[time-nuts] 10MHz to 25MHz

[Bob Camp]

HI

A lot of your evaluation of the term “better” will depend on your intended use. One of the limits on phase noise
is the thermal noise floor. Because of that, starting at a higher frequency will always give you an edge on broadband
phase noise. ADEV / short term stability is linked to the Q of your resonator. In a quartz crystal, maximum Q is 
roughly proportional to frequency. The other limit on Q is blank geometry (size). One other limit is practicality - 
is a $250,000 OCXO that is 1 cubic meter in size appropriate for your application? The answer to that one is 
universally - NO :) Somewhere along the line of larger size and cost, other technologies make more sense. 

So, if better = phase noise floor, 100 MHz is better than 10 MHz. If better = ADEV, 5 MHz in a large package is 
likely better than 100 MHz. Indeed these are only two variables. There are *many* others you could look at. 

Lots of fun 

Bob 



> On Jan 19, 2017, at 7:13 AM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 
> Chris wrote:
> 
>> I have always wondered why we build our "standard" with such a low
>> frequency.   Why not a 100MHz GPSDO?   Why 10MHz
> 
> Quartz crystals work better at lower frequencies, predominantly because they have higher Q.  10MHz was chosen because it is low enough for excellent performance but high enough to be directly useful (since an accident of biology gave us ten fingers, we've created a base-10 world and powers of 10 are favored in almost everything).
> 
> In prior times, 5MHz crystals held this position, and before that, 1MHz.  There is a good argument even today that the best 2.5MHz or 5MHz crystals are better than the best 10MHz crystals, but not by enough to make 2.5MHz or 5MHz standards popular any longer.
> 
> One lonely data point, which proves nothing:  My best crystal oscillator is a Symmetricom clone of the double-oven HP 10811s (it came out of an HP GPSDO, so apparently HP at one time used them interchangeably with the 10811).  That OCXO uses a 5MHz crystal and a frequency doubler to produce its 10MHz output.
> 
> Best Regards,
> 
> Charles
>