On 4/26/22 1:31 PM, ASSI wrote:
On Montag, 25. April 2022 18:27:01 CEST André Balsa wrote:
A PDF of Shera's article can be found here (many thanks to whomever is
hosting this file):
https://www.qsl.net/n9zia/wireless/QST_GPS.pdf
To me, there is no doubt Shera's original design inspired all the following
DIY GPSDO designs in one way or another. Also this remark:
"Figure 6 also suggests that two major causes of frequency
instability—temperature shift and aging—could be predicted and largely
eliminated by tracking the performance of the VCXO for a while to estimate
the aging parameters and by measuring the ambient temperature. The
predicted corrections could be applied to the VCXO independently of the
PLL, which might allow much longer loop filtering time constants to be
used, further reducing GPS jitter. Although this scheme would be ultimately
limited by sources of crystal frequency instability that are random and
inherently unpredictable, it might be interesting to explore."
Establishing aging parameters for a modern non-ovenized crystal is a fools
errand in my experience, at least if you keep the system operational for a
long enough period of time. If you don't, then you'll need to learn the aging
parameters anew or you'll at least have to wait out the retrace before re-
using data from the previous run. When the initial retrace / aging transient
has subsided, a linear model is good enough for short timescales (out to
several days), but the actual logarithmic aging behaviour ensures that the
slope gets very small. I have some systems that are going into their fifth
year of mostly uninterrupted, self-ovenized operation and aging induced
frequency drift is swamped by other influences at the timescales of a
reasonably imaginable control loop, although it is still visible on (much)
longer timescales of course (currently drifting at about 100…200ppb/year).
Feed-forward compensation of temperature fluctuation does work reasonably
well, but you can expect only about one order of magnitude performance
improvement from doing that, maybe two if you manage to get a really close
coupling of the sensor to the actual crystal temperature. IIRC, some TCXO used
to have a second quartz platelet with a special cut to act as a temperature
sensor. It's also possible to interrogate the crystal temperature by exciting
multiple harmonics and looking at their frequency difference, but I don't know
if any commercial applications employ that effect.
That's the MCXO - uses third harmonic and fundamental to measure the
temperature. Q-Tech sells them. Or, more properly, has them in their
catalog and may be happy to quote a price and delivery. The datasheet
revisions are >5 years ago, except for the Space version which was
updated a couple years ago. They're fairly good temp stability (a few
ppb over -40 to +90) and lower power than a OCXO of comparable
performance. The fancy temperature compensation doesn't say anything
about aging, of course. It's unclear whether cycling the temperature up
and down at room temp makes an oscillator age faster than one held at a
constant higher oven temp.
https://q-tech.com/products/mcxo-products/
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