Hi > On Dec 9, 2015, at 9:34 AM, Jim Lux <[email protected]> wrote: > > On 12/9/15 4:37 AM, Bob Camp wrote: >> Hi >> >>> On Dec 8, 2015, at 11:20 PM, Jim Lux <[email protected]> wrote: >>> >>> On 12/8/15 3:31 PM, Bob Camp wrote: >>>> Hi >>>> >>>> Let’s see: >>>> >>>> EFC uses reference out of the OCXO. >>>> EFC comes on the OCXO at no added cost. >>>> 16 bit DAC costs ~$2 to $5 >>>> >>>> Total cost for EFC setup $2 to $5. Net result is a system with >>>> spurs that are how ever far down you wish them to be. (It’s all >>>> about grounding in this case). >>>> >>>> Bob >>>> >>> >>> If the OCXO has steering, the Q of the resonator has to be lower than if >>> the OCXO wasn't steerable. >> >> If the OCXO has an oscillator attached to the crystal, it has a lower Q than >> the crystal it’s self….. >> >> The contribution of a “normal” (relatively narrow band) tuning circuit is >> actually quite small. > > for very high performance oscillators, I'm not sure about that. Here, I'm > thinking about things like USOs where the crystal is in a double vacuum > bottle with multiple heat shields, etc.
Regardless of the oven technology, the Q range on the crystal is pretty much fixed by the blank diameter and the blank design. The USO crystals are no higher or lower Q than those used in other oscillators. The first order impact of the tuning circuit is adding a bit of resistance in series with the crystal. The oscillator circuit does this to a much greater extent. The active components in the oscillator stage have 1/F noise issues to a much greater extent than a narrow bandwidth tuning circuit. Can you crank the tuning range up far enough that a noisy enough reference becomes a problem? Sure you can. Can you tune the oscillator off far enough with a mechanical capacitor that it gets into trouble - yup. What works to your advantage with a fancy oscillator is that it has a very low aging rate. You also can afford to hand select parts. The *required* tuning range for an OCXO typically scales with it’s performance level. A low cost TCXO may indeed need a +/-10 ppm range to tune for 20 years. A high performance OCXO may be equally “happy” with a range below +/- 0.05 ppm. > > There's been several proposals from JHU/APL where a good oscillator is teamed > with a high performance DDS so you don't have to get a crystal at the *exact* > frequency you need. A great idea in my opinion (historically, the crystal > frequency is tied to the channel allocation for your spacecraft, and > non-adjustable frequency makes using spare oscillators from one mission for > another one hard) > > As good an idea as this is, it seems that (very risk averse) folks seem to > stick with the "make lots of oscillators and pick the closest one to the > desired frequency after initial aging". > > The recent GRAIL mission that measured the moon's gravity used two USOs, one > on each spacecraft, with the frequencies slightly different (so they're used > as both Tx source, and LO for Rx for the signal from the other spacecraft). > > A high quality DDS USO would have made this easier in many ways (you could > cherry pick from the dozen or so oscillators for aging and phase noise > properties, rather than also frequency) > > >> >>> >>> So conceivably (if such things were available) you could get a >>> non-steerable OCXO with better (very) close in noise. >> >> Except when you actually wire up that circuit that’s not the outcome. >> >>> >>> And then move the frequency with the DDS. It's fairly straightforward to >>> make a DDS circuit that pushes the spurs and such away from the carrier (at >>> the expense of higher noise farther out). >> >> Which gets you into a variety of spur and noise issues if you want those >> spurs to be below the noise floor of a good OCXO. Getting them into the -130 >> to -150 db down range is far from trivial even >> with the spreading stuff. >> >>> >>> But hey, that's brand new and exotic. >> >> And it pushes the spurs out to where the noise floor should be -170 or -180 >> … hmmm …. > > But there are applications where far out noise isn't as important, for > instance, in a deep space transponder used for ranging. The transponder is > basically a phase locked loop with a very narrow loop bandwidth (a few Hz). > And the receiver on the ground is also very narrow band, so noise that's say, > 10 kHz away, isn't a big deal, compared to noise within a few Hz, which is. > > (ADEV of 4E-16 at tau of 1000 seconds is a typical state of the art > requirement) …. and has been since the 1970’s when I first started talking with JPL people about this :)…. ==== All that said, the real question is — can you change the fabrication of the crystal in ways that improve it’s stability by tuning a long way with DDS rather than a short way with reactance (select parts plus varicap(s)). Bob > > > > >> >> Bob >> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
