Resonance can be thought of as a preferred frequency of vibration. In other words, it is a frequency at which a system operates with reduced limitation. It is exploited in a variety of areas, for example, a good mechanical resonance can be used for the construction of acoustic musical instruments. In this case, we strive to maintain the resonant oscillation in order to enhance the instrument's sustain. On the other hand, we might want to control or limit the resonance, as in an automotive suspension system. In electrical systems, resonance tends to produce either a maximum or a minimum response to current or voltage. As a result, resonant systems can be used to filter out or select specific frequencies across the spectrum. Obvious uses include tuning circuits, oscillators, filters and the like.
There are two basic forms of resonance for electrical circuits: series RLC resonance and parallel RLC resonance. Series resonance tends to be the less complicated of the two. Both types share many similarities but in some respects they are mirror images of each other. Series and parallel resonant circuits both exhibit wide fluctuations in impedance magnitude and phase across the frequency spectrum. In the case of series resonance, the impedance is at a minimum at the resonance frequency. This implies a current maximum if the circuit is driven by a voltage source. In contrast, parallel resonance produces an impedance peak at resonance. This implies a voltage maximum if the circuit is driven by a current source.
1“\(Q\)” as used in this chapter is not to be confused with the \(Q\) used to represent reactive power, although it is related to the \(Q\), or quality factor, of an inductor.