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Noise on AD-FMCOMMS1, RX-TX LO Coupling

Question asked by arvabj on Mar 29, 2016
Latest reply on Apr 1, 2016 by charlyelkhoury


My question is with regards to the "severe clock noise on FMCOMMS1" questions initially posted by douzzer on Feb 11, 2013.


severe clock noise on FMCOMMS1


mhennerich had responded Mar 21, 2013 3:59 AM,


It said

"I recently discovered a cross coupling between the RX and TX LO PLLs. If they are set to exactly the same frequency - and in case there is a slight frequency difference (which is not always the case). You fill see the mixing products of both PLLs on the LO outputs. There are LO filters which are not populated by default -  You can tap the LO output there with your spectrum analyzer. Look at L45 located above the 'HSC 11018 REV B' label on the card.

We have a HW specialist looking into the issue -for now the workaround I use is:

a) Do not tune the PLLs to the same frequency."


We are still seeing this problem with the AD-FMCOMMS1 boards. We are using the HSC 11018 REV-B boards. I am curious to know how this cross coupling between Rx and Tx LO PLLs was resolved on the AD-FMCOMMS1 boards, when they are programmed to the same frequency. Your inputs are greatly appreciated.