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

Andrew Rodland andrew at cleverdomain.org
Sun Sep 14 04:29:36 EDT 2014

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

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

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.

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?


More information about the time-nuts mailing list