Fundamentals of Measurement Technology (12) Prof Wang Boxiong
Fundamentals of Measurement Technology (12) Prof. Wang Boxiong
5. 3 Modulation and demodulation Modulation is a process of controlling or changing a parameter(amplitude, frequency, or phase)of a high-frequency oscillation by use of a measurand, usually a low-frequency signal >Controlled parameter is the amplitude of the high frequency oscillations: amplitude modulation or AM Frequency varied with the measurand frequency modulation, or FM >Controlled parameter is the phase of the oscillations: phase modulation, or PM
5.3 Modulation and demodulation Modulation is a process of controlling or changing a parameter (amplitude, frequency, or phase) of a high-frequency oscillation by use of a measurand, usually a low-frequency signal. ➢Controlled parameter is the amplitude of the high frequency oscillations: amplitude modulation, or AM. ➢Frequency varied with the measurand: frequency modulation, or FM. ➢Controlled parameter is the phase of the oscillations: phase modulation, or PM
5.3 Modulation and demodulation The measurand, usually a low-frequency signal controlling the high frequency oscillation, is called modulating signal, the high-frequenc oscillation used for carrying the low-frequency signal is known as carrier, and the resulting signal from the modulation process, still a high-frequency oscillation, is known as modulated signal In time domain, modulation is a process in which a certain parameter of the carrier is varied with the modulating signal, while in frequency domain, it is a process of phase-shifting
5.3 Modulation and demodulation The measurand, usually a low-frequency signal controlling the high frequency oscillation, is called modulating signal, the high-frequency oscillation used for carrying the low-frequency signal is known as carrier, and the resulting signal from the modulation process, still a high-frequency oscillation, is known as modulated signal. In time domain, modulation is a process in which a certain parameter of the carrier is varied with the modulating signal, while in frequency domain, it is a process of phase-shifting
5.3 Modulation and demodulation Demodulation is the reverse process of modulation, in which the original low frequency signal the measurand. is recovered from the modulated signal MODEM has wide applications in engineering
5.3 Modulation and demodulation Demodulation is the reverse process of modulation, in which the original lowfrequency signal, the measurand, is recovered from the modulated signal. MODEM has wide applications in engineering
5.3. 1 Amplitude modulation and demodulation 1. Amplitude modulation(AM) Amplitude modulation To multiply a high frequency signal(carrier) with the measurand (modulating signal)so that the carrier amplitude is varied with the measurand xm(t)=x(t)cos 2yfot the Fourier transform pair x(O)y(t)eX()*Y() accordingly, we have x()osx1x()*6(+)+1x(0)*(/-f)(625
5.3.1 Amplitude modulation and demodulation 1. Amplitude modulation(AM) Amplitude modulation: To multiply a highfrequency signal (carrier) with the measurand (modulating signal) so that the carrier amplitude is varied with the measurand. x t x t f t m 2 0 ( ) = ( )cos the Fourier transform pair: x(t) y(t) X ( f ) *Y( f ) ,accordingly, we have: ( ) ( ) ( ) ( ) 0 0 * 0 2 1 ( ) * 2 1 x t cos 2f t X f f + f + X f f − f (5.25)