Time Domain
On material: ~40° lag (unloaded coil is ~90°; eddy currents reduce phase)
θ = 0°
Im = 1.00
Depth = 0%TW
t = 0.00s
I/Q Demod
In-phase (cos) + Quadrature (sin) components
What you're seeing:
The instrument "locks" to the carrier frequency and computes two components:
I = Im·cos(θ) and Q = Im·sin(θ).
Those are the phasor's x/y coordinates — extracted from one signal.
⚡ This is NOT a Lissajous figure.
A Lissajous plots two separate signals against each other, producing a fixed geometric shape (an ellipse or figure-8).
Here, a single carrier waveform is mathematically demodulated — multiplied by cos and sin of the reference frequency — to extract its changing phase and amplitude as the probe moves.
That is a fundamentally different operation.
Phasor
Amplitude = length, phase lag = angle
Why it matters:
A defect changes electromagnetic coupling, which changes the effective impedance seen by the coil.
That shows up as a change in amplitude and phase of the coil current.
Scan Position
Probe moving over material with flaw
The probe scans across material. When over good material, signals are stable.
When passing over the flaw, phase and amplitude change.
Impedance Plane (I vs Q)
The "eddy current display" is the moving tip of the phasor
Connect the dots:
As you scan over a flaw, θ and Im both change. The display plots (I, Q) continuously —
tracing the moving tip of a dynamic phasor — and that trajectory helps analysts detect, characterize, and size indications.
⚡ Why "impedance plane" and not "Lissajous display"?
Both look like an X/Y plot, but a Lissajous shows a fixed shape produced by two separate waveforms.
The impedance plane plots the demodulated I and Q components of a single waveform over time,
revealing how the coil's effective impedance changes as the probe moves across a flaw.
That defect trajectory simply cannot be extracted from a Lissajous figure.