Re: [time-nuts] GPS discipline oscillator vs phase lock
Hi
A far more common approach is to let the two oscillators free run and to record something like a local broadcast station.
You then post process all of the data to give you the phase accuracy. One of several gotcha’s is the stability of any
radio link at the level you are looking for.
Bob
On Jun 16, 2017, at 5:01 PM, life speed life_speed@yahoo.com wrote:
I was afraid of that, I guess it doesn't hurt to ask. Perhaps I could implement an ISM band radio link for the purpose of locking the two oscillators. Of course that wouldn't reach a couple miles either.
- Lifespeed
Hi
A lot depends on your definition of “phase locked”. If indeed you are after 0.1 degree at 100 MHz, that gets into the “no can do” range. To put some numbers on it, 0.1 degree at 100 MHz is 2.7 ps. GPS time as received simply is not stable to that level … If you drop back to about 20 degrees, you start to get into the “maybe can do” range.
Bob
On Fri, 16 Jun 2017 17:46:45 -0400
Bob kb8tq kb8tq@n1k.org wrote:
A far more common approach is to let the two oscillators free run and to
record something like a local broadcast station.
You then post process all of the data to give you the phase accuracy. One of
several gotcha’s is the stability of any
radio link at the level you are looking for.
Well, we still don't know what the requirements are.
Though I doubt that a GPSDO cannot deliver the needed stability.
Unless the goal is to build a multi-static radar that can locate
a plane down to sub-cm range, ps level stabilty is not required.
Even something as demanding as VLBI is in the couple ns range.
(But they measure over long intervals (hours) which puts higher
demands on the local oscillator)
Another reference value: Time transfer using calibrated GPS receivers
achieves an absolute accuracy of about 1-2ns(RMS) over base lines of
several 10km with single frequency receivers. Dual frequency receivers
have been reported to do <200ps[1] over 400km (TDEV <60ps up to 5days).
Attila Kinali
[1] "Comparing a GPS time link calibration with an optical fibre
self-calibration with 200ps accuracy", by Jlang et al 2015
http://dx.doi.org/10.1088/0026-1394/52/2/384
--
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering. -- The Doctor
Hi
On Jun 16, 2017, at 6:21 PM, Attila Kinali attila@kinali.ch wrote:
On Fri, 16 Jun 2017 17:46:45 -0400
Bob kb8tq kb8tq@n1k.org wrote:
A far more common approach is to let the two oscillators free run and to
record something like a local broadcast station.
You then post process all of the data to give you the phase accuracy. One of
several gotcha’s is the stability of any
radio link at the level you are looking for.
Well, we still don't know what the requirements are.
…. well, we do. A requirement of 0.1 degree at 100 MHz was stated earlier on.
That’s where the ps stuff comes in.
Bob
Though I doubt that a GPSDO cannot deliver the needed stability.
Unless the goal is to build a multi-static radar that can locate
a plane down to sub-cm range, ps level stabilty is not required.
Even something as demanding as VLBI is in the couple ns range.
(But they measure over long intervals (hours) which puts higher
demands on the local oscillator)
Another reference value: Time transfer using calibrated GPS receivers
achieves an absolute accuracy of about 1-2ns(RMS) over base lines of
several 10km with single frequency receivers. Dual frequency receivers
have been reported to do <200ps[1] over 400km (TDEV <60ps up to 5days).
Attila Kinali
[1] "Comparing a GPS time link calibration with an optical fibre
self-calibration with 200ps accuracy", by Jlang et al 2015
http://dx.doi.org/10.1088/0026-1394/52/2/384
--
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering. -- The Doctor
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On Sat, 17 Jun 2017 00:21:54 +0200
Attila Kinali attila@kinali.ch wrote:
Another reference value: Time transfer using calibrated GPS receivers
achieves an absolute accuracy of about 1-2ns(RMS) over base lines of
several 10km with single frequency receivers. Dual frequency receivers
have been reported to do <200ps[1] over 400km (TDEV <60ps up to 5days).
Yet another reference value:
Using LEA6-T it is possible to achieve sub-cm relative positioning precision
when recording phase data and doing postprocessing over several hour
blocks and with distances of a few km (<5km IIRC). Stability is good
to several months in high-alpine environments.
Attila Kinali
--
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering. -- The Doctor
On Fri, 16 Jun 2017 18:29:31 -0400
Bob kb8tq kb8tq@n1k.org wrote:
Well, we still don't know what the requirements are.
…. well, we do. A requirement of 0.1 degree at 100 MHz was stated earlier on.
That’s where the ps stuff comes in.
Yes, that's a requirement. Not the requirement.
It's like marketing saying we need a 32bit ADC resolution.
Yes, we can do that. No, the ADC will not deliver 32bit
and it doesn't need to either.
Without knowing what the application is, it is very hard
to say whether the 0.1 degree is just a number out of thin
air or really a requirement of the system.
Attila Kinali
--
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering. -- The Doctor
That sounds like phase-locking the oscillators to a local radio transmitter. Not sure there is any difference post-processing vs. real time. - Lifespeed
Hi
A far more common approach is to let the two oscillators free run and to record something like a local broadcast station. You then post process all of the data to give you the phase accuracy. One of several gotcha’s is the stability of any radio link at the level you are looking for.
Bob
Hi
On Jun 16, 2017, at 7:24 PM, life speed life_speed@yahoo.com wrote:
That sounds like phase-locking the oscillators to a local radio transmitter. Not sure there is any difference post-processing vs. real time.
The advantage is that you capture a much wider bandwidth signal than you can lock to. That lets you extract better “instantaneous phase” information. With the narrow band loop normally used for locking, loop dynamics get into the picture. That on top of the RF propagation issues is a bit of a mess. It also is quite possible to capture multiple radio (or TV or …) transmissions and post process against all of them.
The bottom line is still that “many degrees” at 100 MHz is far more practical than “tenth of a degree”. There are very few options if your application really does need roughly a tenth of a degree.
Bob
- Lifespeed
Hi
A far more common approach is to let the two oscillators free run and to record something like a local broadcast station.
You then post process all of the data to give you the phase accuracy. One of several gotcha’s is the stability of any
radio link at the level you are looking for.
Bob
Not too surprising to read locking two crystal oscillators together without using a physical cable is difficult to impossible. Essentially what I am looking for is the phase alignment accuracy (and phase noise) one would get PLL’ing one oscillator to the other using a cable, but over a longer distance. Some modest phase noise degradation might be acceptable, but not an order of magnitude. Clearly not a trivial problem. Yes, the jitter (phase noise) typically accomplished from a PLL phase comparing at 100MHz is better than what one could get “locking” to GPS. It was just a thought, apparently not a realistic one. Thanks for disabusing me of that notion.
Sorry I can’t go into a lot of detail about the overall system block diagram, but this one aspect of the design does just reduce to phase-locking two oscillators over a distance.
Bob, I think I understand your post processing method refers to the reality that all broadcast signals from which phase information could be extracted are modulated, introducing complications that would not be present with a simple carrier.
Lifespeed
Hi
On Jun 16, 2017, at 7:24 PM, life speed life_speed@yahoo.com wrote:
That sounds like phase-locking the oscillators to a local radio transmitter. Not sure there is any difference post-processing vs. real time.
The advantage is that you capture a much wider bandwidth signal than you can lock to. That lets you extract better “instantaneous phase” information. With the narrow band loop normally used for locking, loop dynamics get into the picture. That on top of the RF propagation issues is a bit of a mess. It also is quite possible to capture multiple radio (or TV or …) transmissions and post process against all of them.
The bottom line is still that “many degrees” at 100 MHz is far more practical than “tenth of a degree”. There are very few options if your application really does need roughly a tenth of a degree.
Bob
-
Lifespeed
Hi
A far more common approach is to let the two oscillators free run and to record something like a local broadcast station.
You then post process all of the data to give you the phase accuracy. One of several gotcha’s is the stability of any
radio link at the level you are looking for.
Bob
Le 17 juin 2017 à 07:55, Lifespeed via time-nuts time-nuts@febo.com a écrit :
Not too surprising to read locking two crystal oscillators together without using a physical cable is difficult to impossible. Essentially what I am looking for is the phase alignment accuracy (and phase noise) one would get PLL’ing one oscillator to the other using a cable, but over a longer distance. Some modest phase noise degradation might be acceptable, but not an order of magnitude. Clearly not a trivial problem. Yes, the jitter (phase noise) typically accomplished from a PLL phase comparing at 100MHz is better than what one could get “locking” to GPS. It was just a thought, apparently not a realistic one. Thanks for disabusing me of that notion.
Sorry I can’t go into a lot of detail about the overall system block diagram, but this one aspect of the design does just reduce to phase-locking two oscillators over a distance.
Why not just have ONE frequency generator locked to GPS if you want, and just distribute the output with equal length cables. It would mean a cable roll to store at one end, but you would be assured of phase coherency at both. No?
Bob, I think I understand your post processing method refers to the reality that all broadcast signals from which phase information could be extracted are modulated, introducing complications that would not be present with a simple carrier.
Lifespeed
Hi
On Jun 16, 2017, at 7:24 PM, life speed life_speed@yahoo.com wrote:
That sounds like phase-locking the oscillators to a local radio transmitter. Not sure there is any difference post-processing vs. real time.
The advantage is that you capture a much wider bandwidth signal than you can lock to. That lets you extract better “instantaneous phase” information. With the narrow band loop normally used for locking, loop dynamics get into the picture. That on top of the RF propagation issues is a bit of a mess. It also is quite possible to capture multiple radio (or TV or …) transmissions and post process against all of them.
The bottom line is still that “many degrees” at 100 MHz is far more practical than “tenth of a degree”. There are very few options if your application really does need roughly a tenth of a degree.
Bob
- Lifespeed
Hi
A far more common approach is to let the two oscillators free run and to record something like a local broadcast station.
You then post process all of the data to give you the phase accuracy. One of several gotcha’s is the stability of any
radio link at the level you are looking for.
Bob
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George Bernard Shaw
Hi
If you also need the phase noise of the OCXO’s to be quite good when operating, the PLL approach has some issues. If you
are after -100 doc / Hz sort of numbers at 1 Hz offset at 100 MHz, then a PLL to GPS is not your friend. At GPS will degrade
that by many 10’s of db. If the phase similarity requirement of the two OCXO’s extends out into the 100’s of Hz (you need them
phase coherent to 10 MHz maybe..), you will not be able to do that with any PLL approach. Again, the “record a set of modulated
signals” approach is more likely to do what you need to get done. In some ways the modulation helps in this case. It lets you better
estimate cycle slips and other odd occurrences.
Bob
On Jun 17, 2017, at 1:55 AM, Lifespeed life_speed@yahoo.com wrote:
Not too surprising to read locking two crystal oscillators together without using a physical cable is difficult to impossible. Essentially what I am looking for is the phase alignment accuracy (and phase noise) one would get PLL’ing one oscillator to the other using a cable, but over a longer distance. Some modest phase noise degradation might be acceptable, but not an order of magnitude. Clearly not a trivial problem. Yes, the jitter (phase noise) typically accomplished from a PLL phase comparing at 100MHz is better than what one could get “locking” to GPS. It was just a thought, apparently not a realistic one. Thanks for disabusing me of that notion.
Sorry I can’t go into a lot of detail about the overall system block diagram, but this one aspect of the design does just reduce to phase-locking two oscillators over a distance.
Bob, I think I understand your post processing method refers to the reality that all broadcast signals from which phase information could be extracted are modulated, introducing complications that would not be present with a simple carrier.
Lifespeed
Hi
On Jun 16, 2017, at 7:24 PM, life speed <life_speed@yahoo.com mailto:life_speed@yahoo.com> wrote:
That sounds like phase-locking the oscillators to a local radio transmitter. Not sure there is any difference post-processing vs. real time.
The advantage is that you capture a much wider bandwidth signal than you can lock to. That lets you extract better “instantaneous phase” information. With the narrow band loop normally used for locking, loop dynamics get into the picture. That on top of the RF propagation issues is a bit of a mess. It also is quite possible to capture multiple radio (or TV or …) transmissions and post process against all of them.
The bottom line is still that “many degrees” at 100 MHz is far more practical than “tenth of a degree”. There are very few options if your application really does need roughly a tenth of a degree.
Bob
- Lifespeed
Hi
A far more common approach is to let the two oscillators free run and to record something like a local broadcast station.
You then post process all of the data to give you the phase accuracy. One of several gotcha’s is the stability of any
radio link at the level you are looking for.
Bob