1 / 29

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 .

gaia
Download Presentation

ECE 4371, Fall, 2009

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related