Westbay FasTrack
Software for the Transmission Line Analysis of PCB Tracks
Westbay FasTrack allows the user to model the behaviour of fast risetime digital waveforms on printed circuit board tracks, when the track is long enough to produce transmission line effects. These can cause ringing and distortion of the waveform, unless the track is correctly terminated. EMC (electromagnetic compatibility) problems can then ensue.
Here's an illustration of what can happen, as a pcb track
length is increased.
The Problem
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. How can it be determined whether transmission line effects are present, and whether they will cause problems?
The Solution
Westbay FasTrack provides the digital circuit designer with a stand-alone environment for assessing transmission line effects on a printed circuit board track. Complex calculations of characteristic impedance are carried out automatically by the program, and the pulse waveforms are assembled accurately by using Fourier analysis of the digital waveform. All the user has to do is to describe the track geometry, the waveform, and the load details, to obtain source and load waveforms.
Several configurations of tracks can be analysed, include single, twin and star arrangements. The user may also specify a series of discontinuities on the track, caused by other device capacitances, or via inductances. The load matching resistance can have both a series inductive component, and a dc blocking capacitance.
Program Facilities
Westbay FasTrack provides the user with the following functions:
(Click on a button for further details)
Single Track Model. This analysis allows a single length of track, with a source at one end and a load at the other end, to be modelled. A matching resistance can be placed in series with the source. The load is represented by a shunt resistance and capacitance. A shunt matching resistance can also be placed at the load. The matching resistor can have a series inductance ( to represent lead inductance) and a series dc blocking capacitor. In addition, up to six 'features', or discontinuities, can be placed along the track. These can be used to simulate other devices connected along the track, or vias, and can consist of a series inductance or resistance, or a shunt capacitance.
Twin Track Model. In this model a source feeds two lengths of track, with separately defined lengths, terminating loads and matching resistances.
The Star Arrangement provides a versatile way of examining the problem of feeding multiple lengths of track. It can be shown that only one terminating resistance is required when two branches at the end of a long arm are too short too cause transmission line effects themselves.
Crosstalk Model. When two tracks run in parallel with each other, crosstalk on one track due to a signal on the other may occur when the tracks are placed close together. A model is provided to analyse this situation in transmission line terms.
Waveform Display. Three graphs are plotted, showing source, input and load waveforms. Two periods are always shown, and x-scale expansion to x10 is provided. Voltage scales are autoscaled, but can be manually locked to any scale by the user.
Westbay FasTrack Lite can be downloaded and distributed free of charge. As well as showing the principles involved in transmission line effects on pcb tracks, a complete Help File is included with complete technical background to transmission line effects.
System Requirements
Windows 95 or later. 10Mb hard disk space.
Westbay FasTrack is a trademark of Westbay Technology
Ltd
Windows and Windows 95 are trademarks of Microsoft
Corporation