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

Andrew Rodland andrew at cleverdomain.org
Wed Sep 24 18:14:11 EDT 2014


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> 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>
> 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>
>> 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>
>> > 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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