Signal Integrity
Characteristic resistance of a transmission line measured by analyzing reflected electrical pulses identifies how a signal will behave as it travels through a printed circuit board. This tdr impedance is a critical parameter for high speed digital and radio frequency applications.
Measurement Technique
Equipment for time domain reflectometry sends a fast-rising voltage step down a trace and monitors the echoes that return from any discontinuities. Variations in the width of the trace or the distance to the ground plane cause changes in the tdr impedance that appear as peaks or valleys on the display. Technicians use this tool.
The result is typically expressed in ohms and must match the target value specified in the design files.
Trace Compliance
Matching the output of the driver to the cable and the board trace prevents signal reflections that cause data errors or electromagnetic interference. Maintaining a tight tolerance on tdr impedance is necessary for the proper function of modern interfaces like DDR memory and PCIe buses. Fabrication houses adjust the etching process and dielectric thickness to bring the physical board into compliance with these electrical requirements.
Regular testing of sample coupons ensures that production variability does not degrade the performance of the communication links.
