1. Communication Theory (EC ) charjee l Kumar ECE Department Rajalakshmi Engineering College EC COMMUNICATION THEORY Lecture NotesANNA UNIVERSITY ECE 5TH SEMESTER DETAILS: SUBJECT NAME. EC COMMUNICATION THEORY Lecture Notes for ECE – Fourth (4th) Semester posted by han EC Lecture Notes Syllabus.
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A signal commubication said to be periodic, if it exhibits periodicity. Periodic signal has the property that it is unchanged by a time shift of T. A signal that does not satisfy the above periodicity property is called a non-periodic signal. Periodic signals can be represented using the Fourier Series. Non-periodic signals can be represented using the Fourier Transform. Both Fourier series and Fourier Transform deal with the representation of the signals as a combination of sine and cosine waves.
We get the Fourier series coefficients as follows: A Fourier transform converts the signal from the time domain to the spectral domain. Source Flow of information Destination Figure 1: A simple communication system. Example — Radiotelephony, broadcasting, point-to-point, mobile communications, computer communications, radar and satellite systems.
Transfer Information — Tx in the form of audio, video, texts, computer data and picture through facsimile, telegraph or telex and internet. Broadcasting — Broadcast information to masses, through radio, television or teletext. Transducer — Device that converts one form of energy into another form. Input Lceture — placed at the transmitter which convert an input message into an electrical signal.
Example — Microphone which converts sound energy to electrical energy. Input Transducer Message Electrical Signal. Example — Loudspeaker which converts electrical energy into sound energy. Signal — electrical voltage or current which varies with time and is used to carry message or information from one point to another. Electrical Signal Output Transducer Message.
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Source, Transmitter, Channel, Receiver and Destination. Basic Block Diagram of a Communication System.
Information Source — the communication system exists to send messages. Messages come from voice, data, video and other types of information. Transmitter — Transmit the input message into electrical signals such as voltage or current into electromagnetic waves such as radio waves, microwaves that is suitable for transmission and compatible with the lecturre. Besides, the transmitter also do the modulation and encoding for digital communkcation.
Many links combined will establish a communication networks. There are 5 criteria of a transmission system; Capacity, Performance, Distance, Security and Cost which includes the installation, operation and maintenance. It is also separate the information from the received signal and sent the information to the destination. Basically, a receiver consists of several stages of amplification, frequency conversion and filtering.
Transmission of original information whether analogue or digital, directly into transmission medium is called baseband transmission. Hardware limitations Requires very long antenna Baseband signal is an audio signal of low frequency. For example voice, range of frequency is 0.
Interference with other waves Simultaneous transmission of audio signals will cause interference with each other. This is due to audio signals having the same frequency range and receiver stations cannot distinguish the signals. This process makes the signal suitable for transmission and compatible with the channel.
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Resultant signal — modulated signal 2 types theoru modulation; Analog Modulation and Digital Modulation. Analogue Modulation — to transfer an analogue low lecturf signal over an analogue bandpass channel. Digital Modulation — to transfer a digital bit stream the carrier is a periodic commumication and one of the pulse parameter amplitude, width or position changes according to the audio signal.
To allow efficient transmission — increase transmission speed and distance, eg; By using high frequency carrier signal, the information voice can travel and propagate through the air at greater distances and shorter transmission time Also, high frequency signal is less prone to noise and interference.
PCM systems use repeaters to generate the signal along the transmission path. Frequency modulation is a special case of angle modulation phase notss is another special case in which the angle of the carrier is varied proportionately to the amplitude of the baseband signal.
In angle modulation, the amplitude of the carrier signal is kept constant; hence, angle modulation methods are constant envelope transmissions. The diagram above illustrates amplitude, phase, and frequency modulation by a sine wave.
The remainder of this module will focus on FM for mobile radio. It is an expression of the amount of power in the sidebands. Power content less Same bandwidth Disadvantages – receiver is complex and expensive. Sum of carrier and modulating signal D. AM output across tuned circuit. Current pulses through diode C. The amplitude of the carrier is kept constant while its frequency is varied by the amplitude of the modulating signal. The frequency varies The rate of change of carrier frequency changes is the same as the frequency of the information signal Communicaton amount of carrier frequency changes is proportional fommunication the amplitude of the information signal The amplitude is constant.
One path is direct. The other path contains a degree phase shift unit and a product modulator. Communiction difference between the signals in the two paths produces the NBFM signal. There are two types of frequency modulator circuits.
Carrier frequency is directly varied by the message through voltage-controlled oscillator. This low freq signal passes through the RF choke L1 and is applied across the varactor diode.
Varactor diode behaves as voltage controlled capacitor. When low reverse biased voltage is applied, more capacitance is generated and thus decrease the frequency. The varactor diode changes its capacitance in sympathy with the info signal and therefore changes the total value of the capacitance in the tuned circuit. The changing value of capacitance causes the oscillator freq to increase and decrease under the control of the communication signal. The output is therefore an FM signal.
Crystal control provides frequency stability. The NBFM wave is next multiplied in frequency by using a frequency multiplier so as to produce the desired wideband FM. Any circuit that will convert a frequency variation cokmunication the carrier back into a proportional voltage variation can be used to demodulate or detect FM signals.
A popular method used for FM demodulation is the Frequency discriminator. The slope circuit converts the instantaneous ex2252 variations notse the FM input signal to instantaneous amplitude variations. These amplitude variations are rectified by the envelope detector to provide a DC output voltage which varies in amplitude and polarity with the input signal frequency.
Advantages Disadvantages Better noise immunity Rejection of interfering signals because of capture effect Better transmitter efficiency Excessive use of spectrum More complex and costly circuits.
The Lefture probability of an event A given the occurrence of event B is. If the experiment is flipping a coin.
Then a random variable X can be defined as: A random process is a mapping from the sample space to an ensemble of time ,ecture. X1 t X2 t XN t Sample function t The totality of all sample functions noted called an ensemble For a specific time X tk is a random variable.
Central limit theorem The sum of a large number of independent and identically distributed i. Thermal noise can be closely modeled by Gaussian process.
The effect of the system on X t is simply reflected by the change in mean m and covariance C of X t. It results from the shot effect in the amplifying devices and active device.
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It is caused by random variation in the arrival of electrons or holes at the output of the devices. For diode, the rms shot noise current is given by: The noise power generated by a resistor is given by: It is the idealized form of noise, whose spectrum is independent of the operating frequency.
Ec252 autocorrelation Rw t of white noise is an impulse as shown below. The signal to noise ratio SNR available from the source is given by: Consider that the source is connected to an amplifier with gain G. When noise factor is expressed in decibels, it is called noise theorj.
If the amplifier noise is Pnathen the equivalent noise temperature Te of the amplifier is given by the equation. It generally consists of a pre-selector and an amplifier stage. The pre-selector is a broad motes band-pass filter with adjustable center frequency that is tuned to the desired carrier frequency.
The other functions of the RF section are detecting, band limiting and amplifying the received RF signals. It is the stage of down-converts the received RF frequencies to intermediate frequencies IF which are simply frequencies that fall somewhere cmmunication the RF and information frequencies, hence the name intermediate. This section also includes a local oscillator LO. IF communicatioh intermediate frequency section is the stage where its primary functions are amplification and selectivity.
AM detector section is the stage that demodulates the AM wave and converts it to the original information signal.