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  1. NOISE Instructor: Sam Nanavaty

  2. Noise is the UNDESIRABLE portion of an electrical signal that interferes with the intelligence Instructor: Sam Nanavaty

  3. Why is it important to study the effects of Noise? • Today’s telecom networks handle enormous volume of data • The switching equipment needs to handle high traffic volumes as well • our ability to recover the required data without error is inversely proportional to the magnitude of noise • What steps are taken to minimize the effects of noise? • Special encoding and decoding techniques used to optimize the recovery of the signal • Transmission medium is chosen based on the bandwidth, end to end reliability requirements, anticipated surrounding noise levels and the distance to destination • Elaborate error detection and correction mechanisms utilized in the • communications systems Instructor: Sam Nanavaty

  4. The decibel (abbreviated dB) is the unit used to measure the intensity of a sound.! The smallest audible sound (near total silence) is 0 dB. A sound 10 times more powerful is 10 dB. A sound 1,000 times more powerful than near total silence is 30 dB. Here are some common sounds and their decibel ratings: Normal conversation - 60 dB A rock concert - 120 dB It takes approximate 4 hours of exposure to a 120-dB sound to cause damage to your ears, however 140-dB sound can result in an immediate damage Instructor: Sam Nanavaty

  5. Relative power gain of a device can be expressed as Ap = Po/Pi (Power levels are expressed in Watts) Relative power gain of a device in decibels is Ap(dB) = 10 Log Ap = 10 Log Po/Pi Alternatively, the above equations can be represented as Ap(dB) = 10 Log (Vo2/Ro)/(Vi2/Ri) If (Ro = Ri) Av(dB) = 10 Log (Vo/Vi)2 = 20 Log (Vo/Vi) = 20 Log Av Po and Pi can be substituted with Pfin and Pinit as in Final and initial values of power source Instructor: Sam Nanavaty

  6. Ap is a relative power gain Ap is not necessarily the power gain between output and input Ap can be computed for comparing any two different power levels e.g., You may be asked to compute a relative power gain ratio of an amplifier which has been redesigned so that the maximum output power has increased from .25W and 5W Ap = 5/.25 = 20 and Ap(dB) = 10 x Log Ap = 10 x Log 20 = 13.01 If Ap(dB) = 20 dB and Po = 550mW, compute Pi Ap = 10 Ap(dB)/10 = 100 Po/Pi = 100 Pi = 550/100 = 5.5 mW Instructor: Sam Nanavaty

  7. A preamp has a voltage gain of 28dB. Compute the following: • If Vi = 2 mV then Vo = ? • A v(dB) = 20 Log Av • Av = 10 Av(dB)/20 • Av = 25.11 • If Vi increases from 2 to 5 mV, how many dB has the signal increased? • Av(dB) = 20 Log (Vfin/Vinit) = 20 Log (5/2) = 20 Log 2.5 = 7.95 dB • b) If Vi drops from 2 to 1 mV, how many dB has the signal dropped? • Av(dB) = 20 Log (Vfin/Vinit) = 20 Log (1/2) = 20 Log .5 = -6 dB Instructor: Sam Nanavaty

  8. The absolute power gain is defined as “A unit of gain or loss expressed as an absolute value based on 1 mW of standard reference” Ap(dBm) = 10 Log (P/ 1 mW) This represents an absolute Power gain based on a standard input level of 1 mW in to 50Ω , 600Ω or 900Ω depending on the impedance of the Transmission media. “P” represents Power level which can then be computed as follows: P = 1 mW (10 (Ap(dBm)/10)) In terms of voltage, the above equation can then be represented as (Vrms2/R) = 1 mW (10 (Ap(dBm)/10)) Vrms = √1 mW (10 (Ap(dBm)/10)) x R (where R = standardized value obtained from the manufacturer) Instructor: Sam Nanavaty

  9. 1) If Signal level of 30 MHz test tone measures -30dBm on a spectrum analyzer, Compute the power level P of signal. • P = 1mW x 10 Ap(dBm)/10 = 1 mW x 10 -30dBm/10 = 1 mW x 10 -3 = 1 uW • 2) An rf sinewave generator with o/p impedance of 50Ω is connected to 50Ω • Load using a 50Ω coaxial cable. The generator’s output amplitude level is set to • -12 dBm. An rms voltmeter is used to measure the effective voltage and an oscilloscope is used to display the sine wave. Compute the following: • rms voltage measured by rms voltmeter • Peak voltage Vp of sine wave that should be displayed on the oscilloscope • Peak-to-peak voltage of sinewave that should be displayed on the oscilloscope • a) Vrms = √ 1mW x 10 Ap(dBm) x 50 = 56.17 mV • b) Vp = Vrms/.707 = 79.45 mV • c) Vp-p = 2x Vp = 158.9 mV Instructor: Sam Nanavaty

  10. Signal to Noise ratio: It is a ratio of signal power to Noise power at some point in a Telecom system expressed in decibels (dB) It is typically measured at the receiving end of the communications system BEFORE the detection of signal. SNR = 10 Log (Signal power/ Noise power) dB SNR = 10 Log (Vs/VN)2 = 20 Log (Vs/VN) Instructor: Sam Nanavaty

  11. 1) The noise power at the output of receiver’s IF stage is measured at 45 µW. With receiver tuned to test signal, output power increases to 3.58 mW. Compute the SNR SNR = 10 Log (Signal power/ Noise power) dB = 10 Log (3.58 mW/ 45 µW) = 19 dB 2) A 1 kHZ test tone measured with an oscilloscope at the input of receiver’s FM detector stage. Its peak to peak voltage is 3V. With test tone at transmitter turned off, the noise at same test point is measure with an rms voltmeter. Its value is 640 mV. Compute SNR in dB. SNR = 20 Log (Vs/Vn) = 20 Log ((.707 x Vp-p/2)/Vn) = 20 Log (1.06V/640 mV) = 4.39 dB Instructor: Sam Nanavaty

  12. Noise Factor (F) : It is a measure of How Noisy A Device Is Noise figure (NF) = Noise factor expressed in dB F = (Si/Ni) / (So/No) NF = 10 Log F Instructor: Sam Nanavaty

  13. An input signal of repeater is made of 150 µW of input power and 1.2 µW of Noise power. The repeater contributes an additional 48 µW of noise and has a power gain of 20 dB. Compute • Input SNR : 10 Log (150 µW/ 1.2 µW) = 125 = 20.97 dB • Power gain of 20 dB means Ap = 100 (why?) • So = Si x Ap = 150x100 = 15mW • Output noise = No = Ni x Ap + Nr = 1.2 µW x 100 + 48 µW = 168 µW • So/No = 15 mW/168 µW = 89.3 • 10 Log 89.3 = 19.5 dB • Noise factor = 10 log ((Si/Ni) / (So/No)) = 10 log (125/89.3) = 10 log 1.4 • Noise factor = 1.46 dB Instructor: Sam Nanavaty

  14. Bit Error Rate: Number of bits that are Corrupted or destroyed during transmission E.g., BER of 10-5 indicates that 1 bit out of every 100000 is destroyed during transmission. The factors governing BER are: B/W, SNR, transmission media, Environment surrounding The media, Transmission distance and the transmitter and Receiver performance Instructor: Sam Nanavaty

  15. Noise Types • Atmospheric and Extraterrestrial noise • Gaussian Noise • Crosstalk • Impulse Noise Instructor: Sam Nanavaty

  16. Atmospheric and Extraterrestrial Noise • Lightning: The static discharge generates a wide range of frequencies • Solar Noise: Ionised gases of SUN produce a wide range of frequencies as well. • Cosmic Noise: Distant stars radiate intense level of noise at frequencies that penetrate the earth’s atmosphere. Instructor: Sam Nanavaty

  17. Gaussian Noise: The cumulative effect of all random noise generated over a period of time (it includes all frequencies). Thermal Noise: generated by random motion of free electrons and molecular vibrations in resistive components. The power associated with thermal noise is proportional to both temperature and bandwidth Pn = K x T x BW K = Boltzmann’s constant 1.38x10 -23 T = Absolute temperature of device BW = Circuit bandwidth Instructor: Sam Nanavaty

  18. Shot Noise: Results from the random arrival rate of discrete current carriers at the output electrodes of semiconductor and vaccum tube devices. Noise current associated with shot noise can be computed as In = √ 2qIf In = Shot noise current in rms q = charge of an electron I = DC current flowing through the device f = system bandwidth (Hz) Instructor: Sam Nanavaty

  19. Crosstalk:electrical noise or interference caused by inductive and capacitive coupling of signals from adjacent channels In LANs, the crosstalk noise has greater effect on system Performance than any other types of noise Problem remedied by using UTP or STP. By twisting the cable pairs together, the EMF surrounding the wires cancel out each other. Instructor: Sam Nanavaty

  20. Near end crosstalk: Occurs at transmitting station when strong signals radiating from transmitting pair of wires are coupled in to adjacent weak received signals traveling in opposite direction. Far end crosstalk: Occurs at the far end receiver as a result of adjacent signals traveling in the same direction. Instructor: Sam Nanavaty

  21. Minimizing crosstalk in telecom systems • Using twisted pair of wires • Use of shielding to prevent signals from radiating in to other conductors • Transmitted and received signals over long distance are physically separated • and shielded • 4) Differential amplifiers and receivers are used to reject common-mode signals • 5) Balanced transformers are used with twisted pair media to cancel crosstalk signals coupled equally in both lines • 6) Maximum channels used within a cable are limited to a certain value Instructor: Sam Nanavaty

  22. Impulse Noise: Noise consisting of sudden bursts of irregularly shaped pulses and lasting for a few Microseconds to several hundred milliseconds. • What causes Impulse noise? • Electromechanical switching relays at the C.O. • Electrical motors and appliances, ignition systems • Lightning Instructor: Sam Nanavaty