Pairing the Right Oscillator with the Right Application

Oscillator selection can be challenging, what with so many different frequency and time-keeping components available for many different frequency ranges. Many designers automatically reach for temperature-compensated quartz-oscillator (TCXO) sources; these oscillators have dropped in price over recent years, yet continue to provide excellent signal stability and noise performance while consuming low power levels.

However, EndRun Technologies has found that when applications require more demanding performance levels than are available from TCXOs, many customers opt to specify oven-controlled crystal oscillators (OCXOs) for their system’s reference oscillators. The firm’s four-page application note, “Oscillator Selection Guide,” provides a brief comparison of TCXOs, OCXOs, and even rubidium-based reference oscillators for a variety of different high-frequency applications. These include computer networks and both wired and wireless communications systems.

The application note reviews some of the specifications that are typically used to compare different types of oscillators. For example, a “disciplined” oscillator is one in which the oscillator’s frequency is being controlled by an internal microprocessor based on measurements relative to a CDMA or Global Positioning System (GPS) signal. Holdover accuracy refers to any time error that a transfer standard accumulates when operating without a reference signal and relative to an absolute time reference standard. A typical period for checking the holdover accuracy of an oscillator is one day.

Short-term stability refers to the movement of zero crossings of a source signal relative to those of a reference frequency standard. This is measured over short time intervals, such as less than one second. Short-time stability for an oscillator is usually referred to as its jitter performance. When measured in terms of phase, it is referred to as an oscillator’s phase noise.

The application note reviews some of the performance options available for different types of oscillators. OCXOs, for example, can be specified as medium-stability source, high-stability OCXOs, and ultra stable OCXOs. All of the OCXO source types are high-performance oscillators that use SC-cut crystals for fast warm-up, low aging rate, and reduced sensitivity to ambient temperature fluctuations.

Key differences in performance include differences in temperature stability and in close-in phase-noise performance. The four-page application note provides a brief summary of the different oscillator types and how the different types fit well into different types of applications.

EndRun Technologies, 2270 Northpoint Pkwy., Santa Rosa, CA 95407;(877) 749-3878, FAX: (707) 573-8619