Hi The gotcha is that you have multiple systems working against each other. The crystal in the TCXO has one temperature characteristic. The compensation in the TCXO has a temperature characteristic. They cancel each other out to a limited degree. The residual slope may (or may not) be as shallow as you might think. Your PTC is at an arbitrary point on the residual curve. A somewhat more subtle issue is the gradient between your PTC, the crystal, and the compensation as it cycles.
If the TCXO really isn’t a full TCXO, then some of this goes away. A +/- 2 ppm 0-50C “TCXO" may not have any compensation in it at all. Some 0 to 70C parts are done as 2 ppm 0 to 50 and only compensated at the hot end. They actually may be worse with the PTC than at room... Yes this all assumes an AT cut in the TCXO. That’s a pretty good bet …. Bob > On Jun 4, 2017, at 8:13 AM, jimlux <jim...@earthlink.net> wrote: > > I recall some years ago folks were talking about putting a PTC thermistor on > the TCXO of a FlexRadio SDR1000 to stabilize the frequency as a sort of > poor-man's OCXO. > It's also referenced at > http://www.setileague.org/askdr/xtaloven.htm > where he says "order of magnitude improvement" with no numbers (from 1% to > 0.1% or from 1 ppb to 0.1 ppb?) > > I wonder how well that actually works. > > Say you bought an inexpensive (perhaps non TC) XO and an equally inexpensive > thermistor, glued on on the other, hooked em both up to 3.3 or 5V. > > Yeah, there's issues with room air blowing on it, and tolerances in both the > XO and thermistor, so your absolute frequency accuracy may not be so hot. But > what sort of medium to long term performance can one expect. > > I did some searches, because I'm sure we've discussed this before, but I > couldn't find it. There was some stuff from Oct 2007, but that was in the > context of a more complex circuit, and the thermistor was the sensor. > (discussions of TE devices too) > _______________________________________________ > 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.
