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ECE 4371, Fall, 2009

ECE 4371, Fall, 2009. Zhu Han Department of Electrical and Computer Engineering Class 7 Sep. 15 th , 2009. Overview. Homework Hint Signal to Noise Ratio AM with noise Coherent decoder Non-coherent decoder FM with noise Analysis Threshold effect Preemphasis and deemphasis .

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ECE 4371, Fall, 2009

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  1. ECE 4371, Fall, 2009 Zhu Han Department of Electrical and Computer Engineering Class 7 Sep. 15th, 2009

  2. Overview • Homework Hint • Signal to Noise Ratio • AM with noise • Coherent decoder • Non-coherent decoder • FM with noise • Analysis • Threshold effect • Preemphasis and deemphasis

  3. Signal to Noise Ratio Channel model: additive white Gaussian noise (AWGN) Receiver model: a band-pass filer followed by an ideal demodulator Receiver model • Idealized characteristic of band-pass filtered noise. • The baseband transmission model, assuming a message signal of bandwidth W, used for calculating the channel signal-to-noise ratio. • The PSD of w(t) is denoted by

  4. SNR

  5. Noise in linear receiver using coherent detection • Model of DSB-SC receiver using coherent detection

  6. Noise in linear receiver using coherent detection 59

  7. Noise in linear receiver using coherent detection 60

  8. Noise in AM receiver using envelope detection • Model of AM receiver

  9. Noise in AM receiver using envelope detection • (a) Phasor diagram for AM wave plus narrowband noise for the case of high carrier-to-noise ratio. • (b) Phasor diagram for AM wave plus narrowband noise for the case of low carrier-to-noise ratio.

  10. Noise in AM receiver using envelope detection Waste energy 62

  11. 63

  12. Threshold Effect • Output signal-to-noise ratio of an envelope detector for varying carrier-to-noise ratio.

  13. System Model and Noise Model Discriminator consists of a slope network and an envelope detector.

  14. Signal after bandpass filter • The incoming FM signal s(t) is defined by • At the bandpass filter output

  15. Discriminator Output • Note that the envelope of x(t) is of no interest to us (limiter)

  16. The quadrature appears Noise After Discriminator

  17. Noise After Discriminator cont. • The average output signal power = kf2P Recall nQ(t) nd(t)

  18. Noise After Discriminator cont. • Assume that nQ(t) has ideal low-pass characteristic with bandwidth BT

  19. 71 SNR of FM Bandwidth effect

  20. Single Tone FM SNR

  21. P1 r(t) nQ(t) x(t) P2 0 nI(t) Ac(t) FM Threshold Effect

  22. Example • Illustrating impulse ike components in  (t)  dq (t)/dt produced by changes of 2p in  (t); (a) and (b) are graphs of  (t) and  (t), respectively.

  23. Threshold Effect • Dependence of output signal-to-noise ratio on input carrier-to-noise ratio for FM receiver. In curve I, the average output noise power is calculated assuming an unmodulated carrier. In curve II, the average output noise power is calculated assuming a sinusoidally modulated carrier. Both curves I and II are calculated from theory.

  24. FM Threshold Reduction (tracking filter) • FM threshold extension • FM demodulator with negative feedback

  25. FM Preemphsis and Deemphasis Figure 2.48 (a) Power spectral density of noise at FM receiver output. (b) Power spectral density of a typical message signal. EE 541/451 Fall 2007

  26. Improvement Factor 80

  27. FM Preemphsis and Deemphasis

  28. Comparison of the noise performance of various CW modulation systems. Curve I: Full AM, m= 1. Curve II: DSB-SC, SSB. Curve III: FM, b= 2. Curve IV: FM, b= 5. (Curves III and IV include 13-dB pre-emphasis, de-emphasis improvement..)

  29. Table 2.4 Values of Bn for various CW modulation schemes FM b=2 AM, DSB-SC SSB b=5 Bn 2 1 8 16 Bandwidth Tradeoff In making the comparison, it is informative to keep in mind the transmission bandwidth requirement of the modulation systems in question. Therefore, we define normalized transmission bandwidth as

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