[time-nuts] 1PPS for the beginner

[Chris Caudle]

On Mon, August 13, 2018 9:16 pm, Chris Burford wrote:
> I have a (generic?) GPSDO which contains an Oscilloquartz STAR 4+ OCXO
> that I am using to steer a PRS10 RFS. I'm a little confused on where the
> 1PPS is coming from with respect to the GPSDO.

A GPS disciplined oscillator contains a GPS receiver which outputs 1PPS
based on receiving the GPS signals and calculating the position  + time
equation. That PPS signal is noisy in time, it jitters around relative to
the ideal 1 second period.  The GPSDO implements a long time constant PLL
to synchronize the output of the OCXO to the long term average frequency
and phase of the GPS PPS, so what you see externally is 10MHz directly
from the OCXO, 1 Hz (PPS) which is divided down from the 10MHz OCXO, and
those are controlled by a PLL so that long term the phase of the PPS
divided down from the OCXO follows the PPS calculated by the GPS receiver,
but with lower jitter.

-- 
Chris Caudle