EMI requires a (1) source of energy, (2) a receptor or victim circuit or system, and (3) some coupling path for the energy to get from one place to the other. If there is no energy source, there is no EMI, and if there is no coupling path, there is no EMI. As shown in Figure 2.1, there are four primary coupling modes where energy can transfer from one place to another: inductive, capacitive, radiated, and conducted. Inductive coupling requires a time-varying current source and two “loops”or parallel wires (with return paths), which are magnetically coupled together. Examples might include a power transformer (high di/dt) in a switch-mode power supply coupling to a nearby cable or one “noisy”cable routed in proximity to another. Capacitive coupling requires a time-varying voltage source and two “plates”of metal closely coupled together; these can also be two parallel wires. An example might include a large heat sink of a switch-mode power supply (high dV/dt) that couples to a cable or adjacent PC board.