True Position GPSDP + Rb X72
Hi everyone,I'm new to this list, and I wanted to get the opinion of some mere experienced people on my project idea.I just got a Symmetricom X72 Rb oscillator, and I have a True position GPSDO on the way. I wan't to have a self calibrating 10 MHz signal to synchronize my hobby rf lab equipment.
My idea is to make a GPS disciplined Rubidium oscillator, by removing the crystal oscillator ans substituting the X72 in the GPSDO circuit.I would just have to make a op amp circuit to change the 0 to 4v correction signal from the True Position, to a 0 to 5v signal for the X72.
The X72 seems to work well, 10.000000 MHz on my HP 5350B, last calibrated by the US Navy in 1994. I just ordered the True position, from China, so I won't see that for a while, then I need to get it working.
What do you think? Good idea or waste of time? Will the True Position software be able to cope with the change? I haven't done any programming is at least 20 years, so, I hope I can do this with hard ware.
Also, does anyone have ideas for a low cost 10 MHz 50 ohm distribution amp?
Thanks,Steve
On Tue, 19 Sep 2017 18:09:16 +0000 (UTC)
Stephan Flor via time-nuts time-nuts@febo.com wrote:
Hi everyone,I'm new to this list, and I wanted to get the opinion of some
mere experienced people on my project idea.I just got a Symmetricom X72 Rb
oscillator, and I have a True position GPSDO on the way. I wan't to have a
self calibrating 10 MHz signal to synchronize my hobby rf lab equipment.
My idea is to make a GPS disciplined Rubidium oscillator, by removing the
crystal oscillator ans substituting the X72 in the GPSDO circuit.I would just
have to make a op amp circuit to change the 0 to 4v correction signal from
the True Position, to a 0 to 5v signal for the X72.
Unfortunately, this doesn't work as easily as you thing.
The control loop inside the GPSDO is designed for an OCXO. This means
that its loop constant is in the 10s to 100s of seconds, while for
an GPSDRb you would need 10'000s to 100'000s of seconds. The other,
more criticial issue is the sensitivity of the tuning input. Most
OCXOs have something like 1-10ppm of tuning range. The tuning range
of an Rb is usually 2 to 3 orders of magnitude smaller. In a normal
PI loop, this shouldn't be an issue. But you don't know what the
internal logic does. So at least be careful with that.
The more common way to build such a system is to do a phase comparison
between the 10MHz output of the GPSDO and the Rb, then steer the Rb
using a seperate control loop electronics. One easy way to do this
would be using a uC with a capture compare unit, at least one with 16bit,
better with 24bits and more (e.g. STM32F4xx). Divide both 10MHz outputs
down to roughly 1kHz-10kHz using 74LVC161. With this you can measure
the phase differnce between the two with about 12ns precision (limited
by the uC, not the dividers). Average over 2-3 days and you are good.
For additional precision, you can use an TDC7200 to measure the time
difference with around 50-100ps precision, at which point the GPSDO
noise and the stability of the Rb will be the limiting elements.
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
Hi
On Sep 20, 2017, at 8:39 AM, Attila Kinali attila@kinali.ch wrote:
On Tue, 19 Sep 2017 18:09:16 +0000 (UTC)
Stephan Flor via time-nuts time-nuts@febo.com wrote:
Hi everyone,I'm new to this list, and I wanted to get the opinion of some
mere experienced people on my project idea.I just got a Symmetricom X72 Rb
oscillator, and I have a True position GPSDO on the way. I wan't to have a
self calibrating 10 MHz signal to synchronize my hobby rf lab equipment.
My idea is to make a GPS disciplined Rubidium oscillator, by removing the
crystal oscillator ans substituting the X72 in the GPSDO circuit.I would just
have to make a op amp circuit to change the 0 to 4v correction signal from
the True Position, to a 0 to 5v signal for the X72.
Unfortunately, this doesn't work as easily as you thing.
The control loop inside the GPSDO is designed for an OCXO. This means
that its loop constant is in the 10s to 100s of seconds, while for
an GPSDRb you would need 10'000s to 100'000s of seconds. The other,
more criticial issue is the sensitivity of the tuning input. Most
OCXOs have something like 1-10ppm of tuning range. The tuning range
of an Rb is usually 2 to 3 orders of magnitude smaller. In a normal
PI loop, this shouldn't be an issue. But you don't know what the
internal logic does. So at least be careful with that.
The more common way to build such a system is to do a phase comparison
between the 10MHz output of the GPSDO and the Rb, then steer the Rb
using a seperate control loop electronics. One easy way to do this
would be using a uC with a capture compare unit, at least one with 16bit,
better with 24bits and more (e.g. STM32F4xx). Divide both 10MHz outputs
down to roughly 1kHz-10kHz using 74LVC161. With this you can measure
the phase differnce between the two with about 12ns precision (limited
by the uC, not the dividers). Average over 2-3 days and you are good.
An alternative is to add an offset oscillator to the mix and look at beat
notes. It does not have to be ultra close. An 11 MHz offset signal would take the
12 ns down to an effective 1.2 ns. Odd frequency OCXO’s sell cheap on
eBay.
Bob
For additional precision, you can use an TDC7200 to measure the time
difference with around 50-100ps precision, at which point the GPSDO
noise and the stability of the Rb will be the limiting elements.
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
time-nuts mailing list -- time-nuts@febo.com
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and follow the instructions there.
Thank you for the great feedback. You guys make me realize that I need to learn more, I have some studying to do. I will study while I wait for the True position to arrive. Then I have to get it working. I just wanted a 10 MHz sync for my test equipment that was calibrated. May be the GPSDO is all I need.
Steve
On Wednesday, September 20, 2017 7:28 AM, Bob kb8tq <kb8tq@n1k.org> wrote:
Hi
On Sep 20, 2017, at 8:39 AM, Attila Kinali attila@kinali.ch wrote:
On Tue, 19 Sep 2017 18:09:16 +0000 (UTC)
Stephan Flor via time-nuts time-nuts@febo.com wrote:
Hi everyone,I'm new to this list, and I wanted to get the opinion of some
mere experienced people on my project idea.I just got a Symmetricom X72 Rb
oscillator, and I have a True position GPSDO on the way. I wan't to have a
self calibrating 10 MHz signal to synchronize my hobby rf lab equipment.
My idea is to make a GPS disciplined Rubidium oscillator, by removing the
crystal oscillator ans substituting the X72 in the GPSDO circuit.I would just
have to make a op amp circuit to change the 0 to 4v correction signal from
the True Position, to a 0 to 5v signal for the X72.
Unfortunately, this doesn't work as easily as you thing.
The control loop inside the GPSDO is designed for an OCXO. This means
that its loop constant is in the 10s to 100s of seconds, while for
an GPSDRb you would need 10'000s to 100'000s of seconds. The other,
more criticial issue is the sensitivity of the tuning input. Most
OCXOs have something like 1-10ppm of tuning range. The tuning range
of an Rb is usually 2 to 3 orders of magnitude smaller. In a normal
PI loop, this shouldn't be an issue. But you don't know what the
internal logic does. So at least be careful with that.
The more common way to build such a system is to do a phase comparison
between the 10MHz output of the GPSDO and the Rb, then steer the Rb
using a seperate control loop electronics. One easy way to do this
would be using a uC with a capture compare unit, at least one with 16bit,
better with 24bits and more (e.g. STM32F4xx). Divide both 10MHz outputs
down to roughly 1kHz-10kHz using 74LVC161. With this you can measure
the phase differnce between the two with about 12ns precision (limited
by the uC, not the dividers). Average over 2-3 days and you are good.
An alternative is to add an offset oscillator to the mix and look at beat
notes. It does not have to be ultra close. An 11 MHz offset signal would take the
12 ns down to an effective 1.2 ns. Odd frequency OCXO’s sell cheap on
eBay.
Bob
For additional precision, you can use an TDC7200 to measure the time
difference with around 50-100ps precision, at which point the GPSDO
noise and the stability of the Rb will be the limiting elements.
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
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and follow the instructions there.
On Wed, September 20, 2017 3:44 pm, Stephan Flo via time-nuts wrote:
I just wanted a 10 MHz sync for my test equipment that was calibrated.
May be the GPSDO is all I need.
Even an undisciplined rubidium oscillator is going to be very close to
nominal 10MHz. You have never stated your desired precision that I have
seen. The X72 manual says that the output should be within 1E-9 in under
10 minutes, and within 5E-11 after 30 minutes. Does any of the equipment
you use have finer precision than that?
--
Chris Caudle
On Wed, 20 Sep 2017 10:21:51 -0400
Bob kb8tq kb8tq@n1k.org wrote:
An alternative is to add an offset oscillator to the mix and look at beat
notes. It does not have to be ultra close. An 11 MHz offset signal would take the
12 ns down to an effective 1.2 ns. Odd frequency OCXO’s sell cheap on
I considered the DMTD approach as well, but couldn't come up with a
good idea how to make it easy and reliable to implement.
How would you approach this?
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
Hi
Given the noise levels of typical Rb’s - use XOR gates as the mixers.
Do a either a simple LPF on the output or ignore the HF component
(each approach is a bit of a tradeoff).
Once you have the beat notes, feed them into timer inputs on your uC
chip of choice.
If you do the filters as R/C’s they are cheap. If you just re-clock, you
maybe add a flip flop or two to each channel. Total BOM for the DMTD
part of it would be < $1 not counting the OCXO.
All of the previous comments about “why would you want to do this?”
still apply. Most of the small Rb’s we can get our hands on have really
awful phase noise and spurs. That’s not true of all small Rb’s, but buying
them new in small volume at $2K each is not something most of us
are interested in doing ….
Lots of fun
Bob
On Sep 22, 2017, at 9:37 AM, Attila Kinali attila@kinali.ch wrote:
On Wed, 20 Sep 2017 10:21:51 -0400
Bob kb8tq kb8tq@n1k.org wrote:
An alternative is to add an offset oscillator to the mix and look at beat
notes. It does not have to be ultra close. An 11 MHz offset signal would take the
12 ns down to an effective 1.2 ns. Odd frequency OCXO’s sell cheap on
I considered the DMTD approach as well, but couldn't come up with a
good idea how to make it easy and reliable to implement.
How would you approach this?
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
On Fri, 22 Sep 2017 11:50:29 -0400
Bob kb8tq kb8tq@n1k.org wrote:
Given the noise levels of typical Rb’s - use XOR gates as the mixers.
Right. That makes it a lot more robust than using a diode mixer.
If you do the filters as R/C’s they are cheap. If you just re-clock, you
maybe add a flip flop or two to each channel. Total BOM for the DMTD
part of it would be < $1 not counting the OCXO.
Recklocking shouldn't be necessary. A simple inverter should raise
the signal back to an approriate signal level. After that a division
chain is still needed as otherwise the rate of transitions will be
too high for the uC to handle.
Probably the cheapest way to do this would be to use a 38.4MHz or 38.88MHz
oscillator (available for 1-2$) and using a 74LVC74 to divide it by 4.
That gives 400kHz and 280kHz offset frequency, respectively. Dividing
both beat signals by 256 using two 74LVC161/163 each should get it
down enough to be workable with a modern uC.
All of the previous comments about “why would you want to do this?”
still apply. Most of the small Rb’s we can get our hands on have really
awful phase noise and spurs. That’s not true of all small Rb’s, but buying
them new in small volume at $2K each is not something most of us
are interested in doing ….
Yeah..
If anything, I'd do it the other way. Discipline a Rb to GPS using
a long time constant. Then slave an OCXO onto that using the method above
and use a loop time constant in the order of 1-100s
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
Hi
On Sep 22, 2017, at 3:01 PM, Attila Kinali attila@kinali.ch wrote:
On Fri, 22 Sep 2017 11:50:29 -0400
Bob kb8tq kb8tq@n1k.org wrote:
Given the noise levels of typical Rb’s - use XOR gates as the mixers.
Right. That makes it a lot more robust than using a diode mixer.
If you do the filters as R/C’s they are cheap. If you just re-clock, you
maybe add a flip flop or two to each channel. Total BOM for the DMTD
part of it would be < $1 not counting the OCXO.
Recklocking shouldn't be necessary. A simple inverter should raise
the signal back to an approriate signal level. After that a division
chain is still needed as otherwise the rate of transitions will be
too high for the uC to handle.
The output of the XOR will be at F1+ F2 and at F1-F2. The one you want is the
F1-F2 component. You need to reject the F1+F2 somehow. One would be to
go with a D-FF rather than an XOR in the first place, you still get flaky transition
zones. Next is a filter that rejects the (say) 22MHz and passes the (say) 1 MHz.
The final one would be some sort of flip flop or two that sub samples (at 10 MHz
maybe) and thus rejects the 22 MHz. You still have the flakey transition issue.
Probably the cheapest way to do this would be to use a 38.4MHz or 38.88MHz
oscillator (available for 1-2$) and using a 74LVC74 to divide it by 4.
That gives 400kHz and 280kHz offset frequency, respectively. Dividing
both beat signals by 256 using two 74LVC161/163 each should get it
down enough to be workable with a modern uC.
The hope is that you are dealing with something below 1MHz so heroic efforts
are not needed. An L/C filter is probably cheaper than a bunch of dividers and
this and that.
All of the previous comments about “why would you want to do this?”
still apply. Most of the small Rb’s we can get our hands on have really
awful phase noise and spurs. That’s not true of all small Rb’s, but buying
them new in small volume at $2K each is not something most of us
are interested in doing ….
Yeah..
If anything, I'd do it the other way. Discipline a Rb to GPS using
a long time constant. Then slave an OCXO onto that using the method above
and use a loop time constant in the order of 1-100s
And that’s really the point. You now have a GPS -> OCXO -> Rb -> OCXO combo.
You could tune it up to do some cute stuff. Tuning it will not be at all easy. It’s
going to take good test methods and a lot of time. Is that many years or only
many months of test? Who knows …. If you had to pay somebody to do it all, I’d bet
that buying a Maser would be cheaper than doing this as a one off. Buying a $2K
“less noisy” Rb would be a slam dunk in that case as well.
This does get back to “what do you want to do? If this is just a reference that is going
into a 5334 counter and not much else, the stock GPSDO will do that. The stock Rb
with monthly cal will do that. If you want to do direct multiplication to 40 GHz for
your 3 Hz wide PSK rig … none of it measures up.
Bob
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
time-nuts mailing list -- time-nuts@febo.com
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The highest frequency I can generate is 2.5 GHz. My counter can measure 20 GHz, to 1 HZ. At the moment I'm have a frequency locking problem with the extension module in my HP 8660C, so I can't even get to 2.5 GHz, only 110 MHz at the moment. I gust got some extender boards, so I hope to have it fixed soon. The X72 gives me a solid 10.000000 MHz, I would like to see what I get when I use the Rb as a clock for the signal generator, at 2.5 GHz. I guess I only need to verify the calibration with the GPSDO, when I get it, and get it working. Then I can calibrate everything else.
So, to make a long answer short, I don't think I need the level of accuracy that these devices are capable of. I just have a neurotic need to try and make things as exact as I possibly can. Also, this is just a hobby, and learning experience for me, so I can take my time to work things out.Thank you,Steve
On Thursday, September 21, 2017 12:09 PM, Chris Caudle <chris@chriscaudle.org> wrote:
On Wed, September 20, 2017 3:44 pm, Stephan Flo via time-nuts wrote:
I just wanted a 10 MHz sync for my test equipment that was calibrated.
May be the GPSDO is all I need.
Even an undisciplined rubidium oscillator is going to be very close to
nominal 10MHz. You have never stated your desired precision that I have
seen. The X72 manual says that the output should be within 1E-9 in under
10 minutes, and within 5E-11 after 30 minutes. Does any of the equipment
you use have finer precision than that?
--
Chris Caudle
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.