B-C RF Leakage

I knew that trying to mount the boards on a BBB and mount all of them together would pose some problems...

While measuring a filter response using an A-B-C stack on a single BBB a high baseline was noted with lobing characteristics around the 1600MHz region.  To better understand the cross coupling between LOs among the boards, a spectrum analyzer was connected to the RF input port.  The other board's LO is powered off (VCO power down on the ADF4351) and the aux output is disabled.

You can see the situation is pretty pronounced beyond 1GHz.  Below 700MHz its difficult to identify the LO within the background noise from the BBB and other sources.  This was then followed up by enabling both LO's (at 10.75MHz apart as in the two stage conversion), aux output off,  and measuring each frequency at the RF input port of each board from 800MHz to the limit of my 7L12 spectrum analyzer (stop laughing).  The following figure captures that information.

A fair amount of investigation was done (disabled amplifier block, added 100pF supply bypass capacitors to the mixer,...).  In short, I cannot locate a single source of the leakage (I'm not sure if this is good, bad, or within expected behavior for this type of configuration).  There were many compromises (and errors) made in the layout of these boards.  Part of the goal of this project was to learn and experiment - just go do and learn as opposed to endlessly read, study and simulate.  The next step is to separate the B/C boards on to different BBB's and update the software to control them over the network.