Transmission Line Effects on Digital Signals - a CD Tutorial
Includes Simulation Software
Digital integrated circuits now commonly have rise and fall times of a few nanoseconds or less. Printed circuit board tracks do not transmit signals instantaneously from source to load, but after a propagation time, typically of 1 to 2ns for a track 100 to 200mm in length. In these circumstances a reflection can occur at the load, which can itself be re-reflected from the source. This can cause ringing, and signal distortion, and is caused because the track is behaving as a transmission line.Reflections from the load are prevented when the track is terminated with a resistance equal to the characteristic impedance of the track, which is dependant on the physical geometry of the track and ground plane, and the board material dielectric constant. This involves extra components however, and may not be required in all circumstances.
This tutorial takes the user through around fifty fully narrated slides, explaining the theory behind transmission line behaviour in clear easy to understand steps, and how that is translated into digital pulse behaviour on a pcb track. PCB track structures are explained in terms of their characteristic impedance, and an in-depth series of animated simulations graphically displays pulse behaviour in a variety of circuit conditions.
If you are designing boards, or circuits, using integrated circuits with signal transition times less than 10ns, don't do anything until you've seen this tutorial!
Simulation Package
A simulation software package is included for separate installation,
allowing pulses to be modelled on a single track length under a wide range of termination
and circuit conditions. (Requires Windows 95 or later).
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