[evlatests] phase noise on outer antennas {External}

[Jim Jackson]

Leon , please refresh my memory - what is the loop bandwidth of the L305 PLL?

Jim


-----Original Message-----
From: evlatests <evlatests-bounces at listmgr.nrao.edu> On Behalf Of Leon Abeyta via evlatests
Sent: Wednesday, October 30, 2024 2:38 PM
To: evlatests at listmgr.nrao.edu
Subject: Re: [evlatests] phase noise on outer antennas {External}

Not to mention the additional clean up provided by the YIGs of L301s and L302s.

The optical power is adjustable, but it's actually only 1 dB out-of-spec. It's actually -13.92 dBm.

Leon

On 10/30/2024 2:27 PM, Rob Long via evlatests wrote:
> 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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