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EE104: Lecture 20 Outline

EE104: Lecture 20 Outline. Review of Last Lecture Noise in AM Receivers Single Sideband Modulation Vestigial Sideband Modulation AM Radio and Superheterodyne Receivers. Review of Last Lecture. Generation of AM Waves Square Law and Envelope Detection of AM

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EE104: Lecture 20 Outline

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  1. EE104: Lecture 20 Outline • Review of Last Lecture • Noise in AM Receivers • Single Sideband Modulation • Vestigial Sideband Modulation • AM Radio and Superheterodyne Receivers

  2. Review of Last Lecture • Generation of AM Waves • Square Law and Envelope Detection of AM • Double Side Band Suppressed Carrier • Product Modulators for DSBSC • Coherent Detection for DSBSC: Costas Loop

  3. Noise in AM Receivers • Power in s(t) is .5Ac2Pm • Power in m(t) is .25Ac2Pm • Power in n(t) is .5N0B • SNR=.5Ac2Pm/(N0B) • Power of s(t) over power of n(t) in BW of interest n(t): white LPF s(t)=Accos(2pfct+f)m(t) m´(t)+ n´(t) Product Modulator 1 + Accos(2pfct+f)

  4. Single Sideband • Only transmits upper or lower sideband of AM • Reduces bandwidth by factor of 2 • Transmitted signal can be written in terms of Hilbert transform of m(t) • SSB can introduce distortion at DC LSB USB

  5. Vestigial Sideband • Transmits USB or LSB and vestige of other sideband • Reduces bandwidth by roughly a factor of 2 • Generated using standard AM or DSBSC modulation, then filtering • Standard AM or DSBSC demodulation • VSB used for image transmission in TV signals USB

  6. AM Radio and Superheterodyne Receivers • Multiplexes AM radio signals in frequency • 10 KHz bandwidth, carrier in 530-1610 Khz • Receiver needs tight filtering to remove adjacent signals • LO can radiate out receiver front end • Fix problems by IF processing (Superheterodyne) f1 f2 f3

  7. Main Points • SNR in DSBSC is power of transmit signal over power of noise in the bandwidth of interest. • SSB is a spectrally efficient AM technique with half the BW requirements of standard AM and DSBSC. • VSB similar to SSB, uses slightly more BW for a lower DC distortion. • AM receivers (and others) downconvert to IF for demodulation to avoid filtering/reradiation problems.

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