Consider something like the collection of samples from a PPS. Assume ntpd is not running so the system clock is not bouncing around.
There is some noise with each sample. If you collect several samples, you can average them to get a better number. You probably want 2 numbers rather than 1, a straight line fit, time offset and frequency offset, rather than just a simple average time offset. But if you average over too many samples, the temperature will change or the crystal on your system will drift with age. There is a sweet spot in the middle between too few samples and too many. The graph of goodness vs averaging time is called the Allan Deviation. https://en.wikipedia.org/wiki/Allan_variance http://www.leapsecond.com/pages/adev/adev-why.htm http://www.leapsecond.com/pages/adev-avg/ -------- You can stand on your head, or turn things inside out, or ... If the PC clock is "good", you can collect data on a less good external time source. If your external time source is good, you can collect data on your PC clock. -------- Another way of looking at this problem is that you want your PLL to filter out the high frequency noise, but let the low frequency drift through so the PLL can fix it. There is a sweet spot on the filter bandwidth or time constant. That's at the bottom of the V of the ADEV graph. -------- That all assumes things are nice in a mathematical sense. Normal distribution and such. If you are working with a PC, the lab temperature can change or the PC temperature can change when the CPU does some work. Optimizing the polling interval for good conditions may (will?) not work well when conditions are not nice. -- These are my opinions. I hate spam. _______________________________________________ devel mailing list devel@ntpsec.org http://lists.ntpsec.org/mailman/listinfo/devel
