[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit

[Poul-Henning Kamp]

In message <20061219.235925.2053155698.cfmd at bredband.net>, Magnus Danielson wri
tes:

>It actually uses the CPU builtin counter, which
>will do for the purpose. They could have spent a little more and got better
>single shot resolution out of it, but I suspect they didn't see the need.

They are limited by digital noise inside the box.

I used the "vernier trick" we talked about yesterday on the PPS
input of my first PRS10, trying to determine if the calibration was
still OK.

It was quite obvious that there were 10 MHz overtone noise affecting
the PPS timestamping.  There were both missing codes and jumps
corresponding to at least the 3rd and 5th overtones.

Interestingly, when you take an Oncore PPS and hook it up to the
PPS on a PRS10, and transport the negative sawtooth over the serial
port (early PRS10 firmwares cannot do this) you get incredibly good
performance, because the sawtooth "dithers" the systematics of the
PRS10 out of the picture.

If you feed the PRS10 with a 1PPS derived from a Cesium, you may
end up worse because the input PPS signal parks itself on one of
the unlucky points in the noise-spectrum.

Noise like the oncore sawtooth isn't always a bad thing.

Dithering noise is a very important noise-cancellation technique,
in prof. software defined radios.

They add an analog version of a digitally generated PRNG signal to
the analog input signal, right before the A/D converter and then
subtract it right away again on the digital side in software.

That way any imperfections in the A/D converter gets spread out
over the frequency spectrum rather than appear as sharp spikes,
just like the GPS signal is spread out with a PRNG code.

But similar techniques are very useful in timing as well.

You can increase the resolution of time interval counter down to
the noise limit by adding a sinewave to your input signal on the
analog side remove it again from the counters result and average
long enough.

-- 
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