AREPORT

ON

INDUSTRIAL VISIT

GOVT. ENGINEERING COLLEGE, AJMER

January 2011

Submitted by:

VIPUL KUMAR AGARWAL Roll no.-08EEAEC094

ECE (Final Year)

ACKNOWLEDGEMENT

I wish to express my sincere thanks to DDK, Ajmer(Dordarshan Kendra), Toshniwal Industries Pvt. Ltd.,Ajmer and All India Radio for the successful completion of my Industrial Visit to their prestigious organizations. It was a wonderful experience to work under the supervision of learned teachers and engineers there.

I shall always be grateful and loyal to my teachers who guided us at all times and told us the advantages of Industrial exposure and encouraged us to do so.

I would also like to express my gratitude towards my parents and friends for their support.

VIPUL KUMAR AGARWAL

Roll No. :08EEAEC094

ECE (Final Year)

Govt. Engg. College , Ajmer

ABSTRACT

Industrial Exposure is an important constituent of any curriculum and the B.Tech. course is no exception to this general rule. It helps a student in getting acquainted with the manner in which his knowledge is being practically used outside his institute and this is normally the different form what we has learnt from books theoriticaly. Hence, when one switches from the process of learning to that of implementing his knowledge, he finds an abrupt change. This is exactly why industrial visit session during the B.Tech. Curriculum becomes all the more important. The provision of training offers a very good opportunity to the students to supplement their knowledge and skills by working in actual industrial or corporate environment. This opportunity should be utilized for developing and enhancing our skills as practicing engineers.

This report describes my industrial visits after the Vth Sem B.Tech. session, which I completed at DDK, Ajmer(Dordarshan Kendra), Toshniwal Industries Pvt. Ltd.,Ajmer and All India Radio .

CONTENTS

1. Industrial Visit –Toshniwal Industries Pvt. Ltd.

2. Industrial Visit – All India Radio

3. Industrial Visit – Dordarshan Kendra

INDUSTRIAL VISIT

TO

TOSHNIWAL INDUSTRIES Pvt. Ltd.

PRODUCTS:-

RESISTANCE TEMPERATURE DETECTORS

  

For temperature measurement in the range, -200°C to + 850°C Resistance Temperature Detector (RTD) is preferred to Thermocouple and other sensors because of its higher accuracy, reliability, compact size and faster response. RTDs find use in almost all industries like plastic and rubber processing, food industries, pharmaceuticals, chemical & petrochemical plants, and power plants. It is also used in diesel engines & ships, process control and laboratories for temperature measurement and control. RTDs are available with single or double resistance elements and in 2, 3 or 4 wire circuits. Various types of protection sheaths for protection of the element and screw-in threaded bushes either fixed (welded) or adjustable by screws (for non-pressure application) or by compression fitting (for pressure application) for mounting are available. RTD normally consists of spring-loaded insert, outer protecting sheath with mounting connections and cast aluminum terminal head. The insert assembly consists of Pt 100 element with silver/copper connection leads, insulated by ceramic multibore insulating tube/beads in a brass or stainless steel sheath with ceramic terminal block and brass

terminals attached to a spring loaded arrangement. Features

High accuracy and stability Wide temperature range; -200°C to +850°C Fast response & High insulation resistance Needs only copper cables for long runs.

SPECIFICATIONSA

PPL ICATIONS TECHNICAL DATA

Resistance element Pt 100, 100 Ohms at 0°C

Fundamental interval 38.5 Ohms change in resistance from 0-100°C

Temperature range-50°C to + 500°C(-200°C to + 850°C also available)

No. of elements Single or Double

Insulation resistance 10M Ohms at 500V DC (100 M ohms on request)

Application

They are used in industries, power plants, and metal melting furnaces, salt baths, industrial process control, laboratories and numerous other applications as

Salt Bath : Neutral/Cyanide/Borax/HSS/Chloride/other Molten Metal : Zinc/Magnesium/Aluminium/Copper/Brass/Ferrous Alloys/other Process Furnaces : Glass melting/Cracking/Roasting/Blast/Calcining/Hydrogenising/

High Pressure Stoves. Ovens & Furnace with : Reducing/Oxidising/Sulphurous/Carbonising Plating : Galvanising/ Tinning.

THERMOCOUPLE:-

A thermocouple consists of two dissimilar metallic Wires joined at one end known as hot junction. When the hot junction is heated, an emf is produced which depends on the difference between the temperature of the hot junction and the temperature of free ends known as cold junction. This thermo emf is measured by galvanometric/potentiometer metric/digital instruments, to display temperature.

Features

Most common, convenient and versatile devices used to measure temperature.

Sensing devices for measurement, control and recording of temperature from 200°C to +1800°C.

These are available in two basic designs-

Straight type & Angle Type:

Angle type Thermocouples are used when it is not convenient to use Straight Type Thermocouples or Thermocouple connecting head is desired to be kept away from the place of insertion of Thermocouple so as to avoid attack of harmful fumes and vapors or high temperature.

THERMOCOUPLE ELEMENTS:- Thermocouple wires welded at one end are called elements. Both the positive and negative legs of the thermocouple elements are insulated by ceramic insulators (beads) to avoid short-circuiting and corrosion. A full-length long multibore insulating tube insulates rare metal thermocouple elements. Depending on the use, a thermocouple assembly has one (simplex), two (duplex) or more number of elements.

CONNECTING HEAD:-

The connecting head is cast aluminum or cast iron with detachable cover and is fixed on open end of protecting sheath. Thermocouple wires are terminated on a ceramic terminal block with bass Terminals inside the head. From these terminals, compensating/ extension leads are taken for connections to the measuring or controlling instruments.

PROTECTING SHEATHS:-

Metallic and /or ceramic sheaths (tubes closed at one end) are put over the thermocouple elements for protection against corrosive atmosphere and harmful chemical action, which may be present at the place of use. When the corrosive atmosphere is too severe, an additional, inner gas tight ceramic sheath is provided for protecting the elements from corrosion, especially for the expensive rare metal thermocouples. In the case of angle type thermocouple, this ceramic tube is provided in the hot arm only. General guidelines for protecting sheath for different applications are given in the table below:Note: 1) Protective sheaths of other materials, such as Graphite, Cast Iron (for molten Aluminium), Hastelloy etc. 2) Operating temp. Varies, depending on the atmospheres.

SPECIAL COATINGS

Metallic protective sheaths are also offered with coatings of Tungsten Carbide, Stellite, Teflon Ceramic, glass etc. and sleeve of tantalum, titanium, hastelloy, nickle, zirconium etc.

are also available for resistance to abrasion/chemical corrosion in special process applications.

PROCESS CONNECTIONS

Flange or screwed bush is used for mounting the Thermocouple at the place of its use. Flange in normally adjustable over the protecting sheath or holding tube of ceramic sheath.

Application

They are used in industries, power plants, and metal melting furnaces, salt baths, industrial process control, laboratories and numerous other applications as

Salt Bath : Neutral/Cyanide/Borax/HSS/Chloride/other Molten Metal : Zinc/Magnesium/Aluminium/Copper/Brass/Ferrous Alloys/other Process Furnaces : Glass melting/Cracking/Roasting/Blast/Calcining/Hydrogenising/

High Pressure Stoves. (specify process material). Ovens & Furnace with : Reducing/Oxidising/Sulphurous/Carbonising gaseous

atmosphere (Specify gas details) Plating : Galvanising/ Tinning.

THERMAL IMAGERS:-

High performance thermal imagers have never been this affordable. This rugged. Or, this easy to use … until now.We, at Fluke, are never satisfied leaving the best tools in the hands of the elite, which is why we recently added a new member to our thermal imaging family.

Feature

Superior image quality Delivers the clear, crisp images needed to find problems fast with its 320x240

sensor. Identify even the smallest temperature differences that could indicate problems with

industry-leading thermal sensitivity (NETD).

Automatic alignment (parallax correction) of visual and infrared images with Fluke patented IR-Fusion®

Optional telephoto and wide angle lenses available for added versatility and special applications. (easily installable in the field)

Easy to use Field replaceable batteries give you maximum flexibility no matter where your

work takes you. Intuitive, three-button menu is easy to use—simply navigate with the push of a

thumb. No need to carry pen and paper—record findings by speaking into the imager.

Voice annotations can be recorded with every image you take. Voice comments are saved along with individual images for future reference.

One-handed focus capability, emissivity correction, reflected background temperature compensation, and transmission correction increase the accuracy of measurements in most situations.

Adjustable hand strap for left-or right-handed use. Everything needed to get started is included.

Rugged Optimized for field use in challenging work environments. Engineered and tested to withstand a 2 m (6.5 ft) drop for the ultimate peace of

mind - When was the last time you dropped a tool or piece of equipment? Withstands dust and water—tested to an IP54 rating. Manufactured in the U.S.A.

APPLICATIONS

Inside Electrical distribution & Service (Switchgear & Panels) Motors, Pumps, etc Process- Tanks, Pipes HVAC Outside Electrical Utilities

RHEOSTATS:-

Rheostats are mainly used for regulating electric current in industry and laboratory where these are used as adjustable resistors in electrical circuits. With three terminal provisions, these can be used to vary a D C potential smoothly from zero to maximum.Toshniwal Rheostats consist of resistance wire wound on a former which is made from solid drawn hexagonal steel tube vitreous enameled, ensuring good mechanical strength and sturdiness. The tube is fitted in two end castings insulated with micanite cups.The Brush gear slides on a highly polished square slide rod. The brush is of copper graphite with pigtail connections. Two heat resisting compression spring hold brush family in contact with the resistance wires. Copper graphite brush provides necessary lubrication to prevent wear and tear of wire even at elevated temperatures. The brush is housed in a slider knob of moulded bakelite designed for easy handling. Different designs in a single tube and double tube models are available to meet with demands of different applications.

FEATURES

Industrial quality components.

Trouble-free long service.

Wide range of resistance values and current – carrying capacities.

Tolerance on resistance value + 20% to -5%

H.T. Test 2kV for 1 minute

Temperature rise 375ºC*

Maximum working voltage 500 V

Ambient Temperature 0-45ºC

APPLICATIONS:-

The application of rheostats is in the field circuits of motors or generators for the control of speed or voltage, or in control circuits. The rheostat can adjust generator characteristics, dim lights, and start or control the speed of motors.

OPTICAL PYROMETER:-

The Partial Radiation Pyrometer, ‘PYROPTO’ is a disappearing filament Optical Pyrometer, suitablefor measuring high temperature of incandescent objects, in annealing and hardening furnaces, in firing kilns, for measuring the temperature of molten and flowing metal or glass as well as incandescent blocks during forging, pressing or rolling. The temperature measurement makes use of comparison between the brightness of an electrically heated and calibrated incandescent filament(reference light bulb) in Pyropto and that of the object to be measured.The special advantage offered by the PYROPTO is the light weight of the instrument which houses all components of the complete measuring equipment.

FEATURES ROCHURE/DOCUMENTS

Wide temperature range 700 – 2000 º C or 700 – 3500 º C Fast response and sensitive measuring system Light weight & portable Operates on two 1.5V dry cells

RAYTECH MT4 -

The MiniTemp™ IR thermometer—a new way to solve common problems.Infrared (IR) thermometers have become the preferred tool for diagnostics and inspection where temperature is an indicator. Raytek IR thermometers are valued by professionals in a variety of industries—from industrial maintenance to food service, because they quickly, accurately and safely measure surface temperature! Anywhere that temperature is a factor in process, product quality or diagnostics, a Raytek IR thermometer should be close at hand. IR thermometers are easy to use, and since they do not contact the object being measured, they are the safest way to measure hot, hard-to-reach, or moving parts, while eliminating potential damage and contamination.The Raytek MiniTemp MT4 thermometer incorporate many of the same features found in professional IR thermometers.

What are the benefits of using IR thermometers?Effectively find your problems quickly and safely, saving time and cost. IR thermometers are accurate, usually within one degree. IR thermometers safely read hard-to-reach or inaccessible objects.

FEATURES

Simple to use Instant readings Laser Point (MT4) Large, easy to read display Temperature Range (MT4) -18º to 400ºC (0º to 750ºF) ºC or ºF selectable

APPLICATION-

Electrical Maintenance:

Check for heat created by loose connectors or buildup. Instantly troubleshoot problems in battery banks and power panel terminations, ballasts, switch gears, and fuse connections, and identify hot spots in the output filters on DC battery connections.

Equipment Maintenance:

Check moving parts in motors and gear work for hot spots. Temperature change can indicate developing problems in many types of equipment, from ovens and boilers to freezers. Routine temperature audits of generators and their bearings can prevent expensive repairs.

Building Controls:

Monitor HVAC/R components for quick energy audits and room balancing in a short time.

Troubleshoot engine problems:

Temperature is a vital indicator of how well cooling systems are performing, or whether friction, vibration and other conditions are creating wear in braking systems and bearings.

Process/Product:

Monitor process line equipment. Check the temperature of different products on production lines from rubber tires to plastic and cellophane wrapping to chocolate bars.

INDUSTRIAL VISIT

TO

ALL INDIA RADIO, AJMER

Owned by PRASAR BHARTIBroadcasting Corporation of INDIA

INTRODUCTION:-

High power transmitter in Ajmer was established on 11th December ,1955. initially it was started with 20KW power and connected to AIR Jaipur station. In 1987, its power is raised up to 200KW. Its range covers almost all over Rajasthan. HPT in Ajmer is located at longitude 74°43´00´´ and latitude 26°31´07´´. ALL INDIA RADIO is situated 10 Kms far from the Ajmer Bus Stand at N.H.No.8 in NORTH direction,near GAGWANA village. It is120Kms far from Jaipur in SOUTH direction in the state of RAJASTHAN. A.I.R. is working under the Ministry of INFORMATION AND BROADCASTING. It is broadcasting under PRASAR BHARTI CORPORATION of INDIA. Its range covers almost all over Rajasthan.

PRASAR BHARTI High Power Transmitter Akashwani Ajmer Relay Transmitter From Jaipur

Transmitter Salient Features → M.H.A.S.L. 480Mtrs.

MAKE/TYPE → BEL HMB 140. RADIATED POWER → 2×100 KW.FREQENCY → 603 KHz.WAVELENGTH → 497.51 Mtrs.MAST HEIGHT → 200 Mtrs.ELECTRICAL HEIGHT → 248 Mtrs.MAST BASE IMPEDENCE → 778.4+j50.3 Ohms.SITE AREA → 238656 SQ.MT, → 140 BIGHA.TRANSMISSION HOURS → 05:55 − 09:30. → 11:45 − 15:00. → 17:00 − 23:20.COVERAGE →

• AJMER TO DELHI 215 Kms. • AJMER TO UJJAIN 256 Kms. • AJMER TO UDAIPUR 223 Kms. • AJMER TO BIKANER 197 Kms. • AJMER TO JODHPUR 221 Kms.

POWER SUPPLY → Two 11KV overhead feeder From MADAR and GAGWANA. Two Stand by

Generators.

MODULATION:-

Modulation means to “CHANGE“. In

modulation , some characteristics of carrier waveis changed in accordance with the intensity of the

signal. The resultant wave is called modulated waveor radio wave and contains the audio signal. In India,Amplitude modulation is used in radio broadcasting.

When the amplitude of high frequency carrier wave ischanged in accordance with the intensity of the signal

it is called Amplitude Modulation. In amplitude modulation, only the amplitude of the carrier wave is changed in accordance with the intensity of the carrier signal. However the frequency of the modulated wave remains the same i.e. carrier frequency. In amplitude modulation, the amplitude of the carrier wave changes according to the intensity of the signal and the frequency of the amplitude modulated wave remains the same i.e. carrier frequency fc

Crystal Oscillator:→

To oscillate at a consistent frequency, the crystal is kept in a oven. The temperature of the oven is maintained between 68 to 72o C and the corresponding indication is available in the meter panel. Crystal oven is heated by +12 V. One crystal oscillator with a stand by has been provided. It gives an output of 5 V square wave which is required to drive the Transistor Power Amplifier. The crystal oscillator works between 3 MHz and 6 MHz for different carrier frequencies.Different capacitors are used to select different frequency ranges.

Transistor Power Amplifier:→

Oscillator output is fed to the Transistor Power Amplifier (TRPA). It gives an output of 12Watt across 75 ohms. It works on + 20 V DC,derived from a separate rectifier and regulator. For different operating frequencies, different output filters are selected. (Low Pass Filter).

RF Driver:→

A 4-1000 A tetrode is used as a driver which operates under class AB condition, without drawing any grid current. About 7 to 10 Watts, of power is fed to the grid of the driver through a 75 : 800 ohms RF Transformer, which provides proper impedance matching to the TRPA output and also provides the necessary grid voltage swing to the driver tube.

RF Power Amplifier:→

CQK - 50, condensed vapour cooled tetrode valve is used as a PA stage. High level anode modulation is used, using a class B Modulator stage. The screen of the PA tube is also modulated by a separate tap on modulation transformer. Plate load impedance of the PA stage is about 750 ohms and the output impedance is 120 ohms, and it is matched by L-C components.

PA output circuit:→

PA Output Circuit (HMB-140)

The L-C combination of the output circuit provides the following:→

→ The required load impedance for Class D operation that isthere should be a third harmonic impedance in addition to the fundamental impedance.

→ Matches the plate impedance of 750 ohms to the feeder impedance of 120 ohms at the operating frequency.

→ Filters all the 2nd and 3rd harmonic before the feeder.

CLASS D OPERATION:→

The 100KW BEL Transmitter type HMB 140 uses class D operation in output stage. This is particularly done for increasing the efficiency of the transmitter as the energy saving is one of the main aspects in the design of a transmitter.In class C, the two waveforms are sinusoidal and thus considerable voltage and current exist simultaneous across the tube and it results in power loss as well.

In class D, 17%third harmonic is added to the grid drive to get the plate current waveform. This plate current requires third harmonic impedence in addition to fundamental in the plate circuit of the PA stage. The large voltages and currents do also not exist simultaneously across the tube resulting in lower power loss. The conduction angle in class D is larger and hence for a given DC current the peak current is lower.

AF STAGE

AF Stage (HMB-140) The AF stage supply the audio power required to amplitude modulate the final RF stage. The output of the AF stage is superimposed upon the DC voltage to the RF PA tube via modulation transformer. An Auxiliary winding in the modulation transformer, provides the AF voltage necessary to modulate the screen of the final stage. The modulator stage consists of two CQK-25 ceramic tetrode valves working in push pull class B configuration. The drive stages up to the grid of the modulator are fully transistorized.

High Pass Filter:→

The audio input from the speech rack is fed to active High Pass Filter. It cuts off all frequenciesbelow 60 Hz. Its main function is to suppress the switching transistors from the audio input. This also has the audio attenuator and audio muting relay which will not allow AF to further stage till RF is about 70 kW of power.

AF Pre-amplifier:→

The output of the High Pass Filter is fed to the AF Pre-amplifier, one for each balanced audio line. Signal from the negative feed back network from the secondary of the modulation transformer and the signals from the compensator also are fed to this unit.

AF Pre-Corrector:→

Pre-amplifier output are fed to the AF Pre-correctors. As the final modulator valve in the AF is operating as Class B, its gain will not be uniform for various levels of AF signal. That is the gain of the modulator will be low for low level, input, and high for high level AF input because of the operating characteristics of the vacuum tubes. Hence to compensate for the non linear gain of the modulator. The Pre-corrector amplifies the low level signal highly and high level signal with low gain. Hum compensator is used to have a better signal to noise ratio.

AF Driver:→

Two AF drivers are used to drive the two modulator valves. The driver provides the necessary DC Bias voltage and also AF signal sufficient to modulate 100%. The output of AF driver stage is formed by four transistor in series as it works with a high voltage of about -400 V. the transistors are protected with diodes and zener diodes against high voltages that may result due to internal tube flashovers. There is a potentiometer by which any clipping can be avoided such that the maximum modulation factor will not exceeded.

AF Final Stage:→

AF final stage is equipped with ceramic tetrodes CQK-25. Filament current of this tube is about 210 Amps. at 10V. The filament transformers are of special leakage reactance type and their short circuit current is limited to about 2 to 3 times the normal load current. Hence the filament surge current at the time of switching on will not exceed the maximum limit.

Modulation condenser and modulation choke have been dispensed with due to the special design of the modulation transformer. Special high power varistor is provided across the secondary winding of the modulation transformer to prevent transformer over voltages.

COOLING ARRANGEMENT

Introduction→ In modern A.M. transmitters power valves are used in the PA and modulator stages, which are condensed vapour cooled ceramic tetrodes. In the old generation transmittes, triodes are used in the PA, modulator and exciter stages. Both the tetrodes and triodes tubes are capable of being operated at high voltages (11 kV DC) and large anode current of the order of 50 Amps. They also draw large filament current of about 620 Amps at 24 volt CQK-350. Hence the tubes dissipate large amount of power which require effective cooling.

The tube is installed vertically with the heating connections at the bottom. Handle and transport the tube with utmost care : vibrations and external impacts can cause invisible damage. Avoid sudden movement. Slowly insert the tube in the connection head so that sudden impact is avoided. If the dead weight of the tube is not sufficiently to overcome contact resistance in the connection head, apply gentle pressure. The ceramic parts must be always kept clean. If necessary, they should be cleaned with alcohol or acetone but no circumstances should they be rubbed with emery paper.

The contact surfaces are coated with a heat resistant lubricant film, which does not attack silver. Electrical connections and connection head are provided with contact rings for all electrodes including the anode. The connection head is stationary. It supports and locate the tube, which can be inserted into the connection head only in a certain position. This position is determined by the guide groove on the anode.

Cathodes→

All the ceramic tetrodes used in AIR transmitters are directly heated thoriated tungsten cathode. The filament voltage should not vary beyond +5% of the rated filament voltage. The filament voltage must always be measured at the concentric contact rings using sub-standard volt meter. The cathode cum filament has only a very small resistance when cold. Hence the filament voltage is applied and increased smoothly as per the design of the transmitter. In some transmitters, the filament voltage is applied in steps. In some transmitter, the design of the filament transformer is such, it will restrict the surge current to 3 to 4 times the normal steady current. Screen Voltage→

The screen grid current can become dangerously high, even at normal screen grid voltage, when the anode voltage is lower than that of the screen grid. Hence the screen grid supply will be switched ON only when the anode voltage has become about 40% or so of the anode voltage.

Temperature→

A separate air cooling has been arranged to control the temperature of the ceramic cylinder and all metal ceramic seals in addition to the condensed vapour cooling. There must not be any high frequency on the supply leads. To ensure this filament RF by pass condensers are provided.

Cooling System Used in Transmitter→

In high power A.M. transmitter, lot of power is dissipated in the valve as the input power is not fully converted into output R.F. power due to the efficiency of the amplifier which never reaches 100%. Hence the valves have to be cooled. In addition filaments are drawing large current of the order of 210 Amps at 10 volt for CQK valve. Hence they also have to be cooled. The dissipated heat in the valves also circulates in the concerned cubicle and heat develops there. Hence some kind of cooling has to be provided to the transmitting equipment. Different types of cooling are used in AIR (Ajmer) transmitter at present→

a) Air cooling.b) Condensed vapour cooling.

a) Air Cooling→ At present forced air cooling is used in AIR transmitters. A blower sucks the air through an Air filter and a guided duct system and the forced air is passed on to the required transmitting tubes. There has to be minimum air flow to cool the valves. Hence there will be an air operated Air Flow Switch (Relay) AFR : the AFR will close only when sufficient amount of air has been built up with the blower. Otherwise, AFR will not close and filament cannot be switched on. Sometimes, if the filter is not cleaned, sufficient air may not go out of the blower. Hence the blower needs periodical cleaning.

b) Condensed Vapour Cooling in HMB-140BEL 100 kW MW XTR: →

In BEL solid state transmitter of 100 KW/300 kW MW and 50 KW/100KW/500KW SW transmitters, condensed vapour cooling is used for the PA and modulator valves. Here a circulation of fast flowing stream of de-mineralized water is used. A high velocity water flows through the valve jaket and transforms into vapour due to the dissipation of power in anodes. The tubes are fitted with a specially formed anode which sits in a cylinderical cooler. Due to the fast flow of water, the vapour is condensed to water as soon as they are formed. Hence the cooling efficiency is much higher. The temperature of water coming from the transmitter can theoretically reach about 900 C, but in practice, it is desired to about 700 C in normal programme modulation.

Filaments of the tubes are cooled by forced air by means of a high pressure blower. It also cools the R.F. driver valves, the third harmonic and second harmonic suppression coils.

PARALLEL OPERATION OF TRANSMITTER

Need For Parallel Operation:→

At times it may not be possible to get the required power from the single transmitter for the required coverage of the broadcast service. In such conditions, it is essential to combine two or more transmitters to get the required power. Besides combined operations also facilitate operation of single transmitter in case of failure of one transmitter thereby achieving reliability of the service.

Requirements:→

Like parallel operation of alternators/generators there are three conditions to be satisfied for parallel operation of two transmitters. They are

→ Frequency of the transmitter should be the same.→ The phase of the signal of the transmitters at combiner should be the same.→ The power levels of both the transmitters should be such that the amplitude at the combiner is equal. In order to meet the first condition, it is possible to use one frequency source for both the transmitters. Hence if there is any drift in the frequency, it will be common to both the transmitters. The phase of the signal of the transmitters depends upon the tuning stages which employ active amplifiers. Different circuits give different phase shift. In order to offset this difficulty a phase shifting network is employed in the oscillator circuit. The signal going to one of the transmitters is passed through this network and hence the phase angle can be adjusted.

The third requirement for parallel operation is more stringent and requires constant monitoring for proper operation of combiner to get equal amplitude or power level at the same impedance of the output. Therefore matching the network for offering equal load impedance to both the transmitters is absolutely essential. Different methods are used for getting the proper matching.

Such a network used for combining shall be such that it should →

→ Should offer equal load impedance to both the transmitters.→ Shall be able to continue the operation even if one of the transmitter goes off the air.→ Shall facilitate to dissipate the unbalanced power flowing through the combiner network.

The most common network which is used is a bridged “T” network. It has four reactive networks . Two capacitive and two inductive and all are having impedance’s equal to that of feeder line. The bridged arm also has got one resistive load equal to feeder impedance. This shall take care of the unbalances in the network.

Procedure for tuning and combining:→

Tune each arm of the network for impedance equal to load impedance.

Connect the network and terminate the load impedance.

Measure the load impedance offered at each of the transmitter. It should be equal to load impedance. If not adjust the reactances.

Open and short-circuit the output point of one of the transmitter (in off condition) and measure the load impedance at the other transmitter. It should not change.

Now put on the transmitters with a single oscillator source.

If there is unbalance try to adjust with the phase control of oscillator for minimum unbalance.

Modulate the transmitter slowly to see whether there is unbalance. If so check the audio phase to each of the transmitter.

ANTENNA TUNING UNIT

Introduction:→

Antenna Tuning Unit (ATU) is to match the feeder line impedance to the mast impedance of MW Transmitters for maximum transmission of power. So ATU is located between the mast base and the feeder line and is very close to the mast base. Commonly “Feeder Unit” which is located in the aerial field, houses the ATU. Generally the mast impedance (aerial impedance) is obtained in a complex form i.e. the real part (resistive) and the imaginary part (reactive) component. When the mast impedance is expressed in polar form then negative angle indicates the mast is capacitive and positive angle indicates the mast is inductive. Whether the mast impedance is inductive or capacitive depends on the height of the mast in terms of wave length (). If the height is less than /4, it will be capacitive and inductive if more than /4. This can be measured with impedance bridges.

ANTENNA:→

The signals from ATU are fed to Marconi type antenna. It is 200mtrs high and supported on insulators and guys. In AM transmitter full antenna is used for transmission. The signals propogates parallel to the ground. MW propogates along the surface of the earth. Because of the differaction the wavefront gradually tilts over. As the wave propogates over the earth it tilts over more and the increasing tilts causes greater short circuiting of the electric field component of the wave and hence field strength reduces and it will vanish at some distance.

INDUSTRIAL VISIT

TO

DOORDARSHAN KENDRA, AJMER

THE DOORDARSHAN

Doordarshan the largest (literally Far Vision) is the public television broadcaster of India and a division of Prasar Bharati of broadcasting organizations in the world in terms of the infrastructure of studios and transmitters.

It is a public service broadcaster nominated by the Government of India. Today, Doordarshan present network of 31 Programme Production Centers and 553 transmitters of varying power reaching about 82 percent of country’s population.

THE DOORDARSHAN KENDRA AJMER

AN OVERVIEW

Doordarshan Kendra Ajmer is a part of DD India, the largest television network in the world.

Doordarshan started in 2008 inaugurated by Mrs. Vasundhra Raje. Transmitter used 5KW Signal Coverage Area 70km It transmits DD National Through its home satellite INSAT-2A and DD News

through INSAT-2B.

BROADCASTING IN DOORDARSHAN KENDRA

DDK Ajmer have following main components that manages transmission and maintenance of two channels DD National and DD News.

1. MAIN SWITCHING ROOM (MSR)2. DIGITAL EARTH LINK STATION3. TRANSMITTER

MAIN SWITCHING ROOM (MSR): The MSR stores all the circuitry of the DDK. The monitoring and control of all activities takes place in MSR. It is the MSR which decides what is to go in air. It is the heart of the studio. Most of the switching electronics is kept e.g. camera

base stations, switcher main frame, SPG, Satellite receivers, MW link, DDA and most of the patch panels. Signal is routed through MSR. Signal can be monitored at various stages.

DIGITAL EARTH LINK STATIONSatellite offers the unique possibility to transmit images and sound from almost anywhere on the planet from a fully equipped uplink truck.Earth Station is a very important part of satellite communication system for broadcasting of signals.The earth station is in fully digital domain.

The uplink station has an uplink chain, simulcast transmitters, audio ,video processors, up converters, modulators etc. It is a uplink center from which the signals are fed to Satellite for distribution in a specified area covered by the Satellite.The signal is up linked from the Earth Station and received by many down link centers in TV broad casting via PDA.

Uplinking And DownlinkingThe output of modulator (70MHz) is sent to an up converter. The up converted signals are sent to an HPA. Then this signal is given to a PDA (parabolic dish antenna) for up linking to satellite. The uplinked signal is received again by the same PDA for monitoring purposes. The signal between earth station and satellite are given along line of sight which means there must be a clear path from earth to satellite. The uplink signal is fed from the earth station by a large PDA. The satellite is equipped with its own dish antenna which receives the uplink signals and feeds them to a receiver. The signal is then amplified and changed to a different frequency which is downlink frequency. This is done to prevent interference between uplink and downlink signals. The down linked signal is then again sent to the transmitter which again retransmits it. Each satellite has a transponder and a single antenna receives all signals and another one transmits all signals back.

A satellite transmits signals towards earth in pattern called the satellite footprint of the satellite. The footprint is strongest at centre and the footprint is used to see if the earth station will be suitable for the reception of the desired signal.

MonitoringThe base band segment comprises of baseband subsystems at studio site and base band subsystem at earth station site. This baseband segment processes two video Programmes. The base band segment is monitored and controlled using a PC placed near the base band earth station equipments called base band NMS PC.For monitoring of video programmes professional video monitor, LCD video monitor and audio level monitor are provided in the base band segment. An operator console has one 14” professional video monitor a video audio monitor unit for quantitative monitor of video programmes and a personal computer for centralized merit and contention of earth station sub system.

TRANSMITTERThe last stage is the transmitter which has the antenna and facilities for terrestrial transmission.

Types1. Based on POWER : 1. VLPT / Transposer (10W) 2. LPT (100W, 300W, 500W) 3. HPT (1kW, 10kW,20kW)2. Based on FREQUENCY: 1. VHF 2. UHF

AntennaAn antenna (or aerial) is a transducer designed to transmit or receive electromagnetic waves. In other words, antennas convert electromagnetic waves into electrical currents and vice versa.

A 6.3m diameter antenna with a simplified manual track device features ready erection, ease of maintenance and high reliability. Most of the parts of the panel and antenna structure are made up of aluminum alloy which has corrosion resistance and yield strength.

Antenna Parameters

Antenna Type Limited steer X-Y type Reflector Diameter 6.3m Sky Coverage: X-axis 45 to 90 degrees in steps Y-axis _+40 degree at any position in steps Wind Resistivity 2mm rms for winds upto 60 km/hr Survival upto 200 km/hr Bands- S-Band C-BandFrequency Range 2555MHz to 2635MHz 3700MHz to 4200MHzPolarization Left hand circular Linear and changeableAntenna Gain 41.8db at 2GHz 44.8db at 4GHzVSWR 1.25max 1.30maxVolt Axial Ratio Better than 3db Less than 3dbReciever System G/T 14/db/K at 2600MHz 29.4/db/K at 4GHz

Reflector structure The 6.3 m diameter antenna is made up of 4 quarter segment. Each and every quarter is made up of 10 segments fixed on five trusses. Panels which are fixed to the trusses are made up of fine aluminum expanded mesh strengthened with the help of channel sections and tee sections whose ends are fixed to the backup structure. Trusses are composed of aluminum square tubes and the welded back up made up of hub and 20 trusses.

Mount structure A simple tubular steel space frame makes up most of the mount structure. It allows rotation about x-axis as well as y axis.

The x axis drive rod is connected between the top of the mounted structure and the concrete foundation. The y axis drive rod is connected between the base of the x axis bearing mount and the reflector back up structure on the left hand side as viewed from the rear of the antenna. The mount is rigidly attached to the concrete base which is facing north such that it can survive even in wind speeds up to 200 kemp.

Fixing the feed onto the antenna-

The feed is supported by a set of four pipes called as a quadruped. It is fixed before the whole antenna structure is hoisted, that is, it is fixed on the ground itself before the whole antenna structure is fixed. Care should be taken that the feed is at the exact focus of the reflector. Feed entrances and cable output ports are covered with waterproof Teflon sheet to prevent the entry of moisture into the arrangement.

Treatment-

The reflector is treated in the following order before installation

(a)Etch primer is applied after caustic soda acid treatment(b)Painted with white matt paint

The mount is treated with the following-

(a)A hot dip which galvanizes all steel parts(b)Etch primer treatment(c)White enamel paint is applied as a last coating

CONCLUSION

As per the curriculum of Rajasthan Technical University for III year B.Tech

Degree, I have completed my Industrial visit of 3 days at “ALL INDIA

RADIO,AJMER”, “DOORDARSHAN,AJMER”, “TOSHNIWAL

INDUSTRIES AJMER” and I have learned about their process and

instruments.