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Module 9—Introduction to Wave- Generation and Wave-Shaping

Chapter 1: Pages 1-1 through 1-10

Pages i - ix, 1-1 to 1-10, 1-11 to 1-20, 1-21 to 1-30, 1-31 to 1-40, 1-41 to 1-52, 2-1 to 2-10, 2-11 to 2-20, 2-21 to 2-30, 2-31 to 2-38,

3-1 to 3-10, 3-11 to 3-20, 3-21 to 3-30, 3-31 to 3-40, 3-41 to 3-50, 3-51 to 3-56, 4-1 to 4-10, 4-11 to 4-20, 4-21 to 4-30,

4-31- to 4-40, 4-41 to 4-50, 4-51 to 4-61, Index

**CHAPTER 1 **

**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:

1. State the applications of a resonant circuit.

2. Identify the conditions that exist in a resonant circuit.

3. State and apply the formula for resonant frequency of an AC circuit.

4. State the effect of changes in inductance (L) and capacitance (C) on resonant frequency (fr).

5. Identify the characteristics peculiar to a series resonant circuit.

6. Identify the characteristics peculiar to a parallel resonant circuit.

7. State and apply the formula for Q.

8. State what is meant by the bandwidth of a resonant circuit and compute the bandwidth for a given circuit.

9. Identify the four general types of filters.

10. 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 (XL) and capacitive reactance (XC) have opposite effects on circuit impedance (Z).

1-1

You also learned that if the frequency applied to an LCR circuit causes X

If you realize that X

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 AC 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.

1-2

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 AC 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.

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.

In an AC circuit, an inductor produces inductive reactance which causes the current to lag the voltage by 90
degrees. Because the inductor "reacts" to a changing current, it is known as a reactive component. The opposition
that an inductor presents to AC is called inductive reactance (X_{L}). This opposition is caused by the
inductor "reacting" to the changing current of the AC source. Both the inductance and the frequency determine the
magnitude of this reactance. This relationship is stated by the formula:

X_{L} = 2πfL

Where:

X_{L} = the inductive reactance in ohms

f = the frequency in hertz

L = the inductance in henries

π = 3.1416

As shown in the equation, any increase in frequency, or "f," will cause a corresponding increase of inductive
reactance, or "X_{L}." Therefore, the INDUCTIVE REACTANCE VARIES DIRECTLY WITH THE FREQUENCY. As you can
see, the higher the frequency, the greater the inductive reactance; the lower the frequency, the less the
inductive reactance for a given inductor. This relationship is illustrated in figure 1-2. Increasing values of X_{L}
are plotted in terms of increasing frequency. Starting at the lower left corner with zero frequency, the inductive
reactance is zero. As the frequency is increased (reading to the right), the inductive reactance is shown to
increase in direct proportion.

1-3

Figure 1-2.—Effect of frequency on inductive reactance.

In an AC circuit, a capacitor produces a
reactance which causes the current to lead the voltage by 90 degrees. Because the capacitor "reacts" to a changing
voltage, it is known as a reactive component. The opposition a capacitor presents to AC is called capacitive
reactance (X_{C}). The opposition is caused by the capacitor "reacting" to the changing voltage of the AC
source. The formula for capacitive reactance is:

Where:

X_{C} = the capacitive reactance in ohms

f = the frequency in hertz

C = the capacitance in farads

π = 3.1416

In contrast to the inductive reactance, this equation indicates that the CAPACITIVE REACTANCE VARIES INVERSELY WITH THE FREQUENCY. When f = 0, X

As shown in figure 1-3, the effect of capacitance is opposite to that of inductance.
Remember, capacitance causes the current to lead the voltage by 90 degrees, while inductance causes the current to
lag the voltage by 90 degrees.

1-4

Figure 1-3.—Effect of frequency on capacitive reactance.

In the expression for inductive reactance, X_{L}
= 2πfL, and in the expression for capacitive reactance,

both contain "f" (frequency). Any change of frequency changes the reactance of the circuit components as already explained. So far, nothing has been said about the effect of frequency on resistance. In an Ohm's law relationship, such as R = E/I no "f" is involved. Thus, for all practical purposes, a change of frequency does not affect the resistance of the circuit. If a 60-hertz AC voltage causes 20 milliamperes of current in a resistive circuit, then the same voltage at 2000 hertz, for example, would still cause 20 milliamperes to flow.

NOTE: Remember that the total opposition to AC is called impedance (Z). Impedance is the combination of inductive reactance (X

AC circuits that contain both an
inductor and a capacitor have interesting characteristics because of the opposing effects of L and C. X_{L}
and X_{C} may be treated as reactors which are 180 degrees out of phase. As shown in figure 1-2, the
vector for X_{L}
should be plotted above the baseline; vector for X_{C}, figure 1-3, should be plotted below the baseline.
In a series circuit, the effective reactance, or what is termed the RESULTANT REACTANCE, is the difference between
the individual reactances. As an equation, the resultant reactance is:

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X = X_{L} - X_{C}

Suppose an AC circuit contains an XL of 300 ohms and an XC of 250 ohms. The resultant reactance

X = X_{L} - X_{C} = 300 - 250 = 50 ohms (inductive)

In some cases, the X

Q-1. What is the relationship between frequency and the values of (a) X

Q-2. In an AC circuit that contains both an inductor and a capacitor, what term is used for the difference between the individual reactances?

**RESONANCE**

For every combination of L and C, there is only ONE frequency (in both series and parallel circuits) that causes X

RESONANCE.

Each LCR circuit responds to resonant frequency differently than it does to any other frequency. Because of this, an LCR circuit has the ability to separate frequencies. For example, suppose the TV or radio station you want to see or hear is broadcasting at the resonant frequency. The LC "tuner" in your set can divide the frequencies, picking out the resonant frequency and rejecting the other frequencies. Thus, the tuner selects the station you want and rejects all other stations. If you decide to select another station, you can change the frequency by tuning the resonant circuit to the desired frequency.

As stated before, the frequency at which X_{L} equals X_{C} (in a given circuit) is known as
the resonant frequency of that circuit. Based on this, the following formula has been derived to find the exact
resonant frequency when the values of circuit components are known:

There are two important points to remember about this formula. First, the resonant frequency found when using the formula will cause the reactances (X

An increase in the value of either L or C, or both L and C, will lower the resonant frequency of a given circuit. A decrease in the value of L or C, or both L and C, will raise the resonant frequency of a given circuit.

1-6

The symbol for resonant frequency used in this text is f. Different texts and references may use other symbols for resonant frequency, such as f

been standardized while others have been left to the discretion of the writer. When you study, apply the rules given by the writer of the text or reference; by doing so, you should have no trouble with

nonstandard symbols and designations.

The resonant frequency formula in this text is:

Where:

f_{r} = the resonant frequency in Hertz

L = the inductance in Heries

C = the capacitance in Farads

π = 3.1416

By substituting the constant .159 for the quantity

the formula can be simplified to the following:

Let's use this formula to figure the resonant frequency (f_{r}). The circuit is shown in the practice
tank circuit of figure 1-4.

Figure 1-4.—Practice tank circuit.

1-7

Given:

L = 2mH (2 x 10^{-3} H)

C = 300pF (300 x 10^{-12} F)

Solution:

The important point here is not the formula nor the mathematics. In fact, you may never have to compute a resonant frequency. The important point is for you to see that any given combination of L and C can be resonant at only one frequency; in this case, 205 kHz.

The universal reactance curves of figures 1-2 and 1-3 are joined
in figure 1-5 to show the relative values of X_{L} and X_{L} at resonance, below resonance, and
above resonance.

1-8

Figure 1-5.—Relationship between X_{L} and X_{C} as frequency increases.

First, note that f

Second, look at the area of the curves to the left of f

Lastly, look at the area of the curves to the right of f. This area shows the relative reactances of the circuit at frequencies ABOVE resonance. To these HIGHER frequencies, X

to have some inductance. Because of this, there will be a phase shift.

** RESONANT CIRCUITS**

Resonant circuits may be designed as series resonant or parallel resonant. Each has the ability to discriminate between its resonant frequency and all other frequencies. How this is accomplished by both series- and parallel-LC circuits is the subject of the next section.

NOTE: Practical circuits are often more complex and difficult to understand than simplified versions. Simplified versions contain all of the basic features of a practical circuit, but leave out the nonessential features. For this reason, we will first look at the IDEAL SERIES-RESONANT CIRCUIT— a circuit that really doesn't exist except for our purposes here.

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The ideal series-resonant circuit contains no resistance; it consists of only inductance and capacitance in series with each other and with the source voltage. In this respect, it has the same characteristics of the series circuits you have studied previously. Remember that current is the same in all parts of a series circuit because there is only one path for current.

Each LC circuit responds differently to different input frequencies. In the following paragraphs, we will analyze what happens internally in a series-LC circuit when frequencies at resonance, below resonance, and above resonance are applied. The L and C values in the circuit are those used in the problem just studied under resonant-frequency. The frequencies applied are the three inputs from figure

1-6. Note that the resonant frequency of each of these components is 205 kHz, as figured in the problem.

Figure 1-6.—Output of the resonant circuit.

How the Ideal Series-LC Circuit Responds to the Resonant Frequency (205 kHz)

Given:

Note: You are given the values of X

First, note that
X_{L} and XC are equal. This shows that the circuit is resonant to the applied frequency of 205 kHz.
X_{L} and X_{C} are opposite in effect; therefore, they subtract to zero. (2580 ohms - 2580
ohms = zero.) At resonance, then, X = zero. In our theoretically perfect circuit with zero resistance and zero
reactance, the total opposition to current (Z) must also be zero.

1-10

Introduction to Matter, Energy, and Direct Current, Introduction to Alternating Current and Transformers, Introduction to Circuit Protection, Control, and Measurement, Introduction to Electrical Conductors, Wiring Techniques, and Schematic Reading, Introduction to Generators and Motors, Introduction to Electronic Emission, Tubes, and Power Supplies, Introduction to Solid-State Devices and Power Supplies, Introduction to Amplifiers, Introduction to Wave-Generation and Wave-Shaping Circuits, Introduction to Wave Propagation, Transmission Lines, and Antennas, Microwave Principles, Modulation Principles, Introduction to Number Systems and Logic Circuits, Introduction to Microelectronics, Principles of Synchros, Servos, and Gyros, Introduction to Test Equipment, Radio-Frequency Communications Principles, Radar Principles, The Technician's Handbook, Master Glossary, Test Methods and Practices, Introduction to Digital Computers, Magnetic Recording, Introduction to Fiber Optics