Techniques of Modulation: Pulse Amplitude Modulation, Pulse Width Modulation, Pulse Position Modulation

Abstract

The modulation technique aims at adapting the frequency band of the informative signal to that of the transmission channel. This avoids a great attenuation of certain frequencies on the transmission channel and also reduces the effects of the noise. In addition, the modulation technique, which requires a transposition of the low frequencies towards the high frequencies, is used during the transmission of the informative signal (or useful signal) on long distances: the narrow band transmission. There are several types of modulation according to the nature of the informative signal (analogical or digital) and that of carrier signal (analogical or digital). In fact, the type of modulation to choose depends of practical application. In the transmission in baseband, i.e. on short distances, any frequency transposition is needed. This type of transmission utilizes copper wire, coaxial cable, the twisted pair or optical fiber as physical support; to transport pulse trains. In this article, we consider the narrow band transmission. We choose an informative signal of low frequency analogical nature (for example the human voice) and a carrier signal of high frequency digital nature (for example the clock signal). For frequency transposition, we use modulator with adapted sensitivity. Indeed, we simulate signals modulated in amplitude (PAM, Pulse Amplitude Modulation), in width (PWM, Pulse Width Modulation) and in position (PPM, Pulse Position Modulation). On an illustrative basis, we simulated the case of an audio informative signal. We analyze obtained results from simulation and recall advantages, disadvantages and applicability of each type of modulation. The modulation software used is ISIS from proteus. Let us mention that obtained results from simulation are little different from those of the real world and that because of the performance of the utilized software and other environmental parameters. PAM, PWM and PPM modulations are particularly employed for the analogical transmissions of the signals on optical fibers, in remote control IRE or telemetry.