[evlatests] phase noise on outer antennas {External}

Bruce Mues bmues at nrao.edu
Wed Nov 6 12:24:02 EST 2024 

" 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?"

The attenuation is done in the LO/IF room in the control building.

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 Paul Demorest via evlatests
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
>>>> _______________________________________________
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