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communication system Chapter 6


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communication system Chapter 6

  1. 1. Communication System Ass. Prof. Ibrar Ullah BSc (Electrical Engineering) UET Peshawar MSc (Communication & Electronics Engineering) UET Peshawar PhD (In Progress) Electronics Engineering (Specialization in Wireless Communication) MAJU Islamabad E-Mail: Ph: 03339051548 (0830 to 1300 hrs) 1
  2. 2. Chapter-6 Sampling And Pulse Code Modulation 1. Sampling 2. Signal interpolation 3. Pulse code modulation 2
  3. 3. Sampling 3
  4. 4. Sampling (Cont…) 4
  5. 5. Sampling (Cont…) 5
  6. 6. Signal Interpolation 6
  7. 7. Signal Interpolation (Cont…) 7
  8. 8. Signal Interpolation (Cont…) 8
  9. 9. Signal Interpolation (Cont…) 9
  10. 10. Pulse Code Modulation 10
  11. 11. Pulse Code Modulation (Cont…) 11
  12. 12. Pulse Code Modulation (Cont…) 12
  13. 13. Pulse Code Modulation (Cont…) 13
  14. 14. Noise In any real physical system, when the signal voltage arise at the demodulator, it will be accompanied by a voltage waveform which varies with time in an entirely unpredictable manner. This unpredictable voltage wave form is a random process called noise. Types of Noise Most man made electro-magnetic noise occurs at frequencies below 500 MHz. The most significant of these include: • Hydro lines • Ignition systems • Fluorescent lights • Electric motors Therefore deep space networks are placed out in the desert, far from these sources of interference. 14
  15. 15. Types of Noise (cont..) • There are also a wide range of natural noise sources which cannot be so easily avoided, namely: • Atmospheric noise - lighting < 20 MHz • Solar noise - sun - 11 year sunspot cycle • Cosmic noise - 8 MHz to 1.5 GHz • Thermal or Johnson noise. Due to free electrons striking vibrating ions. • White noise - white noise has a constant spectral density over a specified range of frequencies. Johnson noise is an example of white noise. • Gaussian noise - Gaussian noise is completely random in nature however, the probability of any particular amplitude value follows the normal distribution curve. Johnson noise is Gaussian in nature. • Shot noise - bipolar transistors (caused by random variations in the arrival of electrons or holes at the output electrodes of an amplifying device) • Transit time noise - occurs when the electron transit time across a junction is the same period as the signal. 15 • Of these, only Johnson noise can be readily analyzed and compensated
  16. 16. Noise power • The noise power is given by: Pn = kTB • Where: • k = Boltzman's constant (1.38 x 10-23 J/K) • T = temperature in degrees Kelvin • B = bandwidth in Hz • If the two signals are completely random with respect to each other, such as Johnson noise sources, the total power is the sum of all of the individual powers: 16
  17. 17. Noise power (Cont..) • A Johnson noise of power P = kTB, can be thought of as a noise voltage applied through a resistor, Thevenin equivalent. An example of such a noise source may be a cable or transmission line. The amount of noise power transferred from the source to a load, such as an amplifier input, is a function of the source and load impedances 17
  18. 18. Noise power (Cont..) The rms noise voltage at maximum power transfer is: Observe what happens if the noise resistance is resolved into two components: 18
  19. 19. Noise Figure • The terms used to quantify noise : • Signal to noise ratio: It is either unit-less or specified in dB. The S/N ratio may be specified anywhere within a system. Noise Factor (or Noise Ratio): (unit less) 19
  20. 20. Noise Figure (cont..) • This parameter (i.e. Noise Figure ) is specified in all high performance amplifiers and is measure of how much noise the amplifier itself contributes to the total noise. In a perfect amplifier or system, NF = 0 dB. This discussion does not take into account any noise reduction techniques such as filtering or dynamic emphasis. 20
  21. 21. Noise Figure (cont..) • Friiss' Formula & Amplifier Cascades • It is interesting to examine an amplifier cascade to see how noise builds up in a large communication system. Amplifier gain can be defined as: Therefore the output signal power is: 21
  22. 22. Noise Figure (cont..) and the noise factor (ratio) can be rewritten as: The output noise power can now be written: From this we observe that the input noise is increased by the noise ratio and amplifier gain as it passes through the amplifier. A noiseless amplifier would have a noise ratio (factor) of 1 or noise figure of 0 dB. In this case, the input noise would only be amplified by the gain since the amplifier would not contribute noise. Friiss' Formula 22
  23. 23. Model Paper Communication system CU-510 5th semester Deptt. Of Electrical Engineering, CECOS Univ. Total Time: 3 hrs Q # 1 Time: 30 minutes (1):Is frequency modulation a linear modulation? A) Yes B) No (2): Does Dirichlet’s condition require a signal to be absolutely integrable? A) Yes B) No (3): suitable measure for this signal is ----------- 23
  24. 24. Q#2 (a) (b): Differentiate between Energy and power signals what is correlation. 24
  25. 25. The end 25