1. C H A P T E R 4 AMPLITUDE MODULATIONS AND DEMODULATIONS
Fundamental of Communication Systems ELCT FALL2011

2. Amplitude Modulation (AM) Frequency Modulation (FM)
Baseband Communications: Message signals are directly transmitted without any modification.
Carrier Communications: Uses modulation to shift the frequency spectrum of a signal
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)
Modulation
Amplitude modulation: Amplitude A(t) is proportional to m(t)
Frequency modulation: Frequency is proportional to m(t)
Phase modulation: Phase is proportional to m(t)
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3. DSB-SC Modulation and Demodulation=
Demodulation
Double-sideband, suppressed carrier (DSB-SC) modulation and demodulation.
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4. Example of DSB-SC modulation.
For a baseband signal , find the DSB-SC signal, and identify USB/LSB
Example of DSB-SC modulation.
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5. Nonlinear DSB-SC modulator.
Modulators
Multiplier modulators: Output is proportional to the product of two input signals.
Nonlinear modulators: Achieved by nonlinear devices.
Nonlinear DSB-SC modulator.
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6. Switching modulator for DSB-SC.
Switching modulators: multiplying m(t) by any periodic signal of the fundamental radian frequency ωc
Switching modulator for DSB-SC.
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7. Diode Bridge Modulator
Multiplication of a signal by a square pulse train is in reality a switching operation
(a) Diode-bridge electronic switch. (b) Series-bridge diode modulator. (c) Shunt-bridge diode modulator.
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8. Double Balanced Modulator
Ring Modulator
Double Balanced Modulator
Ring modulation.
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9. Frequency mixer or converter.
Example: Heterodyning Frequency mixing/conversion
Super-heterodyning
Sub-heterodyning
Frequency mixer or converter.
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10. AM signal and its envelope.
Amplitude Modulation (AM)
DSB-SC demodulation requires the receiver to possess a carrier signal that is synchronized with the incoming carrier.
Condition for envelope detection:
A+m(t)≥0 for all t
AM signal and its envelope.
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11. Tone-modulated AM: (a) µ = 0.5; (b) µ = 1.
Tone Modulation
Message Signals m(t) with Zero Offset
mmin=-mmax
Modulation index
Tone-modulated AM: (a) µ = 0.5; (b) µ = 1.
Message Signals m(t) with Zero Offset
mmin≠-mmax
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12. Rectifier detector for AM.
Sideband and Carrier Power
Demodulation of AM Signals
Rectifier
Rectifier detector for AM.
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13. Envelope detector for AM.
Envelope detector for AM.
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14. Bandwidth-Efficient Amplitude Modulations
For a baseband signal m(t) with bandwidth B Hz, DSB modulations require twice the RF bandwidth of transmit
(a) Original message spectrum and (b) the redundant bandwidth consumption in DSB modulations.
To Improve the spectral efficiency of AM
Single-side (SSB) modulation
Remove either LSB or USB
Quadrature amplitude modulation (QAM)
Sending two messages over the same bandwidth of 2B Hz.
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15. SSB spectra from suppressing one DSB sideband.
Amplitude Modulation: Single Sideband (SSB)
Bandpass filtering
SSB spectra from suppressing one DSB sideband.
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16. Transfer function of an ideal π/2 phase shifter (Hilbert transformer).
Transfer function of an ideal π/2 phase shifter (Hilbert transformer).
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17. Expressing SSB spectra in terms of M+(f ) and M−(f ).
Representation of SSB Signals
Expressing SSB spectra in terms of M+(f ) and M−(f ).
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18. SSB spectra for tone modulation.
Tone Modulation: SSB
SSB spectra for tone modulation.
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19. Using the phase-shift method to generate SSB.
SSB Modulation Systems
Phase shift method
Using the phase-shift method to generate SSB.
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20. SSB Modulation Systems
Selective filtering method
Weaver’s method
Two step modulation
Smaller carrier frequency f first to widen the SSB signal
(a) Relative power spectrum of speech signal and (b) the corresponding USB spectrum.
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21. Quadrature Ampltude Modulation (QAM)
QAM offers an attractive to SSB-SC. It can be exactly generated without requiring sharp cutoff bandpass filters.
Cochannel Interference
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22. Vestigial Sideband (VSB)
VSB modulation system, also called asymmetric sideband, is a comprise between DSB and SSB. VSB signals are relatively easy to generate, and their bandwidth is some what greater than that of SSB signals.
Spectra of the modulating signal and corresponding DSB, SSB, and VSB signals.
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23. VSB modulator and demodulator.
Vestigial Sideband (VSB): Modulator and Demodulator
VSB modulator and demodulator.
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24. VSB modulator and receiver filters.
Example:
The carrier frequency of a certain VSB signal is fc=20kHz, and the baseband signal bandwidth is 6kHz. The VSB shaping filter Hi(f) at the input, which cuts off the lower sideband gradually over 2kHz, is shown in (a). Find the output filter Ho(f) required for distortionless reception.
VSB modulator and receiver filters.
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25. Transmitter filter HT (f), receiver front-end filter HR(f ),
VSB Television System
Transmitter filter HT (f), receiver front-end filter HR(f ),
and the receiver output low-pass filter Ho(f ) in VSB Television systems.
Television signal spectra: (a) DSB video signal plus audio; (b) signal transmitted.
VSB bandwidth: 6MHz
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26. Frequency Division Multiplexing (FDM)
Several signals share the band of a channel, each signal is modulated by different carrier frequency. The various carriers are separated to avoid overlap. These carriers are referred as subcarriers.
L-carrier hierarchical long-haul analog telephone frequency division multiplexing system.
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27. Phase-locked loop and its equivalent circuit.
Phase-Locked Loop (PLL)
A device typically used to track the phase and the frequency of the carrier component of an incoming signal.
Basic PLL Operation
VCO is a an oscillator whose frequency can be linearly controlled by an input voltage
The multiplier output is further filtered by the loop filter and then applied to the input of the VCO
The voltage changes the frequency of the VCO and keeps the loop locked by forcing the VCO output to track the face of the input signal
Phase-locked loop and its equivalent circuit.
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28. Generation of coherent demodulation carrier using signal squaring.
Carrier Acquisition in DSB-SC
Signal-Squaring Method
Generation of coherent demodulation carrier using signal squaring.
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29. Carrier Acquisition in DSB-SC
Costas Loop
Costas phase-locked loop for the generation of a coherent demodulation carrier.
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30. Figure 4. 31 Effect of the number of picture elements on resolution
Figure 4.31 Effect of the number of picture elements on resolution. (a) Poor resolution (b) Better resolution.
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31. Figure 4.32 Scanning pattern (raster).
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32. Figure 4. 33 (a) Horizontal deflection signal
Figure 4.33 (a) Horizontal deflection signal. (b) Vertical deflection signal.
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33. Figure 4.34 Television video signal.
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34. Figure 4.35 (a) Television transmitter.
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35. Figure 4.35 (b) Television receiver.
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