Introduction to TDR Waveforms

UPDATED: September 25, 2014

Learning to interpret TDRs waveforms is the first step towards putting this powerful technology to work in your application.

The TDR waveform itself is a plot of a transmission line's electrical properties, such as impedance or voltage, as a function of time or distance. As you have probably already learned, TDR output is based on reflected pulses in the transmission line.

Capacitive and inductive discontinuities create reflections, and show up on the waveform as dips and bumps.

Unlike frequency domain plots, time domain waveforms allow the operator to locate a discontinuity or measure the impedance at a specific point in the transmission line.

In Figure 1 below, a HYPERLABS HL1101 Ruggedized TDR is connected to 1 meter of unterminated 50 ohm coaxial cable. Output is shown in our ZTDR™ software. Horizontal units are in time (ns) and vertical units are in voltage (mV).

Figure 1: Time domain waveform on 1 meter of coax (click for full size)

You can see two distinct "steps" in the waveform. The first, from -250 mV to 0 mV, is called the "incident step". The incident step is the initial short pulse generated by the TDR. It reaches 0 mV (or 50 Ohm) when it enters the TDR instrument and the 50 Ohm cable.

The second step, from 0 mV to 250 mV, shows the signal being reflected almost completely once it reaches the open circuit at far end of the cable.

We will break down the characteristics and common shapes of waveforms in future Application Guides.

It is important to note that the waveform shows voltage as a function of time of distance in meters. The distance is round trip, due to the fact that TDR analyzes reflections in the transmission line.

Time domain waveforms can always be displayed as a function of time. Distance measurements will be accurate if the dielectric constant of the transmission line is known.

Table of Contents