How/where are you going to connect up the OCXO?

Assuming "time source" means NTP server, the CPU is where you want to connect
up a good clock.  You will probably want to unsolder the current crystal and
feed the input pad with the same frequency.  If you have troubles finding an
OSCO at the right frequency, you can get a small board with one of the magic
PLL chips.  I think TAPR has one.  If not, try eBay.

Another approach is to use the CPU as a heater.  It's not as good as a real
OCXO, but much better than a bare crystal.  Cheaper and easier too.
https://github.com/ntpsec/ntpsec/blob/master/contrib/ntpheat
https://blog.ntpsec.org/2017/02/01/heat-it-up.html

It's quite old by now.  Newer modules are usually better.

There are several GPS HATs that plug into a Raspberry Pi.

This is for Linux, but it may provide interesting ideas and it has links to a
couple of GPS HATs.
https://www.ntpsec.org/white-papers/stratum-1-microserver-howto/

Here is David Taylor's HOWTO:
https://www.satsignal.eu/ntp/Raspberry-Pi-quickstart.html

--
These are my opinions.  I hate spam.

On Fri, Apr 24, 2020 at 04:10:46PM -0700, Hal Murray wrote:

I was basically planning on feeding the OCXO and the PPS from a GPS
module into a simple microcontroller. Using a capture-compare module
of the microcontroller, my idea was to tune counters in the
microcontroller using the GPS PPS signal as the reference source and
creating my own GPSDO of sorts. The microcontroller would then create
its own PPS signal that is fed into a GPIO pin of the control
computer, i.e. a Raspberry, and hopefully stay stable even if the GPS
PPS reference disappears.

We used to have a GPS clock with an external antenna, but it died a
few years back and this was basically a shower thought for building
something new and not simply buying some off-the-shelf product.

Cordially,
Andreas Kempe