[time-nuts] Cheap jitter measurements

[Tom Van Baak]

>> Similarly, the box should be able to give me a pulse at a known time.
>
> how do you set up NTP to do that?

Don't know. That's not NTP's job. Any process that can query system time and get/set a GPIO bit will do. The question to be answered is how close to the real time (as in UTC(k), atomic clocks, GPS, etc.) is the fake time running inside the OS / CPU. The way you determine that is to send the fake time out, and/or to send real time in. A low-latency or zero-jitter GPIO pin would be required in either case.

> 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. 

You don't need NTP for this experiment. That's kind of the idea. You run the PC / SBC / R-Pi plain. Or you run it with NTP. Or different versions of NTP or different configs. Or you run it with a better xtal, or you replace the xtal with a GPSDO and DDS. So this isn't intended to be a hack on NTP per-se; it's more of a scientific testbed that you can drop NTP into. You'd get a nice set of phase and ADEV / TDEV / MTIE plots or something. I don't know.

I don't use NTP so take this all with a grain of salt. But from the looks of it, people playing (or developing) NTP fall into the same trap as some GPSDO developers: a focus on the performance of the PLL or other fancy internal colorful plots instead of real measurements of rising edges of electrons at the input and output.

This was easy back in the peek/poke parallel port days. Took a backseat in the serial and USB era. But now that many systems have GPIO ports it should be possible again. Anyway, Gary and Leo, et al. can report eventually what they find. This isn't really an NTP mailing list, but I would think some of the basic concepts of metrology should apply to OS timing as they do to h/w timing.

/tvb