I was having fun performing some modulation measurements on my sig-gen (HP8640B) by using my R3361B as a modulation analyzer ...when started to note a tedious 10KHz AM modulation on all signals, even in CW (I mean with all 8640's modulations settings turned OFF). Using a different S.A. I verified that there is no modulation whatsoever in the 8640's CW signals. The sig-gen is not the culprit.
So I went trough section 4.3.1 of the R3361B's service manual to check the side bands around 0Hz of my S.A.
Pictures show what to expect and the actual I've got.
It seems like my L.O. is AM modulated @10KHz making precise measurements (low SPAN and low RBW) difficult if not impossible..
The problem has been fixed with a full PSU recapping. Here is the result:
