Modulacin Analgica AMFM

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Modulación Analógica (AM-FM)

• Cx Eléctricas 08 E.Tapia

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Modulación de Onda CC (CW)

• Representación en dominios t-f
• Efectos del ruido en los receptores
  correspondientes

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Modulation -Demodulation

• Ix transmission in presence of noise
• Ix bearing signals or baseband signals
• Transmitter-Channel-Receiver
• Frequency shifting on Tx Modulation using a
  carrier
• Frequency shift back on Rx Demodulation

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Modulation

• Carrier is sinusoidal wave
• Amplitude, frequency, or phase are varied with a
  modulating wave - signal

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Amplitude Modulation

• Message signal m(t) and carrier c(t) are
  independent
• Carrier amplitude is varied about a mean value
  (Ac), linearly with m(t)
• Ka is the modulation sensiviy measured in 1/volt

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Some issues on AM

• Overmodulation
• Leads to envelope distortion. The demodulator
  will track a false envelope and information will
  be lost.
• fc gtgtgtgt W the message bandwidth
• Easy envelope visualization and tracking

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Frequency Domain
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Note that

• Mod-Demod are implemented using non-linear
  devices
• Demod are often envelope detectors
• AM Power and AM Bandwith
• Not efficient at power use (tx of c(t))
• Sidebands are related each other gtgtgtgt just one is
  needed
• Hence gtgtgtgt avoid c(t) transmission and duplicate
  sidebands

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Linear Modulation
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DSB-SC- (Double SideBand-Supressed Carrier)
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Coherent Detection
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Note that

• Non coherent detection may lead to null
  quadrature effect
• Need coherent local oscillator at demodulation
  gtgt complexity gtgt the price

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SSB MOdulation

• DSB-SC Filtering for Sideband Removal
• Highly selective filters from cristal oscillators
• Coherent detector gtgt low power pilot carrier
  addition is added at transmission

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VSB Vestigial Sideband Modulation
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More on VSB
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Frequency Modulation (FM)

• ? f is the frequency deviation
• ? is the modulation index defined as ? f /fm

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Which is the FM angle?

• ? ltlt 1 radian is known as narrowband FM
• ? gtgt 1 radian is known as wideband FM

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Noise in CW Modulation

• Chanel Model is AWGN
• Power spectral density is No/2
• Receiver model defined by a bandpass filter and a
  demodulator model

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SNRs

• SNR I (Input)
• Ratio of the average power of the modulated
  signal s(t) to the average power of the filtered
  noise
• SNR o (Output)
• Ratio of the averaged power of the demodulated
  signal to the power of noise measured at the
  receiver output
• SNR c (Channel)
• Ratio of the averaged power of the modulated
  signal to the average power of noise in the
  message bandwith both at the receiver input

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Noise in DSB Coherent Detection

• s(t) is the DSB component of x(t)
• C is system dependent scaling factor
• m(t) sample from stationary process of zero mean
  and S(f)
• Hence compute SNRC, DSB

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Figure of Merit in Coherent Detection

• The quadrature component of noise is rejected in
  coherent detection
• The average power of filtered noise n(t) is
• Same for nI(t)

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Figure .

• The same holds for SSB
• NO way to improve SNR by increasig bandwith use
  in DSB w.r.t SSB
• The effect of modulation is just frequenxy
  shifting

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Noise in AM

• From the SNR at the channel (C, AM) we desire the
  SNR at the output , demodulator envelope

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Phasorial Analysis
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Figures of Merit

• Always ltlt 1for AM envelope receivers
• Equal to 1 for DSB, SSB
• Caused by waste of power on carrir transmission
• Existence of threshold effect

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Threshold effect in AM Detectors
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Noise Effects in FM

• Limiter clipp and round so that amplitude is
  independent of the carrier amplitude at the
  receiver input.

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Noise Model for FM

• R(t) is Rayleigh
• Phase is uniform

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Signal Model for FM
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Signal and Noise in FM
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Discriminator Output

• Provided the carrier to noise is high