Field Guide · term

Also known as: constellation diagram, constellation

A constellation diagram plots a digital signal’s symbols on the IQ plane: the horizontal axis is I and the vertical axis is Q, so each point’s angle is its phase and its distance from the origin is its amplitude.1

clean noisy
A constellation plots symbols on the IQ plane; tight clusters decode reliably, smeared clusters bring errors.

How it works

A clean signal places symbols in tight, well-separated clusters at their ideal points. Noise fuzzes the clusters, a tuning offset rotates them, and clipping distorts them — so the shape of the scatter diagnoses the problem.

Relevance to SDR

GopherTrunk’s constellation panel draws this live, making it the first tool for tuning a clean lock.

Sources

  1. Constellation diagram — Wikipedia, for the IQ-plane representation and how it reflects signal quality. ↩

See also

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