[time-nuts] First few measurements of my Arduino Due GPSDO

Neil Schroeder gigneil at gmail.com
Thu Sep 25 07:34:04 UTC 2014


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 <andrew at cleverdomain.org>
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 <edgecomberts at gmail.com
> <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 <andrew at cleverdomain.org
> <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 <gigneil at gmail.com
> <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 <
> andrew at cleverdomain.org <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
> >> >> _______________________________________________
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