[PDF] SECA1303 – ANALOG COMMUNICATION SYSTEMS

8196_1SECA1303.pdf 1

SCHOOL OF ELECTRICAL AND ELECTRONICS ENGINEERING

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

SECA1303 ± ANALOG COMMUNICATION SYSTEMS

2

UNIT I AMPLITUDE MODULATION AND DEMODULATION

Need for modulation Model of communication system and classification, Representation of AM Modulation index and power calculation Types of AM;DSB-FC: Collector and base modulation circuits, square law modulator- DSB-SC: Balanced modulator circuit using FET SSB: Filter method and phase shift method VSB, Comparison of various AM schemes-AM transmitter: Low level and high level Modulation. Demodulation Envelope detector, Significance of RC time constant-Square law detector.

Introduction to communication systems:

The means it is an act of passing on news, information, feeling etc. There are numerous ways for communication, i.e., Two people may communicate with each other through speech. [Gestures] or graphic symbol. But there is a lot of limitations, mainly [long] distance [For long distance communication] method for Long distance [was accomplished by] such [means] as birds drum beats, smoke signals, [carrier pigeons] and light beams etc. Due to the invention of transistors, IC and other semiconductor devices, today the mode of long distance communication has been accomplished by electrical signals. This is because of the electrical signals can be transmitted over a much longer distance (theoretically any distance in the universe). Today, in our daily lives we are using powerful technologies that allow us to communicate with people around the world The tremendous growth information technology

Examples for Electronic Communication are

1. Telephone

2. Radio Broadcasting

3. Television Broadcasting

4. Radar communication

5. satellite communication: Cover the whole globe carrying voice, text data and images

6. Fax

7. Computer communication

8. Wireless communication

This growth is possible due to high speed transmitter and receiver, development in VLSI technology, high speed microprocessors and high bandwidth transmission media.

What is Communication

Electronic Communication is the process of establishing connection (or link) between two points

(source and destination) for information exchange (or) It is the process of conveying or

transferring messages such as sounds, words, pictures etc. from one point to another point (or) Communication refers to sending, receiving and processing of information by electronic means. 3 Message signal/ information signal/ baseband signal/ signal The basic function of communication system is to transmit a message or information signal from one place to another. The origin of the message is from some information source There are many kinds of information sources like machines as well as people. The message may be in the form of words, code symbols, music, picture etc. But we can classify the message signal into two categories

Analog signal and Digital signal

The nature of the information signal determine the nature and performance of a system

Analog message signal

It is a physical quantity that varies with respect to time in a smooth and continuous manner. When a physical quantity is converted into its equivalent electrical quantity whose magnitude/strength varies with respect to time in a smooth and continuous manner.

Examples

1. An acoustic pressure is produced when we speak and it is converted into an

equivalent electrical signal with the help of a transducer called microphone. This electrical quantity is varying with respect to time in a smooth and continuous manner.

2. The signal intensity at some point in the television image

Digital message The amplitude of an electrical quantity varies with respect to time in a discrete manner not continuously or It is an ordered sequence of symbols (quantity from a set of discrete elements) Example: the keys when you press on a computer keyboard[ the amount of information conveyed in any message is measured in bits] In communication systems, this physical quantities are called as messages carry some sense or meaning and they are converted into equivalent electrical quantity called as message signal or modulating signal or base band signal. This electrical signal is transmitted to a distant place through a communication media (i.e) communication channel. At the receiving end electrical signal is reconverted back into original message.

Time/Frequency Domain Representation of Signals

Electrical signals have both time and frequency domain representations. 4 In the time domain, voltage or current is expressed as a function of time. Most people are relatively comfortable with time domain representations of signals. Signals measured on an oscilloscope are displayed in the time domain and digital information is often conveyed by a voltage as a function of time. Time domain visualization provides information such as shape of the signal and variation in voltage with respect to time. But it did not provide complete information regarding frequency content of a signal. Signals can also be represented by a magnitude and phase as a function of frequency. Signals that repeat periodically in time are represented by a discrete power spectrum. Frequency domain representations are particularly useful when analyzing linear systems. EMC and signal integrity engineers must be able to work with signals represented in both the time and frequency domains. Signal sources and interference are often defined in the time domain. However, system behavior and signal transformations are more convenient and intuitive when working in the frequency domain. Figure Periodic signals in the time and frequency domain. 5 Various stages and block diagram of communication system or elements of a communication system or model of a communication system Communication system is a group (collection) of subsystems such us transmission subsystem, channel and receiver subsystem. They are designed and assembled in a proper manner for sending and receiving information signals. Transmission subsystem: It is a group or collection of individual components and circuits such as information source, input transducer, modulator circuits (mixer, carrier signal generator and band pass filter), amplifiers and transmitting antenna etc. They are designed and assembled in a proper manner to convert the data/ information/ any physical quantity (carries some meaning of sense) into a suitable form for the transmission over the channel Receiver subsystem: It is also a group or collection of individual components and circuits such as receiving antenna, amplifiers, demodulator circuits, low pass filter, output transducer and etc. They are designed and assembled in a proper manner to reconstruct the original data/information from the received signal (from the channel) The three essential components for communication system are

1. Transmitter: lt sends information. For example TV transmitting stations or radio

2. Receiver: It receives information. For example all TV sets and radios are receivers. They

get information from transmitter

3. Communication Channel: This is the path through which the signal propagates from

transmitter to receiver. The basic components of a communication system are information source, input transducer, transmitter, communication channel, receiver, output transducer, and destination. Information source The message produced by the information source is not an electrical in nature. But it may be Sounds (Voice, Music), pictures, words, codes, light, temperature, pressure etc. So we need a transducer, which converts the original physical message into a time varying electrical signal 6 (these signals are called baseband signals or message signal or modulating signal or Audio frequency signal) Similarly at the destination, another transducer is used to convert electrical signal into appropriate message.

Transmitter or sender

Its sends or transmits the information. Hear the message signal is converted into suitable form for the propagation over the communication medium, called modulation or encoding i.e. super imposing (placing) low frequency (A.F) message signal with high frequency (R.F) carrier signal. This is done by modulator circuits. [Since the message signal is low frequency and weak in nature, it cannot be transmitted over longer distance directly] The output of the modulator circuit is called as modulated signal it can be transmitted any distance. For example TV broadcasting stations or Radio broadcasting (Sound) stations. [Note: Explain need for modulation. It is discussed in the next topic]

Receiver

It receives information from the modulated signal available at the transmission channel; the main function of the receiver is to extract the original message signal from the degraded version of the transmitted signal (from the channel). During transmission, the transmitted signal is added with noise (disturbance) Example: TV sets and Radio set are example for receivers.

Transmission channel (medium)

It is the path through which the signal propagates from transmitter to receiver. It may be a pair of

wires, a coaxial cable, microwave links or radio waves or space. Every channel introduces some amount of Transmission loss or attenuation, so the signal power progressively decreases with increasing distance. This is reason that the transmitter signal is degraded.

Losses caused by

Noise Electrical interference Distortion due to non-linearity Electromagnetic discharges such as lightning, power line discharge and etc.

Need for Modulation

The primary purpose of modulation in a communication system is to generate a modulated signal which is well suited to the characteristics of transmission medium. The audio frequency message signal carrying information transmitted for distance directly without modulation. According to inverse square law, the fading of the signal directly proportional to the square of

the distance it travels. So the signal strength decreases naturally. The attenuation of audio signal

is more. A high audio frequency of 15 KHz may needs quarter wave antenna of dimension 5000m. It is impractical Audio signals (if transmitted directly) from various transmitters. May mix up inseparably (it is very difficult to separate ne from other. Hence to separate them, it is necessary to translate them all in to different portions of electromagnetic spectrum, employing modulation 7 Modulation makes the receiver design simple. Without modulation wireless communication is impossible.

Simply we can easily conclude as,

1. To reduce the antenna height

2. To overcome hardware system limitations

3. To reduce the interference, noise & distortions made when we transmit the signals with

nearly same frequency in the audio frequency range (20-20k) Hz.

4. To multiplex the more number of signals

5. To the assignment of channel frequency

6. To narrow banding the signal

7. To reduce the complexity of the transmission system

8. To increase the bandwidth of the signal

1. Reduction in height of the antenna

When free space is used as a communication media, messages are transmitted and received with the help of antennas.

For efficient and easy transmission/ reception of unmodulated signals, the transmitting and

receiving antenna height should be more. In order to reduce height of the antenna, signal must be modulated. For example: In broadcast systems the maximum audio frequency transmitter from a radio station is 5 KHz. If the signals are to be transmitted without modulation, the size of antenna needed for an effective radiation would be of the order of the half of the wavelength, given as

Height of the antenna H = ૃ