Module 12  Modulation Principles
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to 2, Assignment 1, 2
is referred to as the MODULATED WAVE and is the waveform that is transmitted through space. When the modulated wave is received and demodulated, the original component waves (carrier and modulating waves) are reproduced with their respective frequencies, phases, and amplitudes unchanged. Modulation of a carrier can be achieved by any of several methods. Generally, the methods are named for the sinewave characteristic that is altered by the modulation process. In this module, you will study AMPLITUDE MODULATION, which includes CONTINUOUSWAVE MODULATION. You will also learn about two forms of ANGLE MODULATION (FREQUENCY MODULATION and PHASE MODULATION). A special type of modulation, known as PULSE MODULATION, will also be discussed. Before we present the methods involved in developing modulation, you need to study a process that is essential to the modulation of a carrier, known as heterodyning. To help you understand the operation of heterodyning circuits, we will begin with a discussion of LINEAR and NONLINEAR devices. In linear devices, the output rises and falls directly with the input. In nonlinear devices, the output does not rise and fall directly with the input. LINEAR IMPEDANCE Whether the impedance of a device is linear or nonlinear can be determined by comparing the change in current through the device to the change in voltage applied to the device. The simple circuit shown in view (A) of figure 14 is used to explain this process. Figure 14A.  Circuit with one linear impedance. First, the current through the device must be measured as the voltage is varied. Then the current and voltage values can be plotted on a graph, such as the one shown in view (B), to determine the impedance of the device. For example, assume the voltage is varied from 0 to 200 volts in 50volt steps, as shown in view (B). At the first 50volt point, the ammeter reads 0.5 ampere. These ordinates are plotted as point a in view (B). With 100 volts applied, the ammeter reads 1 ampere; this value is plotted as point b. As these steps are continued, the values are plotted as points c and d. These points are connected with a straight line to show the linear relationship between current and voltage. For every change in voltage applied to the device, a proportional change occurs in the current through the device. When the change in current is proportional to the change in applied voltage, the impedance of the device is linear and a straight line is developed in the graph. 111
Figure 14B.  Circuit with one linear impedance. The principle of linear impedance can be extended by connecting two impedance devices in series, as shown in figure 15, view (A). The characteristics of both individual impedances are determined as explained in the preceding section. For example, assume voltmeter V1 shows 50 volts and the ammeter shows 0.5 ampere. Point a in view (B) represents this ordinate. In the same manner, increasing the voltage in increments of 50 volts gives points b, c, and d. Lines Z1 and Z2 show the characteristics of the two impedances. The total voltage of the series combination can be determined by adding the voltages across Z1 and Z2. For example, at 0.5 ampere, point a (50 volts) plus point e (75 volts) produces point i (125 volts). Also, at 1 ampere, point b plus point f produces point j. Line Z1 + Z2 represents the combined voltagecurrent characteristics of the two devices. Figure 15A.  Circuit with two linear impedances. 112
Figure 15B.  Circuit with two linear impedances. View (A) of figure 16 shows two impedances in parallel. View (B) plots the impedances both individually (Z1 and Z2) and combined (Z1 x Z2)/(Z1 + Z2). Note that Z1 and Z2 are not equal. At 100 volts, Z1 has 1 ampere of current plotted at point b and Z2 has 0.5 ampere plotted at point f. The coordinates of the equivalent impedance of the parallel combination are found by adding the current through Z1 to the current through Z2. For example, at 100 volts, point b is added to point f to determine point j (1.5 amperes). 113
Figure 16.  Circuit with parallel linear impedances. Positive or negative voltage values can be used to plot the voltagecurrent graph. Figure 17 shows an example of this situation. First, the voltage versus current is plotted with the battery polarity as shown in view (A). Then the battery polarity is reversed and the remaining voltage versus current points are plotted. As a result, the line shown in view (C) is obtained. Figure 17A.  Linear impedance circuit. 114
Figure 17B.  Linear impedance circuit. Figure 17C.  Linear impedance circuit The battery in view (A) could be replaced with an ac generator, as shown in view (B), to plot the characteristic chart. The same linear voltagecurrent chart would result. Current flow in either direction is directly proportional to the change in voltage. In conclusion, when dc or sinewave voltages are applied to a linear impedance, the current through the impedance will vary directly with a change in the voltage. The device could be a resistor, an aircore inductor, a capacitor, or any other linear device. In other words, if a sinewave generator output is applied to a combination of linear impedances, the resultant current will be a sine wave which is directly proportional to the change in voltage of the generator. The linear impedances do not alter the waveform of the sine wave. The amplitude of the voltage developed across each linear component may vary, or the phase of the wave may shift, but the shape of the wave will remain the same. 115
NONLINEAR IMPEDANCE You have studied that a linear impedance is one in which the resulting current is directly proportional to a change in the applied voltage. A nonlinear impedance is one in which the resulting current is not directly proportional to the change in the applied voltage. View (A) of figure 18 illustrates a circuit which contains a nonlinear impedance (Z), and view (B) shows its voltagecurrent curve. Figure 18A.  Nonlinear impedance circuit. Figure 18B.  Nonlinear impedance circuit. As the applied voltage is varied, ammeter readings which correspond with the various voltages can be recorded. For example, assume that 50 volts yields 0.4 milliampere (point a), 100 volts produces 1 milliampere (point b), and 150 volts causes 2.2 milliamperes (point c). Current through the nonlinear impedance does not vary proportionally with the voltage; the chart is not a straight line. Therefore, Z is a nonlinear impedance; that is, the current through the impedance does not faithfully follow the change in voltage. Various combinations of voltage and current for this particular nonlinear impedance may be obtained by use of this voltagecurrent curve. 116
COMBINED LINEAR AND NONLINEAR IMPEDANCES The series combination of a linear and a nonlinear impedance is illustrated in view (A) of figure 19. The voltagecurrent charts of Z1 and Z2 are shown in view (B). A chart of the combined impedance can be plotted by adding the amount of voltage required to produce a particular current through linear impedance Z1 to the amount of voltage required to produce the same amount of current through nonlinear impedance Z2. The total will be the amount of voltage required to produce that particular current through the series combination. For example, point a (25 volts) is added to point c (50 volts) which yields point e (75 volts); and point b (50 volts) is added to point d (100 volts) which yields point f (150 volts). Intermediate points may be determined in the same manner and the resultant characteristic curve (Z1 + Z2) is obtained for the series combination.
Figure 19A.  Combined linear and nonlinear impedances. Figure 19B.  Combined linear and nonlinear impedances. 117
You should see from this graphic analysis that when a linear impedance is combined with a nonlinear impedance, the resulting characteristic curve is nonlinear. Some examples of nonlinear impedances are crystal diodes, transistors, ironcore transformers, and electron tubes. AC APPLIED TO LINEAR AND NONLINEAR IMPEDANCES Figure 110 illustrates an ac sinewave generator applied to a circuit containing several linear impedances. A sinewave voltage applied to linear impedances will cause a sine wave of current through them. The wave shape across each linear impedance will be identical to the applied waveform.
Figure 110.  Sine wave generator applied to several impedances. The amplitude, on the other hand, may differ from the amplitude of the applied voltage. Furthermore, the phase of the voltage developed by any of the impedances may not be identical to the phase of the voltage across any of the other impedances or the phase of the applied voltage. If an impedance is a reactive component (coil or capacitor), voltage or current may lead or lag, but the wave shape will remain the same. In a linear circuit, the output of the generator is not distorted. The frequency remains the same throughout the entire circuit and no new frequencies are generated. View (A) of figure 111 illustrates a circuit that contains a combination of linear and nonlinear impedances with a sine wave of voltage applied. Impedances Z2, Z3, and Z4 are linear; and Z1 is nonlinear. The result of a linear and nonlinear combination of impedances is a nonlinear waveform. The curve Z, shown in view (B), is the nonlinear curve for the circuit of view (A). Because of the nonlinear impedance, current can flow in the circuit only during the positive alternation of the sinewave generator. If an oscilloscope is connected, as shown in view (A), the waveform across Z3 will not be a sine wave. Figure 112, view (A), illustrates the sine wave from the generator and view (B) shows the waveform across the linear impedance Z3. Notice that the nonlinear impedance Z1 has eliminated the negative half cycles. 118
Figure 111A.  Circuit with nonlinear impedances. Figure 111B.  Circuit with nonlinear impedances. Figure 112A.  Waveform in a circuit with nonlinear impedances. 119
Figure 112B.  Waveform in a circuit with nonlinear impedances. The waveform in view (B) is no longer identical to that of view (A) and the nonlinear impedance network has generated HARMONIC FREQUENCIES. The waveform now consists of the fundamental frequency and its harmonics. (Harmonics were discussed in NEETS, Module 9, Introduction to WaveGeneration and WaveShaping Circuits.) TWO SINE WAVE GENERATORS IN LINEAR CIRCUITS A circuit composed of two sinewave generators, G1 and G2, and two linear impedances, Z1 and Z2, is shown in figure 113. The voltage applied to Z1 and Z2 will be the vector sum of the generator voltages. The sum of the individual instantaneous voltages across each impedance will equal the applied voltages. Figure 113.  Two sinewave generators with linear impedances. If the two generator outputs are of the same frequency, then the waveform across Z1 and Z2 will be a sine wave, as shown in figure 114, views (A) and (B). No new frequencies will be created. Relative amplitude and phase will be determined by the relative values and types of the impedances. 120
NEETS Table of Contents
 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 SolidState Devices and
Power Supplies
 Introduction to Amplifiers
 Introduction to WaveGeneration and WaveShaping
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
 RadioFrequency Communications Principles
 Radar Principles
 The Technician's Handbook, Master Glossary
 Test Methods and Practices
 Introduction to Digital Computers
 Magnetic Recording
 Introduction to Fiber Optics
