GPS Control Loop
25 years ago Brooks Shera developed a GPSDO that had a Frequency accuracy of 1 E-11. A year later in 1998 he published his work in July 11998 in QST. The total package included circuits, PC board and in depth analysis of every part of his design including his filter.. I was one of the first buying the board, GPS receiver and working with an other Time Nut building a couple of units. Using my Tracor 527E,, HP5345A and aHP5061A Cs in parallel to an Austron Loran C system using multiple HP 10811's in all cases observed better than 1 E-11 frequency. All 10811's had previously been tested to have an AV below E-12. Subsequent tests with Austron 1150 and Datum 1000 showed even better results. Over time Brooks added features but the design remained the same. His last effort was to make other DAC's available, but at that time his Cancer had spread to his brain.
Lets remember this was the time of S/A and it is safe to say GPS has improved by a factor of 100. DAC's compared to the Audio DAC are also improved by a factor 50.
Using later circuits we still use his design and Richard McCorkle did a PIC based on his Filter for us.
Very little else has changed. Reviewing commercial units like End Run short term is a function of the OCXO or Rb, long term GPS.
There was a time when Time Nuts had access to quality surplus OCXO's and Rb's at a 'reasonable" price, that tine is gone. Before the time I did get an OSA 8607 I did look at a Morion OCXO, their best with an AV of 2 E-13 at 1 second. The would not respond to my RFQ but thanks to a Time Nut that buys from them regular the answer was 6000 Euro and 1 year delivery after order!
To us Shera is the Gold Standard. worth a review for any one doing their own GPSDO, we are presently focusing on the best GPS receiver, my problem is Ublox 9T is not an option because at 80 I am not able to get safely on the roof. Hire some one is an option but liability may be a problem Bert Kehren
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."
On Fri, Apr 22, 2022 at 3:20 AM ew via time-nuts time-nuts@lists.febo.com
wrote:
25 years ago Brooks Shera developed a GPSDO that had a Frequency accuracy
of 1 E-11. A year later in 1998 he published his work in July 11998 in
QST. The total package included circuits, PC board and in depth analysis
of every part of his design including his filter.. I was one of the first
buying the board, GPS receiver and working with an other Time Nut building
a couple of units. Using my Tracor 527E,, HP5345A and aHP5061A Cs in
parallel to an Austron Loran C system using multiple HP 10811's in all
cases observed better than 1 E-11 frequency. All 10811's had previously
been tested to have an AV below E-12. Subsequent tests with Austron 1150
and Datum 1000 showed even better results. Over time Brooks added features
but the design remained the same. His last effort was to make other DAC's
available, but at that time his Cancer had spread to his brain.
Lets remember this was the time of S/A and it is safe to say GPS has
improved by a factor of 100. DAC's compared to the Audio DAC are also
improved by a factor 50.
Using later circuits we still use his design and Richard McCorkle did a
PIC based on his Filter for us.
Very little else has changed. Reviewing commercial units like End Run
short term is a function of the OCXO or Rb, long term GPS.
There was a time when Time Nuts had access to quality surplus OCXO's and
Rb's at a 'reasonable" price, that tine is gone. Before the time I did get
an OSA 8607 I did look at a Morion OCXO, their best with an AV of 2 E-13 at
1 second. The would not respond to my RFQ but thanks to a Time Nut that
buys from them regular the answer was 6000 Euro and 1 year delivery after
order!
To us Shera is the Gold Standard. worth a review for any one doing their
own GPSDO, we are presently focusing on the best GPS receiver, my problem
is Ublox 9T is not an option because at 80 I am not able to get safely on
the roof. Hire some one is an option but liability may be a problem
Bert
Kehren
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an email to time-nuts-leave@lists.febo.com
To unsubscribe, go to and follow the instructions there.
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.
Regards,
Achim.
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+
Wavetables for the Terratec KOMPLEXER:
http://Synth.Stromeko.net/Downloads.html#KomplexerWaves
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.
On Dienstag, 26. April 2022 23:20:34 CEST Lux, Jim wrote:
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.
Neat. Well, they're certainly not on their stock list (nor the TCXO ultra-
miniature that would drop into a rasPi).
The fancy temperature compensation doesn't say anything
about aging, of course.
It seems that aging is a thing with this particular implementation, seeing
that they can use a reference to tune it out. But they also seem to have
implemented a scheme that determines aging by taking the crystal out of the
synthesis loop for a little while. The data-sheet dooesn't really say if it's
only active at start-up nor whether the continuous correction requires a
reference.
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.
The control loop I implemented for my self-ovenized rasPi can do linear sweeps
of the temperature (usually across the turnover point to determine or refine
the control parameters, you can set both the slope and the span). I have no
hard data, but my impression is that both the sweeping itself and especially
the ramp speed contribute to an accelerated drift that takes a few days to
subside when returning to constant temperature.
Regarts,
Achim.
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+
SD adaptations for KORG EX-800 and Poly-800MkII V0.9:
http://Synth.Stromeko.net/Downloads.html#KorgSDada
Hi
On Apr 27, 2022, at 1:09 PM, ASSI Stromeko@nexgo.de wrote:
On Dienstag, 26. April 2022 23:20:34 CEST Lux, Jim wrote:
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.
Neat. Well, they're certainly not on their stock list (nor the TCXO ultra-
miniature that would drop into a rasPi).
The fancy temperature compensation doesn't say anything
about aging, of course.
It seems that aging is a thing with this particular implementation, seeing
that they can use a reference to tune it out. But they also seem to have
implemented a scheme that determines aging by taking the crystal out of the
synthesis loop for a little while. The data-sheet dooesn't really say if it's
only active at start-up nor whether the continuous correction requires a
reference.
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.
The control loop I implemented for my self-ovenized rasPi can do linear sweeps
of the temperature (usually across the turnover point to determine or refine
the control parameters, you can set both the slope and the span). I have no
hard data, but my impression is that both the sweeping itself and especially
the ramp speed contribute to an accelerated drift that takes a few days to
subside when returning to constant temperature.
As noted in a number of papers, the actual minimum temperature sensitivity
point on an OCXO will not be exactly at the turnover temperature. That will
be the crystal’s “best” but there are a number of other items in the OCXO that
get into the mix as the crystal sensitivity goes to zero. Modern OCXO production
uses temperature test data to set the OCXO to the “best” point.
Bob
Regarts,
Achim.
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+
SD adaptations for KORG EX-800 and Poly-800MkII V0.9:
http://Synth.Stromeko.net/Downloads.html#KorgSDada
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