1.2 Why the Field Approach is Important

All electrical behavior from dc to light can be described in terms of the electric and magnetic fields. For many reasons, a field approach to circuit function is very impractical. This is why engineers heavily rely on circuit theory as a working tool. Our understanding of how a circuit functions is closely related to the circuit symbols we have created and to the language we use. When we use capacitors and inductors in our analysis we think of reactances and we generally ignore the fields and energy storage that are inside of these components. In a typical circuit design, the energy that is moved and stored between traces or between traces and a conducting plane is not considered. In high speed logic, this movement of energy must be considered. In fact, this energy must be controlled and dissipated so that the logic can function. The dissipation of this energy can be a serious problem as it can cause board overheating.

Every component is a conductor geometry of some sort. Fields inside the components determine their performance. In a FET (field-effect transistor), there is an electric field between the source and drain. Fields carry operating power and signals to the components over the connecting traces. Getting these fields to the components on a timely basis is handled by traces. The rou4ing of traces is a problem, and we discuss this in great detail later.

Nature does not read our circuit diagrams or symbols. She approaches a circuit as a conductor geometry that allows the flow of electromagnetic energy. Her one goal is to find a way to store less field energy. We use this one fact to get her to perform electrical tasks. A design usually starts by providing a power supply that is a source of energy. Energy leaves this power source as voltage and current. The energy actually flows in electromagnetic fields that follow in the spaces between conductor pairs. These pathways spread the energy, which leads to various losses. Consider the parallel with the flow of water. A dam stores water that we allow to flow in conduits. These conduits reach our homes to supply water for many uses. In the city, we channel water in storm drains to limit water damage during a storm. These same ideas of flow apply to our circuits. If we cooperate with nature, we can make effective use of this energy flow. Our designs must keep the energy from a storm (radiation) from entering our circuits. We want to control energy paths, so the energy required in one circuit does not interfere with another circuit.

At a sufficiently high frequency, components lose their simple circuit identity. A capacitor, for example, can be viewed as having a series resistance and inductance. The inductance implies that a magnetic field is involved in energy transport. We will see that a simple circuit theory approach may not be effective. If we turn to field theory for help, we will not get exact answers. If we want to appreciate what is actually happening in a circuit, both field theory and circuit theory must be applied to the problem. We must learn how to use the field approach so that we can make good engineering decisions.