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 23 of the total transmitted power resides in a carrier term that is independent of the signal and thu ID: 566092
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Slide1
Chapter 4. Amplitude Modulation
Husheng Li
The University of TennesseeSlide2
AM Signals and Spectra
An AM signal can be written asSlide3
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!)Slide4
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.Slide5
Homework 4
Deadline: Sept. 30, 2013Slide6
Tone Modulation
If the transmitted signal is a single tone signal, then the tone-modulated DSB waveform is given bySlide7
Modulators
Product modulatorsSlide8
Modulators
Square-law and balanced modulators
Spectrum of
V_outSlide9
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.Slide10
Ring Modulator
Another commonly used modulator is the ring modulator, which uses a carrier to cause the diode to switch on and off.Slide11
Switching Modulators
Efficient high-level modulators are arranged so that undesired modulation products never fully develop and need not be filtered out.Slide12
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.Slide13
SSB in Time Domain
In the time domain, the expression of SSB is given bySlide14
Drawbacks of SSBSlide15
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.Slide16
Generation of SSB
Two-step SSB generation phase-shift methodSlide17
V (Vestige) SB
VSB achieves a tradeoff between SSB and DSB, whose signal is passed through the following filterSlide18
Frequency Conversion
The frequency conversion starts with multiplication by a sinusoid.
Frequency converter (mixer)
Satellite transponderSlide19
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.Slide20
Coherent Demodulation of VSB
For VSB, the sum of the vestige side band recovers the original frequency spectrum:Slide21
Detected Signal
For imperfect coherent detection, the detected signal is given by
SummarySlide22
Envelope Detection
An envelope detection can only demodulate signals with a carrier.Slide23
Homework 4
Deadline: Oct. 7, 2013Slide24
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.