File : 1.7 MB, 230 pages
1. Amplitude Modulation
2. Angle and Pulse Modulation
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.
INTRODUCTION to ANGLE AND PULSE MODULATION
In chapter 1 you learned that modulation of a carrier frequency was necessary to allow fast communications between two points. As the volume of transmissions increased, a need for more reliable methods of communication was realized. In this chapter you will study angle modulation and pulse modulation. These two types of modulation have been developed to overcome one of the main disadvantages of amplitude modulation – susceptibility to noise interference. In addition, a special application of pulse type modulation for ranging and detection equipment will be discussed.
ANGLE MODULATION is modulation in which the angle of a sine-wave carrier is varied by a modulating wave. FREQUENCY MODULATION (fm) and PHASE MODULATION (pm) are two types of angle modulation. In frequency modulation the modulating signal causes the carrier frequency to vary. These variations are controlled by both the frequency and the amplitude of the modulating wave. In phase modulation the phase of the carrier is controlled by the modulating waveform. Let’s study these modulation methods for an Understanding of their similarities and differences.
INTRODUCTION TO DEMODULATION
In chapters 1 and 2 you studied how to apply intelligence (modulation) to an rf-carrier wave. Carrier modulation allows the transmission of modulating frequencies without the use of transmission wire as a medium. However, for the communication process to be completed or to be useful, the intelligence must be recovered in its original form at the receiving site. The process of re-creating original modulating frequencies (intelligence) from the rf carrier is referred to as DEMODULATION or DETECTION. Each type of modulation is different and requires different techniques to recover (demodulate) the intelligence. In this chapter we will discuss ways of demodulating AM, cw, fm, phase, and pulse modulation. The circuit in which restoration is achieved is called the DETECTOR or DEMODULATOR (both of these terms are used in NEETS). The term demodulator is used because the demodulation process is considered to be the opposite of modulation. The output of an ideal detector must be an exact reproduction of the modulation existing on the rf wave. Failure to accurately recover this intelligence will result in distortion and degradation of the demodulated signal and intelligence will be lost. The distortion may be in amplitude, frequency, or phase, depending on the nature of the demodulator. A nonlinear device is required for demodulation. This nonlinear device is required to recover the modulating frequencies from the rf envelope. Solid-state detector circuits may be either a pn junction diode or the input junction of a transistor. In electron-tube circuits, either a diode or the grid or plate circuits of a triode electron tube may be used as the nonlinear device.
Download : pdf1