Also known as: NCO, numerically controlled oscillator, DDS

A numerically controlled oscillator (NCO) generates a digital sine and cosine of any programmable frequency from a phase accumulator — a counter that adds a fixed step each sample and looks up the corresponding sine value.¹ It is the software equivalent of a local oscillator and the tunable mixer inside a digital down-converter.

An NCO steps a phase accumulator each sample and looks up the sine — a precisely tunable digital oscillator.

Overview

Changing the accumulator’s step instantly retunes the NCO, which is how an SDR digitally tunes to a channel: multiply the IQ stream by the NCO’s output to shift that channel to baseband before filtering.

Sources

1. Numerically-controlled oscillator — Wikipedia, on phase-accumulator-based digital frequency synthesis. ↩

See also

• Digital down-converter (DDC)
• Local oscillator
• Decimation
• IQ data

Learn it step by step

• Filtering & decimation

From the blog

• SDR Internals, Part 4: DSP foundations — filters, NCO & AGC