1. ECE 4371, Fall, 2009 Zhu Han Department of Electrical and Computer Engineering Class 7 Sep. 15 th, 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. 59 Noise in linear receiver using coherent detection

7. 60 Noise in linear receiver using coherent detection

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. 62 Noise in AM receiver using envelope detection Waste energy

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. Noise After Discriminator The quadrature appears

17. Noise After Discriminator cont. The average output signal power = k f 2 P Recall nQ(t)nQ(t)nd(t)nd(t)

18. Noise After Discriminator cont. Assume that n Q (t) has ideal low-pass characteristic with bandwidth B T

19. SNR of FM 71 Bandwidth effect

20. Single Tone FM SNR

21. FM Threshold Effect nQ(t)nQ(t) r(t)r(t) x(t)x(t) Ac(t)Ac(t) P1P1 0 P2P2 nI(t)nI(t)

22. Example Illustrating impulse ike components in  (t)  d  (t)/dt produced by changes of 2  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. EE 541/451 Fall 2007 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.

26. 80 Improvement Factor

27. FM Preemphsis and Deemphasis

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

29. 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 Table 2.4 Values of B n for various CW modulation schemes FM AM, DSB-SCSSB Bn    Bandwidth Tradeoff