Module 12—Modulation Principles
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AI-1 to AI-6, Index-1 to 2, Assignment 1 , 2
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. Discuss the generation of a sine wave by describing its three characteristics: amplitude, phase, and frequency.
2. Describe the process of heterodyning.
3. Discuss the development of continuous-wave (CW) modulation.
4. Describe the two primary methods of CW communications keying.
5. Discuss the radio frequency (RF) spectrum usage by CW transmissions.
6. Discuss the advantages and disadvantages of CW transmissions.
7. Explain the operation of typical CW transmitter circuitry.
8. Discuss the method of changing sound waves into electrical impulses.
9. Describe the RF usage of an AM signal.
10. Calculate the percent of modulation for an AM signal.
11. Discuss the difference between high- and low-level modulation.
12. Describe the circuit description, operation, advantages, and disadvantages of the following common AM tube/transistor modulating circuits: plate/collector, control grid/base, and cathode/emitter.
13. Discuss the advantages and disadvantages of AM communications.
INTRODUCTION TO MODULATION PRINCIPLES
People have always had the desire to communicate their ideas to others. Communications have not only been desired from a social point of view, but have been an essential element in the building of civilization. Through communications, people have been able to share ideas of mutual benefit to all mankind. Early attempts to maintain communications between distant points were limited by several factors. For example, the relatively short distance sound would carry and the difficulty of hand-carrying messages over great distances hampered effective communications.
As the potential for the uses of electricity were explored, scientists in the United States and England worked to develop the telegraph. The first practical system was established in London, England, in 1838. Just 20 years later, the final link to connect the major countries with electrical communications was completed when a transatlantic submarine cable was connected. Commercial telegraphy was practically worldwide by 1890. The telegraph key, wire lines, and Morse code made possible almost instantaneous communications between points at great distances. Submarine cables solved the problems of transoceanic communications, but communications with ships at sea and mobile forces were still poor.
In 1897 Marconi demonstrated the first practical wireless transmitter. He sent and received messages over a distance of 8 miles. By 1898 he had demonstrated the usefulness of wireless telegraph communications at sea. In 1899 he established a wireless telegraphic link across the English Channel. His company also established general usage of the wireless telegraph between coastal light ships (floating lighthouses) and land. The first successful transatlantic transmissions were achieved in 1902. From that time to the present, radio communication has grown at an extraordinary rate. Early systems transmitted a few words per minute with doubtful reliability. Today, communications systems reliably transmit information across millions of miles.
The desire to communicate directly by voice, at a higher rate of speed than possible through basic telegraphy, led to further research. That research led to the development of MODULATION. Modulation is the ability to impress intelligence upon a TRANSMISSION MEDIUM, such as radio waves. A transmission medium can be described as light, smoke, sound, wire lines, or radio-frequency waves. In this module, you will study the basic principles of modulation and DEMODULATION (removing intelligence from the medium).
In your studies, you will learn about modulation as it applies to radio-frequency communications. To modulate is to impress the characteristics (intelligence) of one waveform onto a second waveform by varying the amplitude, frequency, phase, or other characteristics of the second waveform. First, however, you will review the characteristics and generation of a sine wave. This review will help you to better understand the principles of modulation. Then, an important principle called HETERODYNING (mixing two frequencies across a nonlinear impedance) will be studied and applied to modulation. Nonlinear impedance will be discussed in the heterodyning section. You will also study several methods of modulating a radio-frequency carrier. You will come to a better understanding of the demodulation principle by studying the various circuits used to demodulate a modulated carrier.
Q-1. What is modulation?
Q-2. What is a transmission medium?
Q-3. What is heterodyning?
Q-4. What is demodulation?
SINE WAVE CHARACTERISTICS
The basic alternating waveform for all complex waveforms is the sine wave. Therefore, an understanding of sine wave characteristics and how they can be acted upon is essential for you to understand modulation. You may want to review sine waves in chapter 1 of NEETS, Module 2, Introduction to Alternating Current and Transformers at this point.
GENERATION OF SINE WAVES
Since numbers represent individual items in a group, arrows can be used to represent quantities that have magnitude and direction. This may be done by using an arrow and a number, as illustrated in figure 1-1, view (A). The number represents the magnitude of force and the arrow represents the direction of the
Figure 1-1A.—Vectors representing magnitude and direction.
View (B) illustrates a simpler method of representation. In this method, the length of the arrow is proportional to the magnitude of force, and the direction of force is indicated by the direction of the arrow. Thus, if an arrow 1-inch long represents 50 pounds of force, then an arrow 2-inches long would represent 100 pounds of force. This method of showing both magnitude and direction is called a VECTOR. To more clearly show the relationships between the amplitude, phase, and frequency of a sine wave, we will use vectors.
Figure 1-1B.—Vectors representing magnitude and direction.
Vector Applied to Sine-Wave Generation
As covered, in NEETS, Module 2, Introduction to Alternating Current and Transformers, an alternating current is generated by rotating a coil in the magnetic field between two magnets. As long as the magnetic field is uniform, the output from the coil will be a sine wave, as shown in figure 1-2. This wave shape is called a sine wave because the voltage of the coil depends on its angular position in the magnetic field.
Figure 1-2.—Sine-wave generator.
This relationship can be expressed mathematically by the formula:
You should recall that the trigonometric ratio (inset in figure 1-3) for the sine in a right triangle (a triangle in which one angle is 90 degrees) is:
When an alternating waveform is generated, the coil is represented by a vector which has a length
that is equal to the maximum output voltage (Emax). The output voltage at any given angle can be found by applying the above trigonometric function. Because the output voltage is in direct relationship with the sine of the angle θ, it is commonly called a sine wave.
You can see this relationship more clearly in figure 1-3 where the coil positions in relation to time are represented by the numbers 0 through 12. The corresponding angular displacements, shown as θ, are shown along the horizontal time axis. The induced voltages (V1 through V12) are plotted along this axis. Connecting the induced voltage points, shown in the figure, forms a sine-wave pattern. This relationship can be proven by taking any coil position and applying the trigonometric function to an equivalent right triangle. When the vector is placed horizontally (position 0), the angle θ is 0 degrees. Since e = Emax sine θ, and the sine of 0 degrees is 0, the output voltage is 0 volts, as shown below:
Figure 1-3.—Generation of sine-wave voltage.
At position 2, the sine of 60 degrees is 0.866 and an output of 86.6 volts is developed.
This relationship is plotted through 360 degrees of rotation. A continuous line is drawn through the successive points and is known as 1 CYCLE of a sine wave. If the time axis were extended for a second revolution of the vector plotted, you would see 2 cycles of the sine wave. The 0-degree point of the second cycle would be the same point as the 360-degree point of the first cycle.
Q-5. What waveform is the basis of all complex waveforms?
Q-6. What is the purpose of using vectors?
Q-7. What is the trigonometric ratio for the sine of an angle?
Q-8. What is the mathematical formula for computing the output voltage from a moving coil in a
A sine wave is used to represent values of electrical current or voltage. The greater its height, the greater the value it represents. As you have studied, a sine wave alternately rises above and then falls below the reference line. That part above the line represents a positive value and is referred to as a POSITIVE ALTERNATION. That part of the cycle below the line has a negative value and is referred to as a NEGATIVE ALTERNATION. The maximum value, above or below the reference line, is called the PEAK AMPLITUDE. The value at any given point along the reference line is called the INSTANTANEOUS AMPLITUDE.
PHASE or PHASE ANGLE indicates how much of a cycle has been completed at any given instant. This merely describes the angle that exists between the starting point of the vector and its position at that instant. The number of degrees of vector rotation and the number of degrees of the resultant sine wave that have been completed will be the same. For example, at time position 2 of figure 1-3, the vector has rotated to 60 degrees and 60 degrees of the resultant sine wave has been completed. Therefore, both are said to have an instantaneous phase angle of 60 degrees.
The rate at which the vector rotates determines the FREQUENCY of the sine wave that is generated; that is, the faster the vector rotates, the more cycles completed in a given time period. The basic time period used is 1 second. If a vector completes one revolution per 1 second, the resultant sine wave has a frequency 1 cycle per second (1 hertz). If the rate of rotation is increased to 1,000 revolutions per second, the frequency of the sine wave generated will be 1,000 cycles per second (1 kilohertz).
Another term that is important in the discussion of a sine wave is its duration, or PERIOD. The period of a cycle is the elapsed time from the beginning of a cycle to its completion. If the vector shown in figure 1-3 were to make 1 revolution per second, each cycle of the resultant sine wave would have a period of 1 second. If it were rotating at a speed of 1,000 revolutions per second, each revolution would require 1/1,000 of a second and the period of the resultant sine wave would be 1/1,000 of a second. This illustrates that the period is related to the frequency. As the number of cycles completed in 1 second increases, the period of each cycle will decrease proportionally. This relationship is shown in the following formulas:
The WAVELENGTH of a sine wave is determined by its physical length. During the period a wave is being generated, its leading edge is moving away from the source at 300,000,000 meters per second. The physical length of the sine wave is determined by the amount of time it takes to complete one full cycle. This wavelength is an important factor in determining the size of equipments used to generate and transmit radio frequencies.
To help you understand the magnitude of the distance a wavefront (the initial part of a wave) travels during 1 cycle, we will compute the wavelengths (l;) of several frequencies. Consider a vector that rotates at 1 revolution per second. The resultant sine wave is transmitted into space by an antenna. As the vector moves from its 0-degree starting position, the wavefront begins to travel away from the antenna. When the vector reaches the 360-degree position, and the sine wave is completed, the sine wave is stretched out over 300,000,000 meters. The reason the sine wave is stretched over such a great distance is that the wavefront has been moving away from the antenna at 300,000,000 meters per second. This is shown in the following example:
If a vector were rotating at 1,000 revolutions per second, its period would be 0.001 second. By applying the formula for wavelength, you would find that the wavelength is 300,000 meters:
Since we normally know the frequency of a sine wave instead of its period, the wavelength is easier to find using the frequency:
Thus, for a sine wave with a frequency of 1,000,000 hertz (1 megahertz), the wavelength would be
300 meters, as shown below:
The higher the frequency, the shorter the wavelength of a sine wave. This important relationship between frequency and wavelength is illustrated in table 1-1.
Table 1-1.—Radio frequency versus wavelength
Q-9. What is the instantaneous amplitude of a sine wave?
Q-10. What term describes how much of a cycle has been completed?
Q-11. What determines the frequency of a sine wave?
Q-12. What is the period of a cycle?
Q-13. How do you calculate the wavelength of a sine wave?
Information waveforms are produced by many different sources and are generally quite low in frequency. A good example is the human voice. The frequencies involved in normal speech vary from one individual to another and cover a wide range. This range can be anywhere from a low of about 90 hertz for a deep bass to as high as 10 kilohertz for a high soprano.
The most important speech frequencies almost entirely fall below 3 kilohertz. Higher frequencies merely help to achieve more perfect sound production. The range of frequencies used to transmit voice intelligence over radio circuits depends on the degree of FIDELITY (the ability to faithfully reproduce the input in the output) that is desired. The minimum frequency range that can be used for the transmission of speech is 500 to 2,000 hertz. The average range used on radiotelephone circuits is 250 to 2,750 hertz.
Frequencies contained within the human voice can be transmitted over telephone lines without difficulty, but transmitting them via radio circuits is not practical. This is because of their extremely long wavelengths and the fact that antennas would have to be constructed with long physical dimensions to transmit or radiate these wavelengths. Generally, antennas have radiating elements that are 1/4, 1/2, 1, or more full wavelengths of the frequency to be radiated. The wavelengths of voice frequencies employed on radiotelephone circuits range from 1,200,000 meters at 250 hertz to 109,090 meters at 2,750 hertz. Even a quarter-wave antenna would require a large area, be expensive to construct, and consume enormous amounts of power.
As studied in NEETS, Module 10, Introduction to Wave Propagation, Transmission Lines, and Antennas, radio frequencies do not have the limitations just described for voice frequencies. Radio waves, given a suitable antenna, can often radiate millions of miles into space. Several methods of modulation can be used to impress voices frequencies onto radio waves for transmission through space.
In the modulation process, waves from the information source are impressed onto a radio-frequency sine wave called a CARRIER. This carrier is sufficiently high in frequency to have a wavelength short enough to be radiated from an antenna of practical dimensions. For example, a carrier frequency of 10 megahertz has a wavelength of 30 meters, as shown below:
Construction of an antenna related to that wavelength does not cause any problems.
An information wave is normally referred to as a MODULATING WAVE. When a modulating wave is impressed on a carrier, the voltages of the modulating wave and the carrier are combined in such a manner as to produce a COMPLEX WAVE (a wave composed of two or more parts). This complex wave
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