Also known as: Costas loop

A Costas loop is a phase-locked feedback structure that recovers the phase and frequency of a suppressed carrier, enabling coherent demodulation of PSK and related signals.¹

A Costas loop recovers a PSK carrier by feeding a phase-error estimate back to a controlled oscillator.

How it works

It compares in-phase and quadrature error to drive an oscillator that locks to the carrier, removing residual frequency offset so the constellation stops rotating. It is named for John P. Costas.

Relevance to SDR

Carrier recovery via a Costas loop is essential to decoding phase-modulated systems and to stabilising a constellation that would otherwise spin.

Sources

1. Costas loop — Wikipedia, on the carrier-recovery feedback loop for coherent PSK demodulation. ↩

See also

• Phase-shift keying (PSK)
• Phase
• Demodulation
• CMA equalizer
• PPM frequency correction

Learn it step by step

• Tuning for a clean lock

From the blog

• SDR Internals, Part 7: Symbol timing & sync recovery