On Tue, 31 Oct 2017 21:03:05 +0000 Leo Bodnar <l...@leobodnar.com> wrote:
> The goal was maximum throughput with minimum time offset. > Maximum throughput eventually ended up as "fully saturated full-duplex > 100BASE-TX" and minimum time offset as "below 1 microsecond" > There was nothing on the market below £2-3k that could do that. I think > Microsemi has recently made a server that can do 100kpps+ but I don't know > its price. Hmm? There are at least a dozen how-tos out there that explain how to make an NTP server out of an SBC. And I have seen the one or other being sold as a complete box with batteries included. Basically, all you have to do is use an SBC that runs linux and has a GPIO with an interrupt to act as a PPS input. Attach a GPS receiver and you are almost done. The cheapest option are probably the i.MX233 based ones (go as low as €20). The probably most mentioned option is using a Beaglebone Black. If I had to build something like this today, I would probably go for an OSD3358, which is an AM3358 packaged with memory and power management and allows using a simple 4 layer board. Add a few bits for ethernet and the GPS and you are almost done. > I do want to improve my NTP devices but I do not understand what you are > suggesting. > Why would sawtooth correction matter when there is no GPS signal available at > all? It matters while you have signal. > I am not measuring any frequencies - the whole device runs synchronously hard- > locked to GPS time when it is available and freewheeling when not. You should have a control loop somewhere, which explicitly or implicitly estimates the frequency of the TCXO. The time-nuts archives are full with discussions how to do such control loops and improve hold over performance. Though there weren't many in the last 2-3 years. John Vigs tutorial is also a good start. > Are you saying that if you deprive any PC of any connectivity it will drift > by 4-5ms in 24 hours? Almost. It has to have ntp running and ntp must have had time to discipline the local oscillator. If the PC is then in an environment that will not cause its oscillator to drift more than 10-100ppb per day, then it will stay below 10ms. There are a few ifs there, but it's nothing out of the ordinary. Even ordinary crystal oscillators can be quite stable if they have been running for a while. Just for comparison: decent wrist watches drift less than 1min in half a year. Good ones less than 10s. Attila Kinali -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson _______________________________________________ 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.