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EE104: Lecture 19 Outline

EE104: Lecture 19 Outline. Announcements: HW due today, new HW posted 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: The Costas Loop.

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EE104: Lecture 19 Outline

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  1. EE104: Lecture 19 Outline • Announcements: • HW due today, new HW posted • 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: The Costas Loop

  2. Review of Last Lecture • White Noise • Introduction to Modulation • AM Modulation • When |kam(t)|<1, can recover m(t) from s(t) envelope a(t)=Ac|1+kam(t)|=Ac[1+kam(t)] cos(2pfct) ka s(t)=Ac[1+kam(t)]cos2pfct 1 m(t) + X X

  3. Generation of AM Waves • Multipliers difficult to build in HW • AM waves generated using nonlinear device Accos(2pfct+f) m(t) s(t) Squarer or Switch + BPF

  4. Detection of AM Waves • Entails tradeoff between performance and complexity (cost) • Square law detector squares signal and then passes it through a LPF • Residual distortion proportional to m2(t) • Noncoherent (carrier phase not needed in receiver) • Envelope detector detects envelope of s(t) • Simple circuit (resistors, capacitor, diode) • Only works when |kam(t)|<1 (poor SNR), no distortion. • Noncoherent

  5. Envelope Detector Only works when |kam(t)|<1

  6. Double Sideband Suppressed Carrier (DSBSC) • Remove inefficient constant term • Modulated signal is s(t)=Accos(2pfct)m(t) • Generated by a product modulator • Can also use ring modulator: diodes and inductors m(t) AM Modulator + Accos(2pfct) + s(t) - AM Modulator -m(t)

  7. Coherent Detection of DSBSC • Detector uses another product modulator • Demodulated signal: m´(t)=.5Accos(f2-f1)m(t) • Phase offset: if f2-f1=p/2, m´(t)=0 • Coherent detection (f2f1) required m(t) s(t) m´(t) Product Modulator Product Modulator Channel LPF Accos(2pfct+f1) Accos(2pfct+f2)

  8. Costas Loop Goal: Maintain Df0 DSBSC Demodulator .5Accos(f- f´)m(t) Product Modulator LPF V=sin2(f- f´)/8 Accos(2pfct+f)m(t) Phase Discriminator Accos(2pfct+f´(V)) VCO Df´(V) -90o VDf 0 Acsin(2pfct+f´(V)) Product Modulator LPF .5Acsin(f- f´)m(t)

  9. Main Points • AM demodulated with nonlinear devices (some distortion) or envelope detectors (simple, no distortion, low SNR). • DSBSC eliminates the power inefficiency of standard AM at the expense of significantly higher receiver complexity • Coherent detection is needed for DSBSC: obtaining the carrier phase is one of biggest challenges in all demodulators. • A Costas loop provides phase synchronization using a VCO and feedback control. • SNR ratio in AM increases with signal power and decreases with noise spectral density and signal bandwidth

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