EE4512 Analog and Digital Communications Chapter 6 Chapter 6 Chapter 6 Analog Modulation Analog Modulation and Demodulation and Demodulation EE4512 Analog and Digital Communications Chapter 6 Chapter 6 Chapter 6 Analog Modulation Analog Modulation and Demodulation and Demodulation • • Amplitude Modulation Amplitude Modulation • • Pages 306 Pages 306 - - 309 309 EE4512 Analog and Digital Communications Chapter 6 • • The analytical signal for double sideband, large carrier The analytical signal for double sideband, large carrier amplitude modulation (DSB amplitude modulation (DSB - - LC AM) is: LC AM) is: s s DSB DSB - - LC LC AM AM (t (t ) = ) = A A C C (c + (c + s(t s(t )) )) cos cos (2 (2   f f C C t) t) where where c c is the is the DC bias DC bias or or offset offset and and A A C C is 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(t s(t ) or ) or S(f S(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(f S(f ) = ) = F F (s(t (s(t )) )) The The single single - - sided sided spectrum of the modulated signal is: spectrum of the modulated signal is: F F (A (A C C (c + (c + s(t s(t )) )) cos cos (2 (2   f f C C t t )) = )) = S(f S(f – – f f C C ) ) Power Spectral Density of s(t) Power Spectral Density of s(t) 500 Hz 500 Hz 8 kHz 8 kHz dB dB EE4512 Analog and Digital Communications Chapter 6 • • The The single single - - sided sided (positive frequency axis) spectrum of the (positive frequency axis) spectrum of the modulated signal modulated signal replicates the baseband spectrum as a replicates the baseband spectrum as a double double - - sided sided spectrum about the carrier frequency. spectrum about the carrier frequency. Carrier 25 kHz Carrier 25 kHz Double Double - - sided spectrum sided spectrum Baseband spectrum Baseband spectrum EE4512 Analog and Digital Communications Chapter 6 • • The The double double - - sided modulated spectrum about the carrier sided modulated spectrum about the carrier frequency has an frequency has an lower lower ( ( LSB LSB ) and ) and upper upper ( ( USB USB ) sideband. ) sideband. EE4512 Analog and Digital Communications Chapter 6 • • The modulated DSB The 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 (DSB carrier amplitude modulation (DSB - - SC AM) is: SC AM) is: s s DSB DSB - - SC SC AM AM (t (t ) = ) = A A C C s(t) s(t) cos cos (2 (2   f f C C t) t) where where A A C C is the carrier amplitude. The is the carrier amplitude. The single single - - sided sided spectrum of the modulated signal replicates the baseband spectrum of the modulated signal replicates the baseband spectrum as a double spectrum as a double - - sided spectrum about the carrier sided spectrum about the carrier frequency frequency but 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 (DSB carrier amplitude modulation (DSB - - SC AM) is: SC AM) is: s s DSB DSB - - SC SC AM AM (t (t ) = ) = A A C C s(t) s(t) cos cos (2 (2   f f C C t) t) where where A A C C is the carrier amplitude. The modulated signal is the carrier amplitude. The modulated signal s s DSB DSB - - SC SC AM AM (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 DSB The DSB - - LC AM and the DSB LC AM and the DSB - - SC AM modulated signals SC AM modulated signals have the same have the same sidebands sidebands . . Carrier 25 kHz DSB-LC AM DSB-SC AM No carrier [...]...EE4512 Analog and Digital Communications Chapter 6 • The modulated DSB-LC AM and the DSC-SC AM signals are different EE4512 Analog and Digital Communications Chapter 6 • The modulated DSB-SC AM signal has an envelope that follows the polar baseband signal s(t) but not an outer envelope EE4512 Analog and Digital Communications Chapter 6 Analog Modulation and Demodulation • Coherent Demodulation. .. amplitude of the demodulated signal varies with ∆f: EE4512 Analog and Digital Communications Chapter 6 Analog Modulation and Demodulation • Non-coherent Demodulation of AM Signals • Pages 315-326 Chapter 6 EE4512 Analog and Digital Communications Chapter 6 • The non-coherent AM (DSB-LC) receiver uses an envelope detector implemented as a semiconductor diode and a lowpass filter: The DSB-LC AM analytical signal... 6 EE4512 Analog and Digital Communications Chapter 6 • The DSB-SC AM coherent receiver has a bandpass filter centered at fC and with a bandwidth of twice the bandwidth of s(t) because of the LSB and USB The output of the multiplier is lowpass filtered with a bandwidth equal to the bandwidth of s(t) r(t) = γ sDSB-SC(t) + n(t) • The DSB-SC AM received signal is r(t) = γ sDSB-SC(t) + n(t) The bandpass... EE4512 Analog and Digital Communications Chapter 6 • The non-coherent AM (DSB-LC) receiver is the crystal radio which needs no batteries! Power for the highimpedance ceramic earphone is obtained directly from the transmitted signal For simplicity, the RF BPF is omitted and the audio frequency filter is a simple RC network EE4512 Analog and Digital Communications Chapter 6 Analog Modulation and Demodulation. .. and Demodulation • Frequency Modulation and Phase Modulation • Pages 334-343 Chapter 6 EE4512 Analog and Digital Communications Chapter 6 • The analytical signal for an analog phase modulated (PM) signal is: sPM(t) = AC cos [2π fC t + α s(t)] S&M Eq 6.53 where α is the phase modulation constant rad/V and AC is the carrier amplitude The continuous analog signal s(t) is a baseband signal with the information... S&M Eq 6.3 no(t) has a Gaussian distribution The bandpass filter has a center frequency of fC = 25 kHz and a -3 dB bandwidth of 8 kHz (25 ± 4 kHz) EE4512 Analog and Digital Communications Chapter 5 • The filter noise no(t) has a flat power spectral density within the bandwidth of the bandpass filter: no(t) PSD 21 kHz 29 kHz fC = 25 kHz EE4512 Analog and Digital Communications Chapter 6 • The filter... (offset) EE4512 Analog and Digital Communications Chapter 5 • The envelope detector is a half-wave rectifier and provides a DC bias (c) to the processed DSB-LC AM signal : c = DC bias EE4512 Analog and Digital Communications Chapter 5 • The output of the half-wave diode rectifier is low-pass filtered to remove the carrier frequency and outputs the envelope which is the information: EE4512 Analog and Digital... EE4512 Analog and Digital Communications Chapter 6 • The DSB-LC AM signal wastes at least half the transmitted power because the power in the carrier term has no information: Pcarrier term ≥ Pinfo term η ≤ 0.5 The modulation index m is defined as: m = max [ s(t) + c ] − min [ s(t) + c ] max [ s(t) + c ] + min [ s(t) + c ] S&M Eq 6.30 EE4512 Analog and Digital Communications Chapter 6 • The modulation. .. 1 [S(f − fC ) + S(f + fC )] df 2 S&M Eq 6.8 EE4512 Analog and Digital Communications Chapter 6 • The dual-sided spectral do not overlap (at zero frequency) and the cross terms are zero so that: 2 1 Ptrans = A 2 ∫ [SDSB-SC (f − fC ) + SDSB-SC (f + fC )] df 2 Ptrans A2 = Ps 2 S&M Eq 6.9 where Ps is the average normalized power of s(t) EE4512 Analog and Digital Communications Chapter 6 • The average... modulation index m defines the power efficiency but m must be less than 1 If m > 1 then min [s(t) + c] < 0 and distortion occurs m = max [ s(t) + c ] − min [ s(t) + c ] max [ s(t) + c ] + min [ s(t) + c ] S&M Eq 6.30 EE4512 Analog and Digital Communications Chapter 6 • The average normalized power of the demodulation noiseless DSB-LC AM signal is: Pdemod, noiseless = 2 γ 2 Pinfo term S&M Eq 6.39 Then the . EE4512 Analog and Digital Communications Chapter 6 Chapter 6 Chapter 6 Analog Modulation Analog Modulation and Demodulation and Demodulation EE4512 Analog and Digital Communications. 6 Chapter 6 Analog Modulation Analog Modulation and Demodulation and Demodulation • • Amplitude Modulation Amplitude Modulation • • Pages 306 Pages 306 - - 309 309 EE4512 Analog and Digital Communications. baseband signal s(t) but not an outer envelope envelope . . EE4512 Analog and Digital Communications Chapter 6 Chapter 6 Chapter 6 Analog Modulation Analog Modulation and Demodulation and Demodulation • • Coherent