[time-nuts] 10MHz to 25MHz

Bob Camp kb8tq at n1k.org
Thu Jan 19 16:14:29 EST 2017

> On Jan 19, 2017, at 3:03 PM, Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
> 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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