Notebook

Gate Preparation- Power Systems

Bibi Mohanan December 08, 2015

The major topics in Power Systems for Gate 2016 are as follows:

A. Transmission and Distribution

1. overhead transmission lines

· memorize the expressions for Inductance and Capacitance for both Single Phase and Three Phase Lines.

· practice some problems based on this which involve the computation of GMD and GMR.

2. modelling of Short and Medium Transmission Lines

· Long Transmission Lines is not much important in Steady State Analysis and Short Transmission Line is most important.

· A,B,C,D Parameters of the line

· power expressions in terms of A,B,C,D

· approximate voltage regulation in Short Transmission Line which is same as Transformer.

3. Surges

1. memorize the thevenin equivalent circuit for analysis of surges to calculate the Transmission and Reflection Coefficient for Voltage

2. Voltage Control and Power Factor Correction

4. UG Cables

1. insulation resistance

3. capacitance model for 3 phase 3 core belted cable which has core capacitance and sheath capacitance.

4. Overhead Insulator

voltage distribution across different discs can be asked
remember the expression upto 3 discs.

6. Distribution Systems

applying KVL and finding the currents in different branches.

you can find the Solved question papers of Gate (from 2000 to 2015) Electrical HereGATE - Electrical Engineering 2016 : Solved Papers 2000 - 2015 (English) 13th Edition

Gate Preparation- Power Systems

Reviewed by Bibi Mohanan on December 08, 2015 Rating: 5

Picture and Sound reception in TV

Bibi Mohanan December 04, 2015

TV receiver

The receiving antenna intercepts the radiated picture and sound carrier signals and feeds them to the RF tuner . The receiver is of the heterodyne type and employs two or three stages of intermediate frequency (IF) amplification. The output from the last IF stage is demodulated to recover the video signal. This signal that carries the picture information is

amplified and coupled to the picture tube which converts the electrical signal back into picture elements of the same degree of black and white.

The picture tube shown in Fig. below is very similar to the cathode-ray tube used in an oscilloscope

The glass envelope contains an electron gun structure that produces a beam of electrons aimed at the fluorescent screen. When the Electron beam strikes the screen, light is emitted. The beam is deflected by a pair of deflecting coils mounted on the neck of the picture tube in the same way and rate as the beam scans the target in the camera tube. The amplitudes of the currents in the horizontal and vertical deflecting coils are so adjusted that the entire screen, called raster, gets illuminated because of the fast rate of scanning.

The video signal is fed to the grid or cathode of the picture tube. When the varying

signal voltage makes the control grid less negative, the beam current is increased, making the

spot of light on the screen brighter. More negative grid voltage reduces the brightness. If the

grid voltages is negative enough to cut-off the electron beam current at the picture tube there

will be no light. This state corresponds to black. Thus the video signal illuminates the fluorescent screen from white to black through various shades of grey depending on its amplitude at any instant. This corresponds to the brightness changes encountered by the electron beam of the camera tube while scanning the picture details element by element. The rate at which the spot of light moves is so fast that the eye is unable to follow it and so a complete picture is seen because of the storage capability of the human eye.

Sound reception

The path of the sound signal is common with the picture signal from antenna to the video detector section of the receiver. Here the two signals are separated and fed to their respective channels. The frequency modulated audio signal is demodulated after at least one stage of amplification. The audio output from the FM detector is given due amplification before feeding it to the loudspeaker.

Synchronization

It is essential that the same coordinates be scanned at any instant both at the camera tube target plate and at the raster of the picture tube, otherwise, the picture details would split and get distorted. To ensure perfect synchronization between the scene being televised and the picture produced on the raster, synchronizing pulses are transmitted during the retrace, i.e., fly-back intervals of horizontal and vertical motions of the camera scanning beam. Thus, in addition to carrying picture detail, the radiated signal at the transmitter also contains

synchronizing pulses. These pulses which are distinct for horizontal and vertical motion control, are processed at the receiver and fed to the picture tube sweep circuitry thus ensuring that the receiver picture tube beam is in step with the transmitter camera tube beam.

Receiver controls

The front view of a typical monochrome TV receiver, having various controls is shown in Fig. The channel selector switch is used for selecting the desired channel. The fine tuning control is provided for obtaining best picture details in the selected channel. The hold control is used to get a steady picture in case it rolls up or down. The brightness control varies the beam intensity of the picture tube and is set for optimum average brightness of the picture. The contrast control is actually the gain control of the video amplifier. This can be varied to obtain the desired contrast between the white and black contents of the reproduced picture. The volume and tone controls form part of the audio amplifier in the sound section, and are used for setting the volume and tonal quality of the sound output from the loudspeaker

Picture and Sound reception in TV

Reviewed by Bibi Mohanan on December 04, 2015 Rating: 5

Picture transmission in TV -part 2

Bibi Mohanan December 04, 2015

TV transmitter-Block diagram

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Picture transmission in TV -part 2

Reviewed by Bibi Mohanan on December 04, 2015 Rating: 5

Picture and Sound transmission in TV- part 1

Bibi Mohanan December 04, 2015

Picture transmission

The picture information is optical in character and may be thought of as an assemblage of a large number of bright and dark areas representing picture details. These elementary areas into which the picture details may be broken up are known as ‘picture elements’, which when viewed together, represent the visual information of the scene. At any instant there are almost an infinite number of pieces of information, existing simultaneously, each representing the level of brightness of the scene to the reproduced. In other words the information is a function of two variables, time and space. Ideally then, it would need an infinite number of channels to transmit optical information corresponding to all the picture elements simultaneously. So a method known as scanning is used instead.

In the scanning process, the conversion of optical information to electrical form and its transmission are carried out element by element, one at a time and in a sequential manner to cover the entire scene which is to be televised. Scanning of the elements is done at a very fast rate and this process is repeated a large number of times per second to create an illusion of simultaneous pick-up and transmission of picture details.

A TV camera, the heart of which is a camera tube, is used to convert the optical

Information into a corresponding electrical signal,the amplitude of which varies in accordance with the variations of brightness.

The above Fig. Shows very elementary details of one type of Camera tube (vidicon) to illustrate this principle. An optical image of the scene to be transmitted is focused by a lens assembly on the rectangular glass face-plate of the camera tube. The inner side of the glass face-plate has a transparent conductive coating on which is laid a very thin layer of photoconductive material. The photolayer has a very high resistance when no light falls on it, but decreases depending on the intensity of light falling on it. Thus depending on the light intensity variations in the focused optical image, the conductivity of each element of the photolayer changes accordingly. An electron beam is used to pick-up the picture information

now available on the target plate in terms of varying resistance at each point. The beam is

formed by an electron gun in the TV camera tube. On its way to the inner side of the glass faceplate ,it is deflected by a pair of deflecting coils mounted on the glass envelope and kept mutually perpendicular to each other to achieve scanning of the entire target area.

Scanning is done in the same way as one reads a written page to cover all the words in one line and all the lines on the page (see Fig). To achieve this, the deflecting coils are fed separately from two sweep oscillators which continuously generate saw-tooth waveforms, each operating at a different desired frequency. The magnetic deflection caused by the current in one coil gives horizontal motion to the beam from left to right at a uniform rate and then brings it quickly to the left side to commence the trace of next line. The other coil is used to deflect the beam from top to bottom at a uniform rate and for its quick retrace back to the top of the plate to start this process all over again.

Two simultaneous motions are thus given to the beam, one from left to right across the target plate and the other from top to bottom thereby covering the entire area on which the electrical image of the picture is available. As the beam moves from element to

element, it encounters a different resistance across the target-plate, depending on the resistance of the photoconductive coating. The result is a flow of current which varies in magnitude as the elements are scanned. This current passes through a load resistance R_L, connected to the conductive coating on one side and to a dc supply source on the other. Depending on the magnitude of the current a varying voltage appears across the resistance R_L and this corresponds to the optical information of the picture.

The electrical information obtained from the TV camera tube is generally referred to as video signal (video is Latin for ‘see’). This signal is amplified and then amplitude modulated with the channel picture carrier frequency. The modulated output is fed to the transmitter antenna for radiation along with the sound signal.

Sound transmission

The microphone converts the sound associated with the picture being televised into

proportionate electrical signal, which is normally a voltage. This electrical output, regardlessis a single valued function of time and so needs a single channel for its transmission. The audio signal from the microphone after amplification is frequency modulated, employing the assigned carrier frequency.

In FM, the amplitude of the carrier signal is held constant, whereas its frequency is varied in accordance with amplitude variations of the modulating signal. As shown in the above Fig, output of the sound FM transmitter is finally combined with the AM picture transmitter output, through a combining network, and fed to a common antenna for radiation of energy in the form of electromagnetic waves.

Picture and Sound transmission in TV- part 1

Reviewed by Bibi Mohanan on December 04, 2015 Rating: 5

Television (TV) Basics

Bibi Mohanan December 04, 2015

Television

Tele- vision- To see from a distance
First demonstration -J.L. Baird in UK and C.F. Jenkins in USA around 1927
Technique of mechanical scanning employing rotating discs
CRT- Cathode Ray Tube
First camera tube (the iconoscope)
1930- electromagnetic scanning of both camera and picture tubes and other ancillary circuits

The fundamental aim of a television system is to extend the sense of sight beyond its natural limits, along with the sound associated with the scene being televised. In most television systems, as also in the C.C.I.R. 625 line monochrome system adopted by India, the picture signal is amplitude modulated and sound signal frequency modulated before transmission. The carrier frequencies are suitably spaced and the modulated outputs radiated through a common antenna. Thus each broadcasting station can have its own carrier frequency and the receiver can then be tuned to select any desired station. Figure 1 shows a simplified block representation of a TV transmitter and receiver.

Television Systems

In the absence of any international standards, three monochrome systems grew independently.

1. 525 line American

2. 625 line European

3. 819 line French systems

Three monochrome systems

1. National Television Systems Committee (NTSC)system.

2. PAL ( Phase Alternating Line)

3. SECAM( Sequential couleur Avec Memoire)

Applications of Television

· Public entertainment

· Social education

· Mass communication

· News casts

· Weather reports

· Political organization and campaigns

· Announcements and guidance at public places like airport terminals,

· Sales promotion

· Closed Circuit Television (CCTV) camera signals are made available over cable circuits only to specified destinations

· Special type of CCTV -wired community TV- (Small communities

That fall in the ‘shadow’ of tall geographical features like hills can jointly put up an antenna at

A suitable altitude and distribute the programme to the subscribers’ premises through cable circuits

· Video-telephone or ‘visiphone’.

Equipment

Television broadcasting requires a collection of sophisticated equipment, instruments and

Components that require well trained personnel.

Television studios employ ;

Extensive lighting facilities
Cameras
Microphones,
Control equipment.
Transmitting equipment for Modulation,Amplification And radiation of the signals at the high frequencies used for television.
Support equipment essential in broadcast studios, control rooms and outside includes
Video tape recorders,
Telecine machines,
Special effects equipment +all the apparatus for high quality sound broadcast.