The Beginner’s Guide to GPS Disciplined Oscillators (GPSDO)

The term “GPSDO” is becoming more popular as it’s becoming a more reliable and accurate source of timing for many different applications including GNSS and GPS applications.

GPS Disciplined Oscillators (GPSDO) are powerful devices (sometimes called GPS clocks) that consist of a high-quality stable oscillator and a GPS receiver. The GPSDO works by disciplining (or steering) the oscillator output to a GPS device or GNSS satellite signal via a tracking loop.

GPSDO timing signals are accurate up to nanoseconds and are capable of generating frequency accuracies and stabilities up to parts per trillion. Satellite time signals must be very precise to provide positional accuracy for successful GPS and assured PNT navigation... thus making GPSDOs very valuable.

Let’s dive a little deeper into exactly how GPSDOs work.

GPSDO Basics – Inside the GPSDO

The disciplining mechanism in a GPSDO works in a similar way to a phase-locked loop (PLL). Some GPSDOs utilize PLLs, however many are replaced with a microcontroller to compensate for frequency, temperature, and other environmental parameters.

The real timing magic of a GPSDO happens because of the combined efforts of the GPS signal and the quartz crystal oscillator.

GPS receivers have excellent long-term stability (characterized by Allan deviation). However, their short-term stability is often degraded by many factors including

• The internal resolution of the one pulse per second (1PPS) reference timing circuits
• Signal propagation effects (such as multipath interference)
• Atmospheric conditions
• Other impairments

This is where the high-quality crystal oscillator (often an Oven Controlled Crystal Oscillator OCXO) comes in. OCXOs offer better short-term stability, but are often exposed to thermal, aging, and other longer-term effects.

The combined efforts of the GPS receiver and the OCXO make a GPSDO an excellence source for overall frequency stability.

What's the Difference Between GPSDOs and NCOs?

GPSDOs typically phase-align the internal flywheel oscillator to the GPS signal by using dividers to generate a 1PPS signal from the reference oscillator. 

Then, this 1PPS signal is compared to the GPS-generated 1PPS signal and uses the phase differences to control the local oscillator frequency in small adjustments.

This is the main difference between GPSDOs and Numerically Controlled Oscillators (NCOs). Rather than disciplining an oscillator by frequency adjustments, NCOs typically use a free-running, low-cost crystal oscillator. They also adjust the output phase by digitally lengthening or shortening the output phase many times per second in large phase steps. This assures that the number of phase transitions per second (on average) is aligned to the GPS receiver reference source.

With that said, GPSDOs are guaranteed frequency accuracy... even in the presence of high phase noise and jitter. 

GPS Outages

When a reliable GPS signal becomes unavailable, the GPSDO goes into holdover mode. Holdover mode is where the GPSDO tries to maintain accurate timing using only the internal crystal oscillator.

Sophisticated algorithms are used to compensate for the aging and temperature stability of the oscillator while the GPSDO is in holdover.

GPSDO Applications

GPS Disciplined Oscillators can be used in a variety of applications. For starters, GPSDOs are used as the basis for Coordinated Universal Time (UTC) around the world. UTC is the official accepted standard for time and frequency.

GPSDOs are also used to provide synchronization to wireless base stations and serve well in standards laboratories as an alternative to cesium-based references.

Other common applications for GPSDOs include:

• Assured Positioning, Navigation, and Timing (PNT) applications like passive radar
• Critical Infrastructure
• Cube satellites and LEO Satellite Communication