[time-nuts] 10MHz to 25MHz
csteinmetz at yandex.com
Thu Jan 19 07:13:42 EST 2017
> 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.
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