[evlatests] Birdie at 30.976 GHz

[Rick Perley]

     Just a note to all that this 'birdie' is hardly a critical issue.  
It's actually quite weak (tens of Jy in a 2 MHz channel), is confined to 
a single channel, and is only visible on the shortest spacings.  So -- 
no biggee.

     Rick


On 02/16/2018 10:40 AM, Wes Grammer wrote:
> Thanks Ken. I did confirm my earlier test results: if there's LO 
> leak-through at 15.488 GHz, which seems likely, it's considerably 
> weaker than at 14.720 GHz, by >20 dB. When I get a moment next week, 
> I'll insert a Ku-band LNA ahead of the spectrum analyzer, so I can get 
> an extra 20 dB or so of dynamic range, and maybe see it.
>
> Regardless, I don't think there's much that can be done about it, 
> without redesigning the downconverter; there's no easy fix I can see.
>
> -Wes
>
>
> On 2/15/2018 11:00 AM, Ken Sowinski wrote:
>> On Thu, 15 Feb 2018, Wes Grammer wrote:
>>
>>> Jim, it does seem possible, since this birdie is the second harmonic 
>>> of one of the L301 tuning frequencies (15.488 GHz). I did a bench 
>>> test of harmonic output from the downconverter back in 2015, when a 
>>> similar birdie at 29.44 GHz was found; a PDF is attached. It shows 
>>> the LO-IF leakage for a wide range of the possible LO drive 
>>> frequencies, but the ones applicable to the VLA are between 14.720 
>>> and 16.256 GHz, as they are within the passband of the output filter 
>>> after the LO tripler.
>>>
>>> Interestingly, this new one is right in the center of that band, but 
>>> my earlier measurement showed no detectable IF output at 15.488 GHz 
>>> (which would correspond to a sky frequency of 30.976 GHz). There was 
>>> a substantial spike at 15.744 GHz, however.
>>>
>>> I'll double check this result in the lab today - I happen to have a 
>>> good downconverter in the test set and the equipment set up. Rick, 
>>> in your observation with this detection, was the L301 set to 15.488 
>>> GHz?
>>
>> L301-1 was 15488
>> L301-2 was 13184
>>
>> Ken
>>
>
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