[time-nuts] HP 3586 entirely referenced to 10MHz: A solution II

David I. Emery die at dieconsulting.com
Mon Apr 4 21:33:05 UTC 2011


On Mon, Apr 04, 2011 at 07:19:38AM -0400, Chuck Harris wrote:
> Basically, the higher the division ratio in a PLL synthesizer,
> which is what you are describing, the greater the phase noise.

	No question about that, indeed.

	But I am talking about a very low bandwidth loop (presumably
well under 1 HZ should work) which means the phase noise contribution
from the dividers and reference should be only inside that 1 Hz
bandpass. Outside of that the original crystal oscillator phase noise
should control, and while this won't improve that it also won't make it
any worse.

>
> You can think of it this way: Both the reference, and the oscillator
> being controlled, need to be divided down to some common frequency
> that you feed to the phase detector.  The entire time the counter is
> counting up the cycles to get you a cycle of that common frequency,
> the oscillator is not being disciplined.   It is only after the
> count gets done that the phase detector can compare the two signals
> and create a correction correct for the error in the oscillator.

	True, but I am pretty sure the original crystal oscillator (even
modified with a varactor for tuning) was not phase-noisier than the rest
of the instruments LOs.  It is, after all, a LF crystal oscillator
running at 13 or 17 KHz with presumably a high Q crystal which shouldn't
to the first order have unreasonable phase noise in the band around it. 
The original problem was that this oscillator was not locked to a
reference and could drift a few tenths of a HZ (and maybe even Hz)
randomly with temp - not that it had too much phase noise.


> 
> The DDS is essentially a hardware solution to finding a suitable
> divider ratio to convert one frequency into another.

	I do understand DDSes.
> 
> -Chuck Harris


-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."




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