A GPS Disciplined Frequency Standard

As a radio amateur it is always nice to know that measurements you may make in the course of building and maintaining your station are accurate. Frequency is not the most straightforward measurement to verify and, unlike for some measurements, equipment that you buy off the shelf or second hand within a “hobbyist budget” for frequency measurement cannot necessarily be relied upon.

An accurate frequency and time reference is also useful, some would say essential, when experimenting in some exciting new modes such as WSJT, QRSS, etc.

The GPS system can supply both a time and frequency reference with very good accuracy without costing the earth. Recently Paul G4DCV came across a source of Rockwell Jupiter-T GPS modules. Unfortunately these GPS modules are now obsolete and not easy to find, which is a shame because they are unique amongst GPS modules, having been designed not primarily for generating positioning data, but for providing an accurate time and frequency reference. They feature a 10 kHz reference output locked to GPS timing whereas most GPS modules have only a 1Hz output. This means that they can be employed to control a frequency standard using a much simpler circuit than would be required with a 1Hz output. In addition, the oscillator disciplined by the GPS receiver does not have to be as stable, meaning an oven oscillator module is not necessarily required.

Andy Talbot G4JNT’s excellent web site http://www.g4jnt.com was raided for circuit ideas and here Paul found two circuits of interest. The former is a very simple circuit allowing a 5 or 10 MHz voltage controlled crystal oscillator (VCXO) to be phase-locked to the 10KHz output of the Jupiter-T using just a handful of TTL ICs. The second circuit is an LCD display driver using a PIC16F628 which takes the positioning data which appears at the GPS module’s serial output and converts it into a QRA locator whilst also displaying other information such as the current time and date on an LCD display.

Having built the circuit I’m not 100% sure I have been able to match Andy’s claims for frequency stability. Perhaps I need to find a more stable VCXO or to tweak the loop filter to better match my choice of oscillator. Having said that, I’ve proved it’s stable to within tens of hertz in 1 GHz, and, having counted the digits on my frequency counter, I’ve decided that’ll do nicely for now!