We have an application where we are using internal LOs and the Tx LO is the same frequency as the Rx. First, is there a way to only use 1 LO to drive both Tx and Rx? I don't think so, but thought I'd ask.
The problem we are seeing is the excess phase noise on the Tx signal, around 50% of the time (see spectrum analyzer screen shot of CW signal). Changing frequency or power cycling the unit can clear / cause the high phase noise state. I'm guessing there is some kind of in phase / out of phase interaction between some leaked and divided signal between the LOs which changes randomly when re-locking, but can't say for sure. The poor phase noise signal gives ~2 % worse EVM on the Tx signal.