LESSON 5
Text: What is amplitude modulation and percentage of modulation? Pre-text exercises: 1. Memorize the pronunciation ofthe following words: adjacent, assume, coupling, desirable, sufficiently, evaluate, mill watts, technique, discard, shrinkage 2. Read and translate the text: “WHAT IS AMPLITUDE AND PERCENTAGE OF MODULATION” Amplitude Modulation (AM) is a process in which the amplitude of a radio frequency current is made to vary and modify by impressing an audio frequency current on it. A radio frequency current has a constant amplitude in absence of modulation and this constant amplitude RF carries no information, i.e. no audio intelligence and is of no use to radio telephone (voice communication), but has application in Morse code communication. So, to give intelligence to the RF current, audio signal is impressed/superimposed on the RF current in a non-linear modulator circuit; as a result of which carrier current amplitude begins to rise to a maximum value above and below its original amplitude during the positive cycle of the audio signal and during the negative cycle of the audio signal, it falls to a minimum value. This results in the carrier having two outlines of the audio signal; this is because the variation at instant in the amplitude of the carrier wave is directly proportional to the value of the modulating signal. During amplitude modulation, two side band frequencies are also produced. Upper sideband frequencies equal to the carrier frequency plus audio frequency and lower side band frequency is equal to carrier frequency minus audio frequency. So the amplitude modulated carrier occupies a space in frequency spectrum, the width of which is equal to twice the highest modulating frequency. Percentage of modulation: The degree of modulation in an AM wave is expressed by %ge of maximum deviation from the normal amplitude of the carrier RF wave. The effect of such modulated wave is measured by a receiver's ability to reproduce the signal in distorted or undistorted manner. Why over modulation is not desirable? Over modulation is not desirable, i.e. modulation should not exceed 100 %, because if modulation exceeds 100 % there is an interval during the audio cycle when the RF carrier is removed completely from the air thus producing distortion in the transmission. Side bands are the sum and difference frequencies produced at the transmitter by the modulating frequencies. What is Single Side Band (SSB) transmission? At full modulation the carrier in an AM signal requires two thirds of the power but conveys no information. The second side band can be viewed as redundant (overlooking frequency-selective fading in an ionospheric transmission path, that may distort one side band at times). Interference between several carrier frequencies, resulting in steady audio whistles or 'beats' is another disadvantage of AM. Power may be saved and the band occupied by an AM signal in the frequency spectrum can be halved if only one side band is transmitted without carrier. The result is single side band suppressed carrier signal, called simply single side band signal (SSB) transmission. The carrier must be reintroduced at the receiver in such systems and closely adjusted to the original carrier frequency to avoid signal distortion. The introduced carrier carrier must be within 10 or 20 Hertz of the original carrier frequency for adequate intelligibility of voice signals, and stable oscillators are needed for generation of the local carrier. RF Oscillator This is the stage where the carrier frequency intended to be used is generated by means of Crystal Oscillator Circuitry or capacitance-inductance based Variable Frequency Oscillator (VFO). The RF oscillator is designed to have frequency stability and power delivered from it is of little importance, hence can be operated with low voltage power supply with little dissipation of heat.
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