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EKT 231 COMMUNICATION SYSTEM MEETING LECTURE LABORATORY : 3 HOURS : 2 HOURS LECTURER PUAN NORSUHAIDA AHMAD 04-9798416 [email protected] Other Lecturers Pn Sabarina /Pn Yusnita Microelectronics PM Dr Brijmohan/Pm Dr R. Badlishah Industrial Electronic & Electrical System Cik Junita Electronics & Mechatronics OBJECTIVES To understand and use various terminologies in Communication System. To emphasize on the importance of modulation and demodulation of analog signals along with associated system design issues. To be able to select a channel and assimilate associated peripherals for DAQ and signal processing. To characterize amplitude, double-sideband and single sideband modulated waveforms in the time and the frequency domains. To characterize frequency and phase modulated signals in the time domain and tone modulated signals in the frequency domain. To study the quantization process in a pulse code modulation system in terms of how it is created and how to minimize its effect. To assimilate SNR in various system. Textbook Mullet “Basic Telecommunications : The physical layer", Thomson Learning, 2003. Ziemer/Tranter “Principles of communication systems, modulation and noise, 5th Ed, Wiley,2002 References Wayne Tomasi, “ Electronic Communication Systems Fundamentals Through Advanced” 5th Ed, Prentice Hall, 2004. William Schweber, “ Electronic Communication Systems- A complete Course”, Prentice Hall, 1999. Haykin Simon, “Communication System” 4th Ed. Lathi B.P “Modern Digital & Analog Communication System” Blake “Electronic Communication System” Assessment Final Exam = 50 % Coursework = 50 % Assignments/Quiz =5% Tests = 15 % (test 1, test 2 & test 3) Labs/Tutorials = 30 % Communication System Communication system is IT DEALS in alternatively known as TELECOMMUNICATION It deals in Tele-Communication of Audio Video and/or Data COMMUNICATION means and/or and/or For conveying information and thoughts, we need in depth vocabulary. In depth Vocabulary means: Specific words for specific sense. Primitive communication ….. In primitive times, when vocabulary was in infancy, communication was through facial expressions, vocal sound and body language were used to express, as we do to a dumb-person or, pet animal. Improved primitive communication was basically digital….. Earlier telecommunication incorporated the coded position of torches; had to be in LOS; were error prone. Improved by use of drum beatings; LOS not needed, information could be repeated in speed. Coding information possible. Multi colored flags in different position represented different meanings. Improved coding. Has to be LOS. As vocabulary and technology of printing improved…. began sending of Written or verbal message through man and Written/pictorial message by trend birds. Today.. Perspective is altogether changed. We have ever improving vocabulary, voice and video communication and soft and hard printing technology. Most work, including purchases and bank transactions are executed from home, on net. Cont’d… Hard printing restricted, soft printing encouraged and e_communication promoted. Net is an intelligent, smart and active communication link between electronic gazettes including computers. Internet besides computer, is now accessible on: personal diaries, mobile phone, VoIP phones, TVs etc. Present era is information age Information is available in abundance. This leads to knowledge that helps to develop strategy. It has become possible due to availability of Communication channels, that include data acquisition, transmission and reception. High degree of intelligence and security has been included into these channels to take appropriate decisions. Intonation is the process of communicating quality of feelings/emotions. The process includes pitch in the vocal card enhanced by music, body language with expressions on face and eye. Diagrams, graphs, pictures and photos. intonation is unmasked expression Providing media for tele-communication including multi-media and data communication and Providing controlled QoS. History Evolving of Communication System took centuries of hardships. Techniques developed were mathematically founded and accelerated by material cum fabrication science. Looking to the rate of development, it still seems to be in infancy stage. Now we see the history of advancement. Terminology in Communication Purpose is to familiarize & correlate the terminologies for Communication Systems. Power: a measure of time rate of energy spent or, generated. Units watts scalar. For the instant values of variables voltage v(t) volt and current i(t) ampere in a circuit or it’s any element, the average power in watts consumed is time integral of the product of instant values of voltage and currents over a period of time (t2-t1). P ={ v(t)i(t) dt}/(t2-t1) Watt Divided by over a time “(t2-t1)” Non-zero power P will be non zero for (t2-t1)=T, if and only if the two variables v(t) and i(t) (a) have same periodicity, T and/or (b) not in quadrature phase. Calculation of Power by Graph Tabulate the Multiplication of v(t) and i(t) at different instances of time in seconds. Plot them with time as x-axis. The area under the curve between the time limit corresponds to the integral. It is equal to the work done, in wattsecond. Cont’d… The average power is obtained by divide the area obtained by the time limit. In place of voltage and current, the variables can be torque and angular velocity; force and velocity etc. If the frequency of the two signals is not same, power developed by them over composite period is essentially zero. Alternatively: If the periodicity of the two signals differ, the integral over the composite period will always be zero. This we have seen while computing the coefficients of a wave by Fourier Series. Example: Calculation of Average power v(t)= Vp cos(t+) and i(t) = Ip(t+) 1 P( t ) T T v ( t ) i( t ) d t 0 T 1 T V cos t I cos t d t p p 0 V I p T p T cos t cos t d t 0 T V I p p cos cos 2t d t 2 T 0 If Vp/2 P( t ) =V and Ip/ 2 =I :: effective values V I cos For average Power: V and I should be effective values Average Power by dot product Let V(t) = |V| = iVcos + jVsin and I(t) = |I| = I Icos + j I sin; P(t) =V(t).I(t) =Vcos()xIcos() + Vsin()xIsin() = VI cos(-) Angle between the two vectors same Average Power by complex variable Let V(t) = |V| =Vcos + jVsin and I(t) = |I| =Icos + j I sin ; Complex conjugate of I(t) = I*(t) = |I| - = Icos - j I sin V(t)I*(t) = VI[cos()cos() + sin()sin()] = VI cos(-) Power gain Signal level gain signal gain In Engineering Problems, we have known the term signal gain / mechanical advantage; Examples are chain pulley block, cantilever, gear, amplifier, transformer. Voltage amplifier: Av= Vo/Vi. Cont’d… Transistors current gain: = ic/ib, Chain pulley block: weight lifted/weight applied. Transformer: secondary voltage/primary voltage gear box: output torque/input torque. Power gain It is the ratio of output power over input power. Ap = Po/Pi. If the energy is consumed in doing a work, Power gain is always 1. Example is transformer, chain pulley block, gear boxes etc have power gain less than one. In amplifiers, the apparent power gain may be more than one. The signal power is amplified. DC electric power is transformed into signal power. In signal gain: The advantage or, signal gain may be >1 though the power gain is < 1. At first instance, it appears that there is no apparent relation between signal gain and power gain. It is because the friction of the load in which the power is fed, is not accounted. Power and voltage gain in communication In communication, due to known characteristic impedance of the channel, the power and voltage gains become explicit. It is designated in terms of decibels, dB. Power gain in dB = 10 log (Po/Pi) dB. Voltage gain in dB = 20 log (Vo/Vi) dB. Here if power gain < 1, voltage gain <1. Here the load impedance in which the power of the signal fed, is predefined. Zo is generally the load or, source impedance in communication channels. Zo for a given channel is generally fixed. The source as well as load impedance are matched with the characteristic impedance, Zo. It is because our interest in tele-communication transmission lies in maximum power transfer. The maximum efficiency thus is 50%. This implies that voltage gain and power gain go hand to hand in communication system. Standard characteristic impedances of various channels Audio Applications: Twisted pairs: 600 ohm 100 ohm one twist per 4 cm. Cables TV RF signal delivery Air 75 ohm 50 ohm 377 ohm We are yet to discuss channel’s characteristic impedance as frequency dependant. Power gain in dB =10 log (Po/Pi) dB. Voltage gain in dB = 20 log (Vo/Vi) dB. are absolute gains power ratio Po/Pi = 10,000 = 40 dB Voltage ratio Vo/Vi = 100 = 40 dB. See that Po/Pi = (Vo/Vi)2 Term is power (Po/Pi) dB = 2(Vo/Vi)dB For the power gain 2n = 3n dB. Voltage gain is 2n = 2x3n dB For that power gain 10n = 10n dB Voltage gain of 10n = 2x10n dB In attenuation, gain < 1, the dB gain is negative. Alternatively: (gain in dB/10) Power gain = 10 Voltage gain = 10 (gain in dB/20) Examples: A 64 dB gain means 106.4 = 2.5212x106 watts. An attenuation by 0.01= 10 log(0.01) = -20 dB Examples: Let there be two amplifiers in cascade. Their gains are 13 dB and 10 dB Sum respectively. The overall gain is 13+10 = 23 dB. In terms of ratio: 23 dB = 10(23/10)= 200 13 dB = 10(13/10)= 20 same 10 dB = 10(10/10)= 10 Again 20 x 10 = 200. multiplication Relative dB It is convenient to express signals with some reference such as 1mW power or, 1 V voltage level. This permits input- and output- signals to be expressed in terms of relative dB. When referenced to 1mW, it is written dBm When referenced to 1 V, it is written as dBV Relative dB is not a gain but is termed as gain wrt a reference. 5 watts signal, In relative dB; 10 log(5W/1mW) = 36.99 dBm 500 V signal: In relative dB; 20 log(500/1) = 53.98 dBV Application Reference gain Input to an amplifier is -12 dBV . The gain of the amplifier is 27 dB. The overall gain is 12+27 = 15 dBV. Absolute gain Note here that input signal is relative to 1 V. The gain of the amplifier is absolute. Output signal is again relative to 1 V. Why dB representation? The concept of decibels originated from the term of sound “loudness”. Double (half) the power does not mean double (half) the loudness. It follows the log rule: 10 log(Po/Pi). It permits the large changes to be incorporated in small displays that helps in interpretation. Advantages…. We can estimate the whispers as well as loudness. It enables predicting the characteristics without experimentation: Bode plot. Addition in dB is simple compared to multiplication to get output for a given input through a system. Specification are provided in dB. Communication Process The three basic factors that quantify the communication process: Bandwidth Power Noise Channel: is a media through which communication proceeds. Basically it is a passive. Inclusion of dynamic programming has made it sufficient intelligence to take decisions and act. They can route. . They can detect and correct the error in the signal. They can re-pack the signal-data. Communication networks, channels, of today are more flexible, cost effective and have better quality of service. . The three factors that control the selection of a channel are: . Bandwidth of the channel Its attenuation characteristics Impairment of the signal due to noise and distortion Channel capacity is decided by its susceptibility to noise. Channel Capacity Hartley Shannon proved that Maximum data rate in a channel is governed by its 1. Bandwidth and 2. SNR: signal to noise ratio. An intelligible speech (audible) should have SNR > 10 dB. Ex. A receiver input is 42.2 mW while the noise is 33.3 W. Soln. The SNR = 10 log (42.2 mW/33.3 W) = 31.03 dB. Every body says Noise is detrimental….. What is noise? From where they come? Noise can be an unwanted part of the signal. • It may be added to the signal by the processing equipments, environment conditions of the channel and similar. Part of the signal When it is a part of the signal, a suitable filter can remove the undesired part of the signal. It equally reduces the noise also. The noise outside the desired frequency range of signal can only be removed. When it is external to signal: It can be due to processing equipments or, man made or, created by environment / atmospheric or due to solar-space characteristics. Due to processing equipments White Noise: it theoretically contains all possible frequencies of same amplitude. A time impulse has this characteristic. The shot noise: cause: abrupt variation in current flow/power flow: can be considered as time-impulse. Thermal noise also called Johnson noise also fall under this category. Thermal noise: The thermal noise generated due to random motion of electrons that produces “additional” heat. For the device of unknown resistance, its equivalent resistance is calculated from the heat developed. The voltage level of the noise is then calculated. This noise is temperature dependent. Can be controlled by controlling temperature. Thermal noise The Controlling equation for thermal noise power and voltage are: Pth= kTB watt and eth = [4kTBR] volt where k = Boltzman constant =1.38x10-23 J/°K, T = temperature in Kelvin, B = Bandwidth R = equivalent resistance. Note that The equations Pth = kTB watt and eth = [4kTBR] volt indicate that available noise power from a source does not depend on the value of equivalent resistance but the open circuit rms voltage does. Couch-II Digital and analog communication system, 6/e, pp 579 Noise in active devices Thermal noise developed by transistors, ICs, diodes etc. is difficult to calculate. Therefore they are experimentally found out by the amount of heat developed. The equivalent resistance is calculated. Power flow generates noise Flicker Noise: This noise is inversely proportional to frequency. Has reduced effect at high frequency. Prominent at low frequencies and it affects the base-band signals. Man made noises: automobiles, computers, switching electronics, power switches, commutator Sparking, Switching on and off of electric gazzets and alike. The range is upto 600 MHz. [Kennedy et al,”Electronic communication Systems” 4/e pp14-32] atmospheric Thunder storm/Static discharge, Vicinity of radiating sources, such as corona discharge, spark. Solar radiations. Many unknown sources those radiate energy. Such noises are impulsive and occupy radio frequency spectrum upto 30 MHz. [Kennedy et al,”Electronic communication Systems” 4/e pp14-32] NOISE FACTOR The noise that has been multiplied during transmission of the signal is: NF = SNRinputdB – SNRoutputdB. Ex: If the input SNR is 25 dB and NF = 10, the output SNR in dB is 25 dB + 10 dB =35 dB System generated Distortion An amplifier beside thermal noise, can generate inter modulation signals and harmonics. The later is due to operating point in non linear region of transfer characteristic. A transfer characteristic with y as output and x as input is defined by a power series: y = ao+a1x+a2x2+…..+aixi+……anxn+… The power series : y =ao+a1x+a2x2+…..+aixi+……anxn+… These coefficients are assumed to be constants. Except coefficients of x, other represent non-linearity. y = ao+a1x is linear in mathematical sense, but it is nonlinear in engineering sense. y = a1 x is an LTI system equation in engineering sense. The values of other coefficients provide the measure of non linearity. The non-linearity generates harmonics and intermodulation signals in the output. Generates Harmonic Signals Take the part of the equation: y’ = a2x2. Let x = cos. x2 = cos2 = (1-cos 2)/2 y’ = (a2/2) (1-cos 2) 2nd Harmonic generated Coeff. 2 3rd Harmonic Generated Try for y”=a3x3. The result should be: y”= (a3/2) (cos - cos3) Output depends on the values of coefficients. If say a3 =0; then y”….?? Inter-modulation distortion Let x = (mcos + ncos) (hint take m = dcos and n = dsin) Try both cases. y’ = a2x2 y”= a3x3 The output is inter-modulation distortion. Phase distortion Besides amplitude distortion due to non linear transfer characteristic of the system, there also exists a phase distortion. Phase distortion is due to non linear phase delay vrs frequency characteristic of the system. It does not affect the signal power but does affect the properties of the signal. What does distortions do? Phase distortion results into hazy signal reconstruction. In video, one will miss the sharpness in the figures. Phase distortion does not affect audio. It is due to characteristics of our ear. In Audio, due to amplitude distortion, generation of harmonics distorts intelligence. In video, amplitude distortion does not make too much sense. It is due to characteristic of our eyes. Signal and Noise Signal is a vector quantity. Noise is a random signal from unknown sources. Unwanted noise of signal is also termed as noise. When vectorially added, can randomly add or subtract to the amplitude of the intelligence. The resultant signal may not remain dependable. It can create bit error in digital signaling.