[evlatests] phase noise on outer antennas {External}

[Ken Sowinski]

On Tue, 29 Oct 2024, Todd Hunter via evlatests wrote:

> I wonder if there is an extra EDFA in the fibers to those outermost pads 
> whose amplification level is (no longer) being set correctly?    Is there any 
> monitor point on optical power?

There is for the received digital data and it loks normal, at least for 
ea18.  If there is for LO to the antenna it is hidden with a misleading 
monitor point name in the L304.

Ken


> Todd
>
> On 10/29/24 10:48, 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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