[evlatests] phase noise on outer antennas {External}

[Leon Abeyta]

About 50 Hz.

Leon

On 10/30/2024 2:40 PM, Jim Jackson wrote:
> 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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