[time-nuts] Cheap jitter measurements
kb8tq at n1k.org
Sun Apr 8 16:05:41 EDT 2018
Ok, I’ll bite ….
> On Apr 8, 2018, at 3:36 PM, Tom Van Baak <tvb at 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.
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