[time-nuts] Reference input on HP counters

[jimlux]

On 1/7/18 7:21 AM, Jerry Hancock wrote:
> I’m being a little lazy as I can check the schematics, but in the general case, do HP counters, frequency generators, etc, switch to the external reference when one is available or discipline the internal oscillator?  Are there brands of test equipment that generally discipline vs switch?


By the each.

Particularly for newer equipment it can be somewhat of a hybrid - the 
Keysight 33622, for instance, seems to have some sort of frequency 
locked loop (according to the manual and emails from Keysight).  It 
stays on the internal reference, but the frequency is adjusted to match 
the external reference (whether they adjust the oscillator, or reprogram 
a DDS, I'm not sure).

I've seen test data, though, that seems to imply there's something else 
going on - the phase noise is reduced when fed from a quiet external 
source, and if it was a simple frequency locked loop, I don't think that 
would happen.


> 
> Third question, again generally, when you discipline an oscillator, what impact does this have on the oscillator's phase noise?  I assume this third question depends on the discipline loop (if I’m using the correct term).
> 
> Thanks
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