Are all the devices you're using or considering capable of hardware timestamps? Or are you doing it in software today?
On Wednesday, September 24, 2014, Andrew Rodland <[email protected]> wrote: > Yes indeed, that board is dead. Luckily, though, I had a substitute > (see some conversation around Sep 10 on the other thread about > adapting a UDOO to do the job -- it's a board that has both the Due's > SAM3X and an i.MX chip running Linux, with serial and GPIO shared > between them) and I've made some slower progress using that, mostly > tweaking the control loop for smoother response, and improving the > health-monitoring / holdover logic. > > More excitingly, a board that I ordered from China even before killing > the EtherDue arrived yesterday. It's a Due clone called "Taijiuino" > that brings out the SAM3X's own Ethernet MAC pins, instead of using an > offboard Ethernet controller like the EtherDue does. I'm optimistic > that this will give me much finer control over packet timestamping and > lower the ethernet-induced NTP jitter by an order of magnitude or so, > which would really give me something to show for this project. The > only downside is I have to write the Ethernet driver first! Definitely > hoping to have something to report there. > > On Sun, Sep 14, 2014 at 5:35 PM, Shane Morris <[email protected] > <javascript:;>> wrote: > > I'll be looking for that blog post. By the way, how did the burnt out > > EtherDue go? I remember saying after you had taken your last set of > > pictures that you'd popped it...! > > > > > > On Mon, Sep 15, 2014 at 6:48 AM, Andrew Rodland <[email protected] > <javascript:;>> > > wrote: > > > >> Neil, > >> > >> I'm working on a blog post now, I'm hoping to have it complete by > >> Monday or Tuesday. I'll send a followup here when it's posted. > >> > >> On Sun, Sep 14, 2014 at 11:47 AM, Neil Schroeder <[email protected] > <javascript:;>> > >> wrote: > >> > I have nothing constructive to add at this time but I would truly > enjoy > >> > reviewing your design and build logs/notes. > >> > > >> > > >> > > >> > On Sunday, September 14, 2014, Andrew Rodland < > [email protected] <javascript:;>> > >> > wrote: > >> > > >> >> Hi all, > >> >> > >> >> I've got some figures in from my clock, and I figured I would post > >> >> them here in hopes of getting some eyes on them and some help with > >> >> interpretation. > >> >> > >> >> Reference is a Spectracom NetClock 9183 with OCXO option. Frequency > is > >> >> good to better than 10^-9, PPS is specified as +/- 50ns. > >> >> > >> >> Instrument is an HP 5335A (in lovely condition given that it was > built > >> >> in 1985 according to the serial number) in time difference mode. > >> >> > >> >> My clock is quantized at 10MHz, so you wouldn't expect better than > >> >> 100ns accuracy. But I added -50ns to the offset in software, making > it > >> >> zero in on the edge where the offset is 0 counts 50% of the time and > >> >> -1 count the other 50%. (Dithering provided by noise in the system > and > >> >> the Resolution-T's own sawtooth). This seems to have worked better > >> >> than expected. > >> >> > >> >> (On a side note: this means that the gain of my control loop is > >> >> obviously pretty non-linear inside of 1us. Anything I should read > >> >> about that?) > >> >> > >> >> So far I've done two 24-hour runs, one with PLL and FLL constants at > >> >> 3600s, and one with them at 7200s. > >> >> > >> >> Phase plot: > >> >> 3600s: http://i.imgur.com/LLfYgXe.png > >> >> 7200s: http://i.imgur.com/zUbgNHc.png > >> >> > >> >> Both keep within +/-20ns the majority of the time, which is better > >> >> than I expected given the specs of both clocks. 1us offset is > >> >> deliberately added at the PPS output of my clock to make the 5335A > >> >> happy. > >> >> > >> >> Frequency plot: > >> >> 3600s: http://i.imgur.com/7GoXdoF.png and > >> http://i.imgur.com/rjBa7gf.png > >> >> 7200s: http://i.imgur.com/KcyGT3r.png and > >> http://i.imgur.com/GZH4Pcl.png > >> >> > >> >> Both have similar envelopes that seem to reflect the quantization > more > >> >> than anything (100s averaging shrinks the envelopes by very close to > a > >> >> factor of 100x). 7200s looks like it has some kind of oscillation > with > >> >> 2000s period, which is worth looking into. > >> >> > >> >> MDEV: > >> >> 3600s: http://i.imgur.com/RmAcAwT.png > >> >> 7200s: http://i.imgur.com/xO7aYf9.png > >> >> > >> >> ADEV was a perfectly straight line so I didn't bother. MDEV displays > a > >> >> little more structure, but I'm not really clear on the > interpretation. > >> >> > >> >> TDEV both: http://i.imgur.com/YamRIui.png > >> >> > >> >> I like TDEV. Same information as MDEV but since it turns slope=-1 to > >> >> slope=0 it makes this kind of graph more readable. The two plots are > >> >> within each other's error bars, so any difference between them might > >> >> be imaginary, but they depart at 1000s, which probably corresponds to > >> >> the 2000s oscillation. > >> >> > >> >> I guess I'm seeking general input on where I should go next -- do the > >> >> graphs tell me anything interesting? Should I keep working on the > >> >> control loop even though it already manages to keep things within > half > >> >> a clock tick? Or should I start looking for ways to reduce the > >> >> Ethernet jitter since that's the dominant source of error in the use > >> >> that I care about? > >> >> > >> >> Andrew > >> >> _______________________________________________ > >> >> time-nuts mailing list -- [email protected] <javascript:;> > <javascript:;> > >> >> To unsubscribe, go to > >> >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > >> >> and follow the instructions there. > >> > _______________________________________________ > >> > time-nuts mailing list -- [email protected] <javascript:;> > >> > To unsubscribe, go to > >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > >> > and follow the instructions there. > >> _______________________________________________ > >> time-nuts mailing list -- [email protected] <javascript:;> > >> To unsubscribe, go to > >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > >> and follow the instructions there. > >> > > _______________________________________________ > > time-nuts mailing list -- [email protected] <javascript:;> > > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- [email protected] <javascript:;> > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
