[time-nuts] poor-man's oven
tshoppa at gmail.com
Sun Jun 4 09:45:57 EDT 2017
Bob, at the same time, look at all the guys here who absolutely insist that
the only way to use a double-oven OCXO is to put it in a tightly
temperature controlled environment. "Nuts", yes, but that's why we're here!
I myself have been extremely disappointed with the aging characteristics of
low-end TCXO's. They seem to age even worse than plain old crystal
oscillators, My theory, is this is because the temperature compensation
components are themselves aging more than an AT cut crystal does by itself,
but I've never ripped into one (they're way too tiny to rip into anyway!).
On Sun, Jun 4, 2017 at 9:36 AM, Bob kb8tq <kb8tq at n1k.org> wrote:
> 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
> 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 ….
> > On Jun 4, 2017, at 8:13 AM, jimlux <jimlux at 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
> > 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)
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