C H A P T E R 4 AMPLITUDE MODULATIONS AND DEMODULATIONS Fundamental of Communication Systems ELCT332 FALL2011

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) Fundamental of Communication Systems ELCT332 FALL2011 2

DSB-SC Modulation and Demodulation = Demodulation Double-sideband, suppressed carrier (DSB-SC) modulation and demodulation. Fundamental of Communication Systems ELCT332 FALL2011 3

Example of DSB-SC modulation. For a baseband signal , find the DSB-SC signal, and identify USB/LSB Example of DSB-SC modulation. Fundamental of Communication Systems ELCT332 FALL2011 4

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. Fundamental of Communication Systems ELCT332 FALL2011 5

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. Fundamental of Communication Systems ELCT332 FALL2011 6

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. Fundamental of Communication Systems ELCT332 FALL2011 7

Double Balanced Modulator Ring Modulator Double Balanced Modulator Ring modulation. Fundamental of Communication Systems ELCT332 FALL2011 8

Frequency mixer or converter. Example: Heterodyning Frequency mixing/conversion Super-heterodyning Sub-heterodyning Frequency mixer or converter. Fundamental of Communication Systems ELCT332 FALL2011 9

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. Fundamental of Communication Systems ELCT332 FALL2011 10

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   Fundamental of Communication Systems ELCT332 FALL2011 11

Rectifier detector for AM. Sideband and Carrier Power Demodulation of AM Signals   Rectifier       Rectifier detector for AM. Fundamental of Communication Systems ELCT332 FALL2011 12

Envelope detector for AM.   Envelope detector for AM. Fundamental of Communication Systems ELCT332 FALL2011 13

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. Fundamental of Communication Systems ELCT332 FALL2011 14

SSB spectra from suppressing one DSB sideband. Amplitude Modulation: Single Sideband (SSB) Bandpass filtering SSB spectra from suppressing one DSB sideband. Fundamental of Communication Systems ELCT332 FALL2011 15

Transfer function of an ideal π/2 phase shifter (Hilbert transformer).     Transfer function of an ideal π/2 phase shifter (Hilbert transformer). Fundamental of Communication Systems ELCT332 FALL2011 16

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 ). Fundamental of Communication Systems ELCT332 FALL2011 17

SSB spectra for tone modulation. Tone Modulation: SSB SSB spectra for tone modulation. Fundamental of Communication Systems ELCT332 FALL2011 18

Using the phase-shift method to generate SSB. SSB Modulation Systems Phase shift method Using the phase-shift method to generate SSB. Fundamental of Communication Systems ELCT332 FALL2011 19

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. Fundamental of Communication Systems ELCT332 FALL2011 20

Quadrature Ampltude Modulation (QAM) QAM offers an attractive to SSB-SC. It can be exactly generated without requiring sharp cutoff bandpass filters.   Cochannel Interference Fundamental of Communication Systems ELCT332 FALL2011 21

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. Fundamental of Communication Systems ELCT332 FALL2011 22

VSB modulator and demodulator. Vestigial Sideband (VSB): Modulator and Demodulator VSB modulator and demodulator.         Fundamental of Communication Systems ELCT332 FALL2011 23

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. Fundamental of Communication Systems ELCT332 FALL2011 24

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 Fundamental of Communication Systems ELCT332 FALL2011 25

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. Fundamental of Communication Systems ELCT332 FALL2011 26

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. Fundamental of Communication Systems ELCT332 FALL2011 27

Generation of coherent demodulation carrier using signal squaring. Carrier Acquisition in DSB-SC Signal-Squaring Method Generation of coherent demodulation carrier using signal squaring. Fundamental of Communication Systems ELCT332 FALL2011 28

Carrier Acquisition in DSB-SC Costas Loop Costas phase-locked loop for the generation of a coherent demodulation carrier. Fundamental of Communication Systems ELCT332 FALL2011 29

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. Fundamental of Communication Systems ELCT332 FALL2011

Figure 4.32 Scanning pattern (raster). Fundamental of Communication Systems ELCT332 FALL2011

Figure 4. 33 (a) Horizontal deflection signal Figure 4.33 (a) Horizontal deflection signal. (b) Vertical deflection signal. Fundamental of Communication Systems ELCT332 FALL2011

Figure 4.34 Television video signal. Fundamental of Communication Systems ELCT332 FALL2011

Figure 4.35 (a) Television transmitter. Fundamental of Communication Systems ELCT332 FALL2011

Figure 4.35 (b) Television receiver. Fundamental of Communication Systems ELCT332 FALL2011 35