chapter3_fm

Chapter3_fm - ANALOG MODULATION PART II ANGLE MODULATION What is Angle Modulation In angle modulation information is embedded in the angle of the

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ANALOG MODULATION PART II: ANGLE MODULATION
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2 What is Angle Modulation? In angle modulation, information is embedded in the angle of the carrier. We define the angle of a modulated carrier by the argument of. .. s t ( 29 = A c cos θ t ( 29 ( 29
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3 Phasor Form In the complex plane we have t=1 t=0 t=3 Phasor rotates with nonuniform speed
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4 Angular Velocity Since phase changes nonuniformly vs. time, we can define a rate of change This is what we know as frequency ϖ i = d θ i ( t ) dt s t ( 29 = A c cos 2 π f c t + φ c i t ( 29 1 2 4 3 4 d i dt = 2 f c
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5 Instantaneous Frequency We are used to signals with constant carrier frequency. There are cases where carrier frequency itself changes with time. We can therefore talk about instantaneous frequency defined as f i t ( 29 = 1 2 π d θ i t ( 29 dt
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6 Examples of Inst. Freq. Consider an AM signal Here, the instantaneous frequency is the frequency itself, which is constant s t ( 29 = 1 + km ( t ) [ ] cos 2 π f c t + φ c θ i t ( 29 1 2 4 3 4 d i dt = 2 f c
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7 Impressing a message on the angle of carrier There are two ways to form a an angle modulated signal. Embed it in the phase of the carrier Phase Modulation(PM) Embed it in the frequency of the carrier Frequency Modulation(FM)
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8 Phase Modulation(PM) In PM, carrier angle changes linearly with the message Where 2πf c =angle of unmodulated carrier k p =phase sensitivity in radians/volt s t ( 29 = A c cos θ i t ( 29 ( 29 = A c cos 2 π f c t + k p m t ( 29 ( 29
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9 Frequency Modulation In FM, it is the instantaneous frequency that varies linearly with message amplitude, i.e. f i (t)=f c +k f m(t)
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10 FM Signal We saw that I.F. is the derivative of the phase Therefore, f i t ( 29 = 1 2 π d θ i t ( 29 dt i t ( 29 = 2 f c t + 2 k f m t ( 29 0 t s t ( 29 = A c cos 2 f c t + 2 k f m ( t ) dt 0 t
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11 FM for Tone Signals Consider a sinusoidal message The instantaneous frequency corresponding to its FM version is m ( t ) = A m cos 2 π f m t ( 29 f i t ( 29 = f c + k f m ( t ) = f c resting frequency { + k f A m cos 2 f m t ( 29
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12 Illustrating FM 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 FM message Inst.frequency Moves with the Message amplitude
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13 Frequency Deviation Inst. frequency has upper and lower bounds given by f i t ( 29 = f c + f cos 2 π f m t ( 29 where f = frequency deviation = k f A m then f i max = f c + f f i min = f c - ∆ f
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14 FM Modulation index The equivalent of AM modulation index is β which is also called deviation ratio . It quantifies how much carrier frequency swings relative to message bandwidth β= f W baseband { or f f m tone {
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15 Example:carrier swing A 100 MHz FM carrier is modulated by an audio tone causing 20 KHz frequency deviation. Determine the
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This note was uploaded on 03/09/2010 for the course ECE 7600 taught by Professor Mobaserri during the Spring '10 term at Villanova.

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Chapter3_fm - ANALOG MODULATION PART II ANGLE MODULATION What is Angle Modulation In angle modulation information is embedded in the angle of the

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