[time-nuts] Poor man's oven

Bob kb8tq kb8tq at n1k.org
Mon Jun 5 20:21:10 EDT 2017


That paper is the basis for the MCXO. It is an interesting way to do a TCXO. 
The drift between the two modes makes it a difficult thing to master in an OCXO.
Plating a pair of electrodes (one pair per mode) is also an approach that has been


> On Jun 5, 2017, at 7:20 PM, Chris Caudle <chris at chriscaudle.org> wrote:
> On Mon, June 5, 2017 5:38 pm, Charles Steinmetz wrote:
>> Some years ago, I consulted for a research group that was using a number
>> of non-contact technologies to measure the temperature of oscillating
>> quartz crystals.
> In most cases what you really care about is the stability of the
> frequency, and the temperature of the crystal is just a proxy for that,
> correct?
> I thought there was some effect where different modes of oscillation
> shifted by different amounts with temperature, and if you had two
> oscillation circuits running from the same crystal but different modes,
> you could use the shift in difference frequency between the two modes to
> infer the temperature change.
> Found a reference in the Vig tutorial:
> S. Schodowski, "Resonator Self-Temperature-Sensing Using a
> Dual-Harmonic-Mode Crystal Oscillator," Proc. 43rd Annual Symposium on
> Frequency Control, pp. 2-7, 1989, IEEE Catalog No. 89CH2690-6.
> From page 48 of Vig tutorial version May 2013:
> As is shown in chapter 4, see “Effects of Harmonics on f vs. T,” the f
> vs. T of the fundamental mode of a resonator is different from that of
> the third and higher overtones.  This fact is exploited for
> “self-temperature sensing” in the microcomputer compensated crystal
> oscillator (MCXO). The fundamental (f1) and third overtone (f3)
> frequencies are excited simultaneously (“dual mode” excitation) and a
> beat frequency fb is generated such that fb = 3f1 - f3 (or fb = f1 -
> f3/3). The fb is a monotonic and nearly linear function of
> temperature, as is shown above for a 10 MHz 3rd overtone (3.3. MHz
> fundamental mode) SC-cut resonator.
> The graph shows a line with slope of around 80ppm/deg C.  Not sure what
> that translates to in terms of what you could realistically measure and
> use for frequency compensation.  I guess you could use that information to
> either control an oven or just let the crystal run free and control a
> synthesizer for the used output.
> -- 
> Chris Caudle
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