文档介绍：Fundamentals of Measurement Technology(12)
Prof. Wang Boxiong
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.
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.
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.
Amplitude modulation and demodulation
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.
the Fourier transform pair:
,accordingly, we have:
()
Amplitude modulation and demodulation
Fig. Principle of AM
Amplitude modulation and demodulation
The product of signal x(t) and the carrier is equivalent in frequency domain to a process of shifting the spectrum of x(t) to the position of carrier frequency f0 (Fig. (b)), that is, the AM process is a process of frequency-shifting.
Sinusoidal signal as the m