Holdover
It's been pointed out to me that I didn't understand the function of the 1PPS of a time standard. I confess that somehow I had confused the term to be timing standard; which would be an entirely different thing. But, this is time-nuts, so I should have realized...
Anyway, is there a standard, or at least an accepted practice, for how holdover is handled in a time standard? Not "how it's done", as in algorithms, but what is expected by the user. I can see at least 2 ways: time warping (which would be especially bad if the time standard had gotten ahead in time) and nudging back to the correct time. The case of warping is obvious. But, are there other methods, and is there some standard for how quickly the time output of the time standard, and of course the 1PPS pulse, is nudged back to the correct time?
Bob - AE6RV
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On Tue, 16 Aug 2016 04:35:40 +0000 (UTC)
Bob Stewart bob@evoria.net wrote:
It's been pointed out to me that I didn't understand the function of
the 1PPS of a time standard. I confess that somehow I had confused the
term to be timing standard; which would be an entirely different thing.
But, this is time-nuts, so I should have realized...
Anyway, is there a standard, or at least an accepted practice, for how
holdover is handled in a time standard?
There are many ways how to do that and which one you choose depends
on the application and its requirements. You can find everything between
"jump imediatly" and "just keep the frequency stable and don't care about
alignment".
Attila Kinali
--
Malek's Law:
Any simple idea will be worded in the most complicated way.
Hi Attila,
In my unit, which is a frequency standard, I chose to tell the receiver to stop sending 1PPS pulses when it loses sync to the sats. And since the 1PPS is no longer coming, the PLL does nothing and the DAC doesn't change. (Let's avoid the question of aging correction for now.) So, I'm wondering where to go and what to do if I want to get time from my unit. Clearly I could just tell the receiver to continue to send 1PPS pulses and sync to those - marking the time as unreliable. When the receiver synced back up, then it would warp the time output, the 1PPS would warp in phase, and the PLL would correct the phase error.
So, that's one way, but probably not a desirable way. My interest was in the option of using the OCXO to create the time, which clearly gives a better option when the receiver syncs back up to the sats. Is there a published standard for this, or is this something that everyone (except the newbie) knows so well that it's not worth discussing?
Bob
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GFS GPSDO list:
groups.yahoo.com/neo/groups/GFS-GPSDOs/info
From: Attila Kinali <attila@kinali.ch>
To: Bob Stewart bob@evoria.net; Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Tuesday, August 16, 2016 3:46 PM
Subject: Re: [time-nuts] Holdover
On Tue, 16 Aug 2016 04:35:40 +0000 (UTC)
Bob Stewart bob@evoria.net wrote:
It's been pointed out to me that I didn't understand the function of
the 1PPS of a time standard. I confess that somehow I had confused the
term to be timing standard; which would be an entirely different thing.
But, this is time-nuts, so I should have realized...
Anyway, is there a standard, or at least an accepted practice, for how
holdover is handled in a time standard?
There are many ways how to do that and which one you choose depends
on the application and its requirements. You can find everything between
"jump imediatly" and "just keep the frequency stable and don't care about
alignment".
Attila Kinali
--
Malek's Law:
Any simple idea will be worded in the most complicated way.
On Tue, 16 Aug 2016 21:31:17 +0000 (UTC)
Bob Stewart bob@evoria.net wrote:
So, that's one way, but probably not a desirable way. My interest was in
the option of using the OCXO to create the time, which clearly gives a
better option when the receiver syncs back up to the sats. Is there a
published standard for this, or is this something that everyone (except the
newbie) knows so well that it's not worth discussing?
No, there is no standard[1]. A system like this is always tailored to
the consumer of the PPS pulses and its requirements. Ie you need to
figure out what you want to do with those PPS, then figure out what
the desired behaviour during hold-over and recovery is and from that
you can design the frequency standard.
Attila Kinali
--
Malek's Law:
Any simple idea will be worded in the most complicated way.
Hi Attila,
said "Ie you need to figure out what you want to do with those PPS, then figure out what the desired behaviour during hold-over and recovery is and from that you can design the frequency standard."
What is the most usual method for time standards? I'm satisfied with how my unit works as a frequency standard.
Bob
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GFS GPSDO list:
groups.yahoo.com/neo/groups/GFS-GPSDOs/info
From: Attila Kinali <attila@kinali.ch>
To: Bob Stewart bob@evoria.net
Cc: Discussion of Precise Time and Frequency Measurement time-nuts@febo.com
Sent: Tuesday, August 16, 2016 4:37 PM
Subject: Re: [time-nuts] Holdover
On Tue, 16 Aug 2016 21:31:17 +0000 (UTC)
Bob Stewart bob@evoria.net wrote:
So, that's one way, but probably not a desirable way. My interest was in
the option of using the OCXO to create the time, which clearly gives a
better option when the receiver syncs back up to the sats. Is there a
published standard for this, or is this something that everyone (except the
newbie) knows so well that it's not worth discussing?
No, there is no standard[1]. A system like this is always tailored to
the consumer of the PPS pulses and its requirements. Ie you need to
figure out what you want to do with those PPS, then figure out what
the desired behaviour during hold-over and recovery is and from that
you can design the frequency standard.
Attila Kinali
--
Malek's Law:
Any simple idea will be worded in the most complicated way.
Bob,
On 08/16/2016 11:31 PM, Bob Stewart wrote:
Hi Attila,
In my unit, which is a frequency standard, I chose to tell the receiver to stop sending 1PPS pulses when it loses sync to the sats. And since the 1PPS is no longer coming, the PLL does nothing and the DAC doesn't change. (Let's avoid the question of aging correction for now.) So, I'm wondering where to go and what to do if I want to get time from my unit. Clearly I could just tell the receiver to continue to send 1PPS pulses and sync to those - marking the time as unreliable. When the receiver synced back up, then it would warp the time output, the 1PPS would warp in phase, and the PLL would correct the phase error.
So, that's one way, but probably not a desirable way. My interest was in the option of using the OCXO to create the time, which clearly gives a better option when the receiver syncs back up to the sats. Is there a published standard for this, or is this something that everyone (except the newbie) knows so well that it's not worth discussing?
There is no standard, but a few basic ways to go about which seems
reasonable and used by most is:
- As you go into hold-over, keep producing PPS etc
- As you leave hold-over, attempt to adjust the phase back.
- If your system been in hold-over for a longer time, say that it
reasonably deviates outside of +/- 10 us (or some other limit), alarm
and turn output off
I have selected a somewhat more intricate setup in which you can set a
re-assignment limit, so when the phase error is outside of that limit,
you turn the output off, jumps the phase difference, and then starts to
track in from there. The reason being that at some time deviation, the
time it takes to track in the phase error is too large to be practical
so turning of and jump has less impact.
Cheers,
Magnus
Bob
AE6RV.com
GFS GPSDO list:
groups.yahoo.com/neo/groups/GFS-GPSDOs/info
From: Attila Kinali <attila@kinali.ch>
To: Bob Stewart bob@evoria.net; Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Tuesday, August 16, 2016 3:46 PM
Subject: Re: [time-nuts] Holdover
On Tue, 16 Aug 2016 04:35:40 +0000 (UTC)
Bob Stewart bob@evoria.net wrote:
It's been pointed out to me that I didn't understand the function of
the 1PPS of a time standard. I confess that somehow I had confused the
term to be timing standard; which would be an entirely different thing.
But, this is time-nuts, so I should have realized...
Anyway, is there a standard, or at least an accepted practice, for how
holdover is handled in a time standard?
There are many ways how to do that and which one you choose depends
on the application and its requirements. You can find everything between
"jump imediatly" and "just keep the frequency stable and don't care about
alignment".
Attila Kinali
Thanks Magnus!
These look like good guidelines. I'll see what I can come up with.
Bob
-----------------------------------------------------------------
AE6RV.com
GFS GPSDO list:
groups.yahoo.com/neo/groups/GFS-GPSDOs/info
From: Magnus Danielson <magnus@rubidium.dyndns.org>
To: time-nuts@febo.com
Cc: magnus@rubidium.se
Sent: Tuesday, August 16, 2016 6:49 PM
Subject: Re: [time-nuts] Holdover
Bob,
On 08/16/2016 11:31 PM, Bob Stewart wrote:
Hi Attila,
In my unit, which is a frequency standard, I chose to tell the receiver to stop sending 1PPS pulses when it loses sync to the sats. And since the 1PPS is no longer coming, the PLL does nothing and the DAC doesn't change. (Let's avoid the question of aging correction for now.) So, I'm wondering where to go and what to do if I want to get time from my unit. Clearly I could just tell the receiver to continue to send 1PPS pulses and sync to those - marking the time as unreliable. When the receiver synced back up, then it would warp the time output, the 1PPS would warp in phase, and the PLL would correct the phase error.
So, that's one way, but probably not a desirable way. My interest was in the option of using the OCXO to create the time, which clearly gives a better option when the receiver syncs back up to the sats. Is there a published standard for this, or is this something that everyone (except the newbie) knows so well that it's not worth discussing?
There is no standard, but a few basic ways to go about which seems
reasonable and used by most is:
- As you go into hold-over, keep producing PPS etc
- As you leave hold-over, attempt to adjust the phase back.
- If your system been in hold-over for a longer time, say that it
reasonably deviates outside of +/- 10 us (or some other limit), alarm
and turn output off
I have selected a somewhat more intricate setup in which you can set a
re-assignment limit, so when the phase error is outside of that limit,
you turn the output off, jumps the phase difference, and then starts to
track in from there. The reason being that at some time deviation, the
time it takes to track in the phase error is too large to be practical
so turning of and jump has less impact.
Cheers,
Magnus
Bob
-----------------------------------------------------------------
AE6RV.com
GFS GPSDO list:
groups.yahoo.com/neo/groups/GFS-GPSDOs/info
From: Attila Kinali attila@kinali.ch
To: Bob Stewart bob@evoria.net; Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Tuesday, August 16, 2016 3:46 PM
Subject: Re: [time-nuts] Holdover
On Tue, 16 Aug 2016 04:35:40 +0000 (UTC)
Bob Stewart bob@evoria.net wrote:
It's been pointed out to me that I didn't understand the function of
the 1PPS of a time standard. I confess that somehow I had confused the
term to be timing standard; which would be an entirely different thing.
But, this is time-nuts, so I should have realized...
Anyway, is there a standard, or at least an accepted practice, for how
holdover is handled in a time standard?
There are many ways how to do that and which one you choose depends
on the application and its requirements. You can find everything between
"jump imediatly" and "just keep the frequency stable and don't care about
alignment".
Attila Kinali
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.
What to do during and right after holdover depends on the reason you have a
time standard. If it is for maintaining a lab standard, then just shut
down as you can't perform your primary function. It you have this standard
because you are required to time stamp financial transactions then you have
to keep going until you estimate some error threshold then stop. If you
are using it to aim a telescope then again, stop using it the estimated
error is enough that you'd miss your targets. It depends on the use case.
I remember aiming a telescope when our best source of time was NTP over a
dial-up phone modem in the days before always-on Internet This was in the
1980's and it worked well enough. The normal case was "outage" as the
modem connection was short and only a few times per day. But was good
enough to re-calibrate a local clock
Chris Albertson
Redondo Beach, California
Thanks Chris. Those are more considerations that I hadn't thought of. I begin to see why there's no "standard".
Bob
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From: Chris Albertson <albertson.chris@gmail.com>
To: Bob Stewart bob@evoria.net; Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Tuesday, August 16, 2016 7:59 PM
Subject: Re: [time-nuts] Holdover
What to do during and right after holdover depends on the reason you have a time standard. If it is for maintaining a lab standard, then just shut down as you can't perform your primary function. It you have this standard because you are required to time stamp financial transactions then you have to keep going until you estimate some error threshold then stop. If you are using it to aim a telescope then again, stop using it the estimated error is enough that you'd miss your targets. It depends on the use case.
I remember aiming a telescope when our best source of time was NTP over a dial-up phone modem in the days before always-on Internet This was in the 1980's and it worked well enough. The normal case was "outage" as the modem connection was short and only a few times per day. But was good enough to re-calibrate a local clock Chris Albertson
Redondo Beach, California
Bob,
That is what seem to work well in commercial products, including my designs.
Have you seen the RAI report on GPSDOs? I think we discussed it before,
that will be a relevant reading.
Not all system implements 3), and it is a bit complex, so consider it an
option to add, but not necessarily always used. Sometimes you don't want
to do that.
Cheers,
Magnus
On 08/17/2016 02:08 AM, Bob Stewart wrote:
Thanks Magnus!
These look like good guidelines. I'll see what I can come up with.
Bob
AE6RV.com
GFS GPSDO list:
groups.yahoo.com/neo/groups/GFS-GPSDOs/info
From: Magnus Danielson magnus@rubidium.dyndns.org
To: time-nuts@febo.com
Cc: magnus@rubidium.se
Sent: Tuesday, August 16, 2016 6:49 PM
Subject: Re: [time-nuts] Holdover
Bob,
On 08/16/2016 11:31 PM, Bob Stewart wrote:
Hi Attila,
In my unit, which is a frequency standard, I chose to tell the
receiver to stop sending 1PPS pulses when it loses sync to the sats.
And since the 1PPS is no longer coming, the PLL does nothing and the DAC
doesn't change. (Let's avoid the question of aging correction for
now.) So, I'm wondering where to go and what to do if I want to get
time from my unit. Clearly I could just tell the receiver to continue
to send 1PPS pulses and sync to those - marking the time as unreliable.
When the receiver synced back up, then it would warp the time output,
the 1PPS would warp in phase, and the PLL would correct the phase error.
So, that's one way, but probably not a desirable way. My interest was
in the option of using the OCXO to create the time, which clearly gives
a better option when the receiver syncs back up to the sats. Is there a
published standard for this, or is this something that everyone (except
the newbie) knows so well that it's not worth discussing?
There is no standard, but a few basic ways to go about which seems
reasonable and used by most is:
- As you go into hold-over, keep producing PPS etc
- As you leave hold-over, attempt to adjust the phase back.
- If your system been in hold-over for a longer time, say that it
reasonably deviates outside of +/- 10 us (or some other limit), alarm
and turn output off
I have selected a somewhat more intricate setup in which you can set a
re-assignment limit, so when the phase error is outside of that limit,
you turn the output off, jumps the phase difference, and then starts to
track in from there. The reason being that at some time deviation, the
time it takes to track in the phase error is too large to be practical
so turning of and jump has less impact.
Cheers,
Magnus
Bob
AE6RV.com
GFS GPSDO list:
groups.yahoo.com/neo/groups/GFS-GPSDOs/info
From: Attila Kinali <attila@kinali.ch <mailto:attila@kinali.ch>>
To: Bob Stewart <bob@evoria.net mailto:bob@evoria.net>; Discussion
of precise time and frequency measurement <time-nuts@febo.com
mailto:time-nuts@febo.com>
Sent: Tuesday, August 16, 2016 3:46 PM
Subject: Re: [time-nuts] Holdover
On Tue, 16 Aug 2016 04:35:40 +0000 (UTC)
Bob Stewart <bob@evoria.net mailto:bob@evoria.net> wrote:
It's been pointed out to me that I didn't understand the function of
the 1PPS of a time standard. I confess that somehow I had confused the
term to be timing standard; which would be an entirely different thing.
But, this is time-nuts, so I should have realized...
Anyway, is there a standard, or at least an accepted practice, for how
holdover is handled in a time standard?
There are many ways how to do that and which one you choose depends
on the application and its requirements. You can find everything between
"jump imediatly" and "just keep the frequency stable and don't care about
alignment".
Attila Kinali
time-nuts mailing list -- time-nuts@febo.com mailto:time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.