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Refrence: Dr. sunil joshi sir, Dr. naveen agrwal sir
Batch: 2013-17

Published in: Engineering
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  1. 1. Summer Internship at ALL INDIA RADIO (Indian Government Enterprise) Akashwani, Udaipur Training Report From: 4th June, 2015 To: 18th June, 2015 COLLEGE OF TECHNOLOGY AND ENGINEERING (Department of Electronics and Communication Engineering)
  2. 2. Submitted To: Submitted By: Dr. Navneet Agrwal sir Abhishek Kumar (Assistant Professor) B.Tech 2nd yr. (E.C.E) Placement and Training Incharge (E.C.E) 2013/CTAE/171 ACKNOWLEDGMENT The feeling of euphoria, I have on the successful completion of the summer training at ALL INDIA RADIO (AIR) is expressible. I take this opportunity to express my profound gratitude and indebtedness to permit me to undertake the training in this esteemed organization. I would like to pay heartfelt thanks to the respected Head Dr. Sunil joshi sir for giving the permission to the permission to take the training. My sincere gratitude and thanks to Shri Satish Depal
  3. 3. (Director Engineering), Shri pankaj sir (Assistant Director and Training Superintendent) who has given me the opportunity to perform training at Akashwani Udaipur station. I express my sincere gratefulness to Shri J.K Purbiya(A.E), Shri Arun kumar, Shri D.K. Bhagela, Shri Deepak kuril, Shri B.M. Bhawasar, Shri V.R. Solanki who have spared time to guide me during entire duration of training.
  4. 4. PREFACE Practical training is a way to implement theoretical knowledge to practical use. To become a successful engineer it is necessary to have a sound practical knowledge because it is the only way by which one can acquire proficiency/skill to work successfully industries. It is proven fact that bookish knowledge is not sufficient because things are not as ideal in practical field as they should be. All India Radio is one of the best examples to understand the communication and connectivity in particular of broadcasting systems. It is matter of great pleasure that our college authorities have recommended a practical training of 15 days to supplement our theoretical knowledge acquired in the college .This report is an attempt made to study the overall communication system /related action of AIR, Chetak Circle and Madri station.
  6. 6. INTRODUCTION Radio Broadcasting was pioneered in India by the Madras Presidency Club Radio in 1924. The Club worked a broadcasting service for three years, but owing to financial difficulties gave it up in 1927. In the same year (1927) some enterprising businessmen in Bombay started the Indian Broadcasting Company with stations at Bombay and Calcutta. This company failed in 1930, in 1932 the Government of India took over broadcasting. A separate department known as Indian Broadcasting Service was opened. The Service was later designated 'All India Radio' (AIR) and was placed under a separate Ministry-the Ministry of Information and Broadcasting. The AIR is controlled by a Director General, who is assisted by several Deputy Directors and a Chief Engineer. Broadcasting, in its significance, reach and impact, constitutes the most powerful medium of mass communication in India. Its importance, as a medium of information and education is particularly great in a vast and developing country like India where the reach of the printed word is not very wide or deep. While the total circulation of all the newspapers in India, including both English and Indian language papers, is around 8 million, there are, according to a recent estimate, nearly 400 million (out of a total population of 625 million) potential listeners to All India Radio. Broadcasting in India is a national service, developed and operated by the Government of India. All India Radio (also known
  7. 7. as Akashwani) operates this service, over a network of broadcasting stations located ail over the country. As a national service, catering to the complex needs of a vast country. All India Radio seeks to represent in its national and regional programmers, the attitudes, aspirations and attainments of all Indian people and attempts to reflect, as fully and faithfully as possible, the richness of the Indian scene and the reach of the Indian mind. AIR Network: Starting with 6 broadcasting stations in 1947, the AIR today has a network of 82 broadcasting stations. The 82 radio stations, grouped into five zones, are the following: North Zone: Ajmer, Allahabad, Aligarh, Bikaner, Delhi, Gorakhpur, Jaipur, Jodhpur, Jullundur, Lucknow, Mathura, Rampur, Shimla, Udaipur and Varanasi: East Zone: Agartala, Aizawl, Bhagalpur, Calcutta, Cuttack, Dibrugarh. Gauhati, Imphal, Jeypore, Kohima, Kurseong, Ranchi, Pasighat, Patna, Sambalpur, Shillong, Silchar, Siliguri, Tawang and Tezu ; West Zone : Ahmedabad, Bhopal, Bhuj, Bombay, Gwalior, Indore. Jabalpur, Nagpur, Panaji, Parbani, Pune, Raipur, Rajkot and Sangli; South Zone: Alleppey, Bangalore, Bhadravati, Calicut, Coimbatore, Cuddapah, Dharwar; Gulbarga, Hyderabad, Madras, Mysore, Pondicherry, Port Blair, Tiruchirappalli, Tirunelveli, Trichur, Trivandrum. Vijayawada and Vishakhapatnam; and Kashmir Zone: Jammu, Leh and Srinagar. In addition, there are three auxiliary studio centers at Vado- dara, Darbhanga and Shantiniketan and two Vividh Bharati/commercial centers, one at Chandigarh and the other at Kanpur. These cover all the important cultural and linguistic regions of the country. The expansion of the broadcasting facility remained limited
  8. 8. till independence. In 1947 there were only six radio stations in the country. Today there are as many as 82 AIR stations. With two more stations that will start working soon, India's broadcasting network would cover 89 per cent of the population. Till the end of 1976 radio licenses had reached a colossal figure of nearly 1.74 crores, which fetched revenue of Rs. 23.51 crores. Today the radio network has spread to the remote corners of India. It is now possible to bring sense of unity not only political but also cultural among the diverse traditions that enrich our land. AIR's programmes pattern combines three main elements: a national channel providing programmes of countrywide interest and significance, a zonal service from each of the four metropolitan centers (Delhi, Bombay, Calcutta and Madras); and regional services from individual stations each catering to the needs and interests of its respective area. The principal ingredients of 1AIR's programmes output are Music, Spoken Word, Dramas, and Features. News and Current Affairs, Commentaries and Discussion, Vivid Bharati and its Commercial Service, Farm and Home Broadcasts, Programmes for Special Audiences (like Youth, Women, Children, Industrial Workers and Tribal Population), and Programmes for Overseas Listeners broadcast in the External Services. To enable AIR to reach all sections of the Indian people, its programmes in the Home Service are broadcast in 20 principal languages. In addition, the External Services of AIR beam their programmes to listeners all over the world in 24 languages. New Services: The News Services Division of AIR through its central and regional news bulletins and its current affairs, commentaries and
  9. 9. discussions, provides accurate, objective, speedy and comprehensive coverage of news to listeners at home and abroad. AIR now broadcasts a total of 239 news bulletins a day, with duration of 32 hours 17 minutes. Of these, 67 are Central bulletins broadcast from Delhi in 19 languages, with a daily duration of 10 hours 3 minutes; 57 external bulletins (from Delhi) broadcast in 24 languages for a duration of 7 hours 14 minutes and 15 regional bulletins from 34 regional centers (including the Prade- shik desk in Delhi) broadcast in 22 languages and 34 tribal dialects with a total duration of 15 hours every day. The major sources of news for AIR are its correspondents at home and abroad, the news agencies and the monitoring services, AIR has a total of 206 correspondents. Of these, 111 are part-time. External Services: AIR made its first broadcast to listeners outside India on October I, 1939. Today the External Services of AIR broadcast in 25 languages for about 50 hours daily round-the-clock, reaching listeners in widely scattered areas of the world. PRESENT SETUP Currently there are two complexes in All India Radio, Udaipur. They are: 1. STUDIO CUM OFFICE COMPLEX 2. TRANSMITTER COMPLEX
  10. 10. STUDIO CUM OFFICE COMPLEX, UDAIPUR A broadcasting studio is a room in studio complex which has been specially designed and constructed to serve the purpose of originating broadcasting programs. Whenever any musician sings and we sit in front of a performing musician to listen to him, we enjoy the program by virtue of the superb qualities of our sensory organs namely ears. However, when we listen to the same program over the broadcast chain at our home through domestic receivers, the conditions are entirely different. These changes that we experience are because of the audio processing that are performed in a broadcasting studio. There are three studios at AKASHVANI, UDAIPUR studio complex. They are: • MUSIC STUDIO • TALK STUDIO • PLAYBACK STUDIO Music and talk studio are together known as RECORDING STUDIO. A Recording studio is a facility for sound recording and mixing. Ideally both the recording and the monitoring spaces are specially designed by an acoustician to achieve optimum acoustic properties (acoustic isolation or diffusion or absorption of reflected
  11. 11. sound that could otherwise interface with the sound heard by the listener). Recording studios may be used by recording musicians, voice over dialogue replacement in film, television or animation, Foley or to record their accompanying musical sound tracks. The typical recording studio consists of a room called “Studio” or “Live room”, where instrumentalists and vocalists perform; and the “Control room”, where sound engineers operate professional audio for analog or digital recording to route and manipulate the sound. Following equipment are generally provided in a recording/dubbing room: i) Console tape recorders ii) Console tape decks iii) Recording/dubbing panel having switches jacks and keys etc. The above equipment can be used for the following purpose • For recording of programmes originating from any studio. • For recording of programmes available in the switching. • Consoles in control room. • For dubbing of programmes available on cassette tape. • For editing of programmes. • For mixing and recording of programmes. We can brief the studio arrangements at AIR-UDAIPUR as follows: • MUSIC STUDIO The MUSIC STUDIO is an acoustically treated room attached to a control room. The studio consists of five microphones and sufficient musical instruments. The control room consists of workstations/computers and a control console for adjusting and checking the quality of the program. These arrangements together are used for producing musical programmes. Live musical
  12. 12. programmes can be also conducted here. SONYSOUND FORGE is the software which is commonly used for processing the raw version of the recorded program. The processed version of the recording is saved to the server and then it is made available for broadcasting by scheduling it to the program list using the software VIRTUAL STUDIO. • TALK STUDIO The TALK STUDIO is similar to a music studio with an acoustically treated LIVE ROOM and a CONTROL ROOM. The live room consists of only two microphones. It is equipped with a telephone connection which is a user friendly attribute for recording Phone-in programs. The control room consists of an additional Phone-in console for conducting Phone-in programs. The acoustics of the talk studio is entirely different from a music studio. It is constructed in such a way that the reverberation time is minimised and no echo is experienced. The recording produced and processed at the talk studio is then forwarded to the playback studio for transmission. Talk studio can be also used to produce live chat programs. • PLAYBACK STUDIO A PLAYBACK STUDIO is entirely different from all other studios. It consists of transmission console, microphones, two workstations/computers (Master & Standby). Its main function is co-ordinating the programs, announcements and advertisements. All the recorded programs will be available in the workstations used and the programs are sent to the control room for broadcasting as per the schedule. Before transmission of the first program a tone of 1 kHz and signature tone will be aired. A GPS clock is used both in the studio complex and transmitting section, to avoid time delays.
  14. 14. The Studio console is the major equipment used in the STUDIO CONTROL ROOM. It is with the help of this device the different programmes that are produced and those that are received from other stations routed to air. The various inputs to the console are the programmes from various studios, the programmes that are received using a C BAND receiver which is broadcasted from Delhi and the programmes that are received via an ISDN link from Calicut and Thiruvananthapuram. The Outputs from the console is taken through two master amplifiers among which one is active at a time. This output is directed to the STUDIO TRANSMITTER LINK (STL). CONTROL ROOM AUDIO CONSOLE INPUTS AND OUTPUTS
  15. 15. STUDIO TRANSMITTER LINK: The programs produced at the Studios are not transmitted from the same complex with intention of preventing the problems due to interference and radiation. Instead, the programs are transmitted from the transmission complex which is situated at Avanoor. The high quality sound programmes from AIR studio centre are normally transported to the AIR transmitting centre with the help of a transmission link named as the STUDIO TRANSMITTER LINK (STL). AIR is having three types of STL called STL-01, STL-02 and STL-05. The numbers 01, 02 and 05 describe the number of base band (50Hz – 15 kHz) channels that could be transported. At Thrissur, we are using STL-01 since we are transmitting only one base band channel For quality transmission of the programmes, STL is realised using four methods. They are: • A microwave link • 10W FM transmitter link • ISDN link • BSNL dial up link 1. MICROWAVE
  16. 16. Radio and television broadcast companies originate their signals in studios, but must get them to the transmitter site. In many cities, a nearby hill or mountain holds most of the transmitters. A microwave studio transmitter link (STL) delivers the signal without wires. Positioned at a fixed location and using radio waves, a microwave transmitter sends those waves across space to be received by a microwave receiver at another fixed location. Microwave is broadband, so it can transmit a substantial amount of information from point to point, for use in cell phone and wireless Internet service, with no need for any other equipment between the two fixed locations. The microwave STL system consists of a transmitting system (STL-TX) housed in the studio premises and a receiving system (STL-RX) housed in the AIR transmitting centre. A low loss cable connects the STL TX/RX to the microwave dish antenna of diameter 2m mounted on an approximately 50m tall self- supporting tower at either end. In addition, a VHF service channel in duplex mode is provided at both the ends for voice communication between the AIR studio and transmitting end through a multi-element yagi antenna mounted on the top of the tower. The need for the service channel arises from the fact that there is no RF monitoring facility of the transmitter sound program at STL-TX. The STL system is meant to operate unattended round the clock. The microwave STL TX/RX is powered by an external power supply unit kept adjacent to the STL rack with floating batteries. This unit takes 230V, 50Hz AC and supplies 24V DC to STL TX/RX. The service channel is energised by another external power supply unit placed over that of STL TX/RX. SERVICE CHANNEL (RT 33) (VHF Link) The service channel is mounted at the top of the transmitter and receiver racks. It IS a VHF 1613-88 MHz) trans-receiver. The transmitter output power is 15 watts YAGI( antenna mounted at the
  17. 17. top of the towers on either end is used for the service channel This antenna may be used both in horizontal and vertical polarizations Normally vertical polarization is used The hand set with a press to talk (PM switch is employed at either end for service communication. These units can be revised from the racks and kept at any other convenient location at either end M/s Melton has developed an interface unit with which telephone facilities can be extended to the transmitter sole with this service channel without the use of land lanes. 2. Integrated Services Digital Network (ISDN) Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. It was first defined in 1988 in the CCITT red book. Prior to ISDN, the telephone system was viewed as a way to transport voice, with some special services available for data. The key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. There are several kinds of access interfaces to ISDN defined as Basic Rate Interface (BRI), Primary Rate Interface (PRI), Narrowband ISDN (N-ISDN), and Broadband ISDN (B-ISDN).ISDN is a circuit-switched telephone network system, which also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in potentially better voice quality than an analog phone can provide. For AIR, The ISDN link is facilitated by the BSNL. Air is making use of BROADBAND ISDN. In addition to an STL system ISDN acts as a channel for live broadcasting of AIR programmers. BROADBAND ISDN (B-ISDN): The original or basic version of ISDN employs baseband transmission on copper wires. But B-
  18. 18. ISDN uses broadband transmission at 1.5Mbps for transmission of voice, video and data at the same time. B-ISDN requires fiber optic cables and is less widely available as it has relied mainly on evolution of fiber optics. According to CCITT, B-ISDN is “a service requiring transmission channel capable of supporting transmission rates greater than the primary rate”. SALIENT FEATURES OF ISDN: • ISDN is a fast network • ISDN is a telephone network • Integrated services • Digital network 3. FM TRANSMITTER Here in AIR, UDAIPUR, we use 60W FM transmitter link. The principle of working of a modern FM transmitter is given in the block diagram, The L and R audio signals are converted into the stereo signal by a stereo coder. The stereo signal also called the MULTIPLEXED signals then frequency modulates the VFH oscillator which is a voltage controlled oscillator (VCO) OF THE PHASE LOCKED LOOP (PLL). The PLL is an automatic frequency control system in the FM transmitter is maintained within the specified tolerance limits of +2 KHz. In this arrangement the phase of the VHF oscillator is compared with that of a reference crystal oscillator operating at10MHz. As the VHF oscillator can operate at any assigned in the FM broadcasting band of 87.5MHz-108MHz, the factor N will vary from 8750- 10800Hz.The phases of the output from the two frequency dividers are then compared in a phase comparator and the resultant error amplified rectified and filtered to get a dc error voltage of positive negative polarity which corrects shift in the VHF oscillator frequency. The FM signal obtained at the output of VHF oscillator is then amplified in a VHF power amplifier with an output power
  19. 19. of1.5KW. This amplifier is the basic building block in the series of FM transmitters. It is a wide band amplifier so that no tuning is requires when the operating frequency is changed. 4. BSNL Dial Up Link This link between the studio complex and the transmitter is the least preferred and least used one. If all the other system fails then the program signals are transmitted via the regular BSNL telephone cables. Techniques are used at the transmission station to receive these signals and then it is fed to the modulator.
  21. 21. The air conditioning at the studio complex is done using AC plants. There are 3 AC plants at the studio complex each with a capacity of XX tonnes among which 2 are sufficient for the purpose. These two plants supply conditioned air to two different parts of the studio via AC ducts that runs all over the ceiling. The AC plant consists of Blowers and a water circulation system that carries the hot air out of the building. The hot water is cooled at a cooling tower where water is cooled by dissipation of heat to the surroundings. A fan is also attached to the cooling tower for easy cooling. ANTENNAS Antenna is usually a metallic device (a rod or a wire) used for radiating or receiving electromagnetic waves. The radio frequency power developed in the final stage of a transmitter is delivered through cables/feeders, without themselves consuming any power to the transmitting antenna. The RF energy gets converted into electromagnetic waves and travels in the free space at the speed of light. The receiving antenna picks up the radio waves and delivers useful signal at the input of a receiver for reception of signals. The transmitting and receiving antennae are reciprocal in the sense, any characteristics of the antenna in general applies equally to both. Antennas play a vital role in AIR also since these are the communication links between the various stations and the transmitter complex as well. As the purpose differ the shape, size and specifications varies in case of Antennas. In an AIR station we can see a wide variety of Antenna systems. These include: • A C-band receiver antenna with a dish whose diameter is about 5m. This antenna receives signals from other stations like Delhi. • A DTH receiver antenna with a dish whose diameter is about
  22. 22. 1m. This antenna receives signals from stations like Calicut and Thiruvananthapuram • Yagi antennas are mounted on the top of a mast of height around 45 m. This is the transmitter antenna for the microwave studio transmitter link. And a similar receiver antenna is mounted on a mast of height about 50m. This enables the line of sight communication between the studio and the transmitter. • Similar to microwave transmitter antennas, FM transmitter and receiver antennas are also mounted on the same masts at the studio and transmitter complexes. • Private FM channels have also installed their antennas on the mast at the transmitter complex. • A self-radiating mast of height 122m which itself acts as the antenna is present at the transmitter complex. • Another self-radiating mast is present at the studio complex which was used in earlier times. This mast, having a power dissipation capacity of 20kW has decommissioned and is left as it is. TRANSMITTER COMPLEX, AVANOOR POWER SUPPLY TO THE TRANSMITTER COMPLEX
  23. 23. X TRANSFORMER 500 KVA 11KV/440V X 100KW BEL HMB 140-MW TRANSMITTER A radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current which is applied to
  24. 24. the antenna. When excited by this alternating current, the antenna radiates radio waves. The transmitter combines the information signal to be carried with the radio frequency signal which generates the radio waves, which is often called the carrier. Here in All India Radio, UDAIPUR; the 100KW BEL HMB transmitter performs amplitude modulation in which the information is added to the radio signal by varying its amplitude. This transmitter mainly consists of these parts: • A power supply circuit to transform the input electrical power to the higher voltages needed to produce the required power output. • A quartz crystal oscillator that generates sinusoidal wave at a frequency of 4MHz and associated frequency dividers to divide the high frequency wave to the transmission frequency 630kHz. • A modulator circuit to add the information to be transmitted to the carrier wave produced by the oscillator. This is done by varying some aspect of the carrier wave. The information is provided to the transmitter in the form of an audio signal whose content is the program produced at the studio. Transmitter complex includes the following rooms: • Power supply • HT room • Transmitter • Cooling POWER SUPPLY IN 100kW HMB 140MW TRANSMITTER 1. Main supply to Transmitter: 415 V 3PHASE 50Hz AC 2. Supply voltage to PA and Modulator: HT 11kV (Thyristor Controlled for smooth variation)
  25. 25. 3. Screen Voltage to PA Valve: 800V 4. Screen voltage to Modulator valve: 1070V 5. Plate Voltage to RF Driver: 1900V 6. Grid bias to PA Modulator & RF Driver: -650V 7. Supply voltage to cathode of RF Driver: -600V (Supplied by way Way of a tap on -650V supply) 8. Screen voltage of RF Driver: -100V 9. Thycon unit: +12V DC & -12V DC 10. Audio unit: +24V & +10V 11. Reflectometer: +15V & -15V 12. Control units: • VDDB : +15V Logic Circuits • VDDC : +12V Relays • VDDD : +15V For indication lamps • VDDE : -15V Comparator
  26. 26. Transmitter section includes RF, Audio as well as modulator stages. RF section generates the required operating frequency, here 630 kHz. Audio stage makes the audio to reach up to the power requirement. Modulator stage further modulates the audio with the RF.
  27. 27. RADIO FREQUENCY STAGE (RF STAGE) The RF chain consists of a crystal oscillator followed by a wide band transistorized power Amplifier developing an output of 12-15 watts. This is followed by a driver stage using 4-1000A valve working in class AB condition. This stage delivers a driving voltage of 900V peak to the grid of PA. The PA stage consists of CQK-50 ceramic tetrode valve working as a Class-D Amplifier. The PA stage works into output impedance of 120 ohms delivering a carrier of 100 KW. The stage is also high level anode modulated by the modulator. The stage has been provided with a composite anode circuit to provide for generation of class-D waveforms, harmonic suppression and matching of anode impedance to the output impedance. • CRYSTAL OSCILLATOR: The basic frequency determining unit is the crystal oscillator. Two oscillator circuits are provided one in circuit and one is in standby. The crystal oscillator is a self-contained unit including its power supply and a proportionately controlled oven and gives an output of 5V square wave which is required to transfer the Transistor power Amplifier. The 12 volts DC required for operating the unit as well as the crystal oven is derived from a rectifier and a voltage regulator. The crystal oscillator works between 3MHz and 6MHz for different carrier frequencies. The basic oscillator is a pierce circuit with crystal as the frequency determining element. • TRANSISTOR POWER AMPLIFIER: The power amplifier is a self-contained unit including its
  28. 28. power supply and delivers an output of about 12W and on output impedance of 75 ohms. The stage works as a switching amplifier and is wide band. However, the output filters are to be selected for the frequency ranges 525-1150 kHz & 1150-1650 kHz. The unit works on a 20V DC which is derived by the rectifier. It is followed by the transistorized regulator and series pass transistor. • RF DRIVER: The RF Driver stage provides the driving power required to develop an output of 100 KW to the final amplifier. Moreover, the wave shape required for Class D operation of the final stage is also generated in the driver. 4-1000A tetrode valve is used as a driver valve. The ac plate load impedance of the tube is around 2.4K ohms which needs to the matched to an effective PA grid load of 710 ohms. In order to generate the required Class D driving waveform which is an approximate square wave comprising of fundamental frequency and 20% of the third harmonic grid of the final tube has got a parallel tuned circuit at third harmonic frequency. • RF POWER AMPLIFIER: The final stage RF amplifier consists of a single tube, CQK- 50 beam power tetrode delivering carrier power output of 100 KW. High level anode modulation is used using a class B modulator stage. The screen of the PA tube is also modulated by a tap on modulation transformer. The plate load impedance of the PA stage is about 750 ohms and the output impedance is 120 ohms. The complex PA circuit matches the plate to the output impedance. In addition, the Class D operation of the stage needs third harmonic impedance at the plate. In addition, the output circuit should prevent radiation of harmonic frequencies. The plate circuit provides all these functions.
  29. 29. AUDIO FREQUENCY STAGE (AF STAGE) The AF Stage supply the Audio power required to amplitude modulates the final RF stage. The output of the AF stage is super imposed upon the DC voltage to the RF PA tube via modulation transformer. An auxiliary winding in the modulation transformer, provides 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 inputs from the speech rack are fed to active High Pass Filter. It cuts off all frequencies below 60Hz. Its main function is to supress the switching transistor 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 70kW of power • AF Pre-Amplifier The output of the high pass filter is fed to AF Pre-Amplifier, one for each balanced audio line. Signal from the negative feedback network from the secondary of the modulation transformer and the signals from compensator also are fed to this unit. • AF Pre-Corrector Pre-Amplifier output is 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 signals. That is the gain of the modulator will be low for low level, input, and high for high
  30. 30. level AF input because of the operating characteristics of the vacuum tubes. 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 2 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 transistors in series as it works with a high voltage of about -400V. The transistors are protected with diodes and zener diodes against high voltages that may result due to internal tube flashover. There is a potentiometer by which any clipping can be avoided such that the maximum modulation factor will not exceed. • AF Final Stage AF final stage is equipped with ceramic tetrode CQK 25. Filament current of this tube is about 210 amps at 10V. The filament transformers are of special leakage reactance typeand their short circuit current is limited to about 2-3 times the normal load current. Hence the filament surge current at the time of switching ON will not exceed the maximum limit. A varistor at the screen or spark gaps across the grid are to prevent over voltages. As the modulator valve is condensed vapour cooled tetrodes, deionised water is used for cooling. The valve required about 11.5 L /minute of water. Two water flow switches WF1 and WF2 in the water lines of each of the valves protect against low or no water flow. Thermostats WT1 and WT2 in each water line provides protection against excessive water temperature by tripping the transmitter up to standby if the temperature of the water exceeds 70 degree Celsius. Modulation condenser and modulation choke have been dispensed due to the special design of the modulation transformer.
  31. 31. Special high power varistor is provided across the secondary winding of the modulation transformer to prevent transformer over voltages. CONTROLAND INTERLOCK SYSTEMS IN TRANSMITTER Control and interlocking circuits of the transmitter are to perform four major functions:- • Ensure correct switching sequence. • Safety of the equipment • Safety of the operating personnel. • Indication of the status of the transmitter. In the following paragraph the details regarding the above aspects are dealt briefly: 1. Switching Sequence of Transmitter a) Ventilation b) Filament c) Grid bias or medium tension d) High tension. a. Ventilation: All the transmitters handle large amount of power. Basically the transmitters convert power from AC mains to Radio frequency and Audio frequency energy. The conversion process always results in some loss. The loss in energy is dissipated in the form of heat. The dissipated energy has to be carried away by a suitable medium to keep the raise in temperature of the transmitting equipment within limits. Hence, in order to ensure that the heat generated by the equipment is carried away as soon as it is generated the ventilation equipment need to be switched on first.
  32. 32. Normally the cooling provided in a transmitter could be classified on the following lines: • Cooling for the tube filaments. • Cooling for the tube Anodes. • General cooling of the cubic. • Cooling for coils, condensers, Resistors etc. The cooling equipments comprise of blowers, pumps and heat exchangers. Another important consideration is that during the switching off sequence the cooling equipments should run a little longer to carry away the heat generated in the equipments. This is ensured by providing a time delay for the switch off of the cooling equipment. Normal time delay is of the order of 3 to 6 minutes. The water flow and the air flow provided by the cooling equipments to the various equipments are monitored by means of air flow and water flow switches. In case of failure of water or air flow, these switches provide necessary commands for tripping the transmitter. b. Filaments: All the transmitters invariably employ tubes in their drive and final stages of RF amplifiers and sub modulator and modular stages of AF amplifiers. After ventilation equipments are switched on and requisite air and water flow established, the filament of the tube, the control and interlocking circuits have to take care of the following points. The cold resistance of the filament is very low and hence application of full filament voltage in one strike would result in enormous filament current and may damage the tube filament. Hence it becomes necessary to apply the filament voltage in steps. Various methods adopted are: i. Use of step starter resistance. Here the filament voltage of the tubes is given through a series resistance (called step starter
  33. 33. resistance). The series resistance which limits the initial filament current is shorted and after a time interval by the of a timer switch. ii. Use of special filament transformer which allows slow build-up of the filament voltage. iii. Application of filament voltage in 3 or 4 steps. The emission from the tubes depends upon the temperature of the filament. Generally it takes some time for the filament to reach a steady temperature after it is switched on. Hence, it is not desirable to draw any power from the tube till it attains a stable temperature. This means that the further switching on process has to be suspended till the filament temperature and hence the emission becomes stable. This aspect is taken care of by providing a time delay of 3 to 5 minutes between the filament switching on and the next sequence namely bias switching on. c. Bias and Medium Tension: For obvious reasons the control grid of the tube has to be given the necessary negative bias voltage before its anode voltage can be applied. Hence, after the application of full filament voltage and after the laps of necessary delay for the filament temperature to become stable bias voltage can be switched on. Along with bias generally anode and screen voltages of intermediate stages and driver stages are also switched on. Application of bias and medium tension makes available very high voltages for the various transmitter equipment. Hence, in order to ensure the safety of the personnel access to these equipment should be forbidden before the application of bias and medium tension. This is ensured by providing the interlocking so that the bias and medium tension can be put on only after all the transmitter and other HV equipment doors are closed to prevent access. Connection of load (Antenna/Dummy load)
  34. 34. After the application of ventilation, filament and bias the anode voltage can be switched on. But before the anode voltage can be increased the interlocking circuit is to ensure that the load of the transmitter namely antenna or dummy load is connected to the transmitter. The tuning process of the various RF stages is complete and none of the tuning motors are moving. Application of screen voltage In the case of tetrode tubes, the screen voltage to the tube should not be applied before the application of anode voltage to keep the screen current and screen dissipation within limits. This is taken care of by an interlocking provision that the screen voltage is applied only after the anode voltage reaches a certain predetermined value well above the normal screen voltage. Release of Audio frequency The application of AF signal to the AF stage in the absence of carrier power would result in the operation of modulation transformer with no load connected. This is not desirable. Therefore, the AF signal should be applied to the audio frequency stages only when the RF power amplifier is delivering the nominal power. Normally AF frequency signal to the AF stage is released only when the carrier power is approximately 80% of the normal power. 2. Safety of the equipment The various transmitting equipment and auxiliaries are to be safe guarded against over loads etc. The various safety provisions provided in the transmitter are as follows: • All the existing machineries are provided with switches with
  35. 35. magnetic and thermal overload release. • The air flow and water flow switches and temperature sensors monitors the air flow and water flow of the cooling medium. If the air and water flow fall below a certain predetermined value, it ensures the necessary tripping sequence. • Water levels in the reservoir and water conductivity are monitored constantly. • Momentary release of air flow and water flow switches due to some turbulence for a short duration will not result in the tripping of the transmitter. However, if the fault persists for a few seconds then the tripping will result. • Sometimes thermal sensors are embedded in the filament transformers to monitor its temperature. • The filament voltage of various high power tubes is case of low or high filament voltage tripping of the transmitter filament is initiated. • Circuit breakers associated with various rectifiers such as grid bias, screen voltages etc. Protect the rectifiers and associated equipment against over currents. • All the vital currents of the tubes and stages are monitored and indicated by means of panel meters. This is to monitor abnormality if any on the various operating conditions. • Also current operated over load relays are provided in the cathode, screen grid and anode circuits to protect the tubes and the associated rectifiers in case any of these respective currents exceed a predetermined value. The operation of over load relays are indicated by means of flags or latched lamps • The standing wave ratio on the load side is monitored suitably and signal is used to trip the transmitter anode
  36. 36. voltage in case of VSWR is higher than the predetermined value. • Spark detectors are provided in various cubicles to ensure the tripping sequence in case of sparking to prevent damage to the equipments. • Normally the over currents are counted over a period of time and if number of over currents occur in a short interval the transmitter is tripped up to the filament. • In addition to the above safety provisions spark gaps and varistor provided at various high voltage points offer protection to the equipment against high RF voltage. • In some of the transmitter a crow bar device is provided to short circuit the stored energies in the power supply circuit in case of over load. This provision is to protect the high power tubes. 3. Safety for the personnel Since very high voltages are encountered in transmitters the operating personnel are to be protected by coming into contact with these high voltages accidently. The safety interlocking generally comprises of: • An earthing switch which earths all the high voltage supplies before the access to the cubicles keys are allowed. • A key exchange panel where the key to the transmitter cubicles can be utilized only after the earthing switch is put ON. The earthing switch is interlocked in the bias circuit and hence the operation of the earth switch automatically switches OFF up to bias. This provision ensures that the cubicle doors can be opened only when the bias and medium voltages are switched OFF and earthed through the earthing switch.
  37. 37. • In addition to the above, earth hooks are provided at various parts of the cubicles and high voltage equipment area. The operating professional are too short through these hooks are high voltage points before any work is undertaken in these equipment. • Some of the transmitters are also provided with additional shorting switches in the cubicles which short the supplies in the cubicle as soon as the door is opened. 4. Indication Lamps The indication lamps are provided in the transmitter to indicate the status of switching ON of the transmitter as well as to indicate the occurrence of over load etc. These indication lamps are provided to help the fault diagnosis. COOLING TECHNIQUES 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 transmitter at present. • Air cooling
  38. 38. • Vapour cooling • 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) Vapour Cooling System This system is used in 100 kW BEL Transmitters. For very high power valves and efficient cooling, air cooling is not sufficient. Hence some of the valves like BEL 15000, BEL 75000 etc. are cooled by vapour cooling. (Hence called Vaptron). Here the principle of heat required to convert water into steam at its boiling point is used (Latent heat of steam). The valves are kept in an in-tight water container filtered and de-ionized water. This water has high resistivity and comes in contact with anode. The water containers called "Boilers" are provided with inlet and outlet pipes.
  39. 39. Fig. 1 Block Diagram of Vapor-Cooling System The inlet pipes are interconnected at the bottom to keep the water level same in all the boiler and the outlet pipes are joined at the top which provides passage of the steam to the condensing equipment known as Heat Exchanger. The heat produced at the anode of the vacuum tubes is absorbed by water and gets converted into steam. The steam thus produced goes up through the glass-tube to the steam pipe and to the heat exchanger mounted on top of the transmitter room. The condenser is made of copper tubes. This is a mono-block fine tube moulded from copper in extrusion moulding system and has high terminal conductivity. Steam flows inside the tube and cool air is forced outside through the fins and the action of heat exchange takes place. Now the steam is condensed to water and the cooled water flows down through the water pipe due to gravity back to the boiler. c) Condensed Vapour Cooling in HMB-140BEL 100 kW
  40. 40. MW XTR: In BEL/BBC 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. High velocity water flows through the valve jacket 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 cylindrical 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. The demineralized water is pumped by pumps (one in circuit and one as standby) from the water tank to the PA and modulator tubes through the water piping. At the inlet/outlet of each tube, a double ball valve is provided to facilitate shutting off water supply when the valve is required to be changed. Except for changing the valve, this should be kept in open condition always. (Lever in the horizontal position).
  41. 41. Fig.2 Water Flow Circuit 100 kW HMB 140 ANTENNA TUNING UNIT (ATU) 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
  42. 42. 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. ATU can be designed in a number of ways. The method used may be different in different conditions. Criteria depend on the requirements. Especially when directional antenna system is employed by splitting power to different antenna, the phase angle of the network is the most important parameter. In other cases mostly, simplicity and safety against lightning is important. One of the methods adopted in the past was the reactive component of the mast impedance is neutralised, by putting opposite reactive component of same value in series at mast end side, to make the mast impedance purely resistive (i.e. for inductive mast the series reactance should be capacitive and vice versa). Then the resistive part of the mast impedance can be matched to the feeder line impedance by selecting a suitable matching network. This matching network can be L, T or network, and can be designed as phase lag or phase lead type. In these cases if a capacitor is put in series, there is every possibility of puncturing of capacitors due to lightning. Hence this method is being discouraged. The second method, which is most commonly used now, is first to convert the antenna impedance into a parallel combination. Most of the bridges used to measure the mast impedance measure it in the series form. At AIR, Thrissur we are using the first method and the matching network used is a pi network. Here the mast impedence is 56+81.60j ohms and the feeder impedence is 121.60 ohms. The possibility for puncturing the capacitor bank is minimized by
  43. 43. installing a lightening arrest in the self-radiating mast. This will keep the pi network arrangement intact from the threats of lightening. The self-radiating mast is a part of the ATU. Its base is separated from the ground using porcelain insulator. This prevents the signal from earthing. The mast is held vertical using stay wires. For a certain area, Copper bars are laid radiantly on the earth surface for the sake of proper earthing. AKASHAVANI - In near future…. • DIGITAL RADIO MONDIALE (DRM) DRM is the only universal non-proprietary digital radio system for the short wave, medium wave and long wave AM broadcast bands. Many existing transmitters can be easily modified with an inexpensive upgrade to carry DRM signals, enabling a single tower to broadcast over a large geographic area so that the listeners can receive the same station with near FM quality sound. Commercial and public international broadcasters, as well as national radio networks and local radio stations, have begun transmitting regular DRM broadcasts and special programs. The frequencies required are exposed to highly fluctuating propagation conditions. In this frequency range the waves are influenced by the ionosphere and they are reflected depending on the time of day, season and the relative number on sunspots. Thanks to the digital standards the AAC+ audio coding and the COFDM technology, signal transmission now promises outstanding quality as compared to conventional
  44. 44. AM broadcasting. DRM can use the existing band plan and the frequency grid present in the medium wave, short wave and long wave. The technology therefore facilitates the transmission from analog to digital transmission technology, which can cover large areas end to end at a favourable price. Due to external influences on the transmission conditions, the transmission parameters can be adjusted to match the propagation conditions. One advantage from using DRM plus is significantly lower power consumption of the transmission systems. • DIGITALIZATION Digitalization of program production facilities, transmission facilities and uplink stations has been undertaken to ensure good quality convergence-ready content, which will also support interactive radio services like news on phone, music on demand etc. • FM RADIO STATION AKASHAVANI, Thrissur is about to launch a new FM radio station which is transmitted at a frequency 111 MHz This project which is undergoing the paper level proceedings is expected to be trial running very soon. This project if brought into existence will help in increasing the listenership of AKASHAVANI by means of broadcasting a set of listener friendly programs.
  45. 45. • Computerization of AIR stations and offices is in progress to facilitate online exchange of information and improvement of efficiency. Stations with digital equipments including computerized hard disk based workstations for recording, dubbing, editing and playback facilities etc. are being provided at all the major stations. • Control archives and regional archives have valuable collection of historical and cultural important recordings in their libraries. The audio program material available on analog audio tapes is being converted into digital and stored on CD’s. It is proposed to provide control archive at Delhi and regional archives at four stations of AIR such as Mumbai, Kolkata, Chennai and Hyderabad connected in the network so that the archival material can be shared among the stations. Provision of digitalization of analog material & refurbishing the old recordings at these stations for release in market in CD format is also proposed. • Digitalization and automation of NEWS gathering NEWS production and NEWS broadcast activity has already been introduced at NEWS services division of AIR in Delhi and regional NEWS units. Under IInd plan it is proposed fully digitalize and have NEWS room automation at all the regional NEWS units by providing NEWS automation software, computer server and workstations etc. This facility is further extended to seven more regional stations of AIR. • Up-linking facility used for distribution of programs through satellite is being digitalized by augmenting the existing capital earth stations with digital uplinks. New digital captive
  46. 46. earth stations (Uplinks) have been installed at Varanasi, Rohtak, Leh, Dehradun, Silchar and Aurangabad. Digital downlink facilities have also been installed at most of the stations. • Analog microwave links, used for transportation of programs from studio to transmitter, are proposed to be replaced by digital microwave links. New digital microwave STL equipments have been installed at Cuddaph, Vijayawada, Srinagar, Patna, Jodhpur, Chandigarh, Jaipur, Kota and Aurangabad. Twenty one numbers of Portable MSS terminals have been procured for NEWS gathering in the network. • Digitalization of studio which includes digitalization of switching consoles, audio processor, cables, amplifiers, equalizers, audio ports, jacks etc. CONCLUSION Broadcasting, in its significance, reach and impact, constitutes the most powerful medium of mass communication. In India, All India Radio operates this service, over a network of broadcasting stations located over the country. Starting with 6 broadcasting stations in 1947, the AIR today has a network of 82 broadcasting stations. AIR's programed pattern combines three main elements: a national channel providing programmes of countrywide interest and significance, a zonal service from each of the four metropolitan centers (Delhi, Bombay, Calcutta and Madras); and regional services from individual stations each catering to the needs and interests of its respective area. Currently there are two complexes in AIR UDAIPUR, Studio
  47. 47. cum office complex and the Transmitter complex. In studio complex, there are three studios, MUSIC, TALK and the PLAYBACK. The first two together called to be the recording studio facilitates sound recording and mixing whereas the latter helps in coordinating the programs, announcements and advertisements. The Studio console is the major equipment used in the STUDIO CONTROL ROOM. The various inputs to the console are the programmes from various studios, the programmes that are received using a C BAND receiver which is broadcasted from Delhi and the programmes that are received via an ISDN link from Calicut and Thiruvananthapuram. The Outputs from the console is taken through two master amplifiers among which one is active at a time. This output is directed to the STUDIO TRANSMITTER LINK (STL). This further route the programs to TRANSMITTER at Avanoor. The source to the transmitter complex is also realized using MICROWAVE, FM TRANSMITTER, ISDN or BSNL DIAL UP links. In AIR UDAIPUR, transmitter performs amplitude modulation in which the information is added to the radio signal by varying its amplitude. The transmission frequency is at 630 kHz generated by a quartz crystal oscillator. RF driver stage provides the driving power required to develop an output of 100 KW to the final amplifier. High level anode modulation is used using a class B modulator stage. The AF Stage supply the Audio power required to amplitude modulates the final RF 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. The transmitter complex is also equipped with various control and interlocking systems. Due to the high power evolved, cooling systems are also provided, utilizing ionized air, vapor and condensed vapor cooling techniques. The transmitter complex is also equipped with a HT room for providing the required power supplies.
  48. 48. There is also an ATU to match the feeder line impedance to the mast impedance of MW transmitter for maximum transmission of power, located between mast base and the feeder line. The information in its modulated form is further given to the self- radiating mast provided with proper earthing.