C H A P T E R 4 AMPLITUDE MODULATIONS AND DEMODULATIONS
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)
DSB-SC Modulation and Demodulation Modulation = Demodulation Double-sideband, suppressed carrier (DSB-SC) modulation and demodulation.
Example For a baseband signal , find the DSB-SC signal, and identify USB/LSB Example of DSB-SC modulation.
Modulators Multiplier modulators: Output is proportional to the product of two input signals. Nonlinear modulators: Achieved by nonlinear devices. Nonlinear DSB-SC modulator.
Switching modulators: multiplying m(t) by any periodic signal of the fundamental radian frequency ωc Switching modulator for DSB-SC.
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.
Ring Modulator Double Balanced Modulator Ring modulation.
Example: Heterodyning Frequency mixing/conversion Super-heterodyning Sub-heterodyning Frequency mixer or converter.
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.
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
Sideband and Carrier Power Demodulation of AM Signals Rectifier Rectifier detector for AM.
Envelope Detector Envelope detector for AM.
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.
Amplitude Modulation: Single Sideband (SSB) Bandpass filtering SSB spectra from suppressing one DSB sideband.
Hilbert Transform Transfer function of an ideal π/2 phase shifter (Hilbert transformer).
Representation of SSB Signals Expressing SSB spectra in terms of M+(f ) and M−(f ).
Tone Modulation: SSB SSB spectra for tone modulation.
SSB Modulation Systems Phase shift method Using the phase-shift method to generate SSB.
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.
Quadrature Ampltude Modulation (QAM) QAM offers an attractive to SSB-SC. It can be exactly generated without requiring sharp cutoff bandpass filters. Cochannel Interference
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.
Vestigial Sideband (VSB): Modulator and Demodulator VSB modulator and demodulator.
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.
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
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.
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.
Carrier Acquisition in DSB-SC Signal-Squaring Method Generation of coherent demodulation carrier using signal squaring.
Carrier Acquisition in DSB-SC Costas Loop Costas phase-locked loop for the generation of a coherent demodulation carrier.
Figure 4.31 Effect of the number of picture elements on resolution. (a) Poor resolution (b) Better resolution.
Figure 4.33 (a) Horizontal deflection signal. (b) Vertical deflection signal.