>>>
>>>> Unless it is something buried deep in the firmware of one of the 
>>>> MIBs, I don't think anything in the antenna still generates or 
>>>> operates at 19.2 Hz.  The L305 is at the heart of everything and it 
>>>> generates 20.0 Hz for timing and 10.0 Hz for the noise diodes.
>>>>
>>>> There is a small inverter generating 110 VAC from -48 VDC sealed up 
>>>> inside the MCB rack to run the network switch & VOIP phone off of 
>>>> the battery backed -48V system. Those are free running and not AC 
>>>> line synchronized in any way.  I wouldn't be surprised to see 59.5 
>>>> Hz from one of those but it's pretty isolated from the rest of the 
>>>> RF system.
>>>>
>>>> Cheers,
>>>> Jim
>>>>
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: evlatests <evlatests-bounces at listmgr.nrao.edu> On Behalf Of Ken
>>>> Sowinski via evlatests
>>>> Sent: Thursday, November 14, 2024 8:35 AM
>>>> To: Paul Demorest <pdemores at nrao.edu>
>>>> Cc: Jeffrey Yarnell <jyarnell at nrao.edu>; evlatests at listmgr.nrao.edu;
>>>> Michael LeBlanc <mleblanc at nrao.edu>
>>>> Subject: Re: [evlatests] phase noise on outer antennas {External}
>>>>
>>>> I  await the results from the recent test.  Comments inline.
>>>>
>>>> On Thu, 14 Nov 2024, Paul Demorest via evlatests wrote:
>>>>> Thanks, I got some more 10ms data today to verify, will report back
>>>>> tomorrow.
>>>>>
>>>>> In the meantime, a week or so ago, I recorded raw data from four
>>>>> antennas (ea11, 13, 18, 22) to get a look at higher time resolution.
>>>>> Two inner and two outer antennas, and the only "bad" one in this set
>>>>> is ea18.  I processed this at 0.5 ms to be able to see fluctuation
>>>>> frequencies up to 1 kHz.  Attached plot shows a power spectrum of the
>>>>> phase noise on all 6 baselines, with 0.1 Hz resolution.  A few
>>>>> notable
>>>>> things:
>>>>>
>>>>> 1. The ea18 baselines all show the excess phase noise, as seen 
>>>>> before.
>>>>>     The amplitude starts dropping around 50 Hz and gets down to a
>>>>>     comparable level to the good baselines around 200 Hz. I think 
>>>>> this
>>>>>     is all expected given a PLL bandwidth of 50 Hz previously 
>>>>> noted in
>>>>>     this thread.
>>>>>
>>>>> 2. In addition to the broad-band noise there are some notable 
>>>>> spikes in
>>>>>     the ea18 data at 19.2, 26.2, 29.7, 39.3, and 59.5 Hz. Aside from
>>>> 19.2 Hz is the old wavegiuide transmit/receive cycle frequency.  
>>>> There may still be some remnants of it around in the master lo.  
>>>> The others mean nothing to me.  If 59.5 Hz is related to mains 
>>>> power, I am surprised it is so far from 60 Hz.  Was this during an 
>>>> SEC outage while using the generator?
>>>>
>>>>>     that last one being near 60 Hz power-line freq, none of these 
>>>>> numbers
>>>>>     are especially meaningful to me.  Do they ring a bell for any 
>>>>> of you?
>>>>>
>>>>> 3. The "good" baselines show a few spikes as well, at 4.0, 10.0 
>>>>> and 20.0
>>>>>     Hz.  Though at a much lower level so not really a problem more 
>>>>> of a
>>>>>     curiosity.  I believe 4 Hz is a known correlator artifact 
>>>>> related to
>>>>>     the length of a memory buffer (Ken can probably confirm).  And 10
>>>>> Hz
>>>> The delay buffer is long enough to hold 250 msec of data, but I am 
>>>> hard pressed to see how this would introduce a periodic signal in 
>>>> the sampled data.
>>>>
>>>>>     and harmonics are common throughout the VLA for multiple 
>>>>> reasons, and
>>>>>     have been noted many times before.
>>>>>
>>>>> Cheers,
>>>>> Paul
>>>>>
>>>>> ________________________________________
>>>>> From: evlatests <evlatests-bounces at listmgr.nrao.edu> on behalf of
>>>>> Leon Abeyta via evlatests <evlatests at listmgr.nrao.edu>
>>>>> Sent: Wednesday, November 13, 2024 2:13 PM
>>>>> To: evlatests at listmgr.nrao.edu
>>>>> Cc: Jeffrey Yarnell; Michael LeBlanc
>>>>> Subject: Re: [evlatests] phase noise on outer antennas {External}
>>>>>
>>>>> The patch panel for the 3 farthest antennas were checked today
>>>>> (11/13/24) and they were all without attenuators. Attenuators
>>>>> labelled
>>>>> 2 dB were added to the 3 antennas in the hope that this will improve
>>>>> the phase noise.
>>>>>
>>>>> Leon Abeyta
>>>>>
>>>>>
>>>>> On 11/7/2024 9:12 AM, Ken Sowinski via evlatests wrote:
>>>>>> On Tue, 5 Nov 2024, Jim Jackson via evlatests wrote:
>>>>>>> Thanks Paul.
>>>>>>>
>>>>>>> Usually this has been accomplished by adding an attenuator in the
>>>>>>> optical path.  The easiest place to do that is in the patch panel
>>>>>>> rack in the Control Building right after the L353 transmitters on
>>>>>>> the fiber going out to the antenna.
>>>>>> This suggests that it is happening in the long fiber run or the
>>>>>> detetor at the antenna; not the optical transmitter. Would you
>>>>>> learn anything by comparing the returned signal on the RT phase
>>>>>> fiber with the optical output of the L353?
>>>>>>
>>>>>> Ken
>>>>>>
>>>>>> ]
>>>>>>> The effect Rob and I analyzed years ago was clearly related to the
>>>>>>> optical power level at the input jack of the optical receiver in
>>>>>>> the L304.  I'll check with Leon to confirm exactly what they did.
>>>>>>> I suspect they added optical attenuation at the patch panel in the
>>>>>>> building to get the measured optical power in the L304 within the
>>>>>>> -17 to -20 dBm spec, then reduced the RF attenuation between the
>>>>>>> L304 and
>>>>>>> L305 in the antenna to get the RF power into the L305 at -26.5 dBm
>>>>>>> where it likes to be.
>>>>>>>
>>>>>>> Cheers,
>>>>>>> Jim
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> -----Original Message-----
>>>>>>> From: Paul Demorest <pdemores at nrao.edu>
>>>>>>> Sent: Tuesday, November 5, 2024 4:27 PM
>>>>>>> To: Barry Clark <bclark at nrao.edu>; evlatests <evlatests at nrao.edu>;
>>>>>>> Jim Jackson <jjackson at nrao.edu>
>>>>>>> Subject: Re: [evlatests] phase noise on outer antennas {External}
>>>>>>>
>>>>>>> Thanks Jim.  A couple follow-ups:
>>>>>>>
>>>>>>> I did the test suggested by Ken of comparing a far-North source
>>>>>>> (J1800+7828) with one near the equator (3C273 aka J1229+0203). The
>>>>>>> fringe rate due to earth rotation decreases by about a factor of 5
>>>>>>> (proportional to cos(dec)) between these two.  No difference was
>>>>>>> observed in the amplitude or appearance of the outer antenna phase
>>>>>>> jitter; everything looks about the same.  I think this continues to
>>>>>>> suggest that this is not a correlator-based effect.
>>>>>>>
>>>>>>> I think Barry's suggestion that this effect might occur at the
>>>>>>> transmit side of the LO fiber makes a lot of sense given what we've
>>>>>>> observed so far.  When the ea26 power level was reduced was that a
>>>>>>> change at the transmitter, or was an attenuator installed at the
>>>>>>> antenna?
>>>>>>>
>>>>>>> Also the ea18 optical power is very slightly outside your
>>>>>>> recommended
>>>>>>> range: it's -16.2 dBm, which is now the highest value on the array
>>>>>>> after the ea26 change.  However the other two arm-end antennas are
>>>>>>> much lower (ea21=-18.3 and ea24=-19.4).
>>>>>>>
>>>>>>> Cheers,
>>>>>>> Paul
>>>>>>>
>>>>>>> ________________________________________
>>>>>>> From: evlatests <evlatests-bounces at listmgr.nrao.edu> on behalf of
>>>>>>> Jim Jackson via evlatests <evlatests at listmgr.nrao.edu>
>>>>>>> Sent: Sunday, November 3, 2024 12:03 PM
>>>>>>> To: Barry Clark; evlatests
>>>>>>> Subject: Re: [evlatests] phase noise on outer antennas {External}
>>>>>>>
>>>>>>> I spoke to Leon about this last week.  We know that the Fiber Span
>>>>>>> optical receivers in the L304 do not like too high of an input
>>>>>>> power.  There is a some kind of effect (that looks thermal but we
>>>>>>> were never certain) and results in a slow wandering of phase at the
>>>>>>> output of the L304. This occurred way below the rated max input
>>>>>>> power from the manufacturer but these devices were designed for
>>>>>>> analog CATV transmission where that effect would probably have 
>>>>>>> never been seen.
>>>>>>> Rob Long and I discovered and analyzed this back during initial
>>>>>>> EVLA outfitting and, based on our tests, set the maximum limit on
>>>>>>> optical power at the input jack of those receivers as -18 dBm
>>>>>>> nominal with an acceptable range of -17dBm to -20 dBm. Antenna 26
>>>>>>> looked to be about 3dB hotter than that so it needed to be looked
>>>>>>> at. Sounds like that has been done. For some reason, the alert
>>>>>>> level on the maximum optical power level monitor in the L304 is set
>>>>>>> at -15dBm.  This really should be changed to -17 dBm.
>>>>>>>
>>>>>>> This doesn't explain what is occurring at the end of the arms since
>>>>>>> those antennas look to be in spec as far as optical power is
>>>>>>> concerned.  If this truly is a new effect that hasn't been seen
>>>>>>> before, perhaps some additional lab testing is in order to see if
>>>>>>> something has degraded with age. Perhaps harmonic distortion of the
>>>>>>> signal has gotten worse as components have aged, which would fit 
>>>>>>> with
>>>>>>> Barry's suggestion.   We do still have the big spools of fiber, so
>>>>>>> tests could be setup in the lab.  Originally, we saw the effect by
>>>>>>> comparing the recovered data clock on a deformatter to the original
>>>>>>> clock from the L350 using a double balanced mixer along with a 
>>>>>>> scope
>>>>>>> / dynamic signal analyzer.   That test was fairly easy when the
>>>>>>> deformatters were in the old correlator room during the transition
>>>>>>> but would be much harder now with them inside the WIDAR correlator.
>>>>>>> I suspect the firmware in them now no longer supports the
>>>>>>> transition mode which is what made that possible. In the lab !
>>>>>> we!
>>>>>>>   could simply measure the L304 output against the original source
>>>>>>> fairly easy.
>>>>>>>
>>>>>>> Cheers,
>>>>>>> Jim
>>>>>>>
>>>>>>> -----Original Message-----
>>>>>>> From: evlatests <evlatests-bounces at listmgr.nrao.edu> On Behalf Of
>>>>>>> Barry Clark via evlatests
>>>>>>> Sent: Saturday, November 2, 2024 12:18 PM
>>>>>>> To: evlatests at listmgr.nrao.edu
>>>>>>> Subject: Re: [evlatests] phase noise on outer antennas {External}
>>>>>>>
>>>>>>> The accidental experiment on ea26 suggests to me that the problem
>>>>>>> occurs at or near the laser transmitter, and is triggered by a high
>>>>>>> laser power.  The ends of the arms are susceptible because those
>>>>>>> fibers are driven harder, due to their length.
>>>>>>>
>>>>>>> When hardware misbehaves in curious ways it is always tempting to
>>>>>>> think some sort of thermal effect might be involved. It might be
>>>>>>> worth trying a little extra cooling on the end-of-arm drivers,
>>>>>>> though with little expectation of positive results. More likely is
>>>>>>> the generation of a LO harmonic which is insufficiently suppressed
>>>>>>> at the telescope, and which varies in phase relative to the 
>>>>>>> primary.
>>>>>>>
>>>>>>> During the EVLA design phase, I remember extensive tests on a big
>>>>>>> spool of fiber.  Would these tests have found this effect if it
>>>>>>> were present then?
>>>>>>>
>>>>>>> On 10/31/2024 4:02 PM, Paul Demorest via evlatests wrote:
>>>>>>>> After this change the phase jitter on ea26 has decreased to about
>>>>>>>> the same level as most other antennas (~1 deg RMS at X-band).
>>>>>>>>
>>>>>>>> The outer antennas still look the same (~6 deg on ea18 and ea21,
>>>>>>>> ~3 deg on ea24).
>>>>>>>>
>>>>>>>> -Paul
>>>>>>>>
>>>>>>>> ________________________________________
>>>>>>>> From: Bruce Mues <bmues at nrao.edu>
>>>>>>>> Sent: Thursday, October 31, 2024 3:08 PM
>>>>>>>> To: Rob Long; Paul Demorest; Jim Jackson; Ken Sowinski
>>>>>>>> Cc: evlatests
>>>>>>>> Subject: RE: [evlatests] phase noise on outer antennas
>>>>>>>>
>>>>>>>> The optical power on ea26 was attenuated this afternoon to be
>>>>>>>> within the proper range @ -17.762.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>>
>>>>>>>> Bruce Mues
>>>>>>>> Technical Manager II: Servo-Fiber
>>>>>>>> Work Schedule: Tu-Fr, 6:30am-4:30pm NRAO-VLA New Mexico, USA
>>>>>>>> P: 575-835-7417
>>>>>>>> E: bmues at nrao.edu
>>>>>>>>
>>>>>>>> -----Original Message-----
>>>>>>>> From: evlatests <evlatests-bounces at listmgr.nrao.edu> On Behalf Of
>>>>>>>> Rob Long via evlatests
>>>>>>>> Sent: Wednesday, October 30, 2024 5:00 PM
>>>>>>>> To: Paul Demorest <pdemores at nrao.edu>; Jim Jackson
>>>>>>>> <jjackson at nrao.edu>; Ken Sowinski <ksowinsk at nrao.edu>
>>>>>>>> Cc: evlatests <evlatests at nrao.edu>
>>>>>>>> Subject: Re: [evlatests] phase noise on outer antennas
>>>>>>>>
>>>>>>>> Jim and I looked over the antennas and agree that ea26 could be an
>>>>>>>> optical level problem (power is too high), but the outer antenna
>>>>>>>> levels look normal.
>>>>>>>>
>>>>>>>> Rob
>>>>>>>>
>>>>>>>> On 10/30/2024 4:57 PM, Paul Demorest wrote:
>>>>>>>>> My guess is probably not given how random these variations look..
>>>>>>>>> but we are (for other reasons) planning some tests with these
>>>>>>>>> outer antennas plus VLBA that should be interesting.  Online
>>>>>>>>> fringe rotation will be disabled and the data will be correlated
>>>>>>>>> with difx. If we still see this jitter I think that will mostly
>>>>>>>>> rule out any WIDAR-based cause (hopefully Ken agrees with this
>>>>>>>>> statement ;) Won't have the results for at least a few weeks 
>>>>>>>>> though.
>>>>>>>>>
>>>>>>>>> -Paul
>>>>>>>>>
>>>>>>>>> ________________________________________
>>>>>>>>> From: Rob Long <rlong at nrao.edu>
>>>>>>>>> Sent: Wednesday, October 30, 2024 2:53 PM
>>>>>>>>> To: Jim Jackson; Paul Demorest; Ken Sowinski
>>>>>>>>> Cc: evlatests
>>>>>>>>> Subject: Re: [evlatests] phase noise on outer antennas
>>>>>>>>>
>>>>>>>>> Is there some error being introduced in the fringe rotator for
>>>>>>>>> the
>>>>>>>>> 3 long distance antennas?
>>>>>>>>>
>>>>>>>>> Rob
>>>>>>>>>
>>>>>>>>> On 10/30/2024 2:49 PM, Jim Jackson wrote:
>>>>>>>>>> Thanks, so the slower jitter on those timescales would likely be
>>>>>>>>>> tracked by the L305 oscillator (and synthesizers) and not
>>>>>>>>>> cleaned up in the antenna.
>>>>>>>>>>
>>>>>>>>>> Jim
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> -----Original Message-----
>>>>>>>>>> From: Paul Demorest <pdemores at nrao.edu>
>>>>>>>>>> Sent: Wednesday, October 30, 2024 2:46 PM
>>>>>>>>>> To: Jim Jackson <jjackson at nrao.edu>; Rob Long <rlong at nrao.edu>;
>>>>>>>>>> Ken Sowinski <ksowinsk at nrao.edu>
>>>>>>>>>> Cc: evlatests <evlatests at nrao.edu>
>>>>>>>>>> Subject: Re: [evlatests] phase noise on outer antennas
>>>>>>>>>>
>>>>>>>>>> These observations were sensitive to jitter on 10ms and slower
>>>>>>>>>> timescales.  And when computing RMS I filtered out variations
>>>>>>>>>> slower than 1s to remove atmospheric effects. So the observed
>>>>>>>>>> jitter is on ~1 to 50 Hz scales (does not mean there is not
>>>>>>>>>> faster jitter also, it just gets averaged out in these data).
>>>>>>>>>>
>>>>>>>>>> -Paul
>>>>>>>>>>
>>>>>>>>>> ________________________________________
>>>>>>>>>> From: Jim Jackson <jjackson at nrao.edu>
>>>>>>>>>> Sent: Wednesday, October 30, 2024 2:37 PM
>>>>>>>>>> To: Rob Long; Ken Sowinski; Paul Demorest
>>>>>>>>>> Cc: evlatests
>>>>>>>>>> Subject: RE: [evlatests] phase noise on outer antennas
>>>>>>>>>>
>>>>>>>>>> It would be interesting to know if the nature of this phase
>>>>>>>>>> noise in within or outside the loop bandwidth of the L305 PLL.
>>>>>>>>>> With mention of 10ms dump time this means that jitter is seen
>>>>>>>>>> within the 10ms period - ie. it is faster than 10 ms?
>>>>>>>>>>
>>>>>>>>>> Jim
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> -----Original Message-----
>>>>>>>>>> From: evlatests <evlatests-bounces at listmgr.nrao.edu> On Behalf
>>>>>>>>>> Of Rob Long via evlatests
>>>>>>>>>> Sent: Wednesday, October 30, 2024 2:28 PM
>>>>>>>>>> To: Ken Sowinski <ksowinsk at nrao.edu>; Paul Demorest
>>>>>>>>>> <pdemores at nrao.edu>
>>>>>>>>>> Cc: evlatests <evlatests at nrao.edu>
>>>>>>>>>> Subject: Re: [evlatests] phase noise on outer antennas
>>>>>>>>>>
>>>>>>>>>> Since the LO is actually cleaned up with a VCXO and PLL at the
>>>>>>>>>> antenna, I would think we would see the L305 lose lock if the
>>>>>>>>>> optical levels were too low. If there were some phase variation,
>>>>>>>>>> we should also see the EFC voltage changing as well (indicating
>>>>>>>>>> the reference or VCXO drifting).
>>>>>>>>>>
>>>>>>>>>> Rob
>>>>>>>>>>
>>>>>>>>>> On 10/29/2024 8:48 AM, Ken Sowinski via evlatests wrote:
>>>>>>>>>>> Since you first brought this up I have wondered whether the
>>>>>>>>>>> strict pad dependence implicated a problem with fiber.  Your
>>>>>>>>>>> summary here reinforces that as an explanation. Perhaps a
>>>>>>>>>>> check of LO quality at
>>>>>>>>>>> W32 (it's easier to get to) is in order?
>>>>>>>>>>>
>>>>>>>>>>> Ken
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> On Mon, 28 Oct 2024, Paul Demorest via evlatests wrote:
>>>>>>>>>>>
>>>>>>>>>>>> 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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