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

Andrew Rodland andrew at cleverdomain.org
Sun Sep 14 16:48:20 EDT 2014


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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