5.3. 1 Amplitude modulation and demodulation Fig 5.16 shows another example of AM using a strain-gage bridge Amplifi dc amplifier oc amplifier Strain gage △R@0-10Hz 3000 HZ ig. 5. 16 Application of amplitude modulation (1)
5.3.1 Amplitude modulation and demodulation Fig. 5.16 shows another example of AM using a strain-gage bridge. Fig. 5.16 Application of amplitude modulation (1)
5.3. 1 Amplitude modulation and demodulation train Corrier signa spectrum 10 HZ 3000Hz Amplifier Amplifier input frequency response signal spectrum 2.990Hz 3,010Hz Fig. 5. 16 Application of amplitude modulation (2
5.3.1 Amplitude modulation and demodulation Fig. 5.16 Application of amplitude modulation (2)
5.3. 1 Amplitude modulation and demodulation Demodulation of amplitude modulation Demodulation methods for amplitude modulation synchronizing demodulation, phase-sensible demodulation and rectifying-detection O Synchronizing demodulation In time-domain there exists the following relation x(t cos 2 fot. cos 2myfot=( +x(t cos 4 f,t (5.34 The synchronizing demodulation is simple in method, but it requires a linear multiplier of good quality to avoid signal distortion
5.3.1 Amplitude modulation and demodulation 2. Demodulation of amplitude modulation Demodulation methods for amplitude modulation: synchronizing demodulation, phase-sensible demodulation and rectifying-detection. ① Synchronizing demodulation In time-domain, there exists the following relation: ( ) ( ) x(t) f t x t x t f t f t 0 0 4 0 cos 2 1 2 cos 2 cos 2 = + (5.34) The synchronizing demodulation is simple in method, but it requires a linear multiplier of good quality to avoid signal distortion
5.3. 1 Amplitude modulation and demodulation Modulator MULT AMPL Carrie VVV oscillator MULT Low-pass Demodulator filter Fig. 5. 19 Principle of synchronizing demodulation
5.3.1 Amplitude modulation and demodulation Fig. 5.19 Principle of synchronizing demodulation
5.3. 1 Amplitude modulation and demodulation 2 Rectifying detection Applying a dc bias a to the modulating signal, the biased signal has a positive voltage magnitude(Fig. 5.20(a) The modulated signal xm(t), after the amplitude modulation, will have an envelope with the same shape as the original signal. Through simple(half-wave or full-wave) rectification and filtering, it is possible to recover the original signal. The added dc bias must be removed accurately after the rectification
5.3.1 Amplitude modulation and demodulation ② Rectifying detection Applying a DC bias A to the modulating signal, the biased signal has a positive voltage magnitude(Fig.5.20(a)). The modulated signal xm(t), after the amplitude modulation, will have an envelope with the same shape as the original signal. Through simple (half-wave or full-wave) rectification and filtering, it is possible to recover the original signal. The added DC bias must be removed accurately after the rectification