Electrical Components of Printed Circuit Board Design
The Three Basics
When we think about all the challenging complexities of high-speed designs, it is sometimes hard to believe how few components there really are in the world of electronics. Are you ready for this? There are only three passive components we will ever have to worry about in high-speed design issues. Only three! They are (a) resistors, (b) capacitors, and (c) inductors. Virtually everything we encounter in board designs can be described and analyzed in terms of these three components and their combination and effects.
To go with them, we may need an energy source. If so, we can consider it to be either a pure current source or a pure voltage source (an example of a voltage source might be a battery). And, as we will see later, the fundamental nature of inductance is electromagnetic energy, which relates to EMI and crosstalk. But pure energy sources are simple concepts, and electromagnetic energy is at the heart of inductance. So this does not negate the statement that we need understand only three basic passive components.
We usually consider semiconductor devices to be active components. But even these, in steady state, can be analyzed in terms of a simple energy source and just the three passive components, as we shall see. A clock driver or a logic gate at its "one" or "zero" output level, for example, can be considered to be at a steady state. (Note: A semiconductor device during switching between states, an amplifier with varying input, and circuits with feedback—such as control loops—often, but not always, are a little more difficult to analyze.)
So why are high-speed design issues so difficult? Well, actually, they aren't so difficult if you really understand the behavior of the three basic components and how they interact. The problem is that many people, even many degreed engineers, do not really understand them. And that is a shame, because that single, simple truth is in large part the reason for so much confusion and mystery about high-speed design problems.
This chapter focuses on the three basic components and their fundamental natures. Hydraulic analogs are used as examples. The next chapters will cover their electrical effects (resistance and reactance) and what happens when we combine resistance and reactance to get impedance.
The discussions in this chapter relate to "ideal" components: ideal resistors, capacitors, and inductors. Ideal components are "pure" components. That is, an ideal resistor is a pure resistor with no (parasitic) capacitance or inductance associated with it. An ideal inductor has no resistance or capacitance associated with it, and so on.
In reality, of course, no component is ideal or pure. But once we learn to deal with these ideal components, and their combinations, we will then know how to deal with more realistic situations (e.g., capacitors with lead inductance).