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.