I drew a quick picture of the setup, that I hope is right.? It looks like the reference frequency equation, from the HP 83590A manual, would be correct if you removed the "3.8 GHz" term.? The calculations are based on the fact that the HP 8709A synchronizer input is at 20 MHz, for phase lock.? So, the frequency difference between the "83590A Aux Output" and the "8640A output multiplied by N2" would be 20 MHz at phase lock.
