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 <[email protected]> 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 <[email protected]> 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 < >> [email protected]> >> 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 <[email protected]> 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 <[email protected]> >>> 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 <[email protected]> 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 <[email protected]> >>>> 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 >>>>>>> >>>>>> >>>> >>> >> <image.png><image.png><image.png>_______________________________________________ >>>>>>> time-nuts mailing list -- [email protected] -- To >> unsubscribe >>>>>> send an email to [email protected] >>>>>>> To unsubscribe, go to and follow the instructions there. >>>>>> _______________________________________________ >>>>>> time-nuts mailing list -- [email protected] -- To >> unsubscribe >>>> send >>>>>> an email to [email protected] >>>>>> To unsubscribe, go to and follow the instructions there. >>>>> <image.png><image.png>_______________________________________________ >>>>> time-nuts mailing list -- [email protected] -- To unsubscribe >>>> send an email to [email protected] >>>>> To unsubscribe, go to and follow the instructions there. >>>> _______________________________________________ >>>> time-nuts mailing list -- [email protected] -- To unsubscribe >>> send >>>> an email to [email protected] >>>> To unsubscribe, go to and follow the instructions there. >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] -- To unsubscribe >> send >>> an email to [email protected] >>> To unsubscribe, go to and follow the instructions there. >> _______________________________________________ >> time-nuts mailing list -- [email protected] -- To unsubscribe send >> an email to [email protected] >> To unsubscribe, go to and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send an > email to [email protected] > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
