Configuring PPS with unsynchronised Rubidium Source

The equipment

In the specific case of the author, a high quality Rubidium frequency source was employed by RF engineers as a stable 10MHz signal for improving stability of various RF equipment. In order to provide my PCs with a stable PPS signal, we built a simple counter using standard TTL logic (4 of 74HC390) and amplified the signal for using as input to the serial port.

The result is a very stable PPS signal, however each pulse is not synchronised and there is no indication of which second it represents. Follows is a description on how to use this PPS signal for creating a stable and accurate NTP server.

The operative system

It was decided to employ a PC running a standard freeBSD 5.4. The kernel was recompiled (see the excellent freeBSD handbook at URL: https://www.freebsd.org/) with the following option:

option       PPS_SYNC

As the PPS signal is received on the serial port, the following link was made:

ln -s /dev/cuaa0 /dev/pps

The NTP code was ported using