[time-nuts] Counter Internal Oscillator Importance with External Reference?

Magnus Danielson magnus at rubidium.dyndns.org
Tue May 9 09:50:30 EDT 2017


Hi,

In HP5370A/B you mux sources. In SR620 there is a lockup PLL, as far as 
I remember.

Cheers,
Magnus

On 05/09/2017 01:11 PM, Bob kb8tq wrote:
> Hi
>
> 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.
>
> Bob
>
>> 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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