Hi I do believe you will find that a 3rd overtone will do quite a bit better at 100Hz offset than a fundamental.
Bob On Sep 19, 2010, at 2:56 PM, jimlux wrote: > Magnus Danielson wrote: >> On 09/19/2010 08:23 PM, francesco messineo wrote: >>> On 9/19/10, jimlux<jim...@earthlink.net> wrote: >>>> francesco messineo wrote: >>>>> Hi Mike, >>>>> >>>>> as I said, current plans are for a few frequencies in the 20-50 MHz >>>>> range. The current project needs 20, 22 and 42 MHz oscillators. >>>>> >>>> >>>> >>>> But you're multiplying that up, it will be 20log(N) worse... >>> >>> no, I'm using these as LO for frequency downconversion. 48, 50, 70 MHz >>> to 28 MHz in the first prototype. >> If you use a 10 MHz OCXO as reference, you will have a 20*log10(50/10) and >> 20*log10(70/10) worse phasenoise shift, i.e. 13,98 dB and 16,90 dB higher >> phasenoise than the reference. >> > > which is why some effort at finding a high quality fundamental mode crystal > at the LO frequency might be worthwhile. If you're running into a software > receiver, then the precise LO frequency might not be important as long as you > know what it is. That lets you design an oscillator with no tuning input, so > you can use a higher Q crystal, and you'd just worry about the temperature > not changing too fast. (a TCXO is probably a non-starter from noise > properties here, the fact that it *can* be compensated implies that the Q is > low enough to be moved enough to compensate). > > You could ovenize it, of course. > > Just for a "what can you get from state of the art", you can look at Greg > Weaver's paper > www.pttimeeting.org/archivemeetings/2008papers/paper6.pdf > on the USOs for spaceflight.. > We like to use about 75 MHz, and typical performance is -120dBc/Hz at 100 Hz > offset. I don't recall if these USOs are a lower frequency crystal > multiplied up or a fundamental mode XO at the 75 MHz. > > (scaling the 10811 performance.. -140dBc/Hz at 100Hz, at 10MHz, scaled up to > 75 MHz is about the same: -122dBc/Hz) > > (also, the USO's real thing is long term performance for Allan deviation and > drift, not so much the "phase noise"... it's being used in a measurement > system with a <1Hz bandwidth..) > > _______________________________________________ > 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.
