In message <[EMAIL PROTECTED]>, 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. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
