Hello Paul,
When there is no LO inputs, the zero-crossing small signals can potentially generate spurious, can osicllate inside div-by-2 LO circuit that has flip-flops with a positive feedback loop. Could you check if there are spurious at normal operation condition with LO signal? For example, if you input LO 2GHz, then you will get 1GHz LO(carrier feedthrough) and N*LO (LO Harmonics) on RFout.
Best Regards
Tony
