TDR Fault Distance

Time-domain reflectometry sends a fast step into a cable and times the echo from any impedance discontinuity. Because the pulse travels to the fault and back, the distance is half the round-trip delay times the propagation velocity (the velocity factor of the cable). The reflection sign and size also reveal whether the fault is an open, short, or partial mismatch.

Equations & Parameters ▸

\(d = \dfrac{\text{VF}\cdot c\cdot t}{2} \qquad \text{VF} = \dfrac{1}{\sqrt{\varepsilon_r}} \qquad Z_{\text{fault}} = Z_0\dfrac{1+\rho}{1-\rho}\)

t	Round-trip echo delay (ns) — time from the launched step to the reflected step.
VF	Cable velocity factor (0–1). Enter this or the dielectric constant.
ε_r	Dielectric constant; used to derive VF = 1/√ε_r if VF is blank.
ρ, Z₀	Reflection coefficient and line impedance (Ω), optional — give the fault impedance.

References: Tektronix, TDR Impedance Measurements: A Foundation for Signal Integrity. · D. M. Pozar, Microwave Engineering, 4th ed., Wiley, 2012.

Inputs

Echo delay, t

Round trip

Velocity factor, VF

or εr below

Dielectric, ε_r (alt.)

VF = 1/√εr

Reflection, ρ (optional)

For fault Z

Line impedance, Z₀ (optional)

Default 50

Results

Location

Fault distance—

Velocity factor used—

Fault

Fault impedance—

Type—

Diagram

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