Lab 3: QPSK → Constellation → Impairments¶

Lab 3 introduces digital modulation and visualization through constellation diagrams.

Goal¶

bitstream → QPSK modulation → RF transmission → IQ capture → constellation → quality metrics

Experiment flow¶

flowchart LR
    SRC["Bitstream"]
    MOD["QPSK modulator"]
    TX["Zynq + AD9363"]
    RX["RTL-SDR"]
    IQ["IQ recording"]
    CONST["Constellation plot"]
    METRICS["EVM / phase noise"]

    SRC --> MOD --> TX --> RX --> IQ --> CONST --> METRICS

Typical impairments¶

Impairment	Visual effect
Noise	cloud spreading
Phase offset	rotation
CFO	slow rotation over time
IQ imbalance	ellipse
Clipping	distorted clusters

Demo figure¶

QPSK constellation

Engineering takeaway¶

A constellation shows several impairments at once: noise spreads the clusters, a carrier offset rotates them, IQ imbalance turns them into an ellipse, clipping distorts them. That is why it is usually the first plot to look at when a link misbehaves.