[time-nuts] Basic question regarding comparing two frequencies

[John Ackermann N8UR]

Didier Juges wrote:
> I like the 3586 a lot, it's amazing what you can do with it. However, if you send the audio (beat note) to a computer or other instrument, keep in mind that the BFOs are not phase locked to the reference, they are just free standing crystal oscillators, and they may be off by a few Hz. If you want to use the beat note for high accuracy frequency measurement, it would be a good idea to phase lock the BFOs to the reference (at least the one you are going to use, you don't need to do both).
> 
> The carrier frequency measurement system is independant of the BFOs.

I've measured the BFO frequency in my 3586Cs and while the absolute 
frequencies are off by a Hertz or two (and USB and LSB come from 
separate crystals), they are remarkably stable once the receiver is 
warmed up.  They're derived from an ~1.9 MHz crystal that's divided by a 
large number (IIRC 1000) so any crystal drift is reduced significantly.

Therefore, you don't want to derive frequency directly from the audio 
output tone, but for relative measurements the BFO is stable enough for 
any off-air measurement.  And as Didier notes, the BFO isn't in the 
frequency counter path, so doesn't create an error there.

John