Communication Systems Essay Example For Students

Communication Systems Essay COMMUNICATION SYSTEMSBarriers to Human Communication,XLanguage,XDistanceElectronic CommunicationsThe transmission, reception, and processing of information using electronic circuits. Historymid-19th century V James Clark Maxwell studied electromagnetic wave and predicted that it can be propagated through free space. 1837 V Samuel Morse invented the telegraph. 1876 V Alexander Graham Bell and Thomas A. Watson transmitted human conversation over a functional telephone system. 1888 V Heinrich Hertz radiated electromagnetic energy from a machine he called oscillator. 1894 V Guglielmo Marconi was the first to accomplish wireless transmission. 1908 V Lee DeForest invented the triode vacuum tube. 1933 V Major Howard Armstrong invented frequency modulation. 1948 V William Shockley, Walter Brattain and John Bardeen invented the transistor. Electromagnetic SpectrumElectromagnetic SpectrumThe entire range of frequency. FrequencyThe number of times an alternating current goes through its complete cycle per second is known as its frequency. The international unit of measurement of frequency is hertz, abbreviated Hz. The English unit is cycles per second, abbreviated cps (1 Hz = 1 cps). It is rarely used. To simplify terminology, 1000 Hz is called kilohertz, abbreviated kHz, and 1000000 Hz is called a megahertz, abbreviated MHz. The vibration rate of sound waves in air may also use the term frequency. When middle C is played on a musical instrument, for example, an air disturbance with a frequency of 262 Hz is set up. The lowest tone that can be heard by human beings is about 15 Hz. The highest audible, audio, or sonic tones are usually 12 to 22 kHz (22 kHz for youngsters, 12 kHz for seniors). A microphone is a device or transducer that can change sound waves in air to an equivalent-frequency ac in wires. Frequencies that produce sound waves audible to humans are said to be audio frequencies (AF). Frequencies that can be fed to antennas and will radiate electromagnetic and electrostatic waves in space are considered to be radio frequencies (RF). FCC Band Designations Frequency RangeDesignations30 V 300 HzELF (Extremely Low Frequencies)0.3 V 3 kHzVF (Voice Frequencies)3 V 30 kHzVLF (Very Low Frequencies)30 V 300 kHzLF (Low Frequencies)0.3 V 3 MHzMF (Medium Frequencies)3 V 30 MHzHF (High Frequencies)30 V 300 MHzVHF (Very High Frequencies)0.3 V 3 GHzUHF (Ultra High Frequencies)3 V 30 GHzSHF (Super High Frequencies)30 V 300 GHzEHF (Extremely High Frequencies)0.3 V 3 THzInfrared 3 V 30 THzInfrared 30 V 300 THzInfrared 0.3 V 3 PHzVisible Light3 V 30 PHzUltraviolet 30 V 300 PHzX-rays0.3 V 3 EHzGamma rays3 V 30 EHzCosmic rays, FCC stands for Federal Communications Commission Wavelength, U The length that one cycle of an electromagnetic wave occupies in space, also, the distance between simila r points in a repetitive wave. wherek= velocity factor (equal to 1 in free space)c= velocity of light in free space= 299793000 m/s ,l 3 ,e 108 m/sf= frequency@ high frequencies, wavelength is too short and is usually expressed as Angstrom. Bandwidth and Information CapacityLimitations of Communication Systems,Xnoise,XbandwidthBandwidth, BWunit:Hz (Hertz)A portion of electromagnetic spectrum occupied by a system. Minimum range of frequencies required propagating the source information through the system. must be sufficiently large (wide) to pass all significant information frequencies. difference between the upper and lower limit frequencies. Information CapacityA measure of how much source information can be carried through the system in a given period of time. Hartleys LawInformation Capacity N Bandwidth ,e timeSignal AnalysisFactors Affecting the Signal,XDistortion V signal alteration due to imperfect response of the system to the desired signal. ,XInterference V contamination of extraneous signal usually man-made to a form similar to the desired signal. ,XNoise V random, undesirable high frequency spikes. ,XAttenuation V decrease in signal level. Types of Signals,XBaseband Signal,XModulated SignalTypes of Intelligence Transmitted,XAnalog ,XDigitalAdvantages of Digital Over Analog,Xeasier to multiplex,Ximproved noise immunity,Xbetter performance,Xeasier to interfaceDisadvantages of Digital Over Analog,Xneed for large (wide) bandwidth,Xneed for synchronization,Xneed for additional equipment,Xrestriction to wired topologyKinds of Signal Representations,XTime-Domain V amplitude vs. time,XFrequency-Domain V amplitude vs. frequencyClassifications of Signals:According to Period,XPeriodic V signals that keep on repeating at a regular interval. Autism: Educational and Social Effects EssayResults of Noise,Xhiss/static,Xsnow/confetti,Xbit errors,Xsignal lossKinds of NoiseCorrelated Noise V mutually related to the signal and cannot be present in a circuit unless there is an input signal and is produced by nonlinear amplification. No signal, no noise!Harmonic Distortion V unwanted harmonics of a signal are produced. Intermodulation Distortion V the generation of unwanted sum and difference frequencies (cross products) when two or more signals are amplified in a nonlinear device. Uncorrelated Noise V present regardless of whether there is a signal present or not. Kind of Uncorrelated NoiseExternal Noise V generated outside the device or circuit. Atmospheric Noise V naturally occurring electrical disturbances that originate within the earths atmosphere. Extraterrestrial Noise V consists of electrical signals that originate from outside Earths atmosphere. Solar Noise V directly from the suns heat. Cosmic Noise V from the stars. Man-made Noise V produced by manufactured equipment, such as automotive ignition systems, electric motors and generators. Internal Noise V generated within a device or circuit. Shot Noise V caused by the random arrival of carriers (holes and electrons) at the output element of an electronic device. Transit-Time Noise V shows up as a kind of random noise within the device and is directly proportional to the frequency of operation. Thermal Noise V associated with the rapid and random movement of electrons within a conductor due to thermal agitation. Noise ComputationsNoise Powerthe average noise power is proportional to the absolute temperature of the conductor and to the bandwidth or spectrum of the thermal noise. wherePN= noise power (W)T= Temperature of the conductor (K)B= bandwidth of the noise spectrum (Hz)k= Boltzmanns Constant = Noise VoltagewhereVN= rms noise voltageT= Temperature of the conductor (K)B= bandwidth of the noise spectrum (Hz)k= Boltzmanns Constant = R= equivalent resistance generating the noisefor combinations of resistancesSeries Parallel Power Spectrum Density or Noise Density,Xaverage noise power per Hertz of bandwidth,Xa figure that determines the amount of noise contained in a specified bandwidth. Signal-to-Noise Ratio,Xa relative measure of the desired signal power to the noise power. In decibel form:wherePS= signal powerPN= noise powerVS= signal voltage VN= noise voltage Noise FactorwhereSi= input signal powerNi= input noise powerSo= output signal powerNo= output noise powerNoise Figurefor ideal noiseless networkfor a network that contributes noiseReactance Noise Effects,Xthe significant effect of reactive circuits on noise is their limitation on frequency response. ,Xthe equivalent bandwidth to be used in noise calculations with reactive circuits iswhereB3dB= half power bandwidth Equivalent Noise TemperaturewhereTeq= equivalent noise temperature To= reference absolute temperature = 290 KF= noise factorNoise Due to Amplifiers in CascadeFriiss FormulaOver-all noise factor of n stagesOver-all noise temperature of n stagesOver-all noise resistancewhereA= voltage gain (ratio)G= power gain (ratio)Shot Noise,Xa form of internal noise, which is due to the random variations in current flow in active devices such as tubes, transistors and diodes. WhereiN= rms noise currentq= charge of an electron = B= bandwidth over which the noise is observed I= dc bias current in the deviceProblems:1.What is the shot noise current for a diode with a forward bias of 1.15 mA over a 50-kHz bandwidth?2.An amplifier operating over the frequency range of 455 kHz to 460 kHz has a 200 kC input resistance. What is the rms noise voltage at the input to the amplifier if the ambient temperature is 17XC?3.Two resistors, 5 kC and 20 kC are at 27XC. Calculate the thermal noise power and voltage for a 10 kHz bandwidth. a) for each resistor, b) for their series combination, and c) for their parallel combination. 4.Three matched amplifiers are available to amplify a low level signal, they have the following characteristics. AmplifierPower GainNoise FactorA6 dB1.5B12 dB2C20 dB4The amplifiers are to be connected in cascade. Calculate the lowest overall noise factor obtainable noting the order in which the amplifiers must be connected.