Time Domain
Voltage (reference) + Coil Current (lags by θ)
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 just the phasor's x/y coordinates.
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 change with time. The display plots
(I, Q) continuously, creating a trajectory that helps analysts characterize indications.