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.