Amplitude Modulation (AM) Process Overview

Amplitude Modulation (AM) Process Overview Modulation is the process of modifying the characteristic of one signal in accordance with some characteristic of another signal. In most cases, the information signal, be it voice, video, binary data, or some other information, is normally used to modify a higher-frequency signal known as the carrier. The information signal is usually called the modulating signal, and the higher-frequency signal which is being modulated is called the carrier or modulated wave. The carrier is usually a sine wave, while the information signal can be of any shape, permitting both analog and digital signals to be transmitted. In most cases, the carrier frequency is considerably higher than the highest information frequency to be transmitted. 4.2 Amplitude Modulation (AM) Amplitude modulation is the process of changing the amplitude of a relatively high frequency carrier signal in proportion with the instantaneous value of the modulating signal (information). The carrier frequency remains constant durin g the modulation process but that its amplitude varies in accordance with the modulating signal. An increase in the modulating signal amplitude causes the amplitude of the carrier to increase. Both the positive and negative peaks of the carrier wave vary with the modulating signal. An increase or decrease in the amplitude of the modulating signal causes a corresponding increase or decrease in both the positive and negative peaks of the carrier amplitude. If you interconnect the positive and negative peaks of the carrier waveform with an imaginary line, then you re-create the exact shape of the modulating information signal. This imaginary line on the carrier waveform is known as the envelope, and it is the same as the modulating signal. Amplitude modulation that results in two sidebands and a carrier is often called double sideband amplitude modulation (DSB-AM). In its basic form, amplitude modulation produces a signal with power concentrated at the carrier frequency and in two adja cent sidebands. Each sideband is equal in bandwidth to that of the modulating signal and is a mirror image of the other. Amplitude modulation is inefficient in terms of power usage and much of it is wasted. At least two-thirds of the power is concentrated in the carrier signal, which carries no useful information; the remaining power is split between two identical sidebands, though only one of these is needed since they contain identical information. 4.2.1 Mathematical Representation of AM Suppose we wish to modulate a simple sine wave on a carrier wave. The equation for the carrier wave of frequency fc, taking its phase to be a reference phase of zero, is The equation for the simple sine wave of frequency fm (the signal we wish to broadcast) is Amplitude modulation is performed simply by adding vm(t) to Vc. The amplitude-modulated signal is then The formula for vam(t) above may be written The broadcast signal consists of the carrier wave plus two sinusoidal waves each with a freque ncy slightly different from fc, known as sidebands. For the sinusoidal signals used here, these are at fc + fm and fc − fm. As long as the broadcast (carrier wave) frequencies are sufficiently spaced out so that these side bands do not overlap, stations will not interfere with one another.