[time-nuts] Re: Phase coherence with 2x GPSDO

[Bob kb8tq]

Hi


> On Mar 17, 2022, at 8:57 PM, Krishna Makhija  wrote:
> 
> Good day everyone,
> 
> I ran a few additional tests to figure this issue out. The first test
> involved using one GPS antenna for both units. I found that while phase
> coherence was better in general, there was still a fair bit of disagreement
> between them. I then tried to use the 1PPS from one GPSDO as reference to
> the other one, and looked at the 10 MHz from each unit on a scope. There
> was a fixed phase offset between both 10 MHz signals +- 5-8 ns of error.
> Which means that even if I gave an error-free PPS as reference the
> oscillators would *still* not be in enough agreement for my application.
> Which in turn means implementing a PTP system would be pointless as well.
> 
> On a somewhat related note, does anyone know why the F9T modules only give
> 2.5 ns of timing accuracy? RTK GPS has 1 cm RMS of positioning error which
> should theoretically be 33 ps in timing. Where does the 2.5 ns error come
> from?

On the same “one sigma long average compare two to each other only look at 
the noise basis after corrections”, the F9T does a few hundred ps. Just like 
the 
RTK number, static offsets are not part of that calculation. 

Bob

> 
> On Wed, Mar 9, 2022 at 3:26 PM Mattia Rizzi  wrote:
> 
>> Hi
>> The timestamping in a real implementation should be done in the FPGA or on
>> a SoC integrated in the FPGA. Since you have a SDR you could implement a
>> timestamping core.
>> This timestamping core takes the samples from the ADC and calculates the
>> time of arrival relative to the sampling clock.
>> If you have already a datalink between the drone and the base station, you
>> can use the symbol of the data layer, in that way you can have a lot of
>> freedom in the implementation of the sync algorithm.
>> 
>>> Since there is some
>> non-deterministic computational overhead, you need to take several such
>> measurements and calculate what the mean time of flight would be and
>> correct the offsets accordingly. Is that correct? Please let me know if I
>> am wrong because I really need to fix this issue.
>> 
>> No, in an actual implementation it's deterministic.
>> You're receiving a frame, the timestampjng core gives you a timestamp for
>> each symbol. If you use 802.15.4-CSS, which has 4 MHz chirps as symbol, you
>> can get down to 0.5ns RMS noise *for each symbol* (SNR of around 15 dB),
>> indeed if you make an average with more symbols you can get better results,
>> but there's a limit on how many timestamps you can average (mainly depends
>> on the stability of you sampling clock).
>> 
>> You can run a simple protocol like a stripped down version of PTP, and if
>> you're able to embed it into layer 1 (you've an SDR, you can do it) then
>> it's for free, and you can get a high synchronization rate for free. You
>> can also get two-way ranging for free.
>> 
>> Il Mer 9 Mar 2022, 19:23 Krishna Makhija  ha scritto:
>> 
>>> Hello Mattia,
>>> 
>>> Thanks for the paper. Please correct me if I understood wrong, but it
>> seems
>>> like you are calculating the time of flight between timestamped signals
>> to
>>> synchronize two spatially separated clocks. Since there is some
>>> non-deterministic computational overhead, you need to take several such
>>> measurements and calculate what the mean time of flight would be and
>>> correct the offsets accordingly. Is that correct? Please let me know if I
>>> am wrong because I really need to fix this issue.
>>> 
>>> How would it work if one of the units was moving relative to the other? I
>>> am going to have one of these on a drone and flying around. I admit I
>> never
>>> looked too deep into NTP/PTP but I always figured there would be a
>> constant
>>> variability owing to the steady motion. With GPSDOs as others have
>> pointed
>>> out, there are further errors induced due acceleration (vibrations on a
>>> piezoelectric cause sinusoidal frequency errors), and temperature changes
>>> (the drone rover unit will see more airflow as compared to the base
>> unit),
>>> but as of now I am trying to make sure I can get it to work with that
>>> notwithstanding at least.
>>> 
>>> P.S. The Mini-JLT <
>>> https://www.jackson-labs.com/index.php/products/mini_jlt>
>>> is designed for drones. It has a special mode where it updates its
>> position
>>> in real-time. There is also a gyro with a Kalman filter running on a
>>> real-time operating system which is supposed to reduce the effect of
>>> vibrations. It does not eliminate that effect entirely but it does
>> mitigate
>>> it by a factor of 10.
>>> 
>>> On Mon, Mar 7, 2022 at 2:11 PM Mattia Rizzi 
>>> wrote:
>>> 
 Hi,
 You can definitely get sub-ns precision using a 4-8 MHz bandwidth
>>> wireless
 protocol. You are not limited by the sampling period. In my previous
>>> work I
 used 802.15.4-CSS (chirp based modulation), implemented by me in a SDR.
>>> The
 sampling rate was 32 ns but with crosscorrelation and sample
>>> interpolation

[time-nuts] Re: Phase coherence with 2x GPSDO

[Krishna Makhija]

Good day everyone,

I ran a few additional tests to figure this issue out. The first test
involved using one GPS antenna for both units. I found that while phase
coherence was better in general, there was still a fair bit of disagreement
between them. I then tried to use the 1PPS from one GPSDO as reference to
the other one, and looked at the 10 MHz from each unit on a scope. There
was a fixed phase offset between both 10 MHz signals +- 5-8 ns of error.
Which means that even if I gave an error-free PPS as reference the
oscillators would *still* not be in enough agreement for my application.
Which in turn means implementing a PTP system would be pointless as well.

On a somewhat related note, does anyone know why the F9T modules only give
2.5 ns of timing accuracy? RTK GPS has 1 cm RMS of positioning error which
should theoretically be 33 ps in timing. Where does the 2.5 ns error come
from?

On Wed, Mar 9, 2022 at 3:26 PM Mattia Rizzi  wrote:

> Hi
> The timestamping in a real implementation should be done in the FPGA or on
> a SoC integrated in the FPGA. Since you have a SDR you could implement a
> timestamping core.
> This timestamping core takes the samples from the ADC and calculates the
> time of arrival relative to the sampling clock.
> If you have already a datalink between the drone and the base station, you
> can use the symbol of the data layer, in that way you can have a lot of
> freedom in the implementation of the sync algorithm.
>
> >Since there is some
> non-deterministic computational overhead, you need to take several such
> measurements and calculate what the mean time of flight would be and
> correct the offsets accordingly. Is that correct? Please let me know if I
> am wrong because I really need to fix this issue.
>
> No, in an actual implementation it's deterministic.
> You're receiving a frame, the timestampjng core gives you a timestamp for
> each symbol. If you use 802.15.4-CSS, which has 4 MHz chirps as symbol, you
> can get down to 0.5ns RMS noise *for each symbol* (SNR of around 15 dB),
> indeed if you make an average with more symbols you can get better results,
> but there's a limit on how many timestamps you can average (mainly depends
> on the stability of you sampling clock).
>
> You can run a simple protocol like a stripped down version of PTP, and if
> you're able to embed it into layer 1 (you've an SDR, you can do it) then
> it's for free, and you can get a high synchronization rate for free. You
> can also get two-way ranging for free.
>
> Il Mer 9 Mar 2022, 19:23 Krishna Makhija  ha scritto:
>
> > Hello Mattia,
> >
> > Thanks for the paper. Please correct me if I understood wrong, but it
> seems
> > like you are calculating the time of flight between timestamped signals
> to
> > synchronize two spatially separated clocks. Since there is some
> > non-deterministic computational overhead, you need to take several such
> > measurements and calculate what the mean time of flight would be and
> > correct the offsets accordingly. Is that correct? Please let me know if I
> > am wrong because I really need to fix this issue.
> >
> > How would it work if one of the units was moving relative to the other? I
> > am going to have one of these on a drone and flying around. I admit I
> never
> > looked too deep into NTP/PTP but I always figured there would be a
> constant
> > variability owing to the steady motion. With GPSDOs as others have
> pointed
> > out, there are further errors induced due acceleration (vibrations on a
> > piezoelectric cause sinusoidal frequency errors), and temperature changes
> > (the drone rover unit will see more airflow as compared to the base
> unit),
> > but as of now I am trying to make sure I can get it to work with that
> > notwithstanding at least.
> >
> > P.S. The Mini-JLT <
> > https://www.jackson-labs.com/index.php/products/mini_jlt>
> > is designed for drones. It has a special mode where it updates its
> position
> > in real-time. There is also a gyro with a Kalman filter running on a
> > real-time operating system which is supposed to reduce the effect of
> > vibrations. It does not eliminate that effect entirely but it does
> mitigate
> > it by a factor of 10.
> >
> > On Mon, Mar 7, 2022 at 2:11 PM Mattia Rizzi 
> > wrote:
> >
> > > Hi,
> > > You can definitely get sub-ns precision using a 4-8 MHz bandwidth
> > wireless
> > > protocol. You are not limited by the sampling period. In my previous
> > work I
> > > used 802.15.4-CSS (chirp based modulation), implemented by me in a SDR.
> > The
> > > sampling rate was 32 ns but with crosscorrelation and sample
> > interpolation
> > > you can get down to half ns precision (1 sigma)
> > >
> > > See:
> > >
> > >
> >
> https://www.researchgate.net/publication/261329724_Timestamping_and_ranging_performance_for_IEEE_802154_CSS_systems
> > >
> > >
> > >
> > >
> > >
> > > Il giorno lun 7 mar 2022 alle 19:39 Krishna Makhija <
> km...@virginia.edu>
> > > ha
> > > scritto:
> > >
> > > > Thanks Mattia. What 

[time-nuts] Re: Phase coherence with 2x GPSDO

[Bob kb8tq]

Hi


> On Mar 9, 2022, at 5:00 PM, Hal Murray  wrote:
> 
> 
> kb...@n1k.org said:
>> If your drone is moving at 15 KPH, that???s a bit over 4 m/s. Roughly
>> speaking, you would need updates at > 10 per second to keep  this from
>> dominating the result. This would be true of both survey  and direct time of
>> flight measurements.  
> 
> Do I really need that update rate?  If the velocity changes slowly, both 
> magnitude and direction, then I can extrapolate.

It very much depends on how fast the drone is going and how far you need
to push the location error down in the “mix”. It’s not the only source of error.
Without any “smoothing” you get an update rate at 4 / 0.15 = 26 samples a 
second to be at the stated limit. If you need to push it down to a tenth of 
the combined total, then the straight up number would be 260 samples
a second. 

Is this drone targeting something? If so, 15 KPH would be quite slow. Speed
is very much application dependent. Like a lot of these things, we know 
very little about the details of the system. Thus all we can do is make random
guesses about key parameters. 

Bob

> 
> With access to the drone control signals, I could do a better job of 
> predicting the path.
> 
> The subject is GPSDO so I doubt if you want to be jerking the control voltage 
> around.  Is there any way to take advantage of that?
> 
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> ___
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[time-nuts] Re: Phase coherence with 2x GPSDO

[Hal Murray]

kb...@n1k.org said:
> If your drone is moving at 15 KPH, that???s a bit over 4 m/s. Roughly
> speaking, you would need updates at > 10 per second to keep  this from
> dominating the result. This would be true of both survey  and direct time of
> flight measurements.  

Do I really need that update rate?  If the velocity changes slowly, both 
magnitude and direction, then I can extrapolate.

With access to the drone control signals, I could do a better job of 
predicting the path.

The subject is GPSDO so I doubt if you want to be jerking the control voltage 
around.  Is there any way to take advantage of that?


-- 
These are my opinions.  I hate spam.


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[time-nuts] Re: Phase coherence with 2x GPSDO

[Mattia Rizzi]

Hi
The timestamping in a real implementation should be done in the FPGA or on
a SoC integrated in the FPGA. Since you have a SDR you could implement a
timestamping core.
This timestamping core takes the samples from the ADC and calculates the
time of arrival relative to the sampling clock.
If you have already a datalink between the drone and the base station, you
can use the symbol of the data layer, in that way you can have a lot of
freedom in the implementation of the sync algorithm.

>Since there is some
non-deterministic computational overhead, you need to take several such
measurements and calculate what the mean time of flight would be and
correct the offsets accordingly. Is that correct? Please let me know if I
am wrong because I really need to fix this issue.

No, in an actual implementation it's deterministic.
You're receiving a frame, the timestampjng core gives you a timestamp for
each symbol. If you use 802.15.4-CSS, which has 4 MHz chirps as symbol, you
can get down to 0.5ns RMS noise *for each symbol* (SNR of around 15 dB),
indeed if you make an average with more symbols you can get better results,
but there's a limit on how many timestamps you can average (mainly depends
on the stability of you sampling clock).

You can run a simple protocol like a stripped down version of PTP, and if
you're able to embed it into layer 1 (you've an SDR, you can do it) then
it's for free, and you can get a high synchronization rate for free. You
can also get two-way ranging for free.

Il Mer 9 Mar 2022, 19:23 Krishna Makhija  ha scritto:

> Hello Mattia,
>
> Thanks for the paper. Please correct me if I understood wrong, but it seems
> like you are calculating the time of flight between timestamped signals to
> synchronize two spatially separated clocks. Since there is some
> non-deterministic computational overhead, you need to take several such
> measurements and calculate what the mean time of flight would be and
> correct the offsets accordingly. Is that correct? Please let me know if I
> am wrong because I really need to fix this issue.
>
> How would it work if one of the units was moving relative to the other? I
> am going to have one of these on a drone and flying around. I admit I never
> looked too deep into NTP/PTP but I always figured there would be a constant
> variability owing to the steady motion. With GPSDOs as others have pointed
> out, there are further errors induced due acceleration (vibrations on a
> piezoelectric cause sinusoidal frequency errors), and temperature changes
> (the drone rover unit will see more airflow as compared to the base unit),
> but as of now I am trying to make sure I can get it to work with that
> notwithstanding at least.
>
> P.S. The Mini-JLT <
> https://www.jackson-labs.com/index.php/products/mini_jlt>
> is designed for drones. It has a special mode where it updates its position
> in real-time. There is also a gyro with a Kalman filter running on a
> real-time operating system which is supposed to reduce the effect of
> vibrations. It does not eliminate that effect entirely but it does mitigate
> it by a factor of 10.
>
> On Mon, Mar 7, 2022 at 2:11 PM Mattia Rizzi 
> wrote:
>
> > Hi,
> > You can definitely get sub-ns precision using a 4-8 MHz bandwidth
> wireless
> > protocol. You are not limited by the sampling period. In my previous
> work I
> > used 802.15.4-CSS (chirp based modulation), implemented by me in a SDR.
> The
> > sampling rate was 32 ns but with crosscorrelation and sample
> interpolation
> > you can get down to half ns precision (1 sigma)
> >
> > See:
> >
> >
> https://www.researchgate.net/publication/261329724_Timestamping_and_ranging_performance_for_IEEE_802154_CSS_systems
> >
> >
> >
> >
> >
> > Il giorno lun 7 mar 2022 alle 19:39 Krishna Makhija 
> > ha
> > scritto:
> >
> > > Thanks Mattia. What did you use for your Layer 1? I need to place one
> of
> > > the SDRs on a drone and one on the ground so a fiber or LAN cable is
> > out. I
> > > could use WLAN but can you get sub-nanosecond performance over wi-fi?
> My
> > > initial guess would be no but I am not certain.
> > >
> > > Michael: I've had the same question but I can't see how it could
> > > possibly "know" its own frequency error/uncertainty. What would it
> > > reference to? I'll try asking the manufacturer anyway.
> > >
> > > Regards,
> > > Krishna
> > >
> > > On Mon, Mar 7, 2022 at 11:46 AM Mattia Rizzi 
> > > wrote:
> > >
> > > > Hello Krishna,
> > > > what is your end application? How far away are those boards?
> > > > If each SDR can communicate to each other, you can run PTP over an
> > ad-hoc
> > > > Layer 1.
> > > > I was able to get timestamps out of a 2.4 GHz chirp-based protocol
> with
> > > > less than 0.5ns RMS noise and two-way ranging error down to 10-20
> > > > centimeters, using two SDR.
> > > >
> > > >
> > > > cheers
> > > >
> > > > Il giorno dom 6 mar 2022 alle ore 23:48 Krishna Makhija <
> > > > km...@virginia.edu>
> > > > ha scritto:
> > > >
> > > > > Hello Tom,

[time-nuts] Re: Phase coherence with 2x GPSDO

[Bob kb8tq]

Hi

If you have a clock in motion and want to send it a time signal, 
the speed of light will always get into the process. A nanosecond
is roughly 30 CM. To hit a half nanosecond, you need to know the 
location to within 15 CM. If you are doing this by survey techniques
( = GPS), that would be related to the sum of the location errors on 
both ends. 

If your drone is moving at 15 KPH, that’s a bit over 4 m/s. Roughly 
speaking, you would need updates at > 10 per second to keep 
this from dominating the result. This would be true of both survey 
and direct time of flight measurements. 

If the objective is to to this in real time, that’s a lot to work out. Is 
post processing an alternative in this case? 

Getting a nanosecond ( or even 10’s of nanoseconds ) with NTP is 
not what the protocol is designed to do. With PTP, hitting this level
involves a lot of traffic *and* a stable ( = time invariant ) link between
the devices involved. Even then it struggles as you head below
the 100’s of ns over something other than a hard wired connection. 

Bob

> On Mar 9, 2022, at 8:00 AM, Krishna Makhija  wrote:
> 
> Hello Mattia,
> 
> Thanks for the paper. Please correct me if I understood wrong, but it seems
> like you are calculating the time of flight between timestamped signals to
> synchronize two spatially separated clocks. Since there is some
> non-deterministic computational overhead, you need to take several such
> measurements and calculate what the mean time of flight would be and
> correct the offsets accordingly. Is that correct? Please let me know if I
> am wrong because I really need to fix this issue.
> 
> How would it work if one of the units was moving relative to the other? I
> am going to have one of these on a drone and flying around. I admit I never
> looked too deep into NTP/PTP but I always figured there would be a constant
> variability owing to the steady motion. With GPSDOs as others have pointed
> out, there are further errors induced due acceleration (vibrations on a
> piezoelectric cause sinusoidal frequency errors), and temperature changes
> (the drone rover unit will see more airflow as compared to the base unit),
> but as of now I am trying to make sure I can get it to work with that
> notwithstanding at least.
> 
> P.S. The Mini-JLT 
> is designed for drones. It has a special mode where it updates its position
> in real-time. There is also a gyro with a Kalman filter running on a
> real-time operating system which is supposed to reduce the effect of
> vibrations. It does not eliminate that effect entirely but it does mitigate
> it by a factor of 10.
> 
> On Mon, Mar 7, 2022 at 2:11 PM Mattia Rizzi  wrote:
> 
>> Hi,
>> You can definitely get sub-ns precision using a 4-8 MHz bandwidth wireless
>> protocol. You are not limited by the sampling period. In my previous work I
>> used 802.15.4-CSS (chirp based modulation), implemented by me in a SDR. The
>> sampling rate was 32 ns but with crosscorrelation and sample interpolation
>> you can get down to half ns precision (1 sigma)
>> 
>> See:
>> 
>> https://www.researchgate.net/publication/261329724_Timestamping_and_ranging_performance_for_IEEE_802154_CSS_systems
>> 
>> 
>> 
>> 
>> 
>> Il giorno lun 7 mar 2022 alle 19:39 Krishna Makhija 
>> ha
>> scritto:
>> 
>>> Thanks Mattia. What did you use for your Layer 1? I need to place one of
>>> the SDRs on a drone and one on the ground so a fiber or LAN cable is
>> out. I
>>> could use WLAN but can you get sub-nanosecond performance over wi-fi? My
>>> initial guess would be no but I am not certain.
>>> 
>>> Michael: I've had the same question but I can't see how it could
>>> possibly "know" its own frequency error/uncertainty. What would it
>>> reference to? I'll try asking the manufacturer anyway.
>>> 
>>> Regards,
>>> Krishna
>>> 
>>> On Mon, Mar 7, 2022 at 11:46 AM Mattia Rizzi 
>>> wrote:
>>> 
 Hello Krishna,
 what is your end application? How far away are those boards?
 If each SDR can communicate to each other, you can run PTP over an
>> ad-hoc
 Layer 1.
 I was able to get timestamps out of a 2.4 GHz chirp-based protocol with
 less than 0.5ns RMS noise and two-way ranging error down to 10-20
 centimeters, using two SDR.
 
 
 cheers
 
 Il giorno dom 6 mar 2022 alle ore 23:48 Krishna Makhija <
 km...@virginia.edu>
 ha scritto:
 
> Hello Tom,
> 
> Yes, the GPSDOs are working well. However, when I use each as a
>>> reference
> to a separate radio, I find there is a slow phase change over time
 between
> said radios. I imagine this is expected since there will always be
>> some
> error between two discrete oscillators. However, I am hoping to use
>> the
 PPS
> and FEE metadata to compute what the phase *should* have been in
> post-processing. So far, it is not working out for me. I am wondering
>>> if
> that is even possible or if my 

[time-nuts] Re: Phase coherence with 2x GPSDO

[Krishna Makhija]

Hello Mattia,

Thanks for the paper. Please correct me if I understood wrong, but it seems
like you are calculating the time of flight between timestamped signals to
synchronize two spatially separated clocks. Since there is some
non-deterministic computational overhead, you need to take several such
measurements and calculate what the mean time of flight would be and
correct the offsets accordingly. Is that correct? Please let me know if I
am wrong because I really need to fix this issue.

How would it work if one of the units was moving relative to the other? I
am going to have one of these on a drone and flying around. I admit I never
looked too deep into NTP/PTP but I always figured there would be a constant
variability owing to the steady motion. With GPSDOs as others have pointed
out, there are further errors induced due acceleration (vibrations on a
piezoelectric cause sinusoidal frequency errors), and temperature changes
(the drone rover unit will see more airflow as compared to the base unit),
but as of now I am trying to make sure I can get it to work with that
notwithstanding at least.

P.S. The Mini-JLT 
is designed for drones. It has a special mode where it updates its position
in real-time. There is also a gyro with a Kalman filter running on a
real-time operating system which is supposed to reduce the effect of
vibrations. It does not eliminate that effect entirely but it does mitigate
it by a factor of 10.

On Mon, Mar 7, 2022 at 2:11 PM Mattia Rizzi  wrote:

> Hi,
> You can definitely get sub-ns precision using a 4-8 MHz bandwidth wireless
> protocol. You are not limited by the sampling period. In my previous work I
> used 802.15.4-CSS (chirp based modulation), implemented by me in a SDR. The
> sampling rate was 32 ns but with crosscorrelation and sample interpolation
> you can get down to half ns precision (1 sigma)
>
> See:
>
> https://www.researchgate.net/publication/261329724_Timestamping_and_ranging_performance_for_IEEE_802154_CSS_systems
>
>
>
>
>
> Il giorno lun 7 mar 2022 alle 19:39 Krishna Makhija 
> ha
> scritto:
>
> > Thanks Mattia. What did you use for your Layer 1? I need to place one of
> > the SDRs on a drone and one on the ground so a fiber or LAN cable is
> out. I
> > could use WLAN but can you get sub-nanosecond performance over wi-fi? My
> > initial guess would be no but I am not certain.
> >
> > Michael: I've had the same question but I can't see how it could
> > possibly "know" its own frequency error/uncertainty. What would it
> > reference to? I'll try asking the manufacturer anyway.
> >
> > Regards,
> > Krishna
> >
> > On Mon, Mar 7, 2022 at 11:46 AM Mattia Rizzi 
> > wrote:
> >
> > > Hello Krishna,
> > > what is your end application? How far away are those boards?
> > > If each SDR can communicate to each other, you can run PTP over an
> ad-hoc
> > > Layer 1.
> > > I was able to get timestamps out of a 2.4 GHz chirp-based protocol with
> > > less than 0.5ns RMS noise and two-way ranging error down to 10-20
> > > centimeters, using two SDR.
> > >
> > >
> > > cheers
> > >
> > > Il giorno dom 6 mar 2022 alle ore 23:48 Krishna Makhija <
> > > km...@virginia.edu>
> > > ha scritto:
> > >
> > > > Hello Tom,
> > > >
> > > > Yes, the GPSDOs are working well. However, when I use each as a
> > reference
> > > > to a separate radio, I find there is a slow phase change over time
> > > between
> > > > said radios. I imagine this is expected since there will always be
> some
> > > > error between two discrete oscillators. However, I am hoping to use
> the
> > > PPS
> > > > and FEE metadata to compute what the phase *should* have been in
> > > > post-processing. So far, it is not working out for me. I am wondering
> > if
> > > > that is even possible or if my math is just wrong.
> > > >
> > > > Bob,
> > > >
> > > > The SDRs have an LO running at 150 MHz (~6.66 ns) so a PPS wander of
> +-
> > > 10
> > > > ns is >360 deg. With a common-mode reference I see a small phase
> change
> > > (+-
> > > > 3-4 deg) but that is not an option for my application.
> > > >
> > > > Where does the PPS offset come from? Isn't it from the positioning
> > error?
> > > > Typical GPS receivers have 1-3 m of positioning error which should
> give
> > > > you +- 10 ns. Why is this a "dream" performance? It should be
> expected
> > > from
> > > > any modern GPS receiver.
> > > >
> > > > Thanks for your inputs so far.
> > > >
> > > > Krishna
> > > >
> > > > On Sun, Mar 6, 2022 at 4:30 PM Bob kb8tq  wrote:
> > > >
> > > > > Hi
> > > > >
> > > > > They *are* phase coherant to within 360 * 100/ 10 = 36 degrees. You
> > > > > can get them to *maybe* ten degrees with this and that done here or
> > > > there.
> > > > >
> > > > > If you want them within a degree, no you can’t do that directly
> with
> > > GPS.
> > > > > If your definition of phase coherent is zero degrees, a pair of
> > SDR’s
> > > > off
> > > > > the
> > > > > same buffered clock will have 

[time-nuts] Re: Phase coherence with 2X GPSDO

[Andy Talbot]

Where are the GPS antennas wrt. you 'moving around'?
Could it be that while you were watching the paint drying, you didn't move
much.   But when you moved away, the RF pattern as a result of local
reflections of the signal from the satellites (from you) were enough to
upset the receiver to give the phase shift.


Andy
www.g4jnt.com



On Tue, 8 Mar 2022 at 14:55, Bob kb8tq  wrote:

> HI
>
> > On Mar 8, 2022, at 2:31 AM, ed breya  wrote:
> >
> > Just an interesting observation. Last night, shortly after I wrote about
> my dual GPSDO system, I decided to take a look at it, with the usual scope
> setup looking at both 10 MHz outputs. It looked like one of those
> reasonably good times, where the phase between them seemed very stable
> during fairly long observation time. They were nearly in quadrature,
> drifting very slightly back and forth, over maybe an hour of occasional
> observation. This is akin to the proverbial "watching paint dry" - nothing
> new to report, over a long time, blah blah blah.
> >
> > I went inside for at most a couple minutes to look at something, and
> when I returned to the garage, I noticed they were nearly in-phase, all of
> a sudden. Over the next half hour or so, I could see the phase drift back
> to about where it was before. During that brief moment when I was gone, I
> had missed what was probably a DAC value update, when one of the units
> decided it was time to make a change.
> >
> > The change of one or some LSBs must have had a relatively instantaneous
> effect in the time scale involved, and I missed it, but I did see the
> aftermath. So, you can add that sort of effect in the time/frequency
> situation. The temperatures and drifts of various parts, and the SW
> figuring out what to do about it, tend to go quite slowly, but once the
> decision is made to change a value, there's a step function involved.
> >
> > As I understand, the tuning DAC may be updated once a second at most (if
> comparing to the 1PPS), in small increments, as the DO part tries to keep
> everything right according to the GPS part.
>
> In the case of most GPSDO’s running with most firmware:
>
> The updates are indeed once a second. The math does it’s thing on each PPS
> and comes up with a “new guess” each time. The delta with most DAC’s is in
> the
> “several to many” LSB's range. A lot depends on what’s going on with the
> oscillator
> and the rest of the “stuff”.
>
> While we tend to blame everything on the poor OCXO in the typical OEM
> module,
> that may not be the problem. The Vref or even the tempco of the DAC could
> easily
> be getting into the mix. There are some well known GPSDO’s that a lot of
> us have
> where this is the case.
>
> Since I’m sure “why” will be the next question ….
>
> If the device is targeted at a very long lifespan (10, 20, ….. years ) the
> trim range is
> going to be non-trivial. Just how large it is depends a bit on how
> conservative the
> folks who did the oscillator / GPSDO felt when they did it all up.  Could
> you use
> a $50 Vref and a $90 DAC? That’s really going to destroy your budget ….
>
> Bob
>
>
> > With my simple setup and observations, I can't tell really how big, or
> how often these adjustments are made, but whatever this was, it was quite
> obvious. The most interesting things tend to happen when you're not looking.
> >
> > Ed
> > ___
> > time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe
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[time-nuts] Re: Phase coherence with 2X GPSDO

[Bob kb8tq]

HI

> On Mar 8, 2022, at 2:31 AM, ed breya  wrote:
> 
> Just an interesting observation. Last night, shortly after I wrote about my 
> dual GPSDO system, I decided to take a look at it, with the usual scope setup 
> looking at both 10 MHz outputs. It looked like one of those reasonably good 
> times, where the phase between them seemed very stable during fairly long 
> observation time. They were nearly in quadrature, drifting very slightly back 
> and forth, over maybe an hour of occasional observation. This is akin to the 
> proverbial "watching paint dry" - nothing new to report, over a long time, 
> blah blah blah.
> 
> I went inside for at most a couple minutes to look at something, and when I 
> returned to the garage, I noticed they were nearly in-phase, all of a sudden. 
> Over the next half hour or so, I could see the phase drift back to about 
> where it was before. During that brief moment when I was gone, I had missed 
> what was probably a DAC value update, when one of the units decided it was 
> time to make a change.
> 
> The change of one or some LSBs must have had a relatively instantaneous 
> effect in the time scale involved, and I missed it, but I did see the 
> aftermath. So, you can add that sort of effect in the time/frequency 
> situation. The temperatures and drifts of various parts, and the SW figuring 
> out what to do about it, tend to go quite slowly, but once the decision is 
> made to change a value, there's a step function involved.
> 
> As I understand, the tuning DAC may be updated once a second at most (if 
> comparing to the 1PPS), in small increments, as the DO part tries to keep 
> everything right according to the GPS part.

In the case of most GPSDO’s running with most firmware: 

The updates are indeed once a second. The math does it’s thing on each PPS
and comes up with a “new guess” each time. The delta with most DAC’s is in the
“several to many” LSB's range. A lot depends on what’s going on with the 
oscillator
and the rest of the “stuff”. 

While we tend to blame everything on the poor OCXO in the typical OEM module,
that may not be the problem. The Vref or even the tempco of the DAC could easily
be getting into the mix. There are some well known GPSDO’s that a lot of us have
where this is the case. 

Since I’m sure “why” will be the next question …. 

If the device is targeted at a very long lifespan (10, 20, ….. years ) the trim 
range is 
going to be non-trivial. Just how large it is depends a bit on how conservative 
the
folks who did the oscillator / GPSDO felt when they did it all up.  Could you 
use 
a $50 Vref and a $90 DAC? That’s really going to destroy your budget ….

Bob


> With my simple setup and observations, I can't tell really how big, or how 
> often these adjustments are made, but whatever this was, it was quite 
> obvious. The most interesting things tend to happen when you're not looking.
> 
> Ed
> ___
> time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an 
> email to time-nuts-le...@lists.febo.com
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[time-nuts] Re: Phase coherence with 2X GPSDO

[ed breya]

Just an interesting observation. Last night, shortly after I wrote about 
my dual GPSDO system, I decided to take a look at it, with the usual 
scope setup looking at both 10 MHz outputs. It looked like one of those 
reasonably good times, where the phase between them seemed very stable 
during fairly long observation time. They were nearly in quadrature, 
drifting very slightly back and forth, over maybe an hour of occasional 
observation. This is akin to the proverbial "watching paint dry" - 
nothing new to report, over a long time, blah blah blah.


I went inside for at most a couple minutes to look at something, and 
when I returned to the garage, I noticed they were nearly in-phase, all 
of a sudden. Over the next half hour or so, I could see the phase drift 
back to about where it was before. During that brief moment when I was 
gone, I had missed what was probably a DAC value update, when one of the 
units decided it was time to make a change.


The change of one or some LSBs must have had a relatively instantaneous 
effect in the time scale involved, and I missed it, but I did see the 
aftermath. So, you can add that sort of effect in the time/frequency 
situation. The temperatures and drifts of various parts, and the SW 
figuring out what to do about it, tend to go quite slowly, but once the 
decision is made to change a value, there's a step function involved.


As I understand, the tuning DAC may be updated once a second at most (if 
comparing to the 1PPS), in small increments, as the DO part tries to 
keep everything right according to the GPS part. With my simple setup 
and observations, I can't tell really how big, or how often these 
adjustments are made, but whatever this was, it was quite obvious. The 
most interesting things tend to happen when you're not looking.


Ed
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[time-nuts] Re: Phase coherence with 2x GPSDO

[Tom Van Baak]

> I've had the same question but I can't see how it could possibly "know"
> its own frequency error/uncertainty. What would it reference to?

That's why it's a rough internal *estimate*, not an actual measurement 
against an external reference. Again, it goes back to how a GPSDO works. 
There is the assumption that GPS is "perfect" in the long-term. There is 
the assumption that the OCXO is extremely stable short-term. The two are 
coupled in a slow feedback loop. Thus it is possible to use the OCXO to 
see how bad GPS is, short-term. And it is possible to use GPS to see how 
bad the OCXO is, long-term. This is roughly the nature of the TINT and 
FEE numbers.


I don't of anyone who actually uses these numbers, or trusts the result. 
And the important thing, for this discussion, is that the numbers won't 
improve your accuracy. They are more a diagnostic of how the GPSDO is 
working. As your application is getting clearer, you do not want a GPSDO 
with all its complications and unknowns, not to mention proprietary 
firmware. Instead you want a plain GPS timing receiver and do all your 
own real-time or post-processing.


> I need to place one of the SDRs on a drone and one on the ground.

Will that work? Almost no quartz-based GPSDO are rated for flight. The 
3D acceleration alone will greatly affect the oscillator at ppb levels, 
confusing the GPSDO algorithms. Most GPSDO are designed for static use 
in stable environments. While they are running you do not touch them, 
blow on them, turn them, tilt them, not even by millimeter.


But you should try it to see for yourself. Start a clean phase 
comparison between your two GPSDO on the bench. Wait for steady state. 
Then pick one up, tilt it, wave it, wiggle it, and keep this up for a 
while. See how much your phase coherence suffers and also how quickly 
(or slowly) the GPSDO recovers from these external perturbations. This 
is another reason to use a plain GPS receiver and not a GPSDO.


/tvb
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[time-nuts] Re: Phase coherence with 2x GPSDO

[Mattia Rizzi]

Hi,
You can definitely get sub-ns precision using a 4-8 MHz bandwidth wireless
protocol. You are not limited by the sampling period. In my previous work I
used 802.15.4-CSS (chirp based modulation), implemented by me in a SDR. The
sampling rate was 32 ns but with crosscorrelation and sample interpolation
you can get down to half ns precision (1 sigma)

See:
https://www.researchgate.net/publication/261329724_Timestamping_and_ranging_performance_for_IEEE_802154_CSS_systems





Il giorno lun 7 mar 2022 alle 19:39 Krishna Makhija  ha
scritto:

> Thanks Mattia. What did you use for your Layer 1? I need to place one of
> the SDRs on a drone and one on the ground so a fiber or LAN cable is out. I
> could use WLAN but can you get sub-nanosecond performance over wi-fi? My
> initial guess would be no but I am not certain.
>
> Michael: I've had the same question but I can't see how it could
> possibly "know" its own frequency error/uncertainty. What would it
> reference to? I'll try asking the manufacturer anyway.
>
> Regards,
> Krishna
>
> On Mon, Mar 7, 2022 at 11:46 AM Mattia Rizzi 
> wrote:
>
> > Hello Krishna,
> > what is your end application? How far away are those boards?
> > If each SDR can communicate to each other, you can run PTP over an ad-hoc
> > Layer 1.
> > I was able to get timestamps out of a 2.4 GHz chirp-based protocol with
> > less than 0.5ns RMS noise and two-way ranging error down to 10-20
> > centimeters, using two SDR.
> >
> >
> > cheers
> >
> > Il giorno dom 6 mar 2022 alle ore 23:48 Krishna Makhija <
> > km...@virginia.edu>
> > ha scritto:
> >
> > > Hello Tom,
> > >
> > > Yes, the GPSDOs are working well. However, when I use each as a
> reference
> > > to a separate radio, I find there is a slow phase change over time
> > between
> > > said radios. I imagine this is expected since there will always be some
> > > error between two discrete oscillators. However, I am hoping to use the
> > PPS
> > > and FEE metadata to compute what the phase *should* have been in
> > > post-processing. So far, it is not working out for me. I am wondering
> if
> > > that is even possible or if my math is just wrong.
> > >
> > > Bob,
> > >
> > > The SDRs have an LO running at 150 MHz (~6.66 ns) so a PPS wander of +-
> > 10
> > > ns is >360 deg. With a common-mode reference I see a small phase change
> > (+-
> > > 3-4 deg) but that is not an option for my application.
> > >
> > > Where does the PPS offset come from? Isn't it from the positioning
> error?
> > > Typical GPS receivers have 1-3 m of positioning error which should give
> > > you +- 10 ns. Why is this a "dream" performance? It should be expected
> > from
> > > any modern GPS receiver.
> > >
> > > Thanks for your inputs so far.
> > >
> > > Krishna
> > >
> > > On Sun, Mar 6, 2022 at 4:30 PM Bob kb8tq  wrote:
> > >
> > > > Hi
> > > >
> > > > They *are* phase coherant to within 360 * 100/ 10 = 36 degrees. You
> > > > can get them to *maybe* ten degrees with this and that done here or
> > > there.
> > > >
> > > > If you want them within a degree, no you can’t do that directly with
> > GPS.
> > > > If your definition of phase coherent is zero degrees, a pair of
> SDR’s
> > > off
> > > > the
> > > > same buffered clock will have issues with that definition in the real
> > > > world
> > > > of temperature wandering around …..
> > > >
> > > > Bob
> > > >
> > > > > On Mar 6, 2022, at 2:04 PM, Krishna Makhija 
> > > wrote:
> > > > >
> > > > > Hi Bob,
> > > > >
> > > > > I am currently getting +- 10 ns nominal. The antennas are currently
> > > > almost
> > > > > next to each other (roughly 1-2 inches apart). Yes, they should be
> > > > outside
> > > > > of each of their farfield zones. Here is the PPS offset I am seeing
> > > > during
> > > > > measurement:
> > > > > [image: image.png]
> > > > > And this is the frequency error I am seeing:
> > > > > [image: image.png]
> > > > > Overall, the GPSDOs seem to work pretty well. But the question
> still
> > > > > remains if one can hope to get them to be phase coherent, either in
> > > > > real-time or in post-processing.
> > > > >
> > > > > Jeremy: I bought these items by writing to them. I chose to place a
> > > > > purchase order (since I did it through my organization), but you
> > might
> > > be
> > > > > able to order by talking to them directly and paying using a credit
> > > card.
> > > > >
> > > > > On Sun, Mar 6, 2022 at 1:51 PM Bob kb8tq  wrote:
> > > > >
> > > > >> Hi
> > > > >>
> > > > >> How close are you trying to get?
> > > > >>
> > > > >> How far apart are the GPSDO’s?
> > > > >>
> > > > >> A “run of the mill” number would be out around 100 ns. A “pretty
> > good”
> > > > >> number is in the 20 ns range. A “crazy good” number would be 2 ns.
> > To
> > > > >> do better than this, you likely would need to go to a more exotic
> > > > >> configuration
> > > > >> on the GPSDO.
> > > > >>
> > > > >> Bob
> > > > >>
> > > > >>> On Mar 6, 2022, at 12:55 PM, Krishna Makhija  >
> > > > wrote:
> > > > >>>
> > > > 

[time-nuts] Re: Phase coherence with 2x GPSDO

[Bob kb8tq]

Hi

As soon as you put one device in motion relative to the other
and space them apart, the expected level of alignment between
the PPS outputs will drop. You are adding a number of variables
into the mix.

As noted in other posts, something like a F9T or a Mosiac T is 
a much better way to do this than a single band uBlox in a device
that is targeted at stationary applications. 

Bob

> On Mar 7, 2022, at 12:48 PM, Krishna Makhija  wrote:
> 
> Thanks Mattia. What did you use for your Layer 1? I need to place one of
> the SDRs on a drone and one on the ground so a fiber or LAN cable is out. I
> could use WLAN but can you get sub-nanosecond performance over wi-fi? My
> initial guess would be no but I am not certain.
> 
> Michael: I've had the same question but I can't see how it could
> possibly "know" its own frequency error/uncertainty. What would it
> reference to? I'll try asking the manufacturer anyway.
> 
> Regards,
> Krishna
> 
> On Mon, Mar 7, 2022 at 11:46 AM Mattia Rizzi  wrote:
> 
>> Hello Krishna,
>> what is your end application? How far away are those boards?
>> If each SDR can communicate to each other, you can run PTP over an ad-hoc
>> Layer 1.
>> I was able to get timestamps out of a 2.4 GHz chirp-based protocol with
>> less than 0.5ns RMS noise and two-way ranging error down to 10-20
>> centimeters, using two SDR.
>> 
>> 
>> cheers
>> 
>> Il giorno dom 6 mar 2022 alle ore 23:48 Krishna Makhija <
>> km...@virginia.edu>
>> ha scritto:
>> 
>>> Hello Tom,
>>> 
>>> Yes, the GPSDOs are working well. However, when I use each as a reference
>>> to a separate radio, I find there is a slow phase change over time
>> between
>>> said radios. I imagine this is expected since there will always be some
>>> error between two discrete oscillators. However, I am hoping to use the
>> PPS
>>> and FEE metadata to compute what the phase *should* have been in
>>> post-processing. So far, it is not working out for me. I am wondering if
>>> that is even possible or if my math is just wrong.
>>> 
>>> Bob,
>>> 
>>> The SDRs have an LO running at 150 MHz (~6.66 ns) so a PPS wander of +-
>> 10
>>> ns is >360 deg. With a common-mode reference I see a small phase change
>> (+-
>>> 3-4 deg) but that is not an option for my application.
>>> 
>>> Where does the PPS offset come from? Isn't it from the positioning error?
>>> Typical GPS receivers have 1-3 m of positioning error which should give
>>> you +- 10 ns. Why is this a "dream" performance? It should be expected
>> from
>>> any modern GPS receiver.
>>> 
>>> Thanks for your inputs so far.
>>> 
>>> Krishna
>>> 
>>> On Sun, Mar 6, 2022 at 4:30 PM Bob kb8tq  wrote:
>>> 
 Hi
 
 They *are* phase coherant to within 360 * 100/ 10 = 36 degrees. You
 can get them to *maybe* ten degrees with this and that done here or
>>> there.
 
 If you want them within a degree, no you can’t do that directly with
>> GPS.
 If your definition of phase coherent is zero degrees, a pair of  SDR’s
>>> off
 the
 same buffered clock will have issues with that definition in the real
 world
 of temperature wandering around …..
 
 Bob
 
> On Mar 6, 2022, at 2:04 PM, Krishna Makhija 
>>> wrote:
> 
> Hi Bob,
> 
> I am currently getting +- 10 ns nominal. The antennas are currently
 almost
> next to each other (roughly 1-2 inches apart). Yes, they should be
 outside
> of each of their farfield zones. Here is the PPS offset I am seeing
 during
> measurement:
> [image: image.png]
> And this is the frequency error I am seeing:
> [image: image.png]
> Overall, the GPSDOs seem to work pretty well. But the question still
> remains if one can hope to get them to be phase coherent, either in
> real-time or in post-processing.
> 
> Jeremy: I bought these items by writing to them. I chose to place a
> purchase order (since I did it through my organization), but you
>> might
>>> be
> able to order by talking to them directly and paying using a credit
>>> card.
> 
> On Sun, Mar 6, 2022 at 1:51 PM Bob kb8tq  wrote:
> 
>> Hi
>> 
>> How close are you trying to get?
>> 
>> How far apart are the GPSDO’s?
>> 
>> A “run of the mill” number would be out around 100 ns. A “pretty
>> good”
>> number is in the 20 ns range. A “crazy good” number would be 2 ns.
>> To
>> do better than this, you likely would need to go to a more exotic
>> configuration
>> on the GPSDO.
>> 
>> Bob
>> 
>>> On Mar 6, 2022, at 12:55 PM, Krishna Makhija 
 wrote:
>>> 
>>> Hello,
>>> 
>>> I am new to the whole precision time-keeping game (and to this
>>> mailing
>>> list) so I apologize in advance if my question is too naive or has
>>> been
>>> answered already in your mailing list.
>>> 
>>> Is it possible to have two separate GPSDOs, each with their own
 antennas,
>>> be phase coherent to each other? I 

[time-nuts] Re: Phase coherence with 2x GPSDO

[Krishna Makhija]

Thanks Mattia. What did you use for your Layer 1? I need to place one of
the SDRs on a drone and one on the ground so a fiber or LAN cable is out. I
could use WLAN but can you get sub-nanosecond performance over wi-fi? My
initial guess would be no but I am not certain.

Michael: I've had the same question but I can't see how it could
possibly "know" its own frequency error/uncertainty. What would it
reference to? I'll try asking the manufacturer anyway.

Regards,
Krishna

On Mon, Mar 7, 2022 at 11:46 AM Mattia Rizzi  wrote:

> Hello Krishna,
> what is your end application? How far away are those boards?
> If each SDR can communicate to each other, you can run PTP over an ad-hoc
> Layer 1.
> I was able to get timestamps out of a 2.4 GHz chirp-based protocol with
> less than 0.5ns RMS noise and two-way ranging error down to 10-20
> centimeters, using two SDR.
>
>
> cheers
>
> Il giorno dom 6 mar 2022 alle ore 23:48 Krishna Makhija <
> km...@virginia.edu>
> ha scritto:
>
> > Hello Tom,
> >
> > Yes, the GPSDOs are working well. However, when I use each as a reference
> > to a separate radio, I find there is a slow phase change over time
> between
> > said radios. I imagine this is expected since there will always be some
> > error between two discrete oscillators. However, I am hoping to use the
> PPS
> > and FEE metadata to compute what the phase *should* have been in
> > post-processing. So far, it is not working out for me. I am wondering if
> > that is even possible or if my math is just wrong.
> >
> > Bob,
> >
> > The SDRs have an LO running at 150 MHz (~6.66 ns) so a PPS wander of +-
> 10
> > ns is >360 deg. With a common-mode reference I see a small phase change
> (+-
> > 3-4 deg) but that is not an option for my application.
> >
> > Where does the PPS offset come from? Isn't it from the positioning error?
> > Typical GPS receivers have 1-3 m of positioning error which should give
> > you +- 10 ns. Why is this a "dream" performance? It should be expected
> from
> > any modern GPS receiver.
> >
> > Thanks for your inputs so far.
> >
> > Krishna
> >
> > On Sun, Mar 6, 2022 at 4:30 PM Bob kb8tq  wrote:
> >
> > > Hi
> > >
> > > They *are* phase coherant to within 360 * 100/ 10 = 36 degrees. You
> > > can get them to *maybe* ten degrees with this and that done here or
> > there.
> > >
> > > If you want them within a degree, no you can’t do that directly with
> GPS.
> > > If your definition of phase coherent is zero degrees, a pair of  SDR’s
> > off
> > > the
> > > same buffered clock will have issues with that definition in the real
> > > world
> > > of temperature wandering around …..
> > >
> > > Bob
> > >
> > > > On Mar 6, 2022, at 2:04 PM, Krishna Makhija 
> > wrote:
> > > >
> > > > Hi Bob,
> > > >
> > > > I am currently getting +- 10 ns nominal. The antennas are currently
> > > almost
> > > > next to each other (roughly 1-2 inches apart). Yes, they should be
> > > outside
> > > > of each of their farfield zones. Here is the PPS offset I am seeing
> > > during
> > > > measurement:
> > > > [image: image.png]
> > > > And this is the frequency error I am seeing:
> > > > [image: image.png]
> > > > Overall, the GPSDOs seem to work pretty well. But the question still
> > > > remains if one can hope to get them to be phase coherent, either in
> > > > real-time or in post-processing.
> > > >
> > > > Jeremy: I bought these items by writing to them. I chose to place a
> > > > purchase order (since I did it through my organization), but you
> might
> > be
> > > > able to order by talking to them directly and paying using a credit
> > card.
> > > >
> > > > On Sun, Mar 6, 2022 at 1:51 PM Bob kb8tq  wrote:
> > > >
> > > >> Hi
> > > >>
> > > >> How close are you trying to get?
> > > >>
> > > >> How far apart are the GPSDO’s?
> > > >>
> > > >> A “run of the mill” number would be out around 100 ns. A “pretty
> good”
> > > >> number is in the 20 ns range. A “crazy good” number would be 2 ns.
> To
> > > >> do better than this, you likely would need to go to a more exotic
> > > >> configuration
> > > >> on the GPSDO.
> > > >>
> > > >> Bob
> > > >>
> > > >>> On Mar 6, 2022, at 12:55 PM, Krishna Makhija 
> > > wrote:
> > > >>>
> > > >>> Hello,
> > > >>>
> > > >>> I am new to the whole precision time-keeping game (and to this
> > mailing
> > > >>> list) so I apologize in advance if my question is too naive or has
> > been
> > > >>> answered already in your mailing list.
> > > >>>
> > > >>> Is it possible to have two separate GPSDOs, each with their own
> > > antennas,
> > > >>> be phase coherent to each other? I have a Jackson-Labs Fury
> > > >>>  and a
> > Mini-JLT
> > > >>> . I am using
> > > each
> > > >> to
> > > >>> provide a 10 MHz reference to two separate software-defined radios
> > > >> (SDRs).
> > > >>> In my tests I find that the phase offset between said SDRs has a
> slow
> > > >>> time-varying behavior. I 

[time-nuts] Re: Phase coherence with 2x GPSDO

[Mattia Rizzi]

Hello Krishna,
what is your end application? How far away are those boards?
If each SDR can communicate to each other, you can run PTP over an ad-hoc
Layer 1.
I was able to get timestamps out of a 2.4 GHz chirp-based protocol with
less than 0.5ns RMS noise and two-way ranging error down to 10-20
centimeters, using two SDR.


cheers

Il giorno dom 6 mar 2022 alle ore 23:48 Krishna Makhija 
ha scritto:

> Hello Tom,
>
> Yes, the GPSDOs are working well. However, when I use each as a reference
> to a separate radio, I find there is a slow phase change over time between
> said radios. I imagine this is expected since there will always be some
> error between two discrete oscillators. However, I am hoping to use the PPS
> and FEE metadata to compute what the phase *should* have been in
> post-processing. So far, it is not working out for me. I am wondering if
> that is even possible or if my math is just wrong.
>
> Bob,
>
> The SDRs have an LO running at 150 MHz (~6.66 ns) so a PPS wander of +- 10
> ns is >360 deg. With a common-mode reference I see a small phase change (+-
> 3-4 deg) but that is not an option for my application.
>
> Where does the PPS offset come from? Isn't it from the positioning error?
> Typical GPS receivers have 1-3 m of positioning error which should give
> you +- 10 ns. Why is this a "dream" performance? It should be expected from
> any modern GPS receiver.
>
> Thanks for your inputs so far.
>
> Krishna
>
> On Sun, Mar 6, 2022 at 4:30 PM Bob kb8tq  wrote:
>
> > Hi
> >
> > They *are* phase coherant to within 360 * 100/ 10 = 36 degrees. You
> > can get them to *maybe* ten degrees with this and that done here or
> there.
> >
> > If you want them within a degree, no you can’t do that directly with GPS.
> > If your definition of phase coherent is zero degrees, a pair of  SDR’s
> off
> > the
> > same buffered clock will have issues with that definition in the real
> > world
> > of temperature wandering around …..
> >
> > Bob
> >
> > > On Mar 6, 2022, at 2:04 PM, Krishna Makhija 
> wrote:
> > >
> > > Hi Bob,
> > >
> > > I am currently getting +- 10 ns nominal. The antennas are currently
> > almost
> > > next to each other (roughly 1-2 inches apart). Yes, they should be
> > outside
> > > of each of their farfield zones. Here is the PPS offset I am seeing
> > during
> > > measurement:
> > > [image: image.png]
> > > And this is the frequency error I am seeing:
> > > [image: image.png]
> > > Overall, the GPSDOs seem to work pretty well. But the question still
> > > remains if one can hope to get them to be phase coherent, either in
> > > real-time or in post-processing.
> > >
> > > Jeremy: I bought these items by writing to them. I chose to place a
> > > purchase order (since I did it through my organization), but you might
> be
> > > able to order by talking to them directly and paying using a credit
> card.
> > >
> > > On Sun, Mar 6, 2022 at 1:51 PM Bob kb8tq  wrote:
> > >
> > >> Hi
> > >>
> > >> How close are you trying to get?
> > >>
> > >> How far apart are the GPSDO’s?
> > >>
> > >> A “run of the mill” number would be out around 100 ns. A “pretty good”
> > >> number is in the 20 ns range. A “crazy good” number would be 2 ns. To
> > >> do better than this, you likely would need to go to a more exotic
> > >> configuration
> > >> on the GPSDO.
> > >>
> > >> Bob
> > >>
> > >>> On Mar 6, 2022, at 12:55 PM, Krishna Makhija 
> > wrote:
> > >>>
> > >>> Hello,
> > >>>
> > >>> I am new to the whole precision time-keeping game (and to this
> mailing
> > >>> list) so I apologize in advance if my question is too naive or has
> been
> > >>> answered already in your mailing list.
> > >>>
> > >>> Is it possible to have two separate GPSDOs, each with their own
> > antennas,
> > >>> be phase coherent to each other? I have a Jackson-Labs Fury
> > >>>  and a
> Mini-JLT
> > >>> . I am using
> > each
> > >> to
> > >>> provide a 10 MHz reference to two separate software-defined radios
> > >> (SDRs).
> > >>> In my tests I find that the phase offset between said SDRs has a slow
> > >>> time-varying behavior. I know the frequency errors of the GPSDOs are
> of
> > >> the
> > >>> order of parts per trillion which will show up as slow time-varying
> > phase
> > >>> offsets but I was hoping to use the PPS offsets and instantaneous
> > >> frequency
> > >>> errors that I get from these modules (using SCPI commands) to be able
> > to
> > >>> "back out" or predict what that time-varying phase offset would be.
> Is
> > >> such
> > >>> a thing possible? Currently, the time-varying phase change does not
> > seem
> > >> to
> > >>> follow any discernible pattern and my attempts at backing out the
> phase
> > >>> change do not match my measurements.
> > >>>
> > >>> Here is the math I am using for calculating what I *think *the phase
> > >>> *should* be:
> > >>> [image: image.png]
> > >>> [image: image.png]
> > >>>
> > >>> 

[time-nuts] Re: Phase coherence with 2x GPSDO

[Tom Van Baak]

Hi Krishna,

> Hello Tom,
> Yes, the GPSDOs are working well. However, when I use each as a reference
> to a separate radio, I find there is a slow phase change over time 
between

> said radios. I imagine this is expected since there will always be some
> error between two discrete oscillators.

This is typical; it's not just the oscillators, but also the GPS 
receiver, s/w algorithms, board level components, and environmental 
conditions. How much phase change do you see? And equally important, 
over what span of time?


> However, I am hoping to use the PPS
> and FEE metadata to compute what the phase *should* have been in
> post-processing. So far, it is not working out for me. I am wondering if
> that is even possible or if my math is just wrong.

Sorry. The TINT and FEE values are more like phase and frequency error 
estimates, not offsets that you can use to improve the output of the 
GPSDO. I mean, if it were that easy then the GPSDO would just do it for 
you. So think of them as a rough attempt at "error bars". To give you an 
idea of the actual measured performance of a GPSDO like the Fury, read 
this test report from 2007. Your Fury may differ from mine so take this 
all with a grain of salt.


http://leapsecond.com/pages/fury/

There are also phase noise measurements, in case that is of interest:

http://leapsecond.com/pages/fury/phase.htm

It would help to know your expectation or requirement for phase 
coherence. In a sense all GPS/1PPS signals are phase coherent because 
they are all synchronized to UTC. So usually the question is not if they 
are coherent but how close. And not just how close, but how close for 
how long.


> Where does the PPS offset come from?

For GPSDO that report that sort of thing, it's the time interval between 
the GPS-output 1PPS and the OCXO-derived 1PPS. GPS tends to vary a lot 
from a few ns to a much as tens of ns, but this averages out very well 
over time. The OCXO hardly varies at all short-term but drifts and 
wanders over the long-term. The purpose of a GPSDO is to meld the 
stability (but inaccuracy) of quartz and the accuracy (but instability) 
of GPS.


> Isn't it from the positioning error?

No. In fact many GPSDO run in so-called zero-D mode where the antenna 
position is hardwired into the receiver.


> Typical GPS receivers have 1-3 m of positioning error which should give
> you +- 10 ns. Why is this a "dream" performance?
> It should be expected from any modern GPS receiver.

I don't see where Bob used the word "dream" or "10 ns". Your 
measurements and my plots linked above show the Fury can do ±10 ns. If 
you are expecting 3-4 degrees at 150 MHz then that's ±50 ps so a Fury 
GPSDO is 200× away from your goal.


/tvb
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[time-nuts] Re: Phase coherence with 2x GPSDO

[Michael Wouters]

Dear Krishna

You should probably check with the manufacturer exactly how one should
interpret  eg SYNC:FEE?
The manual says:
"This query returns the Frequency Error Estimate."
but is this a offset due to eg a DAC resolution limitation, or is it
the uncertainty in the frequency?
I am guessing it is the latter.

Regards

On Mon, Mar 7, 2022 at 5:01 AM Krishna Makhija  wrote:
>
> Hello,
>
> I am new to the whole precision time-keeping game (and to this mailing
> list) so I apologize in advance if my question is too naive or has been
> answered already in your mailing list.
>
> Is it possible to have two separate GPSDOs, each with their own antennas,
> be phase coherent to each other? I have a Jackson-Labs Fury
>  and a Mini-JLT
> . I am using each to
> provide a 10 MHz reference to two separate software-defined radios (SDRs).
> In my tests I find that the phase offset between said SDRs has a slow
> time-varying behavior. I know the frequency errors of the GPSDOs are of the
> order of parts per trillion which will show up as slow time-varying phase
> offsets but I was hoping to use the PPS offsets and instantaneous frequency
> errors that I get from these modules (using SCPI commands) to be able to
> "back out" or predict what that time-varying phase offset would be. Is such
> a thing possible? Currently, the time-varying phase change does not seem to
> follow any discernible pattern and my attempts at backing out the phase
> change do not match my measurements.
>
> Here is the math I am using for calculating what I *think *the phase
> *should* be:
> [image: image.png]
> [image: image.png]
>
> [image: image.png]
> Does any of this seem sensible? Any input is appreciated.
>
> TL;DR: Trying to get phase coherence between two separate GPSDOs may not be
> possible but can you use PPS offsets and frequency errors metadata to
> correct for it in post?
>
> Regards,
> Krishna
> ___
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[time-nuts] Re: Phase coherence with 2X GPSDO

[ed breya]

I actually have this kind of setup, with two "identical" GPSDO boards I 
salvaged from some telecom gear long ago. I've yet to be able to 
identify them by make or model. All I know is they use Oncore UT or GT 
RX units, and seem to work, according to the LED indicators and the 10 
MHz outputs.


I built them into one box with a common supply, and common RF in from a 
Wilkinson splitter, fed from an active antenna. When I first got it all 
fired up and working, I was disappointed that the outputs drift in 
relative phase. Sometimes they happen to remain close for long times, 
and  sometimes they obviously slide quickly enough to watch the shift 
over say a half hour or so, observed on a scope. The direction can go 
either way at any time. After I thought about it some, I realized that 
they can't be perfectly aligned, even if there were no sawtooth errors 
from each one's independent situation and activity.


I think the basic limit besides the GPS processing and noise, is the 
discreteness of the frequency control. They use 20 bit DACs for tuning, 
which seems pretty good, but the overall effect of two separate systems, 
each trying to perfect its own deal according to the GPS process, to 
compensate for temperature and their individual characteristics, just 
leads to a certain amount of relative drift. I think the times when they 
seem very stable are when everything happens to be just right, but 
mostly that's not the case. Presuming they actually are working right, 
over the long term, they should both agree on the average frequency, but 
not moment by moment.


So, this is all with two same-type units in the same environment 
conditions, more or less, and getting the same GPS signal, more or less. 
Imagine different units in different locations and conditions.


Ed
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[time-nuts] Re: Phase coherence with 2x GPSDO

[Lux, Jim]

On 3/6/22 1:36 PM, Krishna Makhija wrote:

Hello Tom,

Yes, the GPSDOs are working well. However, when I use each as a reference
to a separate radio, I find there is a slow phase change over time between
said radios. I imagine this is expected since there will always be some
error between two discrete oscillators. However, I am hoping to use the PPS
and FEE metadata to compute what the phase *should* have been in
post-processing. So far, it is not working out for me. I am wondering if
that is even possible or if my math is just wrong.

Bob,

The SDRs have an LO running at 150 MHz (~6.66 ns) so a PPS wander of +- 10
ns is >360 deg. With a common-mode reference I see a small phase change (+-
3-4 deg) but that is not an option for my application.

Where does the PPS offset come from? Isn't it from the positioning error?
Typical GPS receivers have 1-3 m of positioning error which should give
you +- 10 ns. Why is this a "dream" performance? It should be expected from
any modern GPS receiver.

Thanks for your inputs so far.

Krishna


The PPS offset can come from multiple sources - At some point, the GPS 
receiver has an oscillator and counts down to generate the PPS, so any 
given PPS transition is going to be synced to the edges of that internal 
oscillator.  That leads to things like hanging bridges and wander 
between the time quantization of the PPS and the actual GPS epoch.  A 
typical spec is that the 1pps is within 20-50 ns of "true" (e.g. 
corresponding to an internal clock of 50-20 MHz), but the statistics of 
that error are dependent on the receiver.


Very few GPS receivers these days actually lock an internal oscillator 
to the GPS signal - rather they compute observables like code phase and 
epoch phase in terms of the local reference, then compute what the 1pps 
should be (solving for position, etc.) and then set a register so that 
the receiver puts out a pulse at the right time.  On *some* receivers, 
they'll put out estimates of the difference between the 1pps time and 
their current estimate of code "true" 1pps.  After all, the actual 
signal from the satellites are all skewed from each other, so the "true 
epoch" is a mathematical construct.


You've got two 1pps sources - so as a first step, you might hook them up 
to a counter and look at their relative performance.


Indeed, if you are not moving, and you've got two receivers side by side 
(so ionospheric effects, and solid earth tides, and all the other "less 
than a meter" phenomena are common) you should be able to get a 
*calculated* uncertainty in the single digit nanoseconds between the 
receivers.


But you don't just have receivers - you've got GPSDOs, so what you're 
seeing is an oscillator, divided down to 1Hz, with the oscillator pushed 
around to match GPS.  The 1pps from this will have different statistics 
than a 1pps from a bare receiver.  Think of it as the 1pps from GPS 
(with its 20-50 ns somewhat uniformly distributed uncertainty) run 
through a not necessarily linear filter.


You sort of have two choices in front of you:

Understand the nature of the GPSDO filters and be able to de-embed their 
contribution algorithmically - this is not easy.


Measure the instantaneous output of the GPSDO against GPS observables, 
and use some post processing to figure out what it really is. Then you 
can remove the delta between "true" estimated time and "GPSDO estimated 
time"




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[time-nuts] Re: Phase coherence with 2x GPSDO

[Krishna Makhija]

Hello Tom,

Yes, the GPSDOs are working well. However, when I use each as a reference
to a separate radio, I find there is a slow phase change over time between
said radios. I imagine this is expected since there will always be some
error between two discrete oscillators. However, I am hoping to use the PPS
and FEE metadata to compute what the phase *should* have been in
post-processing. So far, it is not working out for me. I am wondering if
that is even possible or if my math is just wrong.

Bob,

The SDRs have an LO running at 150 MHz (~6.66 ns) so a PPS wander of +- 10
ns is >360 deg. With a common-mode reference I see a small phase change (+-
3-4 deg) but that is not an option for my application.

Where does the PPS offset come from? Isn't it from the positioning error?
Typical GPS receivers have 1-3 m of positioning error which should give
you +- 10 ns. Why is this a "dream" performance? It should be expected from
any modern GPS receiver.

Thanks for your inputs so far.

Krishna

On Sun, Mar 6, 2022 at 4:30 PM Bob kb8tq  wrote:

> Hi
>
> They *are* phase coherant to within 360 * 100/ 10 = 36 degrees. You
> can get them to *maybe* ten degrees with this and that done here or there.
>
> If you want them within a degree, no you can’t do that directly with GPS.
> If your definition of phase coherent is zero degrees, a pair of  SDR’s off
> the
> same buffered clock will have issues with that definition in the real
> world
> of temperature wandering around …..
>
> Bob
>
> > On Mar 6, 2022, at 2:04 PM, Krishna Makhija  wrote:
> >
> > Hi Bob,
> >
> > I am currently getting +- 10 ns nominal. The antennas are currently
> almost
> > next to each other (roughly 1-2 inches apart). Yes, they should be
> outside
> > of each of their farfield zones. Here is the PPS offset I am seeing
> during
> > measurement:
> > [image: image.png]
> > And this is the frequency error I am seeing:
> > [image: image.png]
> > Overall, the GPSDOs seem to work pretty well. But the question still
> > remains if one can hope to get them to be phase coherent, either in
> > real-time or in post-processing.
> >
> > Jeremy: I bought these items by writing to them. I chose to place a
> > purchase order (since I did it through my organization), but you might be
> > able to order by talking to them directly and paying using a credit card.
> >
> > On Sun, Mar 6, 2022 at 1:51 PM Bob kb8tq  wrote:
> >
> >> Hi
> >>
> >> How close are you trying to get?
> >>
> >> How far apart are the GPSDO’s?
> >>
> >> A “run of the mill” number would be out around 100 ns. A “pretty good”
> >> number is in the 20 ns range. A “crazy good” number would be 2 ns. To
> >> do better than this, you likely would need to go to a more exotic
> >> configuration
> >> on the GPSDO.
> >>
> >> Bob
> >>
> >>> On Mar 6, 2022, at 12:55 PM, Krishna Makhija 
> wrote:
> >>>
> >>> Hello,
> >>>
> >>> I am new to the whole precision time-keeping game (and to this mailing
> >>> list) so I apologize in advance if my question is too naive or has been
> >>> answered already in your mailing list.
> >>>
> >>> Is it possible to have two separate GPSDOs, each with their own
> antennas,
> >>> be phase coherent to each other? I have a Jackson-Labs Fury
> >>>  and a Mini-JLT
> >>> . I am using
> each
> >> to
> >>> provide a 10 MHz reference to two separate software-defined radios
> >> (SDRs).
> >>> In my tests I find that the phase offset between said SDRs has a slow
> >>> time-varying behavior. I know the frequency errors of the GPSDOs are of
> >> the
> >>> order of parts per trillion which will show up as slow time-varying
> phase
> >>> offsets but I was hoping to use the PPS offsets and instantaneous
> >> frequency
> >>> errors that I get from these modules (using SCPI commands) to be able
> to
> >>> "back out" or predict what that time-varying phase offset would be. Is
> >> such
> >>> a thing possible? Currently, the time-varying phase change does not
> seem
> >> to
> >>> follow any discernible pattern and my attempts at backing out the phase
> >>> change do not match my measurements.
> >>>
> >>> Here is the math I am using for calculating what I *think *the phase
> >>> *should* be:
> >>> [image: image.png]
> >>> [image: image.png]
> >>>
> >>> [image: image.png]
> >>> Does any of this seem sensible? Any input is appreciated.
> >>>
> >>> TL;DR: Trying to get phase coherence between two separate GPSDOs may
> not
> >> be
> >>> possible but can you use PPS offsets and frequency errors metadata to
> >>> correct for it in post?
> >>>
> >>> Regards,
> >>> Krishna
> >>>
> >>
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> >>> time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe
> >> send an email to time-nuts-le...@lists.febo.com
> >>> To unsubscribe, go to and follow the instructions there.
> >> ___
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[time-nuts] Re: Phase coherence with 2x GPSDO

[Bob kb8tq]

Hi

They *are* phase coherant to within 360 * 100/ 10 = 36 degrees. You
can get them to *maybe* ten degrees with this and that done here or there. 

If you want them within a degree, no you can’t do that directly with GPS. 
If your definition of phase coherent is zero degrees, a pair of  SDR’s off the 
same buffered clock will have issues with that definition in the real world 
of temperature wandering around …..

Bob

> On Mar 6, 2022, at 2:04 PM, Krishna Makhija  wrote:
> 
> Hi Bob,
> 
> I am currently getting +- 10 ns nominal. The antennas are currently almost
> next to each other (roughly 1-2 inches apart). Yes, they should be outside
> of each of their farfield zones. Here is the PPS offset I am seeing during
> measurement:
> [image: image.png]
> And this is the frequency error I am seeing:
> [image: image.png]
> Overall, the GPSDOs seem to work pretty well. But the question still
> remains if one can hope to get them to be phase coherent, either in
> real-time or in post-processing.
> 
> Jeremy: I bought these items by writing to them. I chose to place a
> purchase order (since I did it through my organization), but you might be
> able to order by talking to them directly and paying using a credit card.
> 
> On Sun, Mar 6, 2022 at 1:51 PM Bob kb8tq  wrote:
> 
>> Hi
>> 
>> How close are you trying to get?
>> 
>> How far apart are the GPSDO’s?
>> 
>> A “run of the mill” number would be out around 100 ns. A “pretty good”
>> number is in the 20 ns range. A “crazy good” number would be 2 ns. To
>> do better than this, you likely would need to go to a more exotic
>> configuration
>> on the GPSDO.
>> 
>> Bob
>> 
>>> On Mar 6, 2022, at 12:55 PM, Krishna Makhija  wrote:
>>> 
>>> Hello,
>>> 
>>> I am new to the whole precision time-keeping game (and to this mailing
>>> list) so I apologize in advance if my question is too naive or has been
>>> answered already in your mailing list.
>>> 
>>> Is it possible to have two separate GPSDOs, each with their own antennas,
>>> be phase coherent to each other? I have a Jackson-Labs Fury
>>>  and a Mini-JLT
>>> . I am using each
>> to
>>> provide a 10 MHz reference to two separate software-defined radios
>> (SDRs).
>>> In my tests I find that the phase offset between said SDRs has a slow
>>> time-varying behavior. I know the frequency errors of the GPSDOs are of
>> the
>>> order of parts per trillion which will show up as slow time-varying phase
>>> offsets but I was hoping to use the PPS offsets and instantaneous
>> frequency
>>> errors that I get from these modules (using SCPI commands) to be able to
>>> "back out" or predict what that time-varying phase offset would be. Is
>> such
>>> a thing possible? Currently, the time-varying phase change does not seem
>> to
>>> follow any discernible pattern and my attempts at backing out the phase
>>> change do not match my measurements.
>>> 
>>> Here is the math I am using for calculating what I *think *the phase
>>> *should* be:
>>> [image: image.png]
>>> [image: image.png]
>>> 
>>> [image: image.png]
>>> Does any of this seem sensible? Any input is appreciated.
>>> 
>>> TL;DR: Trying to get phase coherence between two separate GPSDOs may not
>> be
>>> possible but can you use PPS offsets and frequency errors metadata to
>>> correct for it in post?
>>> 
>>> Regards,
>>> Krishna
>>> 
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[time-nuts] Re: Phase coherence with 2x GPSDO

[Lux, Jim]

On 3/6/22 10:48 AM, Bob kb8tq wrote:

Hi

How close are you trying to get?

How far apart are the GPSDO’s?

A “run of the mill” number would be out around 100 ns. A “pretty good”
number is in the 20 ns range. A “crazy good” number would be 2 ns. To
do better than this, you likely would need to go to a more exotic configuration
on the GPSDO.



it also depends on whether you can "post process"  as in a receiving 
array application.  For instance, you can let the GPSDO do what it 
wants, but you record the GPS observables against your oscillator for 
each one.  Then, you post process to determine what the GPSDO actually did.


We are doing that for the SunRISE interferometer mission - 6 independent 
SDRs digitizing the signals, with a GNSS receiver running off the same 
clock.  A sort of rudimentary disciplining ensures that all the 
receivers take samples at about the same time, but the fine adjustment 
is done later.  In our case, we are processing the GNSS and clock data 
through GIPSYx, which gives you a time offset and rate at periodic time 
ticks.


So, though we capture data and run everything with (six different) 50 
MHz clocks (20ns resolution), in post processing, we can get down below 
1 ns in ultimate uncertainty. And the data acquisition only needs to be 
within a microsecond (we capture 655.36 microseconds of data, and being 
"off" by a microsecond is only reducing the section with an overlap by 
1/655th)


The trick with SDRs, in general, is *really understanding* the clock 
distribution and processing.  If there's PLLs in the pathway, then you 
need to worry about whether there are uncertainties due to initial 
state, even if the frequency of the oscillator is known perfectly.  For 
example, say you had 2 receivers with a 10MHz oscillator each, and you 
capture data at 100 kHz,  There's 100 possible offsets between the 
samples, depending on the state of the 100:1 divider for the ADC clock.


When you have FPGAs with DPLLs in them to cross clock boundaries, you 
need to be really careful that you understand what's going on there.



In your case, you seem to have continuous recording, so maybe what 
you're seeing is the small scale independent frequency variations of the 
two oscillators.  A GPSDO affects the frequency at say, 10-1000 seconds 
and longer. There will be short term variations on time scales less than 
100 seconds that the GPSDO will likely do nothing about.


If it's possible, what helps is having a free running counter running 
off your oscillator, and you "snapshot" that on each GPS 1pps. That way, 
you are getting a more direct measurement of the oscillator frequency, 
which you can then model/smooth and back out.





Bob


On Mar 6, 2022, at 12:55 PM, Krishna Makhija  wrote:

Hello,

I am new to the whole precision time-keeping game (and to this mailing
list) so I apologize in advance if my question is too naive or has been
answered already in your mailing list.

Is it possible to have two separate GPSDOs, each with their own antennas,
be phase coherent to each other? I have a Jackson-Labs Fury
 and a Mini-JLT
. I am using each to
provide a 10 MHz reference to two separate software-defined radios (SDRs).
In my tests I find that the phase offset between said SDRs has a slow
time-varying behavior. I know the frequency errors of the GPSDOs are of the
order of parts per trillion which will show up as slow time-varying phase
offsets but I was hoping to use the PPS offsets and instantaneous frequency
errors that I get from these modules (using SCPI commands) to be able to
"back out" or predict what that time-varying phase offset would be. Is such
a thing possible? Currently, the time-varying phase change does not seem to
follow any discernible pattern and my attempts at backing out the phase
change do not match my measurements.

Here is the math I am using for calculating what I *think *the phase
*should* be:
[image: image.png]
[image: image.png]

[image: image.png]
Does any of this seem sensible? Any input is appreciated.

TL;DR: Trying to get phase coherence between two separate GPSDOs may not be
possible but can you use PPS offsets and frequency errors metadata to
correct for it in post?

Regards,
Krishna
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[time-nuts] Re: Phase coherence with 2x GPSDO

[Bob kb8tq]

Hi

How close are you trying to get?

How far apart are the GPSDO’s?

A “run of the mill” number would be out around 100 ns. A “pretty good” 
number is in the 20 ns range. A “crazy good” number would be 2 ns. To 
do better than this, you likely would need to go to a more exotic configuration 
on the GPSDO. 

Bob

> On Mar 6, 2022, at 12:55 PM, Krishna Makhija  wrote:
> 
> Hello,
> 
> I am new to the whole precision time-keeping game (and to this mailing
> list) so I apologize in advance if my question is too naive or has been
> answered already in your mailing list.
> 
> Is it possible to have two separate GPSDOs, each with their own antennas,
> be phase coherent to each other? I have a Jackson-Labs Fury
>  and a Mini-JLT
> . I am using each to
> provide a 10 MHz reference to two separate software-defined radios (SDRs).
> In my tests I find that the phase offset between said SDRs has a slow
> time-varying behavior. I know the frequency errors of the GPSDOs are of the
> order of parts per trillion which will show up as slow time-varying phase
> offsets but I was hoping to use the PPS offsets and instantaneous frequency
> errors that I get from these modules (using SCPI commands) to be able to
> "back out" or predict what that time-varying phase offset would be. Is such
> a thing possible? Currently, the time-varying phase change does not seem to
> follow any discernible pattern and my attempts at backing out the phase
> change do not match my measurements.
> 
> Here is the math I am using for calculating what I *think *the phase
> *should* be:
> [image: image.png]
> [image: image.png]
> 
> [image: image.png]
> Does any of this seem sensible? Any input is appreciated.
> 
> TL;DR: Trying to get phase coherence between two separate GPSDOs may not be
> possible but can you use PPS offsets and frequency errors metadata to
> correct for it in post?
> 
> Regards,
> Krishna
> ___
> time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an 
> email to time-nuts-le...@lists.febo.com
> To unsubscribe, go to and follow the instructions there.
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