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Transcript
CHAPTER 1 Part 2 Noise Objectives To differentiate the types of noise To calculate the thermal noise generated by a resistor To calculate the signal-to-noise ratio (SNR) and noise figure for an amplifier Lecture overview Types of noise Thermal noise Signal-to-noise ration (SNR) and noise figure Introduction Noise can be defined 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 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. Block Diagram of Communication System With the Existence of Noise 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. ◦ 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. ◦ Distortion The signal perturbation caused by imperfect response of the system to the desired signal. Disappear when the signal is turnedoff. Can be corrected by the equalizers. Noise Remedies? REDUCE BANDWIDTH INCREASE TRANSMITTER’S POWER LOW NOISE AMPLIFIERS Types of NOISE NOISE INTERNAL EXTERNAL THERMAL NOISE -transistor -diode -resistors MAN MADE NOISE -automobile engine -electric motor -computer SHOT NOISE -electronic system -equipment SPACE NOISE -solar noise -sky noise FLICKER NOISE -tubes ATMOSPHERIC NOISE -Noise blanking -lighting EXTERNAL NOISE ◦ Noise generated outside the device or circuit. ◦ Three primary sources: Atmospheric noise. Extraterrestrial noise Man-made noise EXTERNAL NOISE Atmospheric noise o Naturally occurring electrical disturbances that originate within Earth’s atmosphere. o Is often in the form of impulses that spread energy throughout a wide range of frequencies. EXTERNAL NOISE Extraterrestrial noise o Consists of electrical signals that originate from outside Earth’s atmosphere. o 2 categories: o Solar noise: is generated directly from sun’s heat. o Cosmic noise: are continuously distributed throughout the galaxies. EXTERNAL NOISE Man-made noise o Noise that is produced by mankind. o Predominant sources are sparkproducing mechanisms such as commutators in electric motors, automobile ignition systems, ac powergenerating and switching equipment. o Is impulsive in nature and contains a wide range of frequencies that are propagated through space in the same manner as radio waves. INTERNAL NOISE ◦ Electrical interference generated within a device or circuit. ◦ 3 primary kinds: Shot noise Transit time noise Thermal noise INTERNAL NOISE Shot Noise oCaused by a random arrival of carriers (holes and electrons) at the output of an electronic devices such as diode, field-effect transistor or bipolar transistor. oRandomly varying & superimposed onto any signal present. oSometimes called transistor noise. INTERNAL NOISE Transit-time noise: o Any modification to a stream of carriers as they pass from the input to output of a device produces an irregular, random variation. Thermal Noise Rapid and random movement of electrons within a conductor due to thermal agitation. Is present in all electronic components and communications systems. Is a form of additive noise, meaning that it cannot be eliminated and it increases in intensity with the number of devices in a circuit and with circuit length. Thermal Noise Since it is dependent on temperature, it is also referred to as thermal noise. Thermal noise power is proportional to the product of bandwidth and temperature. N = KTB Where N = noise power (Watts) B = bandwidth (Hz) K = Boltzman’s constant (1.38 x 10-23 joules per kelvin) T = absolute temperature (kelvin) (room temperature = 17oC or 290K) To convert oC to kelvin, add 273oC EXAMPLE 1 Convert the following temperatures to kelvin: 100oC, 0oC and -10oC. Solution: T = oC + 273oC T =100 oC + 273oC = 373K T = 0oC + 273oC = 273K T = -10oC + 273oC = 263K NOISE VOLTAGE 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 N. Let RN = RL = R The noise voltage dropped across R is VN VR 2 2 VN 2 VN 2 Noise power, N KTB 4R R Thus, VN 4 RKTB 2 VN 4 RKTB EXAMPLE 2 For an electronic device operating at a temperature of 170C with a bandwidth of 10 kHz, determine: (a) thermal noise power in watts and dBm (b) rms noise voltage for a 100Ω internal resistance and 100Ω load resistance. SIGNAL-TO-NOISE POWER RATIO Is the ratio of the signal power level to the noise power level. S/N = Ps/Pn where Ps = signal power (watts) Pn = noise power (watts) Or S/N (dB) = 10 log(Ps/Pn) EXAMPLE 3 For an amplifier with an output signal power of 10 W and an output noise power of 0.01 W, determine the signal-to-noise power ratio and express it in dB. NOISE FACTOR AND NOISE FIGURE Noise factor is a ratio of input signal-tonoise power ratio to output signal-tonoise power ratio Noise figure is the noise factor stated in dB Continue… When two or more amplifiers are cascaded as shown in figure below, the total noise factor is the accumulation of the individual noise factors Input Amplifier 1 F1 Ap1 Amplifier 2 F2 Ap2 Amplifier 3 F3 Ap 3 Output Continue… To calculate the total or overall noise factor: ( Fn 1) ( F2 1) ( F3 1) FT F1 ... A1 A1 A2 A1 A2 ... An 1 EXAMPLE 4 For three cascaded amplifier stages, each with noise figures of 3dB and power gains of 10 dB, determine the total noise figure EQUIVALENT NOISE TEMPERATURE Equivalent noise temperature (Te) is a hypothetical value that cannot be directly measured Te = T (F – 1) Where Te = equivalent noise temperature (kelvin) T = environmental temperature (reference value of 290K) F = noise factor (unitless) Similarly, the overall equivalent noise temperature, Te of cascaded amplifiers is given by: Te 2 Te 3 Ten Te Te1 ... A1 A1 A2 A1 A2 ... An 1 EXAMPLE 5 Determine: a. Noise Figure for an equivalent temperature of 75 K (use 290 K for the reference temperature). b. Equivalent noise temperature for a Noise Figure of 6 dB.