Hi Ok, I’ll bite ….
> On Apr 8, 2018, at 3:36 PM, Tom Van Baak <t...@leapsecond.com> wrote: > >>>> What do you mean by "jitter" and what do you really want to do? >>> I mean jitter as NTP defines jitter. Whatever that is. >> >> I think you need to figure out what you want to do so you don't fool >> yourself. >> >> ntpd is a PLL. There is a low pass filter in the control loop. It will >> track the low frequency wander of the source. > > Gary, Hal, Leo, > > My mental model of a black box computer running NTP is that I should be able > to give it a pulse (e.g., via parallel, serial, GPIO) and it tells me what > time it was. Use a GPSDO / Rb / picDIV to generate precise pulses. Compare > the known time of the pulse with the time the box says it was. Repeat many > times, collect data, look at the statistics; just as we would for any clock. > > Similarly, the box should be able to give me a pulse at a known time. how do you set up NTP to do that? > In this case it records the time it thinks the pulse went out, and your GPSDO > / Rb / TIC makes the actual measurement. Again, collect data and look at the > statistics; just as we would for any clock. > > Imagine the black box has two BNC connectors; one accepts an input pulse to > be timed; one outputs a pulse at certain times. This allows a complete > analysis of NTP operation. Should be true for both client or server. If you > get down to nanosecond levels make sure to use equal length cables. > > To me this better than relying on NTP to tell you how NTP is doing, which as > far as I can tell from live plots on the web, is all that most people do. > Instead use real, external, physical measurement. The internal NTP stats are > fine for tracking the performance of the PLL, but don't confuse that with > actual timing. > > So this is why I'm excited to hear Gary wants a Rb timebase and a sub-ns > counter. Someone will finally measure NTP for real, not rely on the internal > numbers of NTP measuring itself. Or at least I hope that's what Gary is up to. In both cases (pulse in and pulse out) the first step is to ask NTP “when was that?”. You still have a pretty big chunk of NTP in the middle of the process …. If NTP only “knows” what is happening (or can control what is happening) to +/- 300 ns. The guts of your data will be limited to the same 300 ns. Bob > > /tvb > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
