PLL to PLL crosstalk

[azaz104]

I have 2 adjacent PLL's working next to each other. Each tuned to 10GHz. with a loop bandwidth of 15MHz ( closed loop BW)

Problem: with some initial phases of the PLL's, the two PLL's start to play a ping pong game and the lock is lost. It's all over the spectrum.
while tuning the phase a bit makes this problem go away.

Any thoughts what's causing this ?

 

[dick_freebird]

I would suspect that ground activity can perturb the
phase detectors, all the internal logic will spike the
grounds continuously and there will be "unfortunate
alignments" possible if their reference and feedback
inputs are all referred to the same ground (many PLLs
do not have differential inputs but an internal de facto
reference to VSS, your ground).

 

[crutschow]

It doesn't take much coupling for 10GHz to travel from one circuit to another. You likely have a decoupling and/or ground problem between the two circuits.