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

Magnus Danielson magnus at rubidium.dyndns.org
Tue May 9 01:55:45 EDT 2017

Hi Tom!

On 05/09/2017 03:02 AM, Tom Knox 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.

When you provide an external reference, the long term stability becomes 
relatively unimportant. Let's view the two cases:

1) Bypass internal oscillator: If you bypass the internal oscillator and 
always supply an external oscillator, don't waste money on the internal 
reference, it has no use.

2) Lock internal oscillator: If the oscillator is PLL steered, the short 
term noise, i.e. phase-noise, is of relevance, but the long-term only 
cares in the aspect that you can maintain lock, and you can probably 
trim the oscillator to every 5 years to ensure lock, but other than that 
you don't need to waste mony on it. If the phase noise is an issue, you 
could possibly see that in the datahseet/performance spec, but for all I 
have seen, only long term performance is given, so I'd say that you 
should not waste your money there either.

So, unless it will operate as a stand-alone, don't waste money on 
internal reference.

Also, there is a reason that you can buy different internal oscillators, 
this is one of them. For labs with their frequency distribution there is 
no need to waste money on stand-alone performance.

Look at the HP5335A for instance. Standard performance is an XO, and you 
need to trim that regularly to have any kind of performance. The high 
stability option is a 10811. That is quite a jump in performance.


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