I read the article by Brooks Shera, W5OJM in the July 1998 issue of QST (Reprint available for download at his website) that describes using a 1 Hertz output from a GPS receiver to discipline a voltage controllled crystal oscillator (VCXO).

There is always a need for an accurate frequency reference. The need for an accurate frequency becomes even more critical when dealing with weak signal work. Especially when frequencies are multiplied up into the UHF and Ghz ranges. Since I'm mostly interested in weak signal work, this became my first project

In the past the common practice was to periodically compare frequncies against the 10Mhz signal from WWV. However, some path distortion shifting occurs so the WWV signal isn't quite as accurate when recieved. GPS satellites have atomic clocks on board and transmit a very accurate 1 hertz signal. Accurate to a few parts in 10e11.

Original design concept
In the orginal design a comercial 5Mhz VCXO is phase locked to the 1Hz GPS signal through the use of a PIC microcontroller and some external counter circuits. Phase is measured by counting pulses from a local 24Mhz crystal oscilltor that occur between the raising edges of the GPS pulse and the 5Mhz signal. Corrections are made through a proportiinal-integral (PI) controller implemented in software.

Possible Redesign
Upon looking over the circuit, I considered that it might be possible to reduce the hardware required. The PIC microcontroller has internal counters and capture/compare features that could be utilized to replace the external counters. After much sketching of schematics and interupt handlers, I came up with a workable idea.

Design goals

1. Most accuracy and stability achievable for cost.
2. Use commonly available parts. Unfortunately, many designs get based on surplus comercial equipment that becomes scarce.
3. Possibly contruct my own temperacture controlled crystal oscillator to hold cost down.