[time-nuts] Designing an embedded precision GPS time

Leo Bodnar leo at leobodnar.com
Wed Nov 1 18:23:05 EDT 2017

Regarding spread spectrum issues:

You might be lucky to find (or have) SSCG implementation that is reasonably stable.  I suspect most are - because it is easy to generate hershey kiss spectrum based on SM, LUT or some sort of multi-level LFSRs.  I don't know what this means - these are some random Internet acronyms so it must be true.

I have several Linux workstations with system board crystals replaced with GPS synchronised synthesiser.  Instead of trying to disable SSCG or spending time analysing it I have tweaked system clock frequency until local time drift of the freewheeling system (with NTP setup with disabled time adjustment and clock correction) was less than 1 microsecond per day.  This took several days and needed better than 10^-11 level frequency adjustments and stability.  With GPS synchronisation this is easily achievable. If I keep NTP local time corrections disabled (basically remove all the servers from ntpd configuration) the local time drift is better than 0.01ppb.  Interestingly, to achieve that what used to be 26.000M crystal has ended up being a system clock that looks more like 26,000,001.7193Hz 


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