Fast Circuit Boards

3Transmission Lines—Part 2

3.1 Introduction

There is no polarity associated with field energy. One field does not cancel another. Consider the wakes of two boats on a lake. The waves will cross each other without any cancellation. Currents and voltages add and subtract in circuit theory and this represents one of the basic differences in how electrical phenomena is viewed. Another problem is that a rise in electric field intensity might occur while the magnetic field intensity is decreasing. This means that the term “rise time” must be used carefully. It would have been better to refer to transition time instead of rise time but that would defy convention. In the discussions that follow, it will be convenient to stress waveforms in terms of voltage. Keep in mind that there will usually be an associated current waveform present. These waveforms are drawn two‐dimensionally and what they represent are three‐dimensional fields. For traces on a circuit board the fields are usually cylindrical. When the path spreads out there are reflections and delays. This spreading of waves is three dimensional and not easily shown on the printed page. When there is spreading, there can be problems. The reader is asked to use his or her imagination to picture the wave action as outlined by the current path. The fact that the field lines are perpendicular to the direction of wave motion adds to the difficulty.

3.2 Energy Sources

Circuit boards can be operated from batteries, utility power, or ...