Transcript Chapter4
Chapter 4. Amplitude Modulation Husheng Li The University of Tennessee AM Signals and Spectra An AM signal can be written as Power of AM The total transmit power is given by We can prove that, at least 50% (often close to 2/3) of the total transmitted power resides in a carrier term that is independent of the signal and thus conveys no message information. (what a waste!) DSB Signal Spectra We set u=1 and suppress the unmodulated component and obtain the modulated signal: DSB conserves power but requires complicated demodulation circuitry, whereas AM requires increased power to permit simple envelope detection. Homework 4 Deadline: Sept. 30, 2013 Tone Modulation If the transmitted signal is a single tone signal, then the tone-modulated DSB waveform is given by Modulators Product modulators Modulators Square-law and balanced modulators Spectrum of V_out Balanced Modulator Perfect square-law devices are rare; highfrequency DSB is obtained in practice using two AM modulation arranged in a balanced configuration to cancel out the carrier. Ring Modulator Another commonly used modulator is the ring modulator, which uses a carrier to cause the diode to switch on and off. Switching Modulators Efficient high-level modulators are arranged so that undesired modulation products never fully develop and need not be filtered out. Suppressed Sideband AM Conventional AM is wasteful of both transmission power and bandwidth. Suppressing the carrier can reduce the transmit power, while suppressing onesideband can reduce the bandwidth. For suppressing the sideband, we have either SSB or VSB. SSB in Time Domain In the time domain, the expression of SSB is given by Drawbacks of SSB Generation of SSB SSB requires perfect filter actions. But a perfect cutoff at f_c cannot be synthesized. Fortunately, many modulating Signals of practical interest have Little or no low-frequency content. Generation of SSB Two-step SSB generation phase-shift method V (Vestige) SB VSB achieves a tradeoff between SSB and DSB, whose signal is passed through the following filter Frequency Conversion The frequency conversion starts with multiplication by a sinusoid. Frequency converter (mixer) Satellite transponder Coherent Detection In coherent detection, the local oscillator of receiver is exactly synchronized with the carrier in both phase and frequency. We can pick off the pilot carrier by using a narrow bandpass filter, which is called homodyne detection. Coherent Demodulation of VSB For VSB, the sum of the vestige side band recovers the original frequency spectrum: Detected Signal For imperfect coherent detection, the detected signal is given by Summary Envelope Detection An envelope detection can only demodulate signals with a carrier. Homework 4 Deadline: Oct. 7, 2013 Quiz 2 Problem 1. Write down the expression of SSB signal in the time domain, if the base band signal is x(t). Problem 2. Explain the principle of coherent demodulation of VSB signals.