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; high- frequency 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 one- sideband 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.