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EE104: Lecture 21 Outline

EE104: Lecture 21 Outline. Review of Last Lecture Vestigial Sideband Modulation AM Radio Introduction to FM FM Bandwidth and Carson’s Rule Narrowband and Wideband FM. Review of Last Lecture. Noise in AM Receivers: DSBSC Single Sideband Modulation. M(f). -B. B. LSB. USB. 0. f c.

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EE104: Lecture 21 Outline

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  1. EE104: Lecture 21 Outline • Review of Last Lecture • Vestigial Sideband Modulation • AM Radio • Introduction to FM • FM Bandwidth and Carson’s Rule • Narrowband and Wideband FM

  2. Review of Last Lecture • Noise in AM Receivers: DSBSC • Single Sideband Modulation M(f) -B B LSB USB 0 fc -fc

  3. Coherent Detection of SSB • Same as detection of DSBSC s(t) v(t) m(t) Product Modulator LPF Accos(2pfct+f) S(f) for USB M(f)=.5M(f) V(f) 1 .5 .5 -fc fc -fc 0 -2fc fc 0 fc -fc 0 2fc

  4. Vestigial Sideband • Transmits upper or lower sideband and vestige of other band • Reduces bandwidth by roughly a factor of 2 • Generated using standard AM or DSBSC modulation, then filtering • Standard AM or DSBSC demodulation USB

  5. 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

  6. Introduction to FM • Information signal encoded in carrier frequency (or phase) • Modulated signal is s(t)=Accos(q(t)) • q(t)=f(m(t)) • Standard FM: q(t)=2pfct+2pkfm(t)dt • Instantaneous frequency: fi=fc+kfm(t) • Signal robust to amplitude variations • Robust to signal reflections and refractions

  7. FM Bandwidth and Carson’s Rule • Frequency Deviation: Df=kf max|m(t)| • Maximum deviation of fi from fc: fi=fc+kfm(t) • Carson’s Rule: • B depends on maximum deviation from fcAND how fast fi changes • Narrowband FM: Df<<BmB2Bm • Wideband FM: Df>>BmB2Df B2Df+2Bm

  8. B2Df WBFM B2Bm NBFM Spectral Analysis • s(t)=Accos(2pfct+2pkfm(t)dt) • Nonlinear function of m(t) • Very hard to analyze with FTs • Let m(t)=cos(2pfmt) • Spectrum S(f) is a sequence of delta functions at multiples of fm from fc S(f) for m(t)=cos(2pfmt) … … f -3fm -4fm fm 3fm fc -2fm 2fm 4fm -fm

  9. Generating FM Signals • NBFM • WBFM • Direct Method: Modulate a VCO with m(t) • Indirect Method: Use a NBFM modulator, followed by a nonlinear device and BPF f(t) s(t) m(t) - Product Modulator 2pkf(·)dt + + Accos(2pfct) Acsin(2pfct) LO -90o

  10. Main Points • VSB similar to SSB, uses slightly more BW for a lower DC distortion. • AM radios and many other systems downconvert to IF before demodulation. • FM modulation encodes information in carrier frequency or phase. More robust to channel impairments. • FM signal bandwidth depends on information signal amplitude. • NBFM is easier to analyze and generate (simple product modulator). WBFM more complicated to analyze and generate.

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