A diagnostic measurement method that launches a fast edge or narrow pulse into a transmission line and records time-resolved reflected signals; by mapping reflection time to distance using the line's propagation velocity, it locates and characterizes impedance discontinuities and distributed impedance profiles.

Principle

A reflection at a point on a transmission line is produced by a local impedance mismatch; the reflection amplitude and phase are proportional to the local reflection coefficient, and the round-trip time of the reflected waveform determines the distance to the discontinuity via distance = (v_p × time)/2, where v_p is the signal propagation velocity on the line.

Demonstration

Illustrative scenario → Situation: A technician must locate a short on a coaxial feed. Recognition: The coax has a known propagation velocity factor. Action: A TDR injects a narrow pulse and the instrument records a reflection peak at measured round‑trip time t. Consequence: Applying distance = (v_p × t)/2 the technician identifies the fault location and isolates the segment for repair.

Misapplication

Assuming reflection amplitude maps linearly to impedance without compensating for frequency-dependent attenuation, dispersion, connector losses or the instrument's impulse response; this yields incorrect estimates of discontinuity severity or location.

Consequence

Correct use yields accurate fault location, impedance profiling and verification of cable or PCB trace integrity; misuse (wrong v_p, insufficient bandwidth, or uncorrected dispersion) produces systematic location errors, missed subtle faults, or mischaracterized impedance profiles.

Reversal

On highly dispersive, lossy or nonlinear lines (long fibre with modal dispersion, very lossy twisted pair, or time‑varying loads) the simple pulse→single‑reflection model breaks down: reflections spread and superpose, resolution falls and specialized processing or frequency‑domain variants (e.g., OTDR/TDT) are required.

Boundary

Clearly within: coaxial, twisted pair and PCB traces treated as distributed transmission lines for which propagation velocity and characteristic impedance are known. Boundary case: very short runs with mixed lumped elements where distributed assumptions are marginal. Clearly outside: measurement of a purely lumped single‑component impedance with no meaningful propagation time.

Semantic Tension

Resolution (short pulse, high bandwidth) versus penetration and dynamic range (longer pulse, more energy): narrower pulses improve spatial resolution but contain less energy and suffer lower dynamic range and higher sensitivity to attenuation.

Synthesis

TDR converts time‑domain reflections into a spatial map of impedance; accurate interpretation requires knowledge of propagation velocity, system bandwidth and dispersion, together with calibration and signal‑processing to avoid amplitude- and time‑domain artifacts.