> On Jan 19, 2017, at 3:03 PM, Scott Stobbe <scott.j.sto...@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 <kb...@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 <csteinm...@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 >>> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/ >> mailman/listinfo/time-nuts >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there.
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