[time-nuts] 10MHz to 25MHz

[Scott Stobbe]

Wouldn't crystal drive level be one of the important specifications for far
out phase noise?

On Thu, Jan 19, 2017 at 1:33 PM, Bob Camp <kb8tq at n1k.org> wrote:

> 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
> >
>
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