Jeff (aka K6JCA) has a great write-up on implementing the Brooks Shera Phase-Locked Loop GPSDO on an Arduino Platform:
This blog post is a continuation of my two earlier GPSDO blog posts. The first one (from a few years back) details a simple Frequency-Locked Loop GPSDO design, based around an Arduino processor. The second (more recent) blog post discusses simulating Brooks Shera’s GPSDO algorithm (from the July, 1998 issue of QST) using The MathWorks Simulink program.
This third blog posts describes my modification of my original Frequency-Locked Loop (FLL) GPSDO to be a Phase-Locked Loop (PLL) GPSDO, and it includes the hardware schematics, Simulink models, and the Arduino code I wrote to implement Brooks Shera’s GSPDO algorithm on an Arduino processor.
More details on K6JCA blog.