Re: [time-nuts] TIC resolution impact on GPSDO's performance
Poul-Henning Kamp wrote:
In message 45905B32.4000301@xtra.co.nz, Dr Bruce Griffiths writes:
As far as I can see on TVB's site, the 8607 is about 5e-13@1000 sec,
so 1ns/1h sounds perfectly good to me.
Only true when one only considers the data available on Tom's site.
Quartzlock have a GPS disciplined BVA OCXO that achieves considerably
better stability, admittedly their carrier phase tracking GPS receiver
has a resolution of a few picoseconds for an integration time of 1 second.
I'm not sure what your point is here... Obviously if you discipline
something with an Oncore, you have to choose a PLL timeconstant that
is optimum for the oscillator you are using. If your oscillator is
very good, the oncore may not be good enough at any timeconstant.
But the 1ns resolution of the Oncore is a fact of life and for all
the "amateur" stuff we are talking about here, 1ns is plenty fine.
Its not the coherence of the sawtooth but the coherence between the
oscillator clocking the timer used to position the PPS signal and the
frequency of the PPS signal itself.
Yes, those are non-coherent, that is why we need the negative sawtooth
in the first place. If they were coherent, we could just have
used a formula.
But they are not sufficiently incoherent as hanging bridges and the
quasi periodic form of the sawtooth error amply demonstrate.
Your choice of the word "coherence" does not help the discussion
here because it implies that the signals are the same frequency,
which is not the case.
But if you insist on using the word coherence, then at least
use it correctly: The hanging bridges happen because the two
signals are not coherent.
Wrong
The hanging bridges occur as the oscillator frequency closely approaches
a harmonic of 1Hz and then drifts away again.
Thus some degree of coherence (at least for the duration of the
"bridge") occurs flattening out the phase error versus time plot.
If the two signals were sufficiently incoherent, hanging bridges would
not occur.
Poul-Henning
You are using a different meaning for coherence than the standard one.
The standard meaning is something like:
Two signals or waves are said to be */coherent/ *if their behaviour at
various times/places is linked in a deterministic way.
Which allows statements like:
Harmonics are coherent with the fundamental.
Two signals do not have to have the same frequency to be coherent, they
can be harmonically related.
Bruce
In message 45906C9A.806@xtra.co.nz, Dr Bruce Griffiths writes:
Poul-Henning
You are using a different meaning for coherence than the standard one.
The standard meaning is something like:
Two signals or waves are said to be */coherent/ *if their behaviour at
various times/places is linked in a deterministic way.
The term coherence originates in optics and was briefly used in
radiotelegraphy and is no almost exclusively used in the context
of lasers and waveguides.
When two signals are coherent, you can predict one from the other.
We are dealing with three signals here:
GPS_PPS: the 1 Hz signal as the Oncore wants to do it.
HW_PPS: the 1 Hz signal as the Oncore can do it.
NEG_SAW: (GPS_PPS - HW_PPS)
None of the above three signals are coherent, you need two out of
three to predict the last one.
And since the first one is a virtual signal, not available except
as the sum of the other two, talking about coherence is especially
pointless.
I think what you are trying to express is that the frequency from
the internal Xtal (at times) is in an overtone of the 1Hz PPS, which
gives rise to hanging bridges.
And to conclude this by now surely far too tiresome debate before
both of us gets booted from the list:
The field in the oncores serial data-stream called negative sawtooth
correction must be taken into account to get optimum performance
out of the receiver. Ignoring it in toto or simply averaging it
will generally not do for OCXO disciplining.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
This discussion is fascinating, and as always it has prompted a number
of other questions for me.
I understand the sawtooth correction is provided to allow correction of
1 PPS timing errors when the processor clock is non-coherent with the
GPS signal (at least not intentionally), because the processor clock is
running at a finite frequency and provides granularity to the PPS
adjustment capability.
I wanted to know if this was available in my Trimble Thunderbolt GPSDO.
I have looked over the entire Thunderbolt manual and I have not seen a
mention about sawtooth correction, so I am not sure if it is available.
Then I realized the Thunderbolt is different from most GPSDO that have
been discussed, in that it uses the 10 MHz from the OCXO as the clock
for the GPS receiver, so in theory, the sawtooth correction should not
be necessary on this type of receiver since the processor clock and the
GPS signals are coherent (maybe I should not use that term...) when the
receiver is tracking.
Yet, the Thunderbolt spec for timing accuracy on the 1 PPS output is
+/- 20 nS at 1 sigma, just like an ordinary GPSDO with stand alone
receiver and what would be about a 25 MHz clock (please confirm).
The Trimble software also displays values referred to as "Timing Output"
for the 1 PPS and the 10 MHz. The PPS value is currently hovering around
-1.xx to -2.xx nS and the 10 MHz value is around +/- 0.0x ppb. I am not
sure what that is based on. The manual simply refers to those as
"Estimate of UTC/GPS offset", but does not give any detail of how they
are computed or what they mean.
The Thunderbolt User's Manual says that the 1 PPS output is the OCXO's
10 MHz divided down. Yet, the block diagram shows the 1 PPS coming from
the CPU and support circuit, not directly coming from the 10 MHz. I
believe the 1 PPS is the 10 MHz divided down, except when the receiver
is doing jam-sync (after power up or recovery, when the GPS_PPS and
HW_PPS are far away from each other)
Going back to what Poul-Henning just said, I believe that for the
Thunderbolt, there actually may be three PPS signals:
GPS_PPS: what the receiver thinks the PPS should be (a theoretical signal)
HW_PPS: the best approximation of the GPS_PPS that the receiver can do
OCXO_PPS: the 10 MHz divided down
In jam-sync mode, the Thunderbolt forces the HW_PPS to within 100nS of
GPS_PPS (the closest it can get using software delays and programmable
dividers I guess, using a 10 MHz clock) without touching the OCXO, so in
that mode, HW_PPS and OCXO_PPS are obviously not the same. Once the two
are within 100 nS, the Thunderbolt switches to discipline mode and fine
tunes the OCXO to get the HW_PPS as close to GPS_PPS as possible (20nS 1
sigma by specification.) It would appear that the receiver should be
able to adjust its OCXO as close to GPS_PPS as the DAC will allow,
without the artificial limit of the processor clock period. So, there
should not be a need for sawtooth correction and there should be no
hanging bridges either.
Then, what are the "Timing Outputs"?
Does this make any sense?
Didier KO4BB
Poul-Henning Kamp wrote:
I think what you are trying to express is that the frequency from
the internal Xtal (at times) is in an overtone of the 1Hz PPS, which
gives rise to hanging bridges.
Poul-Henning
Surely you mean harmonic? An overtone is not necessarily a harmonic.
Indeed I never intended to imply any "coherence" between any of the PPS
signals or sawtooth error signals you listed.
I believe that I used the term coherence in exactly the same way that
David Chu did when discussing averaging errors in the HP5345A counter.
Bruce
Didier Juges wrote:
This discussion is fascinating, and as always it has prompted a number
of other questions for me.
I understand the sawtooth correction is provided to allow correction of
1 PPS timing errors when the processor clock is non-coherent with the
GPS signal (at least not intentionally), because the processor clock is
running at a finite frequency and provides granularity to the PPS
adjustment capability.
I wanted to know if this was available in my Trimble Thunderbolt GPSDO.
I have looked over the entire Thunderbolt manual and I have not seen a
mention about sawtooth correction, so I am not sure if it is available.
Then I realized the Thunderbolt is different from most GPSDO that have
been discussed, in that it uses the 10 MHz from the OCXO as the clock
for the GPS receiver, so in theory, the sawtooth correction should not
be necessary on this type of receiver since the processor clock and the
GPS signals are coherent (maybe I should not use that term...) when the
receiver is tracking.
Yet, the Thunderbolt spec for timing accuracy on the 1 PPS output is
+/- 20 nS at 1 sigma, just like an ordinary GPSDO with stand alone
receiver and what would be about a 25 MHz clock (please confirm).
The Trimble software also displays values referred to as "Timing Output"
for the 1 PPS and the 10 MHz. The PPS value is currently hovering around
-1.xx to -2.xx nS and the 10 MHz value is around +/- 0.0x ppb. I am not
sure what that is based on. The manual simply refers to those as
"Estimate of UTC/GPS offset", but does not give any detail of how they
are computed or what they mean.
The Thunderbolt User's Manual says that the 1 PPS output is the OCXO's
10 MHz divided down. Yet, the block diagram shows the 1 PPS coming from
the CPU and support circuit, not directly coming from the 10 MHz. I
believe the 1 PPS is the 10 MHz divided down, except when the receiver
is doing jam-sync (after power up or recovery, when the GPS_PPS and
HW_PPS are far away from each other)
Going back to what Poul-Henning just said, I believe that for the
Thunderbolt, there actually may be three PPS signals:
GPS_PPS: what the receiver thinks the PPS should be (a theoretical signal)
HW_PPS: the best approximation of the GPS_PPS that the receiver can do
OCXO_PPS: the 10 MHz divided down
In jam-sync mode, the Thunderbolt forces the HW_PPS to within 100nS of
GPS_PPS (the closest it can get using software delays and programmable
dividers I guess, using a 10 MHz clock) without touching the OCXO, so in
that mode, HW_PPS and OCXO_PPS are obviously not the same. Once the two
are within 100 nS, the Thunderbolt switches to discipline mode and fine
tunes the OCXO to get the HW_PPS as close to GPS_PPS as possible (20nS 1
sigma by specification.) It would appear that the receiver should be
able to adjust its OCXO as close to GPS_PPS as the DAC will allow,
without the artificial limit of the processor clock period. So, there
should not be a need for sawtooth correction and there should be no
hanging bridges either.
Then, what are the "Timing Outputs"?
Does this make any sense?
Didier KO4BB
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Didier
Look at:
http://trl.trimble.com/docushare/dsweb/Get/Document-8425/
There is little evidence of sawtooth corrections in graph in the above
document, however the data were taken when SA was switched on.
To confuse matters further the document actually talks about pulse
position quantisation.
http://trl.trimble.com/docushare/dsweb/Get/Document-8424/
Shows differences between 2 Thunderbolts, again there is little evidence
of quantisation but data may have been smoothed.
Have you any reasonably stable crystal oscillator that can be used to
log the time interval between the Thunderbolt PPS output and the divided
down oscillator output?
Plotting these results should quickly show evidence of any sawtooth error.
As you say there shouldn't be any sawtooth error if the receiver timing
is derived from the 10MHz clock which is in turned locked to the PPS output.
However there are other sources of noise in a GPS timing receiver which
could, in an older receiver design like this, easily be as much as 20ns rms.
Bruce
In message 4590BB56.5070809@xtra.co.nz, Dr Bruce Griffiths writes:
Poul-Henning Kamp wrote:
I think what you are trying to express is that the frequency from
the internal Xtal (at times) is in an overtone of the 1Hz PPS, which
gives rise to hanging bridges.
Poul-Henning
Surely you mean harmonic? An overtone is not necessarily a harmonic.
I meant overtone, becuase they are not in any harmonic relationship,
the XO is wandering around whereas the (ideal) PPS is, supposedly,
rock stable.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
From: "Poul-Henning Kamp" phk@phk.freebsd.dk
Subject: Re: [time-nuts] TIC resolution impact on GPSDO's performance
Date: Tue, 26 Dec 2006 08:14:12 +0000
Message-ID: 36958.1167120852@critter.freebsd.dk
In message 4590BB56.5070809@xtra.co.nz, Dr Bruce Griffiths writes:
Poul-Henning Kamp wrote:
I think what you are trying to express is that the frequency from
the internal Xtal (at times) is in an overtone of the 1Hz PPS, which
gives rise to hanging bridges.
Poul-Henning
Surely you mean harmonic? An overtone is not necessarily a harmonic.
I meant overtone, becuase they are not in any harmonic relationship,
the XO is wandering around whereas the (ideal) PPS is, supposedly,
rock stable.
The only real way to describe it is to say that the XO and the PPS is
asynchronous to each other. Those who has freshen up on their greek for
technicians will know that this means "not the same clock" which is quite
accuratly what we have. ITU-T Rec. G.700 has a few interesting sections in it.
The use of words such as harmonic or overtone should not be used since they
is normally used to describe the various frequencies within one signal where
as words as synchronous and asynchronous is to be used mainly for pair of
signals and describes their relative timing. Naturally, for a pair of signals
to be synchronous does not require them to have the same frequency, just a
fixed ratio between those frequencies.
It does happends that a single signal is said to be synchronous, but the
wording which should have been used is isochronous (same clock). Then we have
the lovely words of plesiochronous and mesochronous. :)
Cheers,
Magnus
Poul-Henning Kamp wrote:
In message 4590BB56.5070809@xtra.co.nz, Dr Bruce Griffiths writes:
Poul-Henning Kamp wrote:
I think what you are trying to express is that the frequency from
the internal Xtal (at times) is in an overtone of the 1Hz PPS, which
gives rise to hanging bridges.
Poul-Henning
Surely you mean harmonic? An overtone is not necessarily a harmonic.
I meant overtone, becuase they are not in any harmonic relationship,
the XO is wandering around whereas the (ideal) PPS is, supposedly,
rock stable.
Poul-Henning
Perhaps you should explain what you mean by overtone as it is at odds
with both my understanding of the term as a vibrational mode of an
object (as in acoustics) higher than the fundamental or lowest frequency
vibrational mode and the definition given in Webster's dictionary (My
Oxford dictionary is buried somewhere) which insists that overtone is
synonomous with harmonic.
My understanding is that the phrase "an overtone of a 1Hz" has no real
meaning.
Bruce
From: Dr Bruce Griffiths bruce.griffiths@xtra.co.nz
Subject: Re: [time-nuts] TIC resolution impact on GPSDO's performance
Date: Tue, 26 Dec 2006 23:35:28 +1300
Message-ID: 4590FAF0.2070001@xtra.co.nz
Poul-Henning Kamp wrote:
In message 4590BB56.5070809@xtra.co.nz, Dr Bruce Griffiths writes:
Poul-Henning Kamp wrote:
I think what you are trying to express is that the frequency from
the internal Xtal (at times) is in an overtone of the 1Hz PPS, which
gives rise to hanging bridges.
Poul-Henning
Surely you mean harmonic? An overtone is not necessarily a harmonic.
I meant overtone, becuase they are not in any harmonic relationship,
the XO is wandering around whereas the (ideal) PPS is, supposedly,
rock stable.
Poul-Henning
Perhaps you should explain what you mean by overtone as it is at odds
with both my understanding of the term as a vibrational mode of an
object (as in acoustics) higher than the fundamental or lowest frequency
vibrational mode and the definition given in Webster's dictionary (My
Oxford dictionary is buried somewhere) which insists that overtone is
synonomous with harmonic.
My understanding is that the phrase "an overtone of a 1Hz" has no real
meaning.
coherent
4) said esp of electromagnetic waves: having a definite relationship to each
other: coherent light
harmonic(2)
1a) a tone in a harmonic series
2) in physics, a component frequency of a harmonic motion that is an integral
multiple of the fundamental frequency
overtone
1b) Harmonic(2)
These expressions does not express well what is happening with the XO in
relation to the UTC(GPS)/UTC(USNO) or if you so like PPS_GPS. Only coherent
refers to the case of two signals where as harmonic and overtone (really just
refering to the same meaning) is refering to properties within one signal
mainly (you need special wordings such as "lock <foo> to a harmonic of <bar>",
but then again harmonic only refers to a property of <bar> where as lock is the
word tying <foo> to <bar>).
synchronous
- happening, existing or arising at precisely the same time
2a) going on or operating together at exactly the same rate
b) recurring together - involving or indicating synchronism
- in physics, having the same period or having the same period and phase
asynchronous
not synchronous, proceeding at its own pace; esp relating to or denoting a
computing operation which does not wait for a communication to complete before
processing.
These give indication on the behaviour between two (or more) signals. I beleive
that it is best to point out that the XO is asynchronous to PPS_GPS. However,
both XO and PPS_GPS is isochronous:
isochronous (via isochronal)
- having equal duration
- recurring at regular intervals
This relates to a property of a single signal again. We consider all clocks of
being essentially isochronous except for the perturbations that we as time-nuts
spends endless discussions over how to measure and improve.
The "asynchronous" serial line is actually anisochronous BTW.
The above wordings comes out of my "The New Penguin English Dictionary" but
where applicable coincide with the ITU-T Rec. G.700. Let's use these terms
which have been found usefull by our colleages in the past.
Cheers,
Magnus
Poul,
The field in the oncores serial data-stream called negative
sawtooth correction must be taken into account to get optimum
performance out of the receiver. Ignoring it in toto or
simply averaging it will generally not do for OCXO disciplining.
thank you for your clear words! While my posting has produced a lot of
different points to discuss over, this is basically what I had expected
from a experienced person. I hope you needn't cross your fingers while
writing it down!
Best regards
Ulrich Bangert, DF6JB
-----Ursprüngliche Nachricht-----
Von: time-nuts-bounces@febo.com
[mailto:time-nuts-bounces@febo.com] Im Auftrag von Poul-Henning Kamp
Gesendet: Dienstag, 26. Dezember 2006 03:02
An: Discussion of precise time and frequency measurement
Betreff: Re: [time-nuts] TIC resolution impact on GPSDO's performance
In message 45906C9A.806@xtra.co.nz, Dr Bruce Griffiths writes:
Poul-Henning
You are using a different meaning for coherence than the
standard one.
The standard meaning is something like: Two signals or waves
are said
to be */coherent/ *if their behaviour at various
times/places is linked
in a deterministic way.
The term coherence originates in optics and was briefly used
in radiotelegraphy and is no almost exclusively used in the
context of lasers and waveguides.
When two signals are coherent, you can predict one from the other.
We are dealing with three signals here:
GPS_PPS: the 1 Hz signal as the Oncore wants to do it.
HW_PPS: the 1 Hz signal as the Oncore can do it.
NEG_SAW: (GPS_PPS - HW_PPS)
None of the above three signals are coherent, you need two
out of three to predict the last one.
And since the first one is a virtual signal, not available
except as the sum of the other two, talking about coherence
is especially pointless.
I think what you are trying to express is that the frequency
from the internal Xtal (at times) is in an overtone of the
1Hz PPS, which gives rise to hanging bridges.
And to conclude this by now surely far too tiresome debate
before both of us gets booted from the list:
The field in the oncores serial data-stream called negative
sawtooth correction must be taken into account to get optimum
performance out of the receiver. Ignoring it in toto or
simply averaging it will generally not do for OCXO disciplining.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by
incompetence.
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