[evlatests] phase noise on outer antennas

[Jim Jackson]

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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