Pity the poor man who has (n>1) clocks, for he knows not what time it is.
Dana On Sun, Apr 8, 2018 at 4:29 PM, John Ackermann N8UR <[email protected]> wrote: > I want to jump on Tom's post, and Bob's note at 1:14 on Saturday (that > begins with "Just to be very clear..." They both raise an important point > about measurements. > > With both NTP and GPSDO measurements a lot of folks focus heavily on what > the "black box" is reporting about itself. But self-contained measurements > are really unrelated to actual performance. > > As Bob mentioned, in a GPSDO you can look at tempco, humidco, voltageco, > and all sorts of other things but the overall point of the system is to > make those meaningless: the control loop(s) compensate for them. If those > internal error generators are reduced, it may make the system's work > easier, but that improvement will have no effect on the quality of the > output if the control loop is already properly compensating for it. > > And in NTP, the software reports all sorts of interesting measurements, > but none of them really tell you how close the computer's clock is to a > local reference. As Tom said, the real test is how the time tick coming > out of the box compares with the time tick going into it. > > The bottom line is that no self-contained measurement can tell you actual > performance. The *only* way to do that is to compare your box with an > external reference whose error bounds are known. > > After all, this is why we're time-nuts -- every time you acquire a clock, > you also need to acquire a better clock to test it with. :-) > > John > ---- > > On 04/08/2018 03:36 PM, Tom Van Baak 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. 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. >> >> /tvb >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/m >> ailman/listinfo/time-nuts >> and follow the instructions there. >> >> _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/m > ailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
