Chapter4

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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.
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