Scatter Plot

The dispersion of sample points around their ideal centroids provides an immediate qualitative measure of the Signal-to-Noise Ratio (SNR). In a high-SNR channel, clusters appear as tight, distinct balls. As noise increases, the variance grows, causing clusters to blur and overlap. This visual spread directly correlates with the Error Vector Magnitude (EVM), where a larger cloud radius indicates a higher RMS error between the measured signal and the ideal reference.

A continuous, spinning constellation is a classic visual indicator of a Carrier Frequency Offset (CFO). When the local oscillator at the receiver does not perfectly match the transmitter, the entire IQ plane rotates at a constant angular velocity. Instead of discrete clusters, the scatter plot reveals distinct rings or arcs for each amplitude level. The speed of rotation is proportional to the frequency error in Hertz.

Hardware impairments in direct-conversion receivers manifest as geometric distortions. IQ gain imbalance—where the I and Q branches have unequal gain—stretches the square constellation into a rectangle. IQ quadrature error (non-90-degree phase difference) skews the rectangle into a parallelogram. The scatter plot reveals this as an elliptical or skewed shape rather than a perfect square or circle.

A static rotation of the entire constellation by a fixed multiple of 90 degrees (or 45 degrees for offset QPSK) indicates a phase ambiguity. This is common in blind equalization or non-differential systems where the absolute phase reference is lost. The scatter plot shows perfectly formed clusters, but they are locked to the wrong quadrants. This is resolved via unique words or differential encoding.

When a power amplifier operates near saturation, it compresses the outer amplitude levels. On a scatter plot, this appears as a warping of the outer constellation points inward toward the origin, while inner points remain relatively unaffected. The outer clusters lose their distinct separation and may curve, indicating the need for Digital Pre-Distortion (DPD) to linearize the transmitter.

Coherent interference, such as a continuous wave (CW) tone or another modulated signal, superimposes a secondary pattern on the scatter plot. A CW interferer adds a circular ring offset from the origin, while a second modulated signal creates a complex superposition. The scatter plot reveals these as ghost clusters or a distinct structural background that does not align with the expected ideal constellation points.