[evlatests] Birdie at 30.976 GHz

[Wes Grammer]

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?

-Wes

On 2/14/2018 7:23 PM, jjackson wrote:
> Wes - Is this just another case of the LO birdies caused by the 
> Ka-band downconverters?  It sounds a little higher than I remember but 
> I think there were a number of them that could be produced depending 
> on the L301 setting.
>
> Cheers,
> Jim
>
> On 2/14/2018 4:30 PM, Rick Perley wrote:
>> I have found a strong, unresolved RFI 'birdie' in Ka-band:  at 30.976 
>> GHz.  This is the 242nd harmonic of 128 MHz -- but no other harmonics 
>> are seen within the band.
>>
>> Any idea of what this is from?
>>
>>
>>
>>
>>
>>
>>
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