9 mod analog_am_fm (1)
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    9 mod analog_am_fm (1) 9 mod analog_am_fm (1) Presentation Transcript

    • Modulación Analógica (AM-FM) Cx Eléctricas 09 – E.Tapia
    • Modulación de Onda CC (CW)
      • Representación en dominios t-f
      • Efectos del ruido en los receptores correspondientes
    • 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
    • Modulation
      • Carrier is sinusoidal wave
      • Amplitude, frequency, or phase are varied with a modulating wave - signal
    • 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)
      • K a is the modulation sensiviy measured in 1/volt
    • Some issues on AM
      • Overmodulation
        • Leads to envelope distortion. The demodulator will track a false envelope and information will be lost.
      • f c >>>> W – the message bandwidth
        • Easy envelope visualization and tracking
    • Frequency Domain
    • 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 >>>> just one is needed
        • Hence >>>> avoid c(t) transmission and duplicate sidebands
    • Linear Modulation
    • DSB-SC- (Double SideBand-Supressed Carrier)
    • Coherent Detection
    • Note that
      • Non coherent detection may lead to null quadrature effect
      • Need coherent local oscillator at demodulation >> complexity >> the price
    • SSB MOdulation
      • DSB-SC + Filtering for Sideband Removal
      • Highly selective filters from cristal oscillators
      • Coherent detector >> low power pilot carrier addition is added at transmission
    • VSB – Vestigial Sideband Modulation
    • More on VSB
    • Frequency Modulation (FM)
      •  f is the frequency deviation
      •  is the modulation index defined as  f /f m
    • Which is the FM angle?
      •  << 1 radian is known as narrowband FM
      •  >> 1 radian is known as wideband FM
    • Noise in CW Modulation
      • Chanel Model is AWGN
        • Power spectral density is N o /2
      • Receiver model defined by a bandpass filter and a demodulator model
    • 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
    • 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 SNR C, DSB
    • 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 n I (t)
    • 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
    • Noise in AM
      • From the SNR at the channel (C, AM) we desire the SNR at the output , demodulator – envelope
    • Phasorial Analysis
    • Figures of Merit
      • Always << 1for AM envelope receivers
      • Equal to 1 for DSB, SSB
      • Caused by waste of power on carrir transmission
      • Existence of threshold effect
    • Threshold effect in AM Detectors
    • Noise Effects in FM
      • Limiter: clipp and round so that amplitude is independent of the carrier amplitude at the receiver input.
    • Noise Model for FM
      • R(t) is Rayleigh
      • Phase is uniform
    • Signal Model for FM
    • Signal and Noise in FM
    • Discriminator Output
      • Provided the carrier to noise is high
    • FM Discriminator: S2N
    • Cont’
      • The carrier power has noise quoting effect in FM
      • Recall that
        • The average signal transmitted power is k f 2 P
    • How can we improve S2N in FM?
    • The conclusion
      • FM provides a mechanism for the exchange of improved noise performance by increased transmission bandwidth
      • FM can also reject other FM signals closed to the carrier frequency provided interferent signal are weaker w.r.t. the target FM input
    • Threshold Effect in FM
      • Assumption
        • Carrier to Noise ratio at the discriminator input >> 1
      • Violation to this assumption
        • FM receiver breaks. From breaks to sputtering sounds. The formula does not hold.
    • No signal but Noise
      • Ac >> n I , n Q
      • Ac << n I , n Q
        • P 1 noves to the origin and random phase is observed
      is around
    • Alternatevely
      • Clicks are heard after the low pass filter
    • Threshold Effect
      • As  is decreased the rate of clicks grows
      • Rate of clicks is high threshold occurs
    • Designing an FM System
      • Given D (  )
        • Compute B T
      • Given B T and N 0 (Noise power per unit bandwidth)
        • Determine A C to keep above the threshold
    • FM Threshold Reduction
      • FM demodulator with negative feeback (FMFB) or PLL
    • FM Threshold Reduction The VCO output The phase comparator output
    • FM Threshold Reduction (cont)
    • FM Threshold Reduction (cont)
    • FM Threshold Reduction (cont)
    • Linear Model of the PLL-FM Demodulator
    • PreEmphasis - Deemphasis
      • Pre at transmitter
      • De- at the receiver
    • Pre-emphasis & De-emphasis
      • Pre at transmitter
      • De- at the receiver
    • Conclusions