Introduction

Time Domain Reflectometry (TDR) is a very useful tool for designing transitions such as the transition between coaxial connectors and PC boards. The TDR function will display the impedance over time showing the place and nature of discontinuities that are due to impedance changes. This paper discusses the aspects of TDR measurements that are relevant to the analysis of a transition. The complete range of TDR measurements and applications will not be explored.

What is TDR?

True time domain measurements are made with a pulse generator and an oscilloscope. The TDR tester injects a pulse into a transmission line and the reflections are shown on an oscilloscope. The rise time of the pulse determines the bandwidth of the measurement. The impedance imformation is based on the DC component of the input signal.

Reflections displayed represent discontinuities due to a change in the impedance along a transmission line. The magnitude of the impedance of the discontinuity determines the magnitude of the reflection.
Zero units represents the characteristic impedance of the line, typically 50 ohms.
The direction of the reflection on the display, either positive or negative, is determined by whether the impedance that caused the reflection is inductive or capacitive.