TEMPLE EE 4512 - Analog Modulation and Demodulation

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EE4512 Analog and Digital Communications Chapter 6Chapter 6Chapter 6Analog ModulationAnalog Modulationand Demodulationand DemodulationEE4512 Analog and Digital Communications Chapter 6Chapter 6Chapter 6Analog ModulationAnalog Modulationand Demodulationand Demodulation••Amplitude ModulationAmplitude Modulation••Pages 306Pages 306--309309EE4512 Analog and Digital Communications Chapter 6••The analytical signal for double sideband, large carrier The analytical signal for double sideband, large carrier amplitude modulation (DSBamplitude modulation (DSB--LC AM) is:LC AM) is:ssDSBDSB--LC LC AMAM(t(t) =) =AACC(c + (c + s(ts(t)) )) coscos(2(2ππffCCt)t)where where c c is the is the DC bias DC bias or or offset offset and and AACCis the carrier is the carrier amplitude. The continuous analog signal amplitude. The continuous analog signal s(t)s(t)is a baseband is a baseband signal with the information content (voice or music) to be signal with the information content (voice or music) to be transmitted.transmitted.EE4512 Analog and Digital Communications Chapter 6••The baseband power spectral density (PSD) spectrum of The baseband power spectral density (PSD) spectrum of the information signal the information signal s(ts(t) or ) or S(fS(f) for voice has significant ) for voice has significant components below 500 Hz and a bandwidth of < 8 kHz:components below 500 Hz and a bandwidth of < 8 kHz:S(fS(f) =) =FF(s(t(s(t))))The The singlesingle--sidedsidedspectrum of the modulated signal is:spectrum of the modulated signal is:FF(A(ACC(c + (c + s(ts(t)) )) coscos(2(2ππffCCtt)) = )) = S(fS(f––ffCC))Power Spectral Density of s(t)Power Spectral Density of s(t)500 Hz500 Hz8 kHz8 kHzdBdBEE4512 Analog and Digital Communications Chapter 6••TheThesinglesingle--sidedsided(positive frequency axis) spectrum of the (positive frequency axis) spectrum of the modulated signalmodulated signalreplicates the baseband spectrum as a replicates the baseband spectrum as a doubledouble--sidedsidedspectrum about the carrier frequency.spectrum about the carrier frequency.Carrier 25 kHzCarrier 25 kHzDoubleDouble--sided spectrumsided spectrumBaseband spectrumBaseband spectrumEE4512 Analog and Digital Communications Chapter 6••TheThedoubledouble--sided modulated spectrum about the carrier sided modulated spectrum about the carrier frequency has an frequency has an lowerlower((LSBLSB) and ) and upperupper((USBUSB) sideband.) sideband.EE4512 Analog and Digital Communications Chapter 6••The modulated DSBThe modulated DSB--LC AM signal shows an LC AM signal shows an outer envelope outer envelope that follows the polar baseband signal s(t)that follows the polar baseband signal s(t)..EE4512 Analog and Digital Communications Chapter 6••The analytical signal for double sideband, suppressed The analytical signal for double sideband, suppressed carrier amplitude modulation (DSBcarrier amplitude modulation (DSB--SC AM) is:SC AM) is:ssDSBDSB--SC SC AMAM(t(t) =) =AACCs(t) s(t) coscos(2(2ππffCCt)t)where where AACCis the carrier amplitude. The is the carrier amplitude. The singlesingle--sided sided spectrum of the modulated signal replicates the baseband spectrum of the modulated signal replicates the baseband spectrum as a doublespectrum as a double--sided spectrum about the carrier sided spectrum about the carrier frequencyfrequencybut but without without a carrier component.a carrier component.EE4512 Analog and Digital Communications Chapter 6••The analytical signal for double sideband, suppressed The analytical signal for double sideband, suppressed carrier amplitude modulation (DSBcarrier amplitude modulation (DSB--SC AM) is:SC AM) is:ssDSBDSB--SC SC AMAM(t(t) =) =AACCs(t) s(t) coscos(2(2ππffCCt)t)where where AACCis the carrier amplitude. The modulated signal is the carrier amplitude. The modulated signal ssDSBDSB--SC SC AMAM(t(t) looks similar to s(t) but has a temporal but not ) looks similar to s(t) but has a temporal but not spectral carrier component.spectral carrier component.EE4512 Analog and Digital Communications Chapter 6••The DSBThe DSB--LC AM and the DSBLC AM and the DSB--SC AM modulated signals SC AM modulated signals have the same have the same sidebandssidebands..Carrier 25 kHzDSB-LC AMDSB-SC AM No carrierEE4512 Analog and Digital Communications Chapter 6••The modulated DSBThe modulated DSB--LC AM and the DSCLC AM and the DSC--SC AM signals SC AM signals are different.are different.EE4512 Analog and Digital Communications Chapter 6••The modulated DSBThe modulated DSB--SC AM signal has an SC AM signal has an envelope envelope that that follows the polar baseband signal s(t) but not an outer follows the polar baseband signal s(t) but not an outer envelopeenvelope..EE4512 Analog and Digital Communications Chapter 6Chapter 6Chapter 6Analog ModulationAnalog Modulationand Demodulationand Demodulation••Coherent DemodulationCoherent Demodulationof AM Signalsof AM Signals••Pages 309Pages 309--315315EE4512 Analog and Digital Communications Chapter 6••The DSBThe DSB--SC AM signal can be simulated in SC AM signal can be simulated in SystemVueSystemVue..SVU Figure 1.64 modifiedSVU Figure 1.64 modifiedEE4512 Analog and Digital Communications Chapter 6••The DSBThe DSB--SC AM SC AM coherent receivercoherent receiverhas a has a bandpass filterbandpass filtercentered at fcentered at fCCand with a bandwidth of and with a bandwidth of twice twice the bandwidth the bandwidth of s(t) because of theof s(t) because of theLSB and USBLSB and USB. The. Theoutput of the multiplieroutput of the multiplieris is lowpass filteredlowpass filteredwithwitha bandwidth equal toa bandwidth equal tothe bandwidth of s(t).the bandwidth of s(t).r(t) = r(t) = γγssDSBDSB--SCSC(t(t) + ) + n(tn(t))SVU Figure 1.64 modifiedSVU Figure 1.64 modifiedBandpass filterLowpass filterS&M Figure 6S&M Figure 6--44z(t)EE4512 Analog and Digital Communications Chapter 5••The DSBThe DSB--SC AM received signal is r(t) = SC AM received signal is r(t) = γγssDSBDSB--SCSC(t(t) + ) + n(tn(t).).The bandpass filter passes the modulated signal but filters The bandpass filter passes the modulated signal but filters the noise:the noise:z(tz(t) = ) = γγssDSBDSB--SCSC(t(t) + n) + noo(t) S&M Eq. 6.3(t) S&M Eq. 6.3nnoo(t) has a Gaussian distribution.(t) has a


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TEMPLE EE 4512 - Analog Modulation and Demodulation

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