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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 (frequencyvoltage 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 voltagecontrolled 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 Commerciallevel 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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 Spring '11
 ahmetçakar
 Frequency, Volt, Indirect FM

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