Chapter 4. Amplitude Modulation

Husheng LiThe University of Tennessee

AM Signals and Spectra

An AM signal can be written as

Power of AMThe 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 SpectraWe 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

ModulatorsProduct modulators

ModulatorsSquare-law and balanced modulators

Spectrum of V_out

Balanced ModulatorPerfect 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 ModulatorAnother 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 AMConventional 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 haveLittle or no low-frequency content.

Generation of SSB

Two-step SSB generation phase-shift method

V (Vestige) SBVSB achieves a tradeoff between SSB and DSB,

whose signal is passed through the following filter

Frequency ConversionThe 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 VSBFor VSB, the sum of the vestige side band

recovers the original frequency spectrum:

Detected SignalFor imperfect coherent detection, the detected

signal is given by

Summary

Envelope DetectionAn envelope detection can only demodulate

signals with a carrier.

Homework 4

Deadline: Oct. 7, 2013

Quiz 2Problem 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.