A resonant (or tuned) circuit is fundamental to the operation of a wide variety of electrical and electronic systems in use today. The resonant circuit is a combination of R, L, and C elements having a frequency response characteristic similar to the one appearing in [Fig. 1].
Resonance curve.
Fig. 1: Resonance curve.
Note in the figure that the response is a maximum for the frequency $f_r$, decreasing to the right and left of this frequency. In other words, for a particular range of frequencies, the response will be near or equal to the maximum. The frequencies to the far left or right have very low voltage or current levels and, for all practical purposes, have little effect on the system's response.
The radio or television receiver has a response curve for each broadcast station of the type indicated in [Fig. 1]. When the receiver is set (or tuned) to a particular station, it is set on or near the frequency $f_r$ of [Fig. 1]. Stations transmitting at frequencies to the far right or left of this resonant frequency are not carried through with significant power to affect the program of interest. The tuning process (setting the dial to $f_r$) as described above is the reason for the terminology tuned circuit. When the response is at or near the maximum, the circuit is said to be in a state of resonance.
The concept of resonance is not limited to electrical or electronic systems. If mechanical impulses are applied to a mechanical system at the proper frequency, the system will enter a state of resonance in which sustained vibrations of very large amplitude will develop. The frequency at which this occurs is called the natural frequency of the system.
The classic example of this effect was the Tacoma Narrows Bridge built in 1940 over Puget Sound in Washington State. Four months after the bridge, with its suspended span of 2800 ft, was completed, a 42-mi/h pulsating gale set the bridge into oscillations at its natural frequency. The amplitude of the oscillations increased to the point where the main span broke up and fell into the water below. It has since been replaced by the new Tacoma Narrows Bridge, completed in 1950.
Fig. 2: Suspended Tacoma Narrows Bridge built in 1940.
The resonant electrical circuit must have both inductance and capacitance. In addition, resistance will always be present due either to the lack of ideal elements or to the control offered on the shape of the resonance curve. When resonance occurs due to the application of the proper frequency (fr), the energy absorbed by one reactive element is the same as that released by another reactive element within the system. In other words, energy pulsates from one reactive element to the other. Therefore, once an ideal (pure C, L) system has reached a state of resonance, it requires no further reactive power since it is self-sustaining. In a practical circuit, there is some resistance associated with the reactive elements that will result in the eventual "damping" of the oscillations between reactive elements. There are two types of resonant circuits: series resonant circuits and parallel resonant circuits. Each will be considered in some detail in this chapter.

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