Re: [time-nuts] GPS W/10KHz
The problem with the PLL analog version is the same as with any digital
GPSDO. The saw tooth is present at 10 KHz just like 1 Hz. To the best of my
knowledge there is no GPS receivers out there for less than $ 1000 with out
saw tooth. Timing receivers output the correction value and you can either
with software or a variable delay do correction.
I encourage you to pursue your idea since for more than ten years I had the
privilege to communicate with some of the sharpest minds on that subject
and obviously the missed something.
That also applies for the three of us that have been working intensely on
developing and testing the next generation of GPSDO's that we hope to
introduce to time nuts soon.
In the meantime with the ever increasing price of Tbolts, Shera is worth
a second look. When we worked on the last code release of Shrea using a
Morion we got consistently better than 1 E-11. Yes some of the IC's are hard to
find and that is why I used more readily available parts and did a new
board layout.
Also did one using a $ 1.20 Altera gate array.
Bert Kehren
In a message dated 2/8/2014 12:33:48 P.M. Eastern Standard Time,
albertson.chris@gmail.com writes:
On Sat, Feb 8, 2014 at 3:44 AM, Volker Esper ailer2@t-online.de wrote:
you don't need 10kHz to build a GPS disciplined oscillator. GPSDOs are
build with control loop response times in the range of some hours, so
the loop will be absolutely happy with a 1PPS input.
Of course you are correct. Most GPSDOs are driven with a 1Hz pulse. But I
think maybe the OP is not building just any GPSDO but maybe he is looking
to repair a specific GPSDO that is designed to use the old (and now rare)
Rockwell GPS. If that is the case he needs the old Rockwell or needs
to redesign his system.
If you are starting from scratch to build a new GPSDO it's easier now. All
you need is some kind of a phase detector (74HC4046 ?) and a small uP that
has a good built-in DAC. The uP checks the phase once per second and
adjusts it'sDAC accordingly.
Chris Albertson
Redondo Beach, California
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.
I have made a design with the 10KHz for usage on the OZ7IGY beacon project,
I can mail you the diagram and support you with a PCB if you so whish :)
http://rudius.net/oz2m/ngnb/gps.htm
It have 2 times 10MHz outputs, the stability is less than 5 mHz.
Designed for Rockwell Jupiter GPS receivers TU30, TU40 and TU60 series
Michael, OZ2ELA
-----Oprindelig meddelelse-----
Fra: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] På vegne
af EWKehren@aol.com
Sendt: 8. februar 2014 23:50
Til: time-nuts@febo.com
Emne: Re: [time-nuts] GPS W/10KHz
The problem with the PLL analog version is the same as with any digital
GPSDO. The saw tooth is present at 10 KHz just like 1 Hz. To the best of my
knowledge there is no GPS receivers out there for less than $ 1000 with out
saw tooth. Timing receivers output the correction value and you can either
with software or a variable delay do correction.
I encourage you to pursue your idea since for more than ten years I had the
privilege to communicate with some of the sharpest minds on that subject and
obviously the missed something.
That also applies for the three of us that have been working intensely on
developing and testing the next generation of GPSDO's that we hope to
introduce to time nuts soon.
In the meantime with the ever increasing price of Tbolts, Shera is worth a
second look. When we worked on the last code release of Shrea using a Morion
we got consistently better than 1 E-11. Yes some of the IC's are hard to
find and that is why I used more readily available parts and did a new board
layout.
Also did one using a $ 1.20 Altera gate array.
Bert Kehren
In a message dated 2/8/2014 12:33:48 P.M. Eastern Standard Time,
albertson.chris@gmail.com writes:
On Sat, Feb 8, 2014 at 3:44 AM, Volker Esper ailer2@t-online.de wrote:
you don't need 10kHz to build a GPS disciplined oscillator. GPSDOs
are build with control loop response times in the range of some
hours, so the loop will be absolutely happy with a 1PPS input.
Of course you are correct. Most GPSDOs are driven with a 1Hz pulse. But I
think maybe the OP is not building just any GPSDO but maybe he is looking
to repair a specific GPSDO that is designed to use the old (and now rare)
Rockwell GPS. If that is the case he needs the old Rockwell or needs
to redesign his system.
If you are starting from scratch to build a new GPSDO it's easier now. All
you need is some kind of a phase detector (74HC4046 ?) and a small uP that
has a good built-in DAC. The uP checks the phase once per second and
adjusts it'sDAC accordingly.
Chris Albertson
Redondo Beach, California
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.
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 8 Feb, 2014, at 14:50 , EWKehren@aol.com wrote:
The problem with the PLL analog version is the same as with any digital
GPSDO. The saw tooth is present at 10 KHz just like 1 Hz. To the best of my
knowledge there is no GPS receivers out there for less than $ 1000 with out
saw tooth. Timing receivers output the correction value and you can either
with software or a variable delay do correction.
This is very true, though the sawtooth at a 10 kpps sample rate is going
to a little different than the sawtooth at a 1 pps sample rate. The frequency
of the sawtooth noise will lie somewhere in the Nyquist bandwidth. At a 1 pps
sample rate the frequency of the sawtooth noise will hence be somewhere between
0 Hz and 0.5 Hz, while at 10 kpps the sawtooth frequency will range from 0 Hz
to 5 kHz.
Noise at less than 0.5 Hz is not easy to filter, so you are going to require
the correction from the receiver and/or an integrator with a time constant
that can only be realized digitally. Sawtooth noise over most of a 0 Hz to
5 kHz range, on the other hand, should be eliminated by the analog low pass
filter after the phase detector in the PLL, giving you something nice and clean
coming out. It is only if you get unlucky and the beat frequency between GPS
time and the receiver's oscillator ends up very close to an integer multiple of
10 kHz that you'll see noise at a low enough frequency to leak through into the
control response.
This is interesting because it suggests that very simple GPSDOs using 10 kHz
from the receiver might at times work worse than you are likely to observe in
a single bench measurement as aging (or something) moves the receiver's oscillator
frequency through one of the "bad" frequency errors. Or is there a way to avoid
that altogether (maybe if the receiver does dithering)?
Dennis Ferguson
Dennis Ferguson wrote:
On 8 Feb, 2014, at 14:50 , EWKehren@aol.com wrote:
The problem with the PLL analog version is the same as with any digital
GPSDO. The saw tooth is present at 10 KHz just like 1 Hz. To the best of my
knowledge there is no GPS receivers out there for less than $ 1000 with out
saw tooth. Timing receivers output the correction value and you can either
with software or a variable delay do correction.
This is very true, though the sawtooth at a 10 kpps sample rate is going
to a little different than the sawtooth at a 1 pps sample rate. The frequency
of the sawtooth noise will lie somewhere in the Nyquist bandwidth. At a 1 pps
sample rate the frequency of the sawtooth noise will hence be somewhere between
0 Hz and 0.5 Hz, while at 10 kpps the sawtooth frequency will range from 0 Hz
to 5 kHz.
Noise at less than 0.5 Hz is not easy to filter, so you are going to require
the correction from the receiver and/or an integrator with a time constant
that can only be realized digitally. Sawtooth noise over most of a 0 Hz to
5 kHz range, on the other hand, should be eliminated by the analog low pass
filter after the phase detector in the PLL, giving you something nice and clean
coming out. It is only if you get unlucky and the beat frequency between GPS
time and the receiver's oscillator ends up very close to an integer multiple of
10 kHz that you'll see noise at a low enough frequency to leak through into the
control response.
This is interesting because it suggests that very simple GPSDOs using 10 kHz
from the receiver might at times work worse than you are likely to observe in
a single bench measurement as aging (or something) moves the receiver's oscillator
frequency through one of the "bad" frequency errors. Or is there a way to avoid
that altogether (maybe if the receiver does dithering)?
Dennis Ferguson
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.
Instead of speculating try reading the specifications.
1Hz phase modulation of the 10kHz output is present.
The receiver sawtooth error sample rate is 1Hz not 10kHz.
The 10kHz output signal phase is adjusted at a 1Hz rate by the receiver.
The only advantage of the 10KHz output is that it allows the phase error
of each 10KHz burst to be sampled up to 10,000 times a second.
This may be useful in improving estimates of this phase error due to
limited timing resolution but doesnt remove the 1Hz sawtooth phase error
that is present.
Bruce
On 10 Feb, 2014, at 00:48 , Bruce Griffiths bruce.griffiths@xtra.co.nz wrote:
Dennis Ferguson wrote:
On 8 Feb, 2014, at 14:50 , EWKehren@aol.com wrote:
The problem with the PLL analog version is the same as with any digital
GPSDO. The saw tooth is present at 10 KHz just like 1 Hz. To the best of my
knowledge there is no GPS receivers out there for less than $ 1000 with out
saw tooth. Timing receivers output the correction value and you can either
with software or a variable delay do correction.
This is very true, though the sawtooth at a 10 kpps sample rate is going
to a little different than the sawtooth at a 1 pps sample rate. The frequency
of the sawtooth noise will lie somewhere in the Nyquist bandwidth. At a 1 pps
sample rate the frequency of the sawtooth noise will hence be somewhere between
0 Hz and 0.5 Hz, while at 10 kpps the sawtooth frequency will range from 0 Hz
to 5 kHz.
Noise at less than 0.5 Hz is not easy to filter, so you are going to require
the correction from the receiver and/or an integrator with a time constant
that can only be realized digitally. Sawtooth noise over most of a 0 Hz to
5 kHz range, on the other hand, should be eliminated by the analog low pass
filter after the phase detector in the PLL, giving you something nice and clean
coming out. It is only if you get unlucky and the beat frequency between GPS
time and the receiver's oscillator ends up very close to an integer multiple of
10 kHz that you'll see noise at a low enough frequency to leak through into the
control response.
This is interesting because it suggests that very simple GPSDOs using 10 kHz
from the receiver might at times work worse than you are likely to observe in
a single bench measurement as aging (or something) moves the receiver's oscillator
frequency through one of the "bad" frequency errors. Or is there a way to avoid
that altogether (maybe if the receiver does dithering)?
Dennis Ferguson
Instead of speculating try reading the specifications.
1Hz phase modulation of the 10kHz output is present.
The receiver sawtooth error sample rate is 1Hz not 10kHz.
The 10kHz output signal phase is adjusted at a 1Hz rate by the receiver.
Bruce,
I'm not sure which equipment you want me to read the specifications for, though
I'd be very interested in knowing. What I'm describing is the behaviour of the
timepulse output of the LEA-6T, which can be configured to output edges at any
rate from 1 Hz to 10 MHz. There the only relevant specification I see is the
timepulse output quantization error, which is a constant 21 ns on every output edge
independent of the rate at which the receiver is configured to generate edges. This
should cause exactly the behaviour described above, and as best I can measure by
comparing 1 pps and 50 pps outputs to the divided-down 10 MHz output of a GPSDO is
consistent with how the receiver actually behaves (though my best measurement is none
too good; I need to get a TIC with a resolution better than the 10 ns my Beaglebone
has). If you run the output at 10 kpps you get 10,000 samples of the quantization
error every second and can average it out a lot faster than if you only get one sample
of the quantization error every second. I don't know what a "sawtooth error sample
rate" is if not this.
You seem to be describing a piece of equipment where the sawtooth error is not a
simple consequence of pulse output quantization caused by generating edges with
the receiver's internal free-running clock. I'd be really curious to know what
equipment this is. This page
http://gpsdo.i2phd.com
says he looked for but failed to find any sub-Hz sidebands in the Navman Jupiter
10 kHz output so it doesn't seem like that receiver is the one you are thinking of
either.
Dennis Ferguson
On 10 Feb, 2014, at 00:48 , Bruce Griffiths bruce.griffiths@xtra.co.nz wrote:
Instead of speculating try reading the specifications.
1Hz phase modulation of the 10kHz output is present.
The receiver sawtooth error sample rate is 1Hz not 10kHz.
The 10kHz output signal phase is adjusted at a 1Hz rate by the receiver.
Ah, as soon as I pressed send for the last note I realized what
you were likely telling me.
Yes, the LEA-6T only provides you with a quantization (sawtooth)
correction for 1 pps and no higher rate. At 1 pps you should
pay attention to the digital correction (implying no analog-only
implementation is possible; you minimally need the delay line
thing) since the frequency of the saw tooth is often low enough
to leak into the control response and the correction should make
the sawtooth go away.
All I was pointing out is that at a higher output frequency, like
10 kpps, the frequency of the quantization saw tooth error will
almost always be much higher as well. There's no need for the digital
correction since averaging over a relatively short period, like in
the loop filter of an appropriate analog PLL, will almost always be
sufficient to smooth the sawtooth.
Dennis Ferguson
Dennis Ferguson wrote:
On 10 Feb, 2014, at 00:48 , Bruce Griffithsbruce.griffiths@xtra.co.nz wrote:
Dennis Ferguson wrote:
On 8 Feb, 2014, at 14:50 , EWKehren@aol.com wrote:
The problem with the PLL analog version is the same as with any digital
GPSDO. The saw tooth is present at 10 KHz just like 1 Hz. To the best of my
knowledge there is no GPS receivers out there for less than $ 1000 with out
saw tooth. Timing receivers output the correction value and you can either
with software or a variable delay do correction.
This is very true, though the sawtooth at a 10 kpps sample rate is going
to a little different than the sawtooth at a 1 pps sample rate. The frequency
of the sawtooth noise will lie somewhere in the Nyquist bandwidth. At a 1 pps
sample rate the frequency of the sawtooth noise will hence be somewhere between
0 Hz and 0.5 Hz, while at 10 kpps the sawtooth frequency will range from 0 Hz
to 5 kHz.
Noise at less than 0.5 Hz is not easy to filter, so you are going to require
the correction from the receiver and/or an integrator with a time constant
that can only be realized digitally. Sawtooth noise over most of a 0 Hz to
5 kHz range, on the other hand, should be eliminated by the analog low pass
filter after the phase detector in the PLL, giving you something nice and clean
coming out. It is only if you get unlucky and the beat frequency between GPS
time and the receiver's oscillator ends up very close to an integer multiple of
10 kHz that you'll see noise at a low enough frequency to leak through into the
control response.
This is interesting because it suggests that very simple GPSDOs using 10 kHz
from the receiver might at times work worse than you are likely to observe in
a single bench measurement as aging (or something) moves the receiver's oscillator
frequency through one of the "bad" frequency errors. Or is there a way to avoid
that altogether (maybe if the receiver does dithering)?
Dennis Ferguson
Instead of speculating try reading the specifications.
1Hz phase modulation of the 10kHz output is present.
The receiver sawtooth error sample rate is 1Hz not 10kHz.
The 10kHz output signal phase is adjusted at a 1Hz rate by the receiver.
Bruce,
I'm not sure which equipment you want me to read the specifications for, though
I'd be very interested in knowing. What I'm describing is the behaviour of the
timepulse output of the LEA-6T, which can be configured to output edges at any
rate from 1 Hz to 10 MHz. There the only relevant specification I see is the
timepulse output quantization error, which is a constant 21 ns on every output edge
independent of the rate at which the receiver is configured to generate edges. This
should cause exactly the behaviour described above, and as best I can measure by
comparing 1 pps and 50 pps outputs to the divided-down 10 MHz output of a GPSDO is
consistent with how the receiver actually behaves (though my best measurement is none
too good; I need to get a TIC with a resolution better than the 10 ns my Beaglebone
has). If you run the output at 10 kpps you get 10,000 samples of the quantization
error every second and can average it out a lot faster than if you only get one sample
of the quantization error every second. I don't know what a "sawtooth error sample
rate" is if not this.
Which quantisation error are you referring to?
The quantisation error of the Ublox receiver?
The Ublox datasheets are a good example of how not to write a datasheet.
There is an internal PLL that may smooth the quantisation error somewhat.
The ADEV plot for an 8KHz output certainly exhibits evidence of some
smoothing of the raw GPS phase error.
However for measuring intervals of 10s or more the result is no better
than one would expect for a good GPS timing receiver using a 1 PPS output.
You seem to be describing a piece of equipment where the sawtooth error is not a
simple consequence of pulse output quantization caused by generating edges with
the receiver's internal free-running clock. I'd be really curious to know what
equipment this is. This page
http://gpsdo.i2phd.com
says he looked for but failed to find any sub-Hz sidebands in the Navman Jupiter
10 kHz output so it doesn't seem like that receiver is the one you are thinking of
either.
Singularly unconvincing since the limit of his sideband detection
capability isn't stated.
For the Jupiter receivers the sidebands due to the 1Hz phase modulation
of the 10kHz signal should be 80dB or more below the 10kHz carrier.
Dennis Ferguson
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.
Bruce
Dennis,
All I was pointing out is that at a higher output frequency, like
10 kpps, the frequency of the quantization saw tooth error will
almost always be much higher as well. There's no need for the digital
correction since averaging over a relatively short period, like in
the loop filter of an appropriate analog PLL, will almost always be
sufficient to smooth the sawtooth.
The sawtooth correction is the difference between where the receiver would
wish to place the edge and where its known limited resolution electronics
lets it put the edge.
The receiver wish is based on the timesolution from the last measurement.
In the Jupiter this is done at 1Hz maximum. The sawtooth correction will
apply the same for all 10k (pos or neg) edges in the 10kHz signal during
that one second.
There are effects that are not easily filtered away in the analog domain.
See the archives and
http://www.leapsecond.com/pages/m12/sawtooth.htm
kind regards,
Björn
Dennis Ferguson wrote:
On 10 Feb, 2014, at 00:48 , Bruce Griffithsbruce.griffiths@xtra.co.nz wrote:
Instead of speculating try reading the specifications.
1Hz phase modulation of the 10kHz output is present.
The receiver sawtooth error sample rate is 1Hz not 10kHz.
The 10kHz output signal phase is adjusted at a 1Hz rate by the receiver.
Ah, as soon as I pressed send for the last note I realized what
you were likely telling me.
Yes, the LEA-6T only provides you with a quantization (sawtooth)
correction for 1 pps and no higher rate. At 1 pps you should
pay attention to the digital correction (implying no analog-only
implementation is possible; you minimally need the delay line
thing) since the frequency of the saw tooth is often low enough
to leak into the control response and the correction should make
the sawtooth go away.
All I was pointing out is that at a higher output frequency, like
10 kpps, the frequency of the quantization saw tooth error will
almost always be much higher as well. There's no need for the digital
correction since averaging over a relatively short period, like in
the loop filter of an appropriate analog PLL, will almost always be
sufficient to smooth the sawtooth.
Dennis Ferguson
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.
The internal PLL will smooth out the quantisation error somewhat.
However the phase of the higher frequency outputs isnt adjusted on every
edge by resampling the phase error wrt the GPS signal.
More detail on the internal PLL would be useful.
About all one can glean from the datasheet is that it has a bandwidth of
somewhat less than 1Hz.
Bruce
I was waiting for someone else to mention this. The problem with the sawtooth from a UT+ and similar is that it can go long periods with the phase on one side of zero, switch to going back and forth across zero, and then even start at one side, and do a sawtooth shaped stairstep ramp to the other side. So the phase of your oscillator is going to be pulled all over the place. Long term average is going to look good because it has to average to zero. But short term, the accuracy of your GPSDO may suffer.
OTOH, with one of the newer inexpensive receivers, the sawtooth is only about 10ns total. If your circuit is for frequency averaging, or for phase crossings, it can do pretty good. However, if you have a phase locking system, then there are quantization errors that are shaped a bit different from the UT+, but they still cause you a problem.
Bob - AE6RV
From: ""Björn"" bg@lysator.liu.se
To: Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Monday, February 10, 2014 4:37 PM
Subject: Re: [time-nuts] GPS W/10KHz
Dennis,
The sawtooth correction is the difference between where the receiver would
wish to place the edge and where its known limited resolution electronics
lets it put the edge.
The receiver wish is based on the timesolution from the last measurement.
In the Jupiter this is done at 1Hz maximum. The sawtooth correction will
apply the same for all 10k (pos or neg) edges in the 10kHz signal during
that one second.
There are effects that are not easily filtered away in the analog domain.
See the archives and
http://www.leapsecond.com/pages/m12/sawtooth.htm
kind regards,
Björn
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.