Charles Wyble wrote: > I’m using a raspberry pi with gps hat for my master time source. > Shortly I’ll be having a total of three systems (two using the same > hat, one using the adafruit hat and being a pi2). I’ve got some > interest in multiple way comparison and will follow this thread > shortly.
I'd say three doesn't really get you good enough visibility. It depends somewhat on how good your GPS reception is and how stable the environment, especially temperature. At around five NTP servers with suitable precision you start to see "interesting" things like asymmetric latency in your local network and can more easily throw out the inevitable spurs from degraded GPS reception (unless you have a really good antenna location).
I'd suggest you also log at least the PPS timestamps to correlate to the NTP logging. NTP peer logging will be dominated by network latency and jitter, provided you took care to tune the residual loop error to below 1µs. I'm running a Perl script that also records the CPU temperature and system utilization synchronized with the PPS. All my logging is into files at the moment, which puts some extra stress on the SD card that several no-name cards have not survived for long. I've salvaged an SSD that I plan to connect via an USB to SATA converter and then set up a proper time series database on one the boxes to feed all data into. Alternatively you could log into a tmpfs and rotate onto SD card whenever you've collected a full Flash block.
I currently have seven stratum-1 NTP servers (five different rasPi and two TinkerBoard) on my LAN. I've self-ovenized six of them (the exception is the rasPi 1B+, which simply isn't powerful enought to pull that off) to keep the crystal temperature very near the turnover point of the f vs. T curve, which leaves me with just the jitter and drift of the (apparent) system frequency most of the time. The rasPi crystals (or the interrupt system on the SoC) are a bit noisy with seemingly unprovoked frequency jumps on a not too-long timescale, so that keeps you to within a 5ppb window after removing the drift. The TinkerBoard doesn't have those jumps and I keep both routinely within 1ppb of the expected drift curve. I've experimented with both low and high thermal mass designs, but so far I don't see a difference in timing performance between the two. The high-thermal mass design does smooth out the external temperature swings more effectively, so with further refinements to the oven controller it might eventually provide a usable advantage.
-- Achim. (on the road :-) _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
