[evlatests] 36 GHz 'RFI'

[Dan Mertely]

I got to look at this on antenna 22 and indeed found a
spike at around 36280 MHz and 34875 MHz.  The cause was
found to be oscillating LNA in the X-band FE.  This has
been seen before.

The FE has been written up for repair.

Details:

With LO1 @ 15234 MHz, RFI was seen at 9414 MHz @ -44 dBm
at the FE output, corresponding to the 36 GHz RFI, and
at 10821 MHz @ -55 dBm, corresponding to the 34 GHz RFI.

When the Ka band Feed was covered with foam the signals
dropped down by 30 dB.

When the X-band feed was covered with foam, the signals
dropped significantly.

When the X-band FE bias cards were pulled, the RFI went
away.  (RCP for the lower signal, LCP for the upper signal)

-Mert

Rick Perley wrote:
>     I got some time to look more closely at the 36280 MHz 'birdie' 
> reported on yesterday. 
>    
>     It is only on antenna 22, in both RCP and LCP, but stronger in the 
> former.  It is seen on all baselines between 22 and other antennas, but 
> on no other baselines -- even those comprising adjacent antennas.  The 
> strength of the 'line' is much stronger on short baselines to 22 -- 
> indicating that fringe winding is operating. 
> 
>     So this would presumably be an internal signal. 
> 
>    
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