[time-nuts] Big Picture View

John Ackermann N8UR jra at febo.com
Wed Dec 12 13:48:11 EST 2007


Keith E. Brandt, M.D. wrote:
>    As a relative newbie to the who time-nut scene, I'm still working on
>    the big picture of how all the hardware fits together. I've
>    successfully put together a FreeBSD box with a Garmin 18 LVC as an NTP
>    server and am getting consistent microsecond accuracy. I've been
>    looking at the next step of adding a stable oscillator to the system.
>    I'm not seeing how to integrate this into the system.
>    The way I understand it, the oscillator is there to provide a very
>    stable frequency reference, regardless of the type used --> VXCO,
>    rubidium, cesium, etc. What I'm not seeing is how you link the
>    frequency reference to UTC. For NTP, I'm guessing that you'd have to
>    sync a PPS pulse with a known time source such as GPS. Am I on the
>    right track? Pointers to references appreciated.
>    73, Keith

Hi Keith --

Frequency isn't linked to UTC; phase is.  What you're doing with an 
external oscillator is simply providing the computer with a stable clock 
frequency.  The PPS signal from the GPS provides the phase information 
that tells NTP when the second marker is.  The stable clock frequency 
just makes it easier for NTP to hold the second marker at the proper point.

Even if the reference frequency is inaccurate (and that may be the 
question your asking), NTP is still better off as it can correct for a 
frequency offset more easily than it can a clock that is erratic.

Hope this helps.

73,
John



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