TDR (Time Domain Reflectometry)

Notes

Modern practice usually computes TDR from VNA S-parameters rather than measuring it directly, since the two are Fourier pairs.

Overview

TDR (time domain reflectometry) is a measurement technique that sends a fast voltage step down a transmission path and watches the reflections that come back, revealing how the impedance changes along the path and where. It is the standard way to check controlled-impedance traces, connectors, cables and package routing for high-speed signals. Because a TDR trace shows impedance against distance, faults and discontinuities can be located as well as measured.

How it works

A TDR instrument, often a sampling oscilloscope with a TDR module, launches a step with a very fast edge into the device under test through a reference impedance, usually 50 ohms. Wherever the impedance changes, part of the step is reflected. The ratio of reflected to incident amplitude is the reflection coefficient, rho, which equals (ZL minus Z0) divided by (ZL plus Z0). It runs from minus one for a short circuit, through zero for a perfect match, to plus one for an open circuit, so the instrument can convert each reflection directly into ohms.

Time on the display corresponds to distance: a reflection arriving later comes from further along the line, and the round-trip delay together with the propagation speed in the material gives its position. The shape of each feature also tells a story. Shunt capacitance, as at a via pad, pulls the trace down briefly, while series inductance, as in a long bond wire or connector pin, pushes it up.

Resolution depends on the system rise time: two discontinuities closer together than about half of it merge into one feature. A very fast step can exaggerate small discontinuities that real signals with slower edges would hardly notice, so instruments often filter the step to match the edge rates of the actual circuit. Differential TDR drives both lines of a pair with opposite steps to measure differential impedance.

Examples

• Measuring a controlled-impedance trace on a printed board to confirm that it is within its tolerance.
• Locating a capacitive via stub on a server backplane that is degrading a high-speed SerDes channel.
• Checking a high-speed USB port with differential TDR, using a slower 400 ps step, against its 90 ohm differential impedance target.
• Finding an open circuit inside a flip-chip package by comparing the TDR trace of a failing pin with a good reference unit.

Why it matters

Signal integrity engineers use TDR to confirm that boards, packages and cables meet their impedance targets before high-speed links are brought up, and to find the via, connector or stub causing a problem. Board makers use IPC test methods based on TDR to verify controlled-impedance boards, and failure analysis teams use it to locate opens and shorts in packages without cutting them open.

Related terms

• VNA
• Return loss
• S-parameters
• Impedance discontinuity

Where it appears on the Hub

Used on 11 pages of the Hub. In each group, the pages that mention it most come first.