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CHAPTER 5

CHAPTER 5. 5.1 Noise 5.2 Transmission Media & EM Propagations. Introduction. Define as undesired random variations that interface with the desired signal and inhibit communication. Where does noise originate in a communication system? Channel @ transmission medium

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CHAPTER 5

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  1. CHAPTER 5 5.1 Noise 5.2 Transmission Media & EM Propagations

  2. Introduction Define as • undesired random variations that interface with the desired signal and inhibit communication. Where does noise originate in a communication system? • Channel @ transmission medium • Devices @ Equipments

  3. Cont’d... Noise Effect • One of the main limiting factor in obtaining high performance of a communication system. • Decrease the quality of the receiving signal.

  4. Block Diagram of Communication System With the Existence of Noise

  5. Cont’d... • Noise, interference and distortion • Noise • Refers to random and unpredictable electrical signals produced by natural process. • Superimposed on information bearing signal, the message partially corrupted or totally erased. • Can be reduced by filtering but can’t totally eliminated.

  6. Cont’d... • Interference • A contamination by extraneous signals from human sources (e.g. from other Tx, power lines, machineries) • Often occurred in radio system whose Rx antenna intercept several signals at the same time.

  7. Cont’d... • Distortion • The signal perturbation caused by imperfect response of the system to the desired signal. • Disappear when the signal us turned-off. • Can be corrected by the equalizers.

  8. Noise Remedies? REDUCE BANDWIDTH INCREASE TRANSMITTER’S POWER LOW NOISE AMPLIFIERS

  9. Types of NOISE

  10. Cont’d... External Noise • Noise generated outside the electronic equipment used. • Source can be terrestrial or extraterrestrial (E.g. the earth, the moon, the sun, the galaxies). • Do not effect the entire communication frequency spectrum but affect certain frequencies at certain times and locations. • Types: Man made noise, space noise, atmospheric noise.

  11. Cont’d... • Man made noise • Produced by mankind • Source : Spark-producing mechanisms • Impulsive in nature & contains a wide range of frequencies propagated through space. • Sometimes called industrial noise (metropolitan & industrial area).

  12. Cont’d... b. Space noise • The sun is a powerful source of radiation. • Stars also radiate noise called cosmic, stellar or sky noise. • Important at higher frequencies (VHF and above) because atmospheric noise dominates at lower frequencies.

  13. Cont’d... c. Atmospheric noise • The principle source is lightning ( a static electricity discharge. • Can propagate for a long distances through space. • The lightning energy relatively low frequency (up to several MHz).

  14. Cont’d... • Electronic noise generated by the passive and active components incorporated in the designs of communications equipment. • Types : Shot noise, flicker noise, thermal noise. Internal Noise

  15. Cont’d... • Shot Noise • Caused by a random arrival of carriers (holes and electrons) at the output of an electronic devices. • Randomly varying & superimposed onto any signal present. • Sometimes called transistor noise.

  16. Cont’d... • Flicker noise • Excess noise that related to dc current flow through imperfect conductors. • The real nature of flicker noise not yet fully understood.

  17. Thermal Noise • This type of noise arise due to the random motion of free electrons in the conducting medium such as resistor. • Each free electron inside a resistor is in motion due to its thermal energy. • The path of electron motion is random and zig-zag due to collision with the lattice structure.

  18. Cont’d... • The net effect of the motion of all electrons constitutes an electric current flowing through the resistor. • It causes the rate of arrival of electron at either end of a resistor to vary randomly and thereby varies the resistor’s potential difference. That is the direction of current flow is random and has a zero mean value.

  19. Cont’d... • Resistors and the resistance within all electronic devices are constantly producing noise voltage Vn(t). • Since it is dependent on temperature, it is also referred to as thermal noise.

  20. Where Pn = noise power (Watt) k = Boltzman constant (1.38 x 10-23J/K) T = conductor temperature (K) [Add 273 to C] B = Bandwidth of system (Hz) Watt • Thermal noise also known as Johnson noise or white noise. • In 1928, J.B. Johnson founded that Noise Power is direct proportionally with temperature and bandwidth. • Noise spectrum density is constant for all value of frequency to 1012 Hz. Pn = k T B

  21. From the study of circuit theory, the relationship between source resistor and matched load under maximum power transfer is when Rn = RL . • The total of noise source power is Pn.

  22. Known as Rn = RL = R, Therefore voltage at RL is

  23. Example 1 • A receiver has a BW of 10 kHz with the 4.14 x 10-17 W noise power. A resistor that matches the receiver input impedance is connected across its antenna terminals. Calculate the resistor’s temperature in Celsius.

  24. Example 2 • A 1 kΩ resistor is connected across 1 kΩ antenna input of a television receiver. The BW of the receiver is 5 MHz and the resistor at the room temperature 293 K. Calculate the noise power and noise voltage applied to the receiver input.

  25. How to Quantifying the Noise? • The presence of noise degrades the performance of analog and digital communication. • The extent to which noise affects the performance of communication systems is measured by the output signal to noise power ratio or SNR (for analog communication systems) and probability of error (for digital communication systems).

  26. Cont’d... • The signal quality at the input of the receiver is characterized by the input signal to noise ratio. Because of the noise sources within the receiver, which is introduced during the filtering and amplification processes, the SNR at the output of the receiver will be lower than at the input of the receiver. • This degradation in the signal quality is characterized in terms of noise equivalent bandwidth, N0, effective noise temperature, Te. and noise figure,F

  27. Noise Calculation • SNR is ratio of signal power, S to noise power, N. • Noise Factor, F • Noise Figure, NF

  28. Noise Calculation In Amplifier o Two types of model - Noise amplifier Model. - Noiseless amplifier model.

  29. Analysis ofNoise AmplifierModel

  30. Analysis ofNoiseless AmplifierModel

  31. SNR0 <<< SNRi As known as Noise Factor, Noise Temperature,

  32. antenna F3, Te3 F1, Te1 Si Ni Ti F2, G2, Te2 So No G1 G3 Nai1 Nai2 Nai3 demodulator amplifier pre-amplifier Analysis of Cascade Stages • Consider three two ports in cascade S1 N1 S2 N2 Stage 1 Stage 2 Stage 3

  33. Stage 1

  34. Stage 2

  35. Stage 3

  36. Noise Factor, F

  37. Known as the overall noise factor, FTOTAL

  38. And we can calculate noise temperature, Te

  39. It can also be shown that the overall noise figure, F and the effective noise temperature, Teof n networks in cascade is given by:

  40. Transmission Loss, Attenuator • Every transmission medium will produce power loss. Pout < Pin. • Power loss or attenuated is given by the following equation:

  41. Where ℓ = transmission medium length α = attenuated constant Cont’d... • We also can calculate by using this following equation;

  42. Example 3 Determine: • Noise Figure for an equivalent temperature of 75 K (use 290 K for the reference temperature). • Equivalent noise temperature for a Noise Figure of 6 dB.

  43. Example 4 For three cascaded amplifier stages, each with noise figure of 3dB and power gain of 10 dB, determine the total noise figure.

  44. Additional Notes • Noise can be divided into two general categories: correlated and uncorrelated noise • Correlation implies a relationship between the signal and the noise. • Correlated noise: exists only when a signal is present. • Uncorrelated noise: present all the time whether there is signal or not.

  45. Correlated Noise • Exists only when a signal is present. • Is a form of internal noise that is correlated to the signal. • Produced by nonlinear amplification and includes harmonic and intermodulationdistortion (both of which are forms of nonlinear distortion/amplitude distortion). • Nonlinear distortion creates unwanted frequencies that interfere with the signal and degrade performance.

  46. Harmonic Distortion • Harmonics: integer multiples of the original signal • HD  occurs when unwanted harmonics of a signal are produced through nonlinear amplification (nonlinear mixing). In other words, the creation of harmonics of the fundamental frequency of a sine wave input to a system. • Various degrees of harmonic distortion. (2nd order HD, 3rd order HD…) • 2nd order HD: ratio of the rms amplitude of the 2nd harmonic to rms amplitude of the fundamental. • 3rd order HD:…

  47. Harmonic Distortion cont’d… • More meaningful measurement is the Total Harmonic Distortion (THD):- the ratio of the quadratic sum of the rms values of all the higher harmonics to the rms value of the fundamental. %THD = percent total harmonic distortion vhigher = quadratic sum of the rms voltages of the harmonics above the fundamental frequency vfundamental = rms voltage of the fundamental frequency Lower THD percentages are better…

  48. Intermodulation Distortion • Generation of unwanted sum and difference frequencies produced when two or more signals mix in a nonlinear device. • The sum and difference frequencies are called cross products. • Unwanted cross-product frequencies can interfere with the information signals in a circuit or with the information signals in other circuits. Note: Refer example 1-13, page 27 (text book)

  49. Example (HD) • Determine: • 2nd, 3rd and 12th harmonics for a 1-kHz repetitive wave. • Percent of 2nd order, 3rd order and THD for a fundamental frequency with an amplitude of 8 Vrms, a second harmonic amplitude of 0.2 Vrms and a third harmonic amplitude of 0.1 Vrms.

  50. QUIZ 3 • For an electronic device operating at a temperature of 17ºC with a bandwidth of 10 kHz, determine: • Thermal noise power in watts and dBm. • RMS noise voltage for a 100-Ω internal resistance and a 100-Ω load resistance. • An amplifier consists of three identical stages in tandem. Each stage having equal input and output impedances. For each stages, the power gain is 8 dB when correctly matched and the noise figure is 6dB. Calculate the overall power gain and noise figure of the amplifier.

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