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Modulación Analógica (AM-FM)

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

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Modulación Analógica (AM-FM)

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  1. Modulación Analógica (AM-FM) Cx Eléctricas 09 – E.Tapia

  2. Modulación de Onda CC (CW) • Representación en dominios t-f • Efectos del ruido en los receptores correspondientes

  3. 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

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

  5. 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

  6. Some issues on AM • Overmodulation • Leads to envelope distortion. The demodulator will track a false envelope and information will be lost. • fc >>>> W – the message bandwidth • Easy envelope visualization and tracking

  7. Frequency Domain

  8. 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

  9. Linear Modulation

  10. DSB-SC- (Double SideBand-Supressed Carrier)

  11. Coherent Detection

  12. Note that • Non coherent detection may lead to null quadrature effect • Need coherent local oscillator at demodulation >> complexity >> the price

  13. 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

  14. VSB – Vestigial Sideband Modulation

  15. More on VSB

  16. Frequency Modulation (FM) •  f is the frequency deviation •  is the modulation index defined as  f /fm

  17. Which is the FM angle? •  << 1 radian is known as narrowband FM •  >> 1 radian is known as wideband FM

  18. 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

  19. SNRs • SNRI (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

  20. 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

  21. 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)

  22. 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

  23. Noise in AM • From the SNR at the channel (C, AM) we desire the SNR at the output , demodulator – envelope

  24. Phasorial Analysis

  25. 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

  26. Threshold effect in AM Detectors

  27. Noise Effects in FM • Limiter: clipp and round so that amplitude is independent of the carrier amplitude at the receiver input.

  28. Noise Model for FM • R(t) is Rayleigh • Phase is uniform

  29. Signal Model for FM

  30. Signal and Noise in FM

  31. Discriminator Output • Provided the carrier to noise is high

  32. FM Discriminator: S2N

  33. Cont’ • The carrier power has noise quoting effect in FM • Recall that • The average signal transmitted power is kf2P

  34. How can we improve S2N in FM?

  35. 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

  36. 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.

  37. No signal but Noise • Ac >> nI , nQ • Ac << nI , nQ • P1 noves to the origin and random phase is observed is around

  38. Alternatevely • Clicks are heard after the low pass filter

  39. Threshold Effect • As  is decreased the rate of clicks grows • Rate of clicks is high threshold occurs

  40. Designing an FM System • Given D () • Compute BT • Given BT and N0 (Noise power per unit bandwidth) • Determine AC to keep above the threshold

  41. FM Threshold Reduction • FM demodulator with negative feeback (FMFB) or PLL

  42. FM Threshold Reduction The VCO output The phase comparator output

  43. FM Threshold Reduction (cont)

  44. FM Threshold Reduction (cont)

  45. FM Threshold Reduction (cont)

  46. Linear Model of the PLL-FM Demodulator

  47. PreEmphasis - Deemphasis • Pre at transmitter • De- at the receiver

  48. Pre-emphasis & De-emphasis • Pre at transmitter • De- at the receiver

  49. Conclusions

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