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]> 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:;> > 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.
