Numerically-controlled oscillator

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A numerically controlled oscillator or digitally controlled oscillator (DCO) is an electronic system for synthesizing a range of frequencies from a fixed timebase.

[edit] Capabilities and limitations

Unlike a phase-locked loop-based analog frequency synthesiser, it is capable of synthesizing a very wide range of precise frequency ratios. However, it is limited to producing frequencies that are less than half the timebase frequency, due to Nyquist's theorem. The name is by analogy with "voltage-controlled oscillator".

Note: the term "digitally controlled oscillator" has also been used to describe the combination of a voltage-controlled oscillator driven by a control signal from a digital-to-analog converter. This article is about the more recent design.

[edit] Operation

A DCO generally consists of a digital waveform generator that increments a phase counter by a per-sample increment. This phase is then looked up in a waveform table to create a sine waveform. If an analog output is desired, this waveform is sent to a digital-to-analog converter to produce an analog waveform, and then filtered by an analog filter to remove aliasing and DAC artifacts. Alternatively, the digital waveform may be used directly as an input for further digital signal processing.

In this way, the frequency ratios that can be produced are limited only to the precision of the arithmetic used to compute the phase. At the same time, DCOs are phase- and frequency-agile, and can trivially be modified to produce phase-modulated or frequency-modulated outputs, or quadrature outputs.

A DCO can consist of a digital counter such as the 82C53 chip. These produce square wave signals that are very accurate. The square wave can then be modified by standard analog waveshaping techniques. DCOs have been used as cheap replacements for VCOs in some synthesizers. In these applications, their greatest benefit is stability along with having no frequency drift.