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

[Tim Shoppa]

Lacking a GPS simulator (do any of us have one?) for testing loop response,
it is very interesting to do a "Holdover test". Unhook or block the GPS
antenna for an hour or 24 hours, hook it back up, and make a graph of time
offset and tuning word as it recovers.

If you see the tuning word (this is what Z3801 owners call the "EFC"
number) doing something very far from optimal damping, then you can start
tweaking PLL parameters.

Tim N3QE

On Sun, Sep 14, 2014 at 4:29 AM, 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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