Frequency Control Explained: Crystals, OCXO, GPSDO and Rubidium

Frequency control means giving an electronic system a stable and accurate frequency reference, so that radio, telecom, measurement instruments and timing systems work as intended. This guide covers the most common solutions and how to choose the right level for your application.

What is frequency control?

Almost all electronics that communicate or keep time need a clock. That clock is usually based on a quartz crystal that vibrates at a very specific frequency. The problem is that the frequency is affected by temperature, ageing and supply voltage. Frequency control is the set of methods and components that keep the frequency stable despite this.

The most common frequency control products

• Quartz crystals: the foundation of most frequency sources. The cut angle determines the properties, see SC-cut vs AT-cut quartz crystals.
• Crystal oscillators: crystal and electronics in one package, ready to use.
• TCXO: temperature-compensated oscillator for good stability at low cost and power.
• OCXO: oven-controlled oscillator with very high stability and low phase noise. See What is an OCXO?
• VCXO: voltage-controlled oscillator that can be fine-tuned, for example in a control loop.
• GPS disciplined oscillators (GPSDO): an OCXO steered to GNSS time for long-term accuracy, see What is a GPSDO?
• Rubidium frequency standards: an atomic reference with very low ageing and long holdover.

How to choose the right level

A rule of thumb: start with what your application actually requires, not with the best component available.

• Basic timing: a standard crystal oscillator or TCXO is often enough.
• Low phase noise and stability over temperature: choose an OCXO.
• Traceability to GNSS time and long-term accuracy: choose a GPSDO.
• Long holdover or operation without a satellite signal: choose a rubidium standard. See the comparison in GPSDO vs rubidium frequency standard.

What is holdover?

Holdover is how long a reference keeps its accuracy after the external signal, such as GPS, is lost. It is often the most important difference between solutions. Read more in What is holdover in timing systems?

Frequency control products from Vikronix

Vikronix supplies frequency control products from Quartzlock, HCD Research, RFX Group and other manufacturers to engineering teams in Sweden, Norway, Denmark and Finland, with local support from Malmö. Browse our product range or get in touch and we will help you choose the right solution for your application.