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 <kb...@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 <km...@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 <kb...@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 <km...@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 > > >>> > > >> > > > <image.png><image.png><image.png>_______________________________________________ > > >>> 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. > > >> _______________________________________________ > > >> 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. > > > <image.png><image.png>_______________________________________________ > > > 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. > > _______________________________________________ > > 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. > _______________________________________________ > 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. _______________________________________________ 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.
