All students are required to take a College Seminar during the fall semester of their first year. For more information about the College Seminar Program, click on “What are the College Seminars?” on the navigation bar to the left. Course descriptions for this year’s CSems can be found by clicking on “For Students,” and then on “Courses.” Overview College Seminar Director / Resource Person / Advisory and Support Committee. The College Seminar will be further supported by a director, a resource person, and an advisory board. These individuals will ensure that courses offered as College Seminars meet the articulated learning goals, provide support and guidance to faculty as they develop and revise their courses, distribute grants, and facilitate the development and provision of summer workshops and the resource bank.
The College Seminar is a unique one-semester course experience shared by all sophomores majoring in the College of Arts and Letters. The course offers students an introduction to the diversity and distinctive focus of Arts and Letters at the University of Notre Dame. Specific sections of the College Seminar vary in their topics and texts (i.e., there will not be a shared reading list across sections), but all feature an interdisciplinary approach, commitment to engaging important questions, employment of major works, and emphasis on the development of oral skills. Every College Seminar syllabus will include works that approach the topic from the perspective of each of the three divisions of the College: the Arts, Humanities, and Social Sciences. Acting on their own or in groups, faculty are encouraged to develop new versions of the College Seminar or to choose from among an ever-expanding bank of existing versions.
One advantage of the flexibility in both topic and text allowed by the College Seminar is that it allows faculty to develop and teach courses consistent with their own interests and expertise. At the same time, faculty will stretch beyond disciplinary boundaries by developing courses that employ works from fields outside of their own. Resources will be available to assist faculty in this task, as well as to encourage and support the development of new courses, collaboration between faculty across divisions, and the enhancement of specific pedagogical skills.
Principles and Learning Goals
This document represents an initial attempt to identify the basic components that might characterize and define the College Seminar. Important details remain to be fleshed out. We envision classes of no more than 18, and ideally 15, students. The course format should focus on discussion and oral communication. In this section we identify some of the major principles and goals that would guide the development of the College Seminar
The College Seminars are reading- and writing-intensive courses for first-year students. Topics vary from year to year, but all the seminars are discussion-oriented and strive to engage students in examining questions and debates that are of general concern both within and outside the framework of any particular field or profession. As such, they are likely to address fundamental issues that exceed as well as establish the boundaries of any department or group of disciplines. The seminars require and support careful reading of texts and critical engagement with ideas and emphasize the need to write clear and cogent prose as a necessary condition for good interpretive essays. Course materials include books and essays but also films, material objects, social practices, scientific observations and experiments.
INTRODUCTION
In olden days the important (or) useful message or information is to be communicated by means of so many processes such as by using Birds, Horses, by Persons to Person and soon so forth. Depending on the importance of the information the communication procedure is to be adopted. This takes more time to pass information to 2nd sem seminer.docothers. Because of this an important technique is introduced i.e., Radio Broadcasting. Radio Broadcasting is a process of spreading information in all directions with the help of radio waves. The information broadcast may be music, drama or new originating from a Broadcasting studio.
By using this facility we can receive useful messages by so many numbers of persons at a time with a single transmitting signal. This information may transfer over a long distances with a large transparency. By adopting this system we have so many advantages than the older transmission procedures. In this there is no leakage of signal takes place. Large number of people can receive the message at a time. This system is having reliability, durability, flexible and also effective. By introducing a small desired changes it can be utilized for so many purposes.
Radio Broadcasting and Reception:
A very useful application of radio waves is found in radio communication. It refers to the sending, receiving and processing of information by means of radio waves. The three main
operations performed in radio-communication are:
1. Generation of radio waves:
In this process a modulated carrier voltage, obtained by modulating the intelligence over the carrier, is fed to the transmitting antenna which radiates it in space. This function of generation of radio waves is performed by the radio transmitter and its associated transmitting antenna.
2. Propagation of modulated carrier wave:
Electromagnetic waves radiated from a transmitting antenna travel outward in all directions and reach the receiving antenna after propagating through the medium.
3. Radio reception:
It is performed by a receiving antenna by intercepting a portion of the radio waves. It is followed by a radio receiver which selects, amplifies and detects the signal so received. A loudspeaker converts the detected electrical signal into sound there by reproducing the information.
Fig.1 illustrates the block diagram of the steps involved at the transmitting and receiving stages.
TRANSMITTER
The function of a transmitter is to convert audible sound waves into electrical impulses which are amplified and modulated and sent out into space by transmitting antenna.
The first part of fig8.1 shows the various parts of a transmitter and is reproduced here for convenience in fig 8.2. it consists of an R.F. section and an A.F. section. The outputs of these both sections are fed to the power amplifier which not only increases the power to the requisite level but also carries out modulation. The output of power amplifier is given to the transmitting antenna for transmission into space.
1) R.F Oscillator: The transmitter is required to be operated at a constant radio frequency. A Crystal controlled master oscillator is used to generate the stable carrier voltage of desired frequency. 2) Buffer Amplifier: The purpose of buffer amplifier is to isolate master oscillator from the influence of modulation done at a later stage. It also amplifies the oscillator output and is generally operated as class A amplifier. 3) A.F.Amplifier: the intelligence(speech or music) to be transmitted is converted into equivalent electrical signals by microphone. The level of audio signals so produced is raised by audio frequency amplifier which is always operated under class A conditions. 4) Modulating Amplifier. It is a class B push-pull amplifier and is used to increase the audio power to a level suitable for modulation. Its power output is generally comparable to the power amplifier. 5) Power Amplifier: The buffer stage is followed by a power amplifier where the carrier gets amplitude modulated. It is a class C tuned modulated amplifier. The modulated signal produced here is of sufficient so as derive the antenna 6) Transmitting Antenna: The modulated carrier output is sent over feeder wires to the transmitting antenna. It converts R.F. currents into electromagnetic energy and radiates into space.
Some of the functions are discussed below Microphone:
The microphone is a device for conversion of sound pressure variations into corresponding electrical variations. For this, the sound waves set up mechanical vibrations in some moving element to generate small AF voltages which can be amplified. Microphones are classified as constant velocity or constant amplitude types depending upon whether the voltage generated is proportional to the velocity or amplitude of the moving elements. Of the five types of commonly used microphones the carbon microphone, the crystal microphone and the capacitor microphones are constant amplitude type whereas the moving coil and ribbon type of microphones are constant velocity microphones. A pickup is an electrical device for reproducing the sound recorded on a disc. The mechanical vibrations of the stylus attached to the pickup as it follows the groove undulations are converted into corresponding electrical variations.
There are two types commonly used in disc playback equipment.
1. Magnetic pick-up
2. Crystal pick-u
p.
The principle of operation of each type resembles the operation of the corresponding type of microphone. A tone arm is used to hold the pick-up in position and to guide the pick-up cartridge properly through the grooves so as to keep it tangential to the grooves so as to keep it tangential to the groove at the point of contact.
POWER AMPLIFIER:
A transistor amplifier, which raises the power level of the signal that have Audio Frequency range is known as Transistor Power Amplifier. Strictly speaking the power amplifier does not actually amplify power. In fact, it takes power from the D.C supply
connected to the output circuit and converts it into useful A.C signal power which is fed to the load.
There are different types Power Amplifiers are available depending upon the transistor mode of operating conditions
.
1) Class A Power Amplifier.
2) Class B Power Amplifier.
3) Class AB Power Amplifier.
4) Class C Power Amplifier.
1) Class A Power Amplifier:
The Power Amplifier is known as Class A Power Amplifier when the input is changing from 0 to 360 o then the output is also changing from 0 to 360 o. There fore this Power Amplifier must be biased in such a way that no part of the input signal is cutoff. The output wave shape is similar to the input wave shape, these amplifiers have minimum distortion. Thus the output current is never zero during the complete cycle of the input signal. The efficiency of this Power Amplifier is only 50%.
In this amplifier we use the transformer for impudence matching at input and output
But it is some disadvantages.
1) transformer coupling tends to introduce a special type of distortion.
2) Transformer is bulky.
3) Poor frequency response.
4) Harmonic distortion is high.
2)Class B Power Amplifier:
The Power Amplifier is known as Class A Power Amplifier when the input is changing from 0 to 360o then the output is also changing from 0 to 180o. The efficiency of this Power Amplifier is 78.5%.
MODULATION:
Audio signal(information) is not suitable for direct transmission over long distances due to its low frequency range, even if it is converted into an electrical signal. In order to transmit information in a sinusoidal wave form, it is necessary that some characteristics of a high frequency wave called carrier wave is changed in some manner.
Def: Modulation: The process of combining an audio frequency signal with a radio frequency carrier wave is known as modulation.
Need for modulation:
1) Short Range: The energy of a wave depends up on its frequency. The greater the
frequency of the wave the greater is the energy for the wave. Therefore audio signals have short range.
2) Mutual interference: if low frequency signals are radiated directly mutual
interference will cause all of them ineffective.
3) Poor Efficiency: Radiation of audio-frequency signals directly into space is not practicable because the efficiency of radiation is quite poor in low frequency range.
4) Practical antenna height: The size of Antennas required for their efficient would be large. The height of the transmitting antenna should be equal to the wave length of the wave.
Example: For transmitting an audio signal of frequency 20KHz the height of the antenna needed is equal to.
Wave length= velocity of radio waves
Frequency (Hz)
=3X108
20000
= 15 Km.
5) For wireless communication, modulation is only the choice.
6) Energy content is also very small at low frequency range.
TYPES OF MODULATION:
There are different types modulations techniques available depending upon changing the characteristics of carrier wave. They are
1. Amplitude Modulation
2. Frequency Modulation
3. Phase Modulation
1) Amplitude Modulation:
When the frequency of the carrier is varied in proportional to the instantaneous amplitude of the modulating voltage it is called amplitude modulation. In this process the frequency, phases are remain unchanged.
The figure shows graphically the principle of amplitude modulation. The first Fig shows the audio frequency(AF) modulating signal. The next Fig shows the high frequency carrier wave of constant amplitude. The next Fig shows the resultant wave called Amplitude Modulated wave.
2) Frequency Modulation:
When the frequency of the carrier is varied in proportional to the instantaneous amplitude of the modulating signal it is called Frequency Modulation. In this process the amplitude and phase of signal are remain unchanged.
The first two Figures represent the modulating signal and carrier waves respectively. The frequency modulated signal is represented in last Figure.
ANTENNAS:
A structure which is capable of radiating electromagnetic waves or receives them is known as Antenna or Arial.
Radiation pattern: The radiation pattern of an antenna is a graphical representation of the radiation of the antenna as a function of field strength E the radiation pattern is called field strength pattern. If the radiation in a given direction is expressed in terms of power per unit solid angle the resulting pattern is known as power pattern.
Different radiation patterns for dipole are shown below.
Gain: The directive gain in a given direction is defined as the ratio of the radiation
Intensity in that direction to the average radiated power.
G= (4πr2/w)p
where p=radiated power.
Directivity: The maximum directive gain of an antenna system is called directivity is a
Function of angle.
Efficiency: Efficiency of an antenna is defined as the ratio of the radiated power to the
total input power.
η = Rr/(Rr+Rs)
where Rs = Space resistance
Rr=radiated resistance
Beam width: It is the angle measured on the radiation pattern between the points where the radiated power has fallen to half its maximum value or the radiated voltage has fallen to 0.707 times its maximum value.
Band width: the band of frequencies converted by a particular antenna is known as its bandwidth.
Radiation resistance: It is defined as the imaginary resistance which will consume the same amount of heat as radiated by the antenna. It is also known as input impedance of an antenna.
Antenna Arrays: an antenna array is a system of similar antennas, similarly oriented to achieve great directivity.
Types of Antennas:
There are different types of Antennas depending upon capable of radiating electromagnetic waves or receiving electromagnetic waves.
1. Isotropic Antennas
2. Folded dipole Antenna
3. Yagi Antennas
4. Rhombic Antennas
5. Log periodic Antennas
6. Microwave Antennas
7. Parabolic reflector Antennas
8. lens Antennas
Isotropic Antennas:
An antenna which radiated energy uniformely in all directions is called isotropic Antennas.
2. Folded dipole Antennas:
The folded dipole is made of two half wave Antennas joined at the ends with open at the center to connect at transmission line. The spacing between the two conductors is small.
This antenna has same directive gain and signal pickup as a straight dipole Antennas but it impedance is equal to 300 Ωs. If the two elements have the same diameter they will carry equal radiating current. This Antenna is used for wide band of radiations or transmissions.
3) Yagi Antennas:
The antenna widely used with T.V. receivers is the folded dipole with one reflector and one director. This is known as Yagi or Yagiuda Antennas. The driven element is the (dipole) actual antenna where as reflector and directors are parasitic elements. Due to reflector and director currents are induced and field patterns are form. It is as shown.
The reflector length l1 is usually chosen as a little more than driven element. So that it is resonated lower frequency than the dipole. Usually l1 will be 0.58λ. On the other hand the director length l3 is chosen as small it is approximately 0.45λ. Due to the parasitic elements the energy is reflected to right side of the dipole. Radiation patter on right side is bigger than the left side.
A Yagi Antenna with large number of directors is used for receptioning fringe areas. It is used as a very high frequency TV receiving Antenna besides it is also used as an high frequency transmitting antenna.
4) Rhombic Antennas:
Four antenna elements of suitable lengths forming the sides of the Rhombus is called Rhombic Antennas. It operates with traveling wave. It is a non resonant type Antenna. This Antenna has the advantage of providing satisfactory performance over a broad frequency range(3-30MHz). This antenna is arranged above the ground horizontally with the help of four poles. Rhombic Antennas can be used both for transmission and reception
5) Log periodic Antennas:
This is a wideband very high frequency antenna or an array of dipole antennas arranged as shown in the figure.
All dimensions increases in a geometric proportions the distance from origin.
6)Microwave Antennas:
Transmitting and receiving antennas for use in the 0.3 to 3GHs are called microwave Antennas. They are extensively used for direction finding and microwave communication links. In micro wave frequency range interference is also more this must be avoided. And the power required in microwave range is also more. These conditions are to be satisfied with microwave Antennas.
7) Parabolic reflector Antennas
The parabola is a plane curve defined as the locus of a point which moves so that it’s distance from another point(focus) plus(+) its distance from a straight line is constant. These geometric properties yield excellent microwave light reflector.
Practical reflector employing the properties of the parabola will be a three dimensional surface, obtained by revolving the parabola about the axis AB. The resulting geometric surface is the paraboloid often called parabolic reflector or microwave dish. When it is used as a transmitting antenna, source is placed at the focus. All waves coming from the source and reflected by the parabola will travel the same distance by the time they reached the straight line CD. All these waves (reflected) are in phase. Therefore the radiation is very strong when it is used for reception, the receiving antenna element is placed at the focus. All rays reflected from the parabola pass through the antenna element. The parabolic reflected provides high gain.
This antenna can be used for both reception and transmission. Under reception antenna element is placed at the focal point. Therefore all rays passing from the lens are concentrated at the antenna element.
Lens antennas are made of polystyrene. These Antennas are used upto 10GHz. Lens Antennas are employed to correct the curved wave front. These are preferred than parabolic
reflectors at millimeter frequencies. The disadvantages of the lens are greater, bulk expensive and design is difficult.
Propagation of Radio waves
There are a number of mechanisms, depending mainly on frequency, by which radio waves may travel from the transmitting to receiving antenna. The more important of these are:
1) Ground or surface waves
2) Space or troposphere waves
3) Sky waves.
Waves radiated at zero or lower angles with respect to the earth’s surface travel along the earth as a ground or surface wave. These waves are greatly affected by the electrical properties of the ground over which they move. They travel along the earth’s surface in the same way as an electromagnetic wave travels along a transmission line. Ground wave propagation is useful only at low frequencies because high frequency waves are largely absorbed by the ground. All broadcast signals received in the day time are through ground waves. Below 500 KHz, ground waves can be used for communications upto distances of about 1500 Km. from the transmitter.
Space wave represents energy that travels from the transmitting to the receiving antenna within, the first 15Kms. Over the surface of the earth i.e., in the earth’s troposphere. The space wave has two components:
i) The direct or line of sight waves, that travel directly from transmitter to receiver.
ii) The earth reflected wave, that travels from the transmitting antenna to the earth and reflected to the receiving antenna.
Space wave energy may also reach the receiver as a result of reflection or refraction produced by variations in the electrical characteristics of troposphere and by diffraction around the curvature of earth, hills etc. Space wave propagation becomes important in the VHF band i.e., above about 30 MHz. Television transmission, frequency modulation and radar etc. are systems of this methods of propagation.
Waves radiated at high angles may reach the receiver by virtue of a downward bend in their travel that occurs while they are moving through the ionized regions of the earth’s upper atmosphere, called the ionosphere. Such waves are called the sky waves.
In fig 8.5 are shown the various ways of propagation of the radio waves. Each of these mechanisms of wave propagation has been discussed separately in the following sections.
Propagation of Ground or Surface waves.
In surface wave propagation, the radio waves are guided along the surface of the earth because of the electrical discontinuity that exists between the earth and the atmosphere and follow a curved surface from the transmitter to the receiver. The wave can exist when the transmitting and receiving antennas are close to the surface of the earth. The surface wave is vertically polarized i.e., the electric vector of the wave is vertical. If there is any horizontal component of electric field, it will be short circuited by the earth.
Due to electric field, the surface wave, in this passage, induces charges on the earth, which travel with the wave and thus constitute a current. In carrying this induced current, the earth behaves like a leaky capacitor and so can be represented by a resistance shunted by a capacitance. The characteristics of the earth as conductor ca, therefore, be described in terms of the conductivity σ and dielectric constant K.
As the ground wave passes over the surface of the earth, energy is consumed due to flow of charges through the earth’s resistance and hence the wave is weakened.
The wave is also attenuated due to the tilting of electric vector due to diffraction effects. As the wave propagates, it tilts more and more and hence a greater part of electric component is short circuited by the earth, resulting in the reduction of field strength. Consequently at some distance from the antenna, the wave lies down and dies.
Thus the transmission distance over which the surface wave may be utilized effectively depends upon the energy losses in the earth, which in turn depends upon the conductivity of the surface. The attenuation of the wave is less while traveling over moist soil or sea water than over dry land. The energy losses also depend upon frequency of the wave, being greater as the frequency increases. At frequencies, above about 2 MHz, the attenuation is so high that the transmission distance becomes essentially negligible. Therefore, at higher frequencies, used for frequency modulation(FM),television and radar etc. the ground wave is increasingly absorbed and becomes insignificant beyond a distance of few kilometers from the transmitter.
Surface wave has practical importance for local radio broadcasts. AM radio broadcast in the medium frequency band (550-1600 KHz) is transmited primarily via surface wave transmission has the merit of being extremely reliable propagation mode that is not subject to appreciable variations with time or with atmospheric conditions.
Sommerfeld has shown that the surface wave field intensity at a distance d from the transmitting antenna, assuming the earth as flat, is given by the relation
E= Ae/d
Where A is called the sommerfeld reduction factor which depends upon the conductivity and dielectric constant of the earth and e is the field strength of the wave at a unit distance from the transmitting antenna.
Space wave propagation:
This type of propagation also called the tropospheric propagation. It is used at frequencies above about30 Mc/s, where neither surface wave nor sky wave propagation is possible. In this frequency range, the surface wave amplitude is reduced in strength by absorption as to become virtually useless within a few hundred meter, because due to short wavelength more charges are induced in the earth per unit length and their flow causes heavy dissipation of energy. The sky wave is also not reflected back to earth by the ionosphere at such a high frequency.
In this frequency range, useful propagation can be achieved byspace waves. Fig8.6 illustrates the manner of space wave propagation, assuming the earth as flat i.e., neglecting the earth curvature. It may be seen from fig 8.6 that energy reaches the receiver in to two ways:
i) By a ray traveling directly between transmitting and receiving antenna over a path TR.
ii) By a ray traveling over path TOR which is reflected from the earth at O.
The field strength at the receiving antenna R is the vector sum of fields represented by these two rays. As both the rays travel through space, they suffer negligible attenuation and their amplitudes can be supposed to be inversely proportional to the distance from the transmitter.
Effect of Earth’s Curvature on Space Wave Propagation:
When the receiving and transmitting antennas are above the horizon of the curved earth, the space wave propagation is in the manner shown in fig. the field strength at the receiving antenna is still the vector sum of direct and earth reflected waves but now the effective antenna heights are H’ and h’, above the tangent line,t,t’, in place oh H and h.
The curvature of the earth also causes the earth reflected wave to be diverging rather than plane wave. Consequently, the earth reflected wave at the receiver is weaker than if the reflection were from a flat surface. This effect is negligible when the angle of incidence of the incident wave at the earth’s surface is moderately large, it increases as the angle of incidence becomes less.
Another consequence of the earth’s curvature is that is the receiving antenna R of moderate height is at a considerable greater distance from the transmitting the wave by either means i.e., either direct wave or earth reflected wave. This situation has been shown in Fig.
There are, however, certain methods by which transmitted energy can be received beyond the direct line of sight, known as shadow zone. These methods have been discussed in the next section.
Atmospheric Effects on Space Wave Propagation:
The atmosphere, through which the space wave propagation takes place, influences the transmission to a significant degree. Some distance beyond the direct line of sight can be converted due to refraction and reflection of radio waves by the earth’s atmosphere(troposphere) and diffraction by the roughness of the earth’s surface.
Near the surface of the earth, the dielectric constant and hence the refractive index of the atmosphere usually decreases linearly with height. It is called the standard atmosphere. If, any how, the moisture content of the air at the earth surface is very high but decreases sharply with increasing height, then the refractive index of the air decreases with height much more rapidly than normal. It is called the region of the duct.
It causes the bending of radio waves moving through the atmosphere towards the earth’s surface in accordance with Snell’s law. Hence the rays initially moving parallel to the earth’s surface get refracted in the atmosphere and follow a path having slightly downward curvature. This downward curvature of the path permits the direct ray to reach slightly beyond the horizon (shadow zone). The radio waves may then be reflected back from the earth to follow a series of hoops, as shown in Fig. this phenomenon is known as duct propagation. such duct are generally found over the ocean and less frequency over lands.
In addition to the refraction of waves occurring in the troposphere, there also occurs reflection at places of abrupt changes in the dielectric constant. The effect of tropospheric reflection is to extend the range of propagation by hundreds of kilometers. In addition to it, the inhomogeneties in the atmosphere give rise to scattering of radio signals. By using highly directional high gain antennas and large transmitted power, tropospheric scatter propagation can be used to establish a link well beyond the radio horizon.
Space wave propagation is affected by atmospheric conditions more seriously at frequencies in the microwave region. Rain, cloud and fog result in serious at attenuation of waves at frequencies above about 30GHz. Hail has a very little effect except at frequencies above about 100 GHz, while snow has a negligible effect at all frequencies.
Sky Wave Propagation:
In this propagation radio wave is reflected back to earth form ionized region in the upper atmosphere called Ionosphere.
Ionosphere and its effects:
The Ionosphere is the upper portion of the atmosphere which absorbs large quantity of radiant energy form the sun, thus becoming heated and ionized(in the form of the +ve,-ve ions)
This happens about 60Km from the earth’s surface. The ionization density varies with height and becomes maximum. The levels at which the electron density reaches a maximum are terned as layers. These layers are named as D,E F1,F2 in order of height.
The D layer is the lowest existing at an average height of 70Km with an average thickness of 10 Km. it disappears at night due to lower ionization levels. It reflects vary low frequency and low frequency waves and absorbs medium frequency and high frequency waves. The E layer is next in height, existing about 100Km with a thickness of about 25Km.Due to recombination of ions into molecules it disappears at night. The main effects of the E layer are to aid medium frequency waves. It reflects some high frequency waves in day time. The Es layer is a thin layer of very high ionization density, some times making an appearance with the E layer. It is also called the sporadic E layer.
The F1 layer exists at a height of 180Km in a day time and combines with the F2 layer at night. It’s day time thickness is about 20Km. it is the only ionized layer which always remain ionized irrespective of hours of the day or seasons of the yerars.
The higher layer F2 is formed by the ionization of ultra violet and X-rays. It is approximate thickness can be 200 Km and its Height ranges from 250 to 450 Km in day time.
At night it falls to a height of about 300Km where it combines with F1. Therefore all high frequency waves will be reflected from F2 layer.
RECEIVER
Characteristics of receiver:
There are three main characteristics by which the quality of a receiver can be judged. These are also known as the performance characteristics or the specifications of a receiver. They are
1) Selectivity
2) Sensitivity
3) Fidelity
4) Noise level
1) Selectivity: Selectivity of a receiver is its ability to select a desired signal frequency without any objectionable interference from other neighboring stations. It is a measure of the extent to which the receiver can reject all other neighboring stations and accept only the desired station. A good, selective receiver will select the desired station and reject all other unwanted stations. The selectivity is generally expressed in the form of a curve shown in Fig. In this curve, the strength of the input signal at the resonant frequency required to produce a given output is taken as the reference and the strength of the modulated carrier at neighboring frequencies required to produce the same output is plotted on the vertical axis. The sharper the selectivity curve, the more selective a receiver is.
The selectivity of a receiver increases with the number of tuned RF stages in the circuit. A very selective receiver allows only a limited band of frequencies to pass through. As such, many of the side bands do not appear in the output and the quality of reproduction of the receiver suffers.
2) Sensitivity:
sensitivity of a receiver is its ability to respond to weak signals. This is expressed as the minimum voltage or power that must be applied to the input of the receiver for getting a standard output of 0.5W in the loudspeaker or in the resistive load substituted for the loudspeaker. For measuring sensitivity, the input RF signal must be modulated 30% at an AF of 400Hz. Moreover, the RF signal voltage is applied through an artificial antenna called a dummy antenna. This dummy antenna creates the same input conditions for a receiver as a real antenna installed on the roof top. It can be made by connecting an inductance(coil) of in series.
The sensitivity of a receiver is expressed in micro volts. The smaller the input in micro volts the greater is the sensitivity of a receiver. A high grade broadcast receiver will have a sensitivity of less than 10 microvolts
3) Fidelity:
fidelity is the ability of a receiver to reproduce faithfully all the audio frequencies with which the carrier is modulated. This is generally expressed as a frequency response curve
The fidelity of a broadcast receiver mainly depends on the response of the audio stage but the tuned RF stages also limit the response of the receiver by not allowing the higher sidebands to be reproduced properly. Selectivity and fidelity in a receiver are opposed to each other. Modern broadcast receivers are capable of reproducing properly all modulation frequencies from 30 Hz to about 8 kHz. However, the international regulations do not allow a bandwidth of more than 5 or 6kHz for broadcast purposes.
4) Noise level:
the noise level of a receiver is another important characteristic particularly at shortwave bands. Noise produced in the receiver by its own circuits should be sufficiently low so that even the weakest signals received can suppress this noise.
TYPES OF RECEIVERS:
The basic requirements of selection, detection and sound reproduction for a receiver can be met by a tuned LC circuit for selection, a diode(or a crystal) for detection, and a pair of headphones for sound reproduction. This was the earliest and the simplest form of a receiver and was called a straight receiver. It did not require a power supply and provided no amplification for the radio frequencies or the detected audio frequencies. It could be used for listening only the local stations. The crystal set described earlier is a form of straight receiver.
TRF RECEIVER:
The invention of the vacuum tubes or radio valves made it possible to amplify the detected audio frequencies to drive a loudspeaker. With the development of RF amplification techniques, it was possible to add one or two stages of RF amplification before detection so that a sufficiently strong RF signal could be delivered for detection by the diode. Thus, with RF amplification before detection and AF amplification after detection, a new type of receiver known as the TRF or tuned radio frequency receiver came into existence. A TRF was the first practical type of receiver to be designed and constructed. A block diagram of
A TRF receiver tends to be selective and its fidelity is not so good. Also, the sensitivity of a TRF receiver varies with the received frequency. In spite of this, a TRF receiver still finds good use in many special applications in the field of radio communication.
However, the TRF receiver has been largely replaced by the modern super heterodyne receiver which possesses many advantages over all other types of radio receivers.
SUPERHETERODYNE RECEIVER:
A radio receiver is a device which picks up any desired radio frequency signal, i.e., modulated carrier voltage, transmitted by the transmitter and converts it into an audible sound wave.
Most of the radio receivers are of super heterodyne type. The meaning of the term heterodyne is to mix while super heterodyne stands for ‘supersonic heterodyne’ which means the production of beat frequencies above the range of hearing. In super heterodyne receivers, often referred to as ‘superhet’, all incoming carrier frequencies are converted to a lower fixed value called the intermediate frequency (I.F.). Thus the heterodyne process involves a simple change of carrier frequency which is achieved by heterodyning or mixing the incoming R.F. signal with oscillations produced by a local oscillator. The difference frequency (I.F.) is still in the inaudible R.F. range but it is fixed for a receiver which enables the amplifier circuit to operate with maximum sensitivity, selectivity and stability.
The name super heterodyne is generally applied to receivers in which the frequency change is produced before A.F. detection. Receivers using double frequency changes (two I.F.) are usually called double super heterodyne receivers
Functions of different stages of a superhet are discussed below:
1) Antenna or Aerial- The receiver antenna intercepts the electromagnetic waves, converts them into R.F. voltage and delivers into the receiver input by means of feeder wire, where a parallel tuned circuit responds only to voltages at the desired carrier frequency. The voltage so picked up is fed to the input of R.F. amplifier stage.
2) R.F.Amplifier Stage- The voltage developed across the capacitor of the input tuned circuit should be amplified to detect weak signals. It is done by using a R.F. voltage amplifier. It raises the level of signal voltage before it is fed to the mixer. It also improves the signal/noise ratio.
3) Mixer or Frequency Converter stage- The frequency changing section of the receiver is required to convert the frequency of all incoming carriers to the intermediate frequency(I.F.) value which is most common cases is 455 kHz. To obtain it, the radio frequency signal voltage of frequency fs and a local oscillator voltage fo are heterodyned or mixed in the mixer stage. The output current of the mixer has the frequency components.
In transistor receiver, self oscillating mixers are commonly employed.
4) I.F.Amplier: I.F. signals produced by mixer stage are amplified by I.F. amplifier which have one or more stages of amplification and designed to amplify only a narrow band of frequencies around a fixed central frequency.
5) Detector and A.G.C: Output of I.F. amplifier is fed to the detector ot separate the modulating signal from the carrier wave. In addition to converting I.F. signals into audio, this stage also provides AGC bias to preceeding stages. A linear diode detector is usually employed for this purpose.
6) Audio Frequency amplifier: The audio frequency signal obtained at the output of detector is of insufficient amplitude. It is, therefore, fed to A.F. amplifier to provide additional amplification. Usually one stage of audio voltage amplifier is used which is followed by one or more stages of audio power amplifier.
7) Loudspeaker: Through an impedance matching transformer, the amplified audio output voltage of audio power amplifier is fed to the loudspeaker. It is an electromechanical device that is used to convert the audio currents into sound waves. Thus the original programme is reproduced.
LOUDSPEAKER
A Loudspeaker is a device which converts electrical energy into sound energy.
Audio frequency currents from an AF amplifier are converted into sound waves of corresponding frequency and amplitude by the loudspeaker. In principle, a loudspeaker is the converse of a microphone and the two instruments perform complementary function.
A loud speaker is the voice of any electronic entertainment equipment and, as such, it should be able to reproduce, as faithfully as possible, the original sound from the broadcasting studios. A good loudspeaker should be able to reproduce all sounds equally well irrespective of their amplitude, frequency and waveform.
The principle employed for converting electrical energy into mechanical motion is different in different types of loudspeakers. They are:
1) Dynamic speakers
2) Dual Loudspeakers
3) Horn Loudspeakers
4) Column Loudspeakers
1) Dynamic speakers:
The most commonly used type is the dynamic or the moving coil type of loudspeaker.
The operation of dynamic loudspeaker is similar to that of an electric motor. The audio currents passing through the voice coil placed in a strong magnetic field produce a vibrating motion in the voice coil. This to-and-fro motion of the voice coil is transferred to the cone or diaphragm attached to the voice coil. The vibratory motion of the cone produces sound waves in the air which reaching the eardrum, produce the sensation of sound. Fig shows the construction of dynamic loud speaker.
2) Dual Loudspeakers:
A single cone type speaker is not able to provide uniform response and adequate output power. A loudspeaker mechanism with a heavy and large diameter called woofer can reproduce low frequencies efficiently but fails to reproduce the higher frequencies properly. A tweeters performs much better at the high frequency range of audio frequencies. If the frequency range is split into two ranges at a frequency called the cross-over frequency.
The combination of Woofers and tweeters are used with cross-over networks for getting a uniform frequency response over the entire frequency range.
3) Horn Loudspeakers:
Large-scale reproduction of sound involving several acoustical watts requires high power audio amplifier. For this reason they employed a Loudspeakers. The Horn Loudspeakers are used for large-scale reproduction of sound. These Loudspeakers are high efficiency reproducers with good frequency response even at low frequencies. The efficiency is 50% higher when it compared the dynamic type Loudspeakers.
4) Column Loudspeakers:
Column Loudspeakers consisting of a number of dynamic speakers mounted in a cabinet are used where a narrow directivity pattern in the vertical plane is required. In fact, column loudspeakers are useful in all cases of sound reinforcement where the sound must be confined to a narrow beam in the vertical plane in order to increase the ratio of direct to reflected sound.
CONCLUSION
By proper usage of Radio Broad casting system there are so many advantages in our human life’s. This system consists of only audio signals due to this there is no high harmful to human life’s through this. Information can be transferred over long distance messages are also transferred through this. At the time of freedom fighting movement also this system placed a smart role. This is one type of entertainment path.
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