[time-nuts] Counter Internal Oscillator Importance with External Reference?
kb8tq at n1k.org
Tue May 9 07:11:27 EDT 2017
If you grab the schematic of pretty much any of these counters, you find a fairly common approach.
Somewhere inside is a VHF oscillator. The internal TCXO, OCXO, or Rb acts as a phase lock source
for that VHF oscillator. Typical PLL bandwidths are pretty low (10’s of Hz). When you put in an external
reference, it acts as the reference to the same PLL.
This does a couple of things. You take care of spurs on the external reference. That is the same
thing you do locking up a VHF oscillator to your GPSD for microwave work. You get the long term
accuracy (1 second and out) of the external reference. Your phase noise (and thus broadband mask
jitter) is improved.
Does every counter on the planet work this way? - of course not. Somewhere somebody did it a
different way. As long as they *did* do it this way, the internal reference does not matter once you
switch to an external standard. Even if they did it in some other way, if they did it right, the same
statement would apply.
> On May 8, 2017, at 9:02 PM, Tom Knox <actast at hotmail.com> wrote:
> Hi All;
> How important is the standard, medium, or high stability reference in counters like the 53230A, or FCA3120 when locked to an ultra low phase noise external reference? Particularly when making measurements and adjustments on other ultra high performance references?
> I know Agilent sold 53132A counters with an option (H01 I believe) that bypassed the internal reference completely when and external reference is applied, but I think standard configuration on most counter is to discipline the internal reference.
> Thanks for your thoughts.
> Thomas Knox
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