GPSDO for the Ham Shack: A 10 MHz Frequency Reference Explained

A GPSDO is a small box that does a quiet but remarkable job: it takes the time signals broadcast by navigation satellites and uses them to keep a local oscillator exactly on frequency. For a radio amateur, the result is a 10 MHz reference that is accurate enough to calibrate every other oscillator in the shack, to lock a transverter on the microwave bands or to keep an SDR on frequency to a tiny fraction of a hertz. Most operators never need one, because modern transceivers are accurate enough for HF and for FT8. But if you work on 23 cm and above, on EME, or on weak-signal modes where you need to know your frequency very well, a GPSDO is one of the best investments you can make. This guide explains how it works, how the oscillators inside radios compare, and where a reference makes a practical difference.

What a GPSDO Is

The Wikipedia article on the GPS-disciplined oscillator defines a GPS clock, or GPSDO, as a combination of a GPS receiver and a high-quality, stable oscillator, such as a quartz or rubidium oscillator, whose output is controlled to agree with the signals broadcast by GPS or other satellite navigation systems. The reason it works is that the satellite time signals have to be accurate to provide positional accuracy for navigation: the article says the signals are accurate to nanoseconds and provide a good reference for timing applications. A typical unit has an antenna input, a 10 MHz output and a one-pulse-per-second (1PPS) output, and sometimes a serial or USB interface for status.

The two parts complement each other. The article explains that GPS receivers have excellent long-term stability at averaging times of several hours, but their short-term stability is degraded by limits of the internal timing circuits and by effects such as multipath and atmospheric conditions. A good quartz oscillator, on the other hand, has excellent short-term stability but drifts over time. The GPSDO combines them: the oscillator provides a clean signal from moment to moment, and the satellite signal corrects its long-term drift.

How the Disciplining Works

The article describes the mechanism as similar to a phase-locked loop. The unit divides the oscillator down to a 1PPS signal, compares it with the 1PPS signal from the GPS receiver and uses the difference to make small adjustments to the oscillator frequency. In most units the loop filter is replaced by a microcontroller that uses software to compensate for the phase and frequency changes and for the learned effects of aging, temperature and other environmental parameters. If the satellite signal is lost, for example when the antenna is disconnected, the unit goes into holdover, where it tries to maintain accurate timing with the oscillator alone, using the algorithms that it has learned during the locked period.

For the user, the consequence is simple. After a warm-up of some minutes to hours, depending on the oscillator, the output is locked to the satellites and its accuracy is far better than any free-running oscillator you can afford. The accuracy is a property of the lock, and the quality of the oscillator determines how clean the signal is and how well it holds when the lock is lost.

Oscillators in the Shack: A Comparison

To understand what a GPSDO improves, look at what a radio normally has. The Wikipedia article on crystal ovens gives a comparison of oscillator types, with the stability expressed as a fraction and the ageing per ten years.

The OCXO keeps the quartz crystal at a constant temperature in a small oven, which, according to the article, prevents changes in the frequency caused by variations in ambient temperature, and so improves the stability. The cost is that it needs more power than other types and is physically larger, so it is not used in portable equipment. A GPSDO usually has an OCXO or a good TCXO inside, which the satellites then discipline.

Why ppm Matters More As the Frequency Goes Up

Frequency error is measured in parts per million, and a fixed error in ppm gives a larger error in hertz at a higher frequency. One part per million is 14 Hz at 14 MHz, but 144 Hz at 144 MHz, 1296 Hz at 1296 MHz and over 10 kHz on 10 GHz. This explains why a transceiver that is perfectly good on HF, where the error is a few tens of hertz, can drift out of the passband of a narrow signal on the microwave bands, where the same ppm error is thousands of hertz. A transverter, which converts the signal of an HF radio up to a microwave band, multiplies the error of its local oscillator by the full frequency, and many amateurs lock it to an external 10 MHz reference for this reason.

• Microwave and 23 cm work. A narrow-band signal on 1296 MHz can only be found if both stations are accurate to within a small fraction of the signal bandwidth. Our guide to the 23 cm band shows how the requirements change at higher frequencies.
• EME. Moonbounce contacts use very narrow-band weak-signal modes, in which a frequency error can make a decoded signal disappear. See our EME beginner’s guide for the modes and the Doppler considerations.
• Satellite work. On a geostationary satellite such as QO-100, the uplink and downlink frequencies must be accurate for the narrow transponder to work well, as described in the QO-100 guide, and many operators lock their equipment to a GPS reference for this reason.
• SDR receivers. A software defined radio has an internal clock that determines its frequency accuracy. A clean external 10 MHz reference improves the frequency readout and the stability of the display, which matters for weak-signal observation and for calibrating other equipment.

Other Uses of the Reference

A GPSDO also gives you a method to calibrate a transceiver. With a counter that is locked to the reference, you can measure the frequency of the radio’s transmitter and see its error directly. Our article on transceiver frequency calibration describes methods that use broadcast time signals, and a GPSDO is a more accurate and convenient alternative. The 1PPS output is a precise time signal, and combined with a computer program it can provide a reference for the system clock, which matters for FT8 and related modes. Our guide to FT8 time synchronisation explains how large the time error can be before decoding fails, and a GPS-based time source removes the problem.

Choosing and Installing One

1. Decide what you need. For calibration and a single transverter, a modest unit with a 10 MHz output is enough. For several devices, choose one with multiple outputs or add a distribution amplifier.
2. Check the oscillator type and the specified holdover and phase noise. An OCXO-based unit has better short-term stability and warms up more slowly, and a TCXO-based unit is smaller and cheaper.
3. Mount the GPS antenna with a clear view of the sky, away from your transmitting antennas and on a path that is not shared with a noisy cable. GPS signals are very weak, and a strong transmitter nearby can overload the receiver, so keep the antenna away from your own antennas and use a filter if needed.
4. Allow warm-up and lock time. The first lock after a cold start can take from minutes to hours before the output reaches its best accuracy, particularly with an OCXO. Leave the unit on, since a reference that is never turned off is best.
5. Connect it with good cables. Use a good coaxial cable for the 10 MHz output with the correct impedance, normally 50 ohms, and avoid long runs that pick up noise. Feed the reference input of the transverter or SDR directly, and check the level that the input expects.
6. Check the lock indicator. Most units have a light or a status signal that shows the lock. Do not trust the reference until the unit says that it is locked.

A Simple Check: Is My Radio Accurate?

Before you decide that you need a reference, measure what you have. Connect the reference to a frequency counter, or to a receiver that can be calibrated against it, and compare it with the radio. If the transmitter on 28 MHz is 40 Hz off, that is about 1.4 ppm, and the same error on 144 MHz would be about 200 Hz, which is within the tolerance of most weak-signal modes at VHF but starts to matter on higher bands. Many amateurs find that their TCXO radios are within a few ppm, which is fine for HF and for FT8 work, since the software decodes within a tolerance of tens of hertz. The result of the measurement tells you where the real need is. If the error is small, a GPSDO is not necessary on HF, and the money is better spent elsewhere. If you are building a transverter for a microwave band, the answer is different, and the reference is worth having from the beginning.

Distributing the Reference

If you have several devices that can use a 10 MHz input, such as a transverter, an SDR and a frequency counter, one output may not be enough. A small distribution amplifier splits the signal into several outputs of the same level with isolation between them, so that a fault in one device does not disturb the others. Keep the cables short and of equal quality, label each output and terminate unused ones with a 50 ohm load, as a loose output can reflect the signal and upset the others. Keep the reference cables away from transmitting feedlines, because a strong RF field can couple into them and add spurious signals to the otherwise clean reference.

Limits and Cautions

A GPSDO is accurate in frequency, but it is not a magic cure for everything. The reference does not improve the phase noise of a poor local oscillator in the transverter, or the quality of the receiver. It does not help if the rest of the chain has a different problem, such as a drifting filter or an unstable power supply. The satellite signal can be disturbed by interference, and in the case of strong jamming or loss of reception the unit falls back on holdover, with a reduced accuracy that depends on the oscillator and its time since the last lock. For most hams the best approach is to treat the unit as a reference to check against and to lock one or two key devices to it, not to distribute it everywhere in the shack without a reason.

The Bottom Line

A GPSDO combines a GPS receiver with a good quartz oscillator and steers the oscillator to the satellite time signals, producing a 10 MHz reference whose long-term accuracy comes from the satellites and whose short-term cleanness comes from the oscillator. A typical radio has a TCXO with a ppm-level error, and a ppm error grows in hertz with frequency: 14 Hz at 14 MHz, but more than 10 kHz at 10 GHz. That is why microwave, EME, satellite and SDR users value the reference. Put the antenna in the clear, give the unit time to lock and use it to calibrate and to lock the devices where accuracy matters. For HF and FT8 on a modern radio, it is a luxury; for the weak-signal bands it is a very useful tool.

Frequently Asked Questions

What is a GPSDO?

A GPS-disciplined oscillator is a combination of a GPS receiver and a stable oscillator, such as a quartz or rubidium oscillator, whose output is controlled to agree with the satellite signals.

Do I need a GPSDO for HF or FT8?

Usually not. Modern transceivers with a TCXO are accurate enough for HF and FT8. A reference becomes valuable on the VHF, UHF and microwave bands and for calibration.

What is the difference between a TCXO and an OCXO?

A TCXO compensates for temperature changes electronically. An OCXO keeps the crystal in a small oven at a constant temperature, which gives better stability but needs more power and warm-up time.

Why does 1 ppm matter more at higher frequencies?

A given error in parts per million gives a larger error in hertz at a higher frequency. 1 ppm is 14 Hz at 14 MHz but about 1.3 kHz at 1296 MHz.

Where should I put the GPS antenna?

With a clear view of the sky and away from your transmitting antennas and noise sources. The GPS signal is very weak and strong nearby signals can interfere.

What is holdover?

It is the state in which the GPSDO has lost the satellite signal and keeps timing using only its internal oscillator, with algorithms that compensate for its ageing and temperature behaviour.