[time-nuts] My TICC came in the mail yesterday
John Ackermann N8UR
jra at febo.com
Fri Feb 24 19:08:23 EST 2017
Hi Andrew --
There seems to be more than a little magic involved in getting sane
three-corner measurements. I've gotten best results when the run is
long enough to have many data points per tau, and also that results when
you're noise limited tend to go imaginary. Finally, I think things work
best when the three sources have similar noise processes, e.g., looking
at 3x OCXO or 3x Rb or whatever.
I'd love any of the experts to jump in on this, as I've not done much
beyond basic experiments, and I don't have the maths to comprehend the
On 02/24/2017 12:59 PM, Andrew Rodland wrote:
> Which means, after a bit of scrounging for some BNC to SMA adapters, I
> have some plots worth using (more or less) of my clock!
> Background info: my clock's main purpose is to be a GPS-disciplined
> NTP server on an Arduino Due clone board. As such, accuracy beyond
> tens to hundreds of microseconds isn't really relevant. But for purely
> time-nut reasons, it has an Rb oscillator (cheap surplus X72), and for
> similar reasons it has a PPS output generated by the CPU timer. I
> didn't hack the Due apart to replace the crystal, so the CPU clock
> (84MHz) is asynchronous from the Rb, which has some limitations, but
> also introduces a nice little bit of dither
> The TICC is set up with 10MHz from a Spectracom NetClock, chA from a
> (probably insufficiently thermally stabilized) LTE-Lite, and chB from
> my clock. Output is in Timelab mode.
> A representative bit of the phase plot: http://i.imgur.com/cRXv9ia.png
> You can see all the quantization noise on my clock, but also that in
> the ~100s region, it does better than the LTE-Lite. You can also see
> the nice smooth (at short time) plot of the LTE-Lite which gives me
> some good faith in the TICC.
> ADEV plot so far: http://i.imgur.com/DLb15rt.png
> Timelab loses the thread a little bit and comes up with negative
> computed deviations for my clock for some tau. Not sure how much of
> that is due to instrument limitations, and how much is due to the
> noise being not-really-independent, since all three clocks are GPS
> receivers, with rather nearby antennas. Still, more than I've ever
> seen before!
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