QADF4155, PLL doesn’t lock properly in some condition the due to harmonic
content of the RF in.
In the datasheet there is no spec about the harmonic content allowed in the RF
in and some VCOs have problems to lock in low temperature (where probably the
harmonics get higher).
AWe show the RF sensitivity in fig 4, but we don’t test for higher than 8GHz, a
good test to do would be to look at the N divider o/p to see if the RFin is
choosing a harmonic of the VCO signal, if the N divider o/p is say twice the
expected value, then one could deduce that the VCO 2nd harmonic is high enough
to drive the N divider as opposed to the fundamental.
I would advise measuring the level of the fundamental and harmonics at RFin and
possibly increase the value of the ac coupling cap if required.