On 07/23/2011 12:07 AM, Bob Camp wrote:
Hi
The easy way is to take a pps off of your external oscillator and feed that
into a port on your NTP server. Let NTP tell you where that pps is. Don't let
NTP lock to the pps, just let it report it's position.
After that all you need to do is write some code to read the location of the
pulse and implement a *long* time constant loop. Taking the 1 ms number and a
1x10^-10 goal, the time constant would need to be around 4 months. There are a
few minor details about drift of the local reference and how valid 1 ms is over
long time periods. A reasonable GPSDO will likely be much more stable and much
more accurate.
To get it into the range of being practical, you have to get the NTP setup into
single digit microseconds. In the us range you still would not beat the GPSDO,
but at least you would have a useful device. 1 us can be done on a LAN with
NTP. It's tough to do better than 1 ms over the net. Since PTP suffers the same
issues over the net that NTP does, it's not a lot of help in this situation.
There are people already hacking an OCXO into the system clock of their
NTP server.
Now, look at the correction log (frequency and phase) of the system,
low-pass filter that and use it to steer a secondary loop steering the
oscillator. It should be fairly easy and not require much of hacking to
achieve.
Cheers,
Magnus
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