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

Magnus Danielson cfmd at bredband.net
Tue Dec 19 18:43:15 EST 2006


From: "Poul-Henning Kamp" <phk at phk.freebsd.dk>
Subject: Re: [time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit
Date: Tue, 19 Dec 2006 23:17:03 +0000
Message-ID: <48601.1166570223 at critter.freebsd.dk>

> 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.

Ofcourse they are. It is bleeding obvious. If they would have cared more about
it they should not have stuck the PPS as another pin in that connector for
starters. But then again, I think it is sufficient for what I perceive to be
their market and more power to them for it.

> 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.

Interesting. Signal integrity is really crutial in acheiving the full potential
of such an interpolator.

> 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.

A little noise or signal into such non-linearities do improve quantization
noise. Known fact and still valid. 

> 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.

I.e. don't forget to phasemodulate up some jitter on the PPS! :D

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

Indeed.

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

... and in tones of audio-gear etc. etc. etc. and your GPS receiver!!!

Dithering is a technique one should learn to handle for quantization noise and
similar small-scale non-linearities. What type of measure which is quantized
this way is really of a lesser important thing since the concept and related
math is more or less the same. It is just a method, which sometimes applies
itself without the explicit intent.

> 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.

Actually, the thermal noise make a hell of a good quantization dithering
signal for the often 1.5 bit A/D. The integrate & dump is where there is bits
showing up.

> But similar techniques are very useful in timing as well.

Indeed.

> 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.

Indeed.

Cheers,
Magnus



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