How does time-domain reflectometry work?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

Time-domain reflectometry launches a fast step or impulse into a line and observes reflections; the reflected voltage gives local impedance Z = Z₀·(1 + ρ)/(1 − ρ) versus time, and time converts to distance using velocity. VNAs provide equivalent results by inverse-transforming frequency-domain data, with gating to isolate a discontinuity.^[1]^[2]

What resolution can TDR achieve?

Two discontinuities are resolved if separated by about half the system rise time times velocity: Δx ≈ v·t_r/2. With 35 ps rise time on FR-4 (v ≈ 0.15 mm/ps), resolution is about 2.6 mm. On a VNA, response resolution depends on the frequency span and window.^[1]

What are VNA time-domain modes?

Low-pass mode (harmonically related frequencies from near DC) gives impulse and step responses that read as impedance; band-pass mode works with any frequency range but gives only magnitude of reflections. Windowing trades sidelobes against resolution; gating removes connector or fixture effects in time and transforms back.^[1]

What is TDR used for?

Verifying PCB controlled impedance, locating cable faults, characterizing connectors and transitions, and inspecting via stubs. See stackup design.

Frequently asked questions

Does TDR show distance directly?

Yes, given the propagation velocity.

What causes a capacitive dip?

A local impedance drop such as a large pad or via capacitance.

Can a VNA do TDR?

Yes, through time-domain transforms of S-parameter data.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“Time Domain Analysis Using a Network Analyzer (Application Note 1287-12)”. Keysight Technologies. application note.
“Signal and Power Integrity, Simplified, 3rd ed.”. E. Bogatin, Prentice Hall. 2018.

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