[time-nuts] Poor man's oven
chris at chriscaudle.org
Mon Jun 5 19:20:31 EDT 2017
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,
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 220.127.116.11 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.
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