On 2014-03-18, Martin Burnicki <martin.burni...@meinberg.de> wrote: > Magnus Danielson wrote: >> On 17/03/14 09:48, Martin Burnicki wrote: >>> You'd need hardware (FPGA?) which can be clocked at 1 GHz, and even in >>> the hardware signal processing you'd need to account for a number of >>> signal propagation delays which you can eventually ignore at lower clock >>> rates. >>> >>> So of course the effort becomes much higher if you want more accuracy, >>> but this is always the case, even if you compare NTP to the "time" >>> protocol, or PTP to NTP. >> >> You don't need to count at 1 GHz, you can achieve the resolution with >> *much* lower frequencies. One pair of counters I have achieve 2,7 ps >> single-shot resolution using 90 MHz clock. Interpolators does the trick. >> There is many ways to interpolate. > > Agreed. I just thought the way to use a higher counter clock is more > obvious. All depends on how accurate and precise you can get your > timestamps, and this is probably easier with network packet timestampers > at both sides of a cable than with a wireless time transfer method like > GPS which usually suffers from delays which can't easily be measured, > like ionospheric delays. And yes, I know that this can be improved if > you receive 2 GPS frequencies instead of only the L1. ;-)
Unless it is a straight wire from one machine to the other, there are lots of unconstrained delays by wire as well-- all the switches, etc between you and the other end. Much worse than the ionosphere. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
