[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 <km5es at virginia.edu> 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 <mattia.rizzi at gmail.com> 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 <
>> km5es at 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 <kb8tq at n1k.org> 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 <km5es at virginia.edu>
>>> 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 <kb8tq at n1k.org> 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 <km5es at virginia.edu>
>>>> 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
>>>>>>> <https://www.jackson-labs.com/index.php/products/fury> and a
>>> Mini-JLT
>>>>>>> <https://www.jackson-labs.com/index.php/products/fury>. 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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