Introduction

A GPS disciplined oscillator (GPSDO) is designed to get the best of both worlds: the short-term frequency stability of a free-running, low-noise quartz oscillator and the long-term timing performance delivered by GPS. Obviously there is region between short- and long-term where the performance of a GPSDO is necessarily a compromise between the two sources.

For any given measurement averaging interval (tau) the frequency stability of a GPSDO is a combination of many internal limiting noise sources: OCXO instability, GPS receiver timing jitter, DAC and voltage reference noise, temperature variations, power supply noise and instability, buffer amplifier noise, etc. Depending on the tau, one or more of these noise sources may dominate.

Without detailed analysis, it's not always easy to determine exactly how much of noise each source contributes. But one of the easiest experiments to perform is to contrast the frequency stability of a GPSDO in two modes: free-running and locked. This reveals a great deal about the quality and implementation of the GPSDO. For each GPSDO, and in each mode, measurements were made against a very stable 10 MHz reference.

I had hoped to make detailed measurements on a dozen or more models of GPSDO that I have lying around the lab, but due to a busy schedule in 2007, I have data on only 4. Still, until I get around to measuring the rest of them, these four are quite interesting.

HP Z3801A SmartClock GPSDO

This is a classic HP Z3801A SmartClock (similar to 58503A and 58503B), a gold standard among the ham radio and amateur time & frequency crowd due to its superb performance and availability on the surplus market. This is one of the better units I've measured. Links: Z3801A Frequency Standard

Jackson Labs Fury GPSDO

This is a recent GPSDO designed by Jackson Labs. This model is the OCXO version in the nice aluminum case. Links: Jackson Labs and Fury Performance Review

Trimble Thunderbolt GPSDO (OCXO version)

This version of the Thunderbolt has an OCXO (unlike the model shown on the Trimble web page). These are also recently available on the surplus market.

James Miller Simple GPSDO

This is about the simplest design possible for a GPSDO; with no DAC, no Vref, no time interval counter, no microprocessor. Just an XOR gate acting as a phase comparator aginst the 10 kPPS output of a Jupiter GPS engine. In spite of this simplicity, the performance is amazingly good. Links: GPS Stabilised 10 MHz Oscillator

Composite Plot

Here they all are on the same plot; each GPSDO has a different color, with bold for locked and light for unlocked.

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