Capacitance Functions and Effects
The capacitance between closely spaced power and ground planes on a good-sized PCB can be fairly significant. We will see later that this might be used to our advantage in high-speed designs. If the planes are spaced further apart, however, the available capacitance drops quickly. We cannot just blindly assume the capacitance formed by the planes will work to our advantage. Some conditions do apply.
Capacitors provide several different important functions in a circuit. Their ability to store DC charge and to offer relatively low impedance to alternating frequency signals make them useful in power supply circuits and bypass applications. We can think of them as functioning as little batteries here. Since they tend to block DC but allow AC to pass, they are used to "couple" circuit stages together. The signal can pass from one stage to the next, but DC bias voltages are isolated to their own stage. Since the amount of charge that accumulates on their storage plates is a function of time and current flow, they can be used to "time" various circuit functions, such as the length of time a digital camera shutter is open or a circuit gate is open to accept a signal. We will see later that capacitors can be combined in a unique way with inductors to control frequency. Thus they are an important part of tuning and filter circuits.
But sometimes capacitors show up where we don't want or expect them to. For example, consider two traces routed side by side. Their edges can form the plates of a capacitor, and a signal can be coupled from one trace to the other, creating unwanted noise on the other trace. This is an important cause of crosstalk. Wirewound resistors have so much capacitive coupling between their wire turns that they are virtually useless, and almost never used, in higher frequency circuits. Unwanted and undesirable capacitive effects are among the top three problems in high-speed PCB designs (the other two are inductance and EMI).