Amplitude Modulation |
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Amplitude modulation uses the instantaneous amplitude of a modulating signal (voice, music, data, etc.) to directly vary the amplitude of a carrier signal. Modulation index, m, is used to describe the ratio of maximum voltage to minimum voltage in the modulated signal. If the modulating signal is equal in magnitude to the carrier, then m = 1 and the modulated signal varies from a scaled maximum of unity down to zero (see figure below). When m = 0, no modulation of the carrier is performed. If m is greater than 1, the carrier is actually cut off for some period of time, and unwanted harmonics are created at the transmitter output. The pictures here are of my restored Crosley 03CB radio, which was manufactured in 1941, before commercial FM broadcast radio was still in the experimental phase of development. This radio tuned the 540 to 1700 kHz AM band and the 16 to 49 meter shortwave band.
In the frequency domain, the carrier frequency is flanked on both sides by mirror image copies of the modulating signal. ΩM1 = Ωc ± ωm1, ΩM2 = Ωc ± Ωm2 AM General Equation Let the carrier be xc(t) = Xc·sin (Ωct), and the modulating signal be xm(t) = Xm·sin (ωmt) Then x(t) = Xc·[1+m·sin (Ωmt)]·sin (Ωct) Modulation Index In the following examples, the carrier frequency is nine times the modulation frequency. Red (dashed) lines represent the modulation envelope. Blue (solid) lines represent the modulated carrier. Note: AM waveforms created with MathCAD 4.0 software. Related Pages on RF Cafe - Quadrature (I/Q) Modulator Sideband Suppression - Modulation Principles, AM Modulation, NEETS - Modulation Principles, FM Modulation, NEETS |
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