TUNED CIRCUITS

LEARNING OBJECTIVES

Learning objectives are stated at the beginning of each chapter. These learning objectives serve as a preview of the information you are expected to learn in the chapter. The comprehensive check questions are based on the objectives. By successfully completing the OCC/ECC, you indicate that you have met the objectives and have learned the information. The learning objectives are listed below.

Upon completion of this chapter, you will be able to:

• State the applications of a resonant circuit.
• Identify the conditions that exist in a resonant circuit.
• State and apply the formula for resonant frequency of an a.c. circuit.
• State the effect of changes in inductance (L) and capacitance (C) on resonant frequency (f_r).
• Identify the characteristics peculiar to a series resonant circuit.
• Identify the characteristics peculiar to a parallel resonant circuit.
• State and apply the formula for Q.
• State what is meant by the bandwidth of a resonant circuit and compute the bandwidth for a given circuit.
• Identify the four general types of filters.
• Identify how the series- and parallel-resonant circuit can be used as a bandpass or a band-reject filter.

INTRODUCTION TO TUNED CIRCUITS

When your radio or television set is turned on, many events take place within the "receiver" before you hear the sound or see the picture being sent by the transmitting station.

Many different signals reach the antenna of a radio receiver at the same time. To select a station, the listener adjusts the tuning dial on the radio receiver until the desired station is heard. Within the radio or TV receiver, the actual "selecting" of the desired signal and the rejecting of the unwanted signals are accomplished by what is called a TUNED CIRCUIT. A tuned circuit consists of a coil and a capacitor connected in series or parallel. Later in this chapter you will see the application and advantages of both series- and parallel-tuned circuits. Whenever the characteristics of inductance and capacitance are found in a tuned circuit, the phenomenon as RESONANCE takes place.

You learned earlier in the Navy Electricity and Electronics Training Series, Module 2, chapter 4, that inductive reactance (X_L) and capacitive reactance (X_C) have opposite effects on circuit impedance (Z).

You also learned that if the frequency applied to an LCR circuit causes X_L and X_C to be equal, the circuit is RESONANT.

If you realize that X_L and X_C can be equal ONLY at ONE FREQUENCY (the resonant frequency), then you will have learned the most important single fact about resonant circuits. This fact is the principle that enables tuned circuits in the radio receiver to select one particular frequency and reject all others. This is the reason why so much emphasis is placed on X_L and X_C in the discussions that follow.

Examine figure 1-1. Notice that a basic tuned circuit consists of a coil and a capacitor, connected either in series, view (A), or in parallel, view (B). The resistance (R) in the circuit is usually limited to the inherent resistance of the components (particularly the resistance of the coil). For our purposes we are going to disregard this small resistance in future diagrams and explanations.

Figure 1-1A. - Basic tuned circuits.SERIES TUNED CIRCUIT

Figure 1-1B. - Basic tuned circuits.PARALLEL TUNED CIRCUIT

You have already learned how a coil and a capacitor in an a.c. circuit perform. This action will be the basis of the following discussion about tuned circuits.

Why should you study tuned circuits? Because the tuned circuit that has been described above is used in just about every electronic device, from remote-controlled model airplanes to the most sophisticated space satellite.

You can assume, if you are going to be involved in electricity or electronics, that you will need to have a good working knowledge of tuned circuits and how they are used in electronic and electrical circuits.

REVIEW OF SERIES/PARALLEL A.C. CIRCUITS

First we will review the effects of frequency on a circuit which contains resistance, inductance, and capacitance. This review recaps what you previously learned in the Inductive and Capacitive Reactance chapter in module 2 of the NEETS.

FREQUENCY EFFECTS ON RLC CIRCUITS

Perhaps the most often used control of a radio or television set is the station or channel selector. Of course, the volume, tone, and picture quality controls are adjusted to suit the individual's taste, but very often they are not adjusted when the station is changed. What goes on behind this station selecting? In this chapter, you will learn the basic principles that account for the ability of circuits to "tune" to the desired station.