[evlatests] phase noise on outer antennas {External}

[Leon Abeyta]

It's "OPT_PWR".  Typically -18 dBm.

Leon


On 10/29/2024 9:09 AM, Ken Sowinski via evlatests wrote:
> 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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