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ECE4371_class6

ECE4371_class6 - ECE4371 Fall 2009 Zhu Han Department of...

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Unformatted text preview: ECE4371, Fall, 2009 Zhu Han Department of Electrical and Computer Engineering Class 6 Sep. 10 th , 2007 FM Modulator and Demodulator FM Modulator and Demodulator FM modulator – Direct FM – Indirect FM FM demodulator – Direct: use frequency discriminator (frequency-voltage converter) – Ratio detector – Zero crossing detector – Indirect: using PLL Superheterodyne receiver FM broadcasting and Satellite radio FM Direct Modulator FM Direct Modulator Direct FM – Carrier frequency is directly varied by the message through voltage-controlled oscillator (VCO) – VCO: output frequency changes linearly with input voltage – A simple VCO: implemented by variable capacitor – Capacitor Microphone FM generator FM Direct Modulator cont. FM Direct Modulator cont. Direct method is simple, low cost, but lack of high stability & accuracy, low power application, unstable at the carrier frequency Modern VCOs are usually implemented as PLL IC Why VCO generates FM signal? Capacitance changes with the applied voltage: ( ) ( ) C t C Cm t = + ∆ 2 LC oscillator frequency: 1 1 ( ) 2 2 ( ) 1 1 ( ) ( ) 2 2 ( ) 2 ( ) i f t LC LC L Cm t C m t O t C LC f C f m t C f f m t π π π = = + ∆ ∆ =- + ∆ ≈- =- ∆ Indirect FM Indirect FM Generate NBFM first, then NBFM is frequency multiplied for targeted Δf. Good for the requirement of stable carrier frequency Commercial-level FM broadcasting equipment all use indirect FM A typical indirect FM implementation: Armstrong FM Block diagram of indirect FM Indirect FM cont. Indirect FM cont. First, generate NBFM signal with a very small β 1 1 1 1 ( ) cos(2 ) sin(2 )sin(2 ) c c m v t A f t A f t f t π β π π =- m(t) Indirect FM cont. Indirect FM cont. Then, apply frequency multiplier to magnify β – Instantaneous frequency is multiplied by n – So do carrier frequency, Δf, and β – What about bandwidth? right left i i f n f = Analysis of Indirect FM Analysis of Indirect FM 1 2 1 2 2. Nonlinear device outputs frequencies: ( ) ( ) ( ) ( ) ( ) f n o n nf nk m t v t a v t a v t a v t + = + + + + L L 1 1 1. Input: ( ) cos 2 2 ( ) , max | ( ) | where ( ) ( ), 1 t c f f i f v t A f t k m d k m t...
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ECE4371_class6 - ECE4371 Fall 2009 Zhu Han Department of...

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