05-04-2016, 04:44 PM
VOCATIONAL TRAINING AT DOORDARSHAN KENDRA, RANCHI, JHARKHAND report
INTRODUCTION OF DURDARSHAN, RANCHI, JHARKHAND:
During 1975-76, the Satellite Instruction Television Experiment (SITE) project started with an aim to provide education to rural people in six states namely Rajasthan, Madhya Pradesh, Bihar, Andhra Pradesh, Karnataka and Orissa. Under SITE continuity plan during 1976-82, six terrestrial transmitters were installed at different places and Muzzaffarpur was one of them. When INSAT-IB was installed, some more terrestrial transmitters with programme generating facilities were installed during 1982 to produce “Area specific programmes” and Ranchi came into being under the scheme.
Doordarshan Kendra, Ranchi was commissioned on priority basis on 25th September 1984 to accelerate the socio-economic and cultural development of the people including tribal of this area.
The Kendra is situated on Ratu Road over an area of 2.75 acres. It accommodates one double storied transmitter building, one double storied studio building and director’s office in a separate building.
The Kendra produced “Area Specific Programme” till 2nd April 2002 catering to a single ethno-cultural zone of Jharkhand. On 2nd April 2002, the Kendra became a regional unit with its coverage extending to entire Jharkhand. On this very day, another remarkable event in the form of telecast of regional news began from the Kendra. These two events were inaugurated by the Hon’ble Minister of Information and Broadcasting of Government of India Smt. Sushma Swaraj, programmes catering to needs of women, children, youth, tribal and on fields like agriculture, health, music and dance, literature, industrial development were got prioritized emphasizing the state’s economy, culture and tradition.
Form the day of its commissioning the Kendra has made remarkable progress in producing programmes of high standard with state’s rich talents, innovative and well researched programmes like kalyani, krishi darshan,flagship programmes are being introduced from time to time with increase in the duration of transmission time of the Kendra.
History:
The first primitive radio transmitters (called Hertzian oscillators) were built by German physicist Heinrich Hertz in 1887 during his pioneering investigations of radio waves. These generated radio waves by a high voltage spark between two conductors. These spark-gap transmitters were used during the first three decades of radio (1887-1917), called the wireless telegraphy era. Short-lived competing techniques came into use after the turn of the century, such as the Alexanderson alternator and Poulsen Arc transmitters. But all these early technologies were replaced by vacuum tube transmitters in the 1920s, because they were inexpensive and produced continuous waves, which could be modulated to transmit audio (sound) using amplitude modulation (AM) and frequency modulation (FM). This made possible commercial radio broadcasting, which began about 1920. The development of radar before and during World War 2 was a great stimulus to the evolution of high frequency transmitters in the UHF and microwave ranges, using new devices such as themagnetron and traveling wave tube. In recent years, the need to conserve crowded radio spectrum bandwidth has driven the development of new types of transmitters such as spread spectrum .
Description:
A transmitter can be a separate piece of electronic equipment, or an electrical circuit within another electronic device. A transmitter andreceiver combined in one unit is called a transceiver. The term transmitter is often abbreviated "XMTR" or "TX" in technical documents. The purpose of most transmitters is radio communication of information over a distance. The information is provided to the transmitter in the form of an electronic signal, such as an audio (sound) signal from a microphone, a video (TV) signal from a TV camera, or inwireless networking devices a digital signal from a computer. 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. This process is called modulation. The information can be added to the carrier in several different ways, in different types of transmitter. In an amplitude modulation (AM) transmitter, the information is added to the radio signal by varying its amplitude (strength). In a frequency modulation (FM) transmitter, it is added by varying the radio signal's frequency slightly. Many other types of modulation are used.
The antenna may be enclosed inside the case or attached to the outside of the transmitter, as in portable devices such as cell phones,walkie-talkies, and auto keyless remotes. In more powerful transmitters, the antenna may be located on top of a building or on a separate tower, and connected to the transmitter by a feed line (transmission line).
Crystal Oscillator and Buffer Stage:
The crystal oscillator with buffer stage is generally kept together and is shielded by a metal cover to isolate from other circuits.
This crystal oscillator employs a pentode tube 6 AU 6 or its equivalent, connected as a triode. The frequency of oscillation is controlled by a quartz crystal and by a variable trimmer capacitor.
The frequency of the medium wave transmitter should be highly stable. For medium wave transmitter operating in the range of 540 kHz to 1602 kHz, the variation of a transmitter frequency should be within a tolerance of + 10 Hertz. To maintain a high stability of the transmitter frequency it is necessary that the oscillator should oscillate at a particular frequency against variations in voltage and ambient temperature. Hence the crystal is kept in a constant temperature ovens whose temperature is controlled by a thermostat and maintained at a 75o + 1.5o C.
The oscillator frequency changes considerably under initial transient condition, that is when power is switched ON. However, it is essential to keep it always ready at a stable condition. To facilitate this a separate power supply is provided to feed the oven which can be switched ON and OFF with the help of a snap switch S3 (Oven) located on the AE panel of the transmitter. Two crystal units X1 and X2 housed separately in different ovens Z1 and Z2 viz. a normal and a stand by unit are provided. Either one of them can be selected by means of change over switch S2. However, both the ovens Z1 and Z2 are kept ON all the time.
Exciter:
This stage is operated as a class - C amplifier, employing air cooled tetrode type BEL 400 and drives P.A. stage. Screen supply is taken from plate supply. The output is a tuned circuit consists of a fixed capacitor C 29 (Value of C29 depends on the operating frequency) and coil L3. L3 is having a flipper, through it, fine tuning can be made.
This stage is modulated about 10 to 20%. A small secondary tap from the modulation transformer supplies the necessary audio and super-imposes on the DC Plate supply. When the triodes are anode modulated, the grid must be overdriven in the carrier condition in order that the drive level will be adequate to sustain the peak anode current at 100% modulation. Alternatively the drive must be modulated. Hence the 10 to 20% modulation. With tetrode the same effect is achieved by modulating the screen enabling the anode current peaks to be attained with the same drive level as that required for the carrier only condition. To some extent this ceases the grid dissipation limit.
Power Amplifier Stage:
This is a class - C power amplifier obtaining the required output by means of three parallel connected forced air cooled, directly heated triode tubes type BEL 3000. As a triode tube is used in this stage, neutralization technique is adopted to neutralize, the grid-plate capacitance. The output circuit is formed by PI () section and 'L' section made up of coils and condensers. There is a variable coil to tune the output. A second harmonic filter is connected at the output which attenuates the harmonics. This filter is a simple L C circuit tuned to the second harmonic frequency. The output circuit also matches the plate impedance of about 1100 ohms to the feeder impedance of 230 ohms, which is carried out at the time of installation of the transmitter using Impedance Bridge.
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. In addition, variable capacitor is used for varying the frequency of the crystal within a few cycles. The oscillator frequency is divided by 2, 4, or 8 which is selected by jumpering the appropriate terminals. The oscillator Unit gives 3 outputs, one each for RF output, RF Monitoring and RF output indication.
High Pass Filter:
The audio input from the speech rack is fed to active High Pass Filter. It cuts off all frequencies below 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:
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 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.
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 litres/min. 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 provide protection against excessive water temp. by tripping the transmitter up to stand-by if the temperature of the water exceeds 70o C.
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.
INTRODUCTION OF DURDARSHAN, RANCHI, JHARKHAND:
During 1975-76, the Satellite Instruction Television Experiment (SITE) project started with an aim to provide education to rural people in six states namely Rajasthan, Madhya Pradesh, Bihar, Andhra Pradesh, Karnataka and Orissa. Under SITE continuity plan during 1976-82, six terrestrial transmitters were installed at different places and Muzzaffarpur was one of them. When INSAT-IB was installed, some more terrestrial transmitters with programme generating facilities were installed during 1982 to produce “Area specific programmes” and Ranchi came into being under the scheme.
Doordarshan Kendra, Ranchi was commissioned on priority basis on 25th September 1984 to accelerate the socio-economic and cultural development of the people including tribal of this area.
The Kendra is situated on Ratu Road over an area of 2.75 acres. It accommodates one double storied transmitter building, one double storied studio building and director’s office in a separate building.
The Kendra produced “Area Specific Programme” till 2nd April 2002 catering to a single ethno-cultural zone of Jharkhand. On 2nd April 2002, the Kendra became a regional unit with its coverage extending to entire Jharkhand. On this very day, another remarkable event in the form of telecast of regional news began from the Kendra. These two events were inaugurated by the Hon’ble Minister of Information and Broadcasting of Government of India Smt. Sushma Swaraj, programmes catering to needs of women, children, youth, tribal and on fields like agriculture, health, music and dance, literature, industrial development were got prioritized emphasizing the state’s economy, culture and tradition.
Form the day of its commissioning the Kendra has made remarkable progress in producing programmes of high standard with state’s rich talents, innovative and well researched programmes like kalyani, krishi darshan,flagship programmes are being introduced from time to time with increase in the duration of transmission time of the Kendra.
History:
The first primitive radio transmitters (called Hertzian oscillators) were built by German physicist Heinrich Hertz in 1887 during his pioneering investigations of radio waves. These generated radio waves by a high voltage spark between two conductors. These spark-gap transmitters were used during the first three decades of radio (1887-1917), called the wireless telegraphy era. Short-lived competing techniques came into use after the turn of the century, such as the Alexanderson alternator and Poulsen Arc transmitters. But all these early technologies were replaced by vacuum tube transmitters in the 1920s, because they were inexpensive and produced continuous waves, which could be modulated to transmit audio (sound) using amplitude modulation (AM) and frequency modulation (FM). This made possible commercial radio broadcasting, which began about 1920. The development of radar before and during World War 2 was a great stimulus to the evolution of high frequency transmitters in the UHF and microwave ranges, using new devices such as themagnetron and traveling wave tube. In recent years, the need to conserve crowded radio spectrum bandwidth has driven the development of new types of transmitters such as spread spectrum .
Description:
A transmitter can be a separate piece of electronic equipment, or an electrical circuit within another electronic device. A transmitter andreceiver combined in one unit is called a transceiver. The term transmitter is often abbreviated "XMTR" or "TX" in technical documents. The purpose of most transmitters is radio communication of information over a distance. The information is provided to the transmitter in the form of an electronic signal, such as an audio (sound) signal from a microphone, a video (TV) signal from a TV camera, or inwireless networking devices a digital signal from a computer. 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. This process is called modulation. The information can be added to the carrier in several different ways, in different types of transmitter. In an amplitude modulation (AM) transmitter, the information is added to the radio signal by varying its amplitude (strength). In a frequency modulation (FM) transmitter, it is added by varying the radio signal's frequency slightly. Many other types of modulation are used.
The antenna may be enclosed inside the case or attached to the outside of the transmitter, as in portable devices such as cell phones,walkie-talkies, and auto keyless remotes. In more powerful transmitters, the antenna may be located on top of a building or on a separate tower, and connected to the transmitter by a feed line (transmission line).
Crystal Oscillator and Buffer Stage:
The crystal oscillator with buffer stage is generally kept together and is shielded by a metal cover to isolate from other circuits.
This crystal oscillator employs a pentode tube 6 AU 6 or its equivalent, connected as a triode. The frequency of oscillation is controlled by a quartz crystal and by a variable trimmer capacitor.
The frequency of the medium wave transmitter should be highly stable. For medium wave transmitter operating in the range of 540 kHz to 1602 kHz, the variation of a transmitter frequency should be within a tolerance of + 10 Hertz. To maintain a high stability of the transmitter frequency it is necessary that the oscillator should oscillate at a particular frequency against variations in voltage and ambient temperature. Hence the crystal is kept in a constant temperature ovens whose temperature is controlled by a thermostat and maintained at a 75o + 1.5o C.
The oscillator frequency changes considerably under initial transient condition, that is when power is switched ON. However, it is essential to keep it always ready at a stable condition. To facilitate this a separate power supply is provided to feed the oven which can be switched ON and OFF with the help of a snap switch S3 (Oven) located on the AE panel of the transmitter. Two crystal units X1 and X2 housed separately in different ovens Z1 and Z2 viz. a normal and a stand by unit are provided. Either one of them can be selected by means of change over switch S2. However, both the ovens Z1 and Z2 are kept ON all the time.
Exciter:
This stage is operated as a class - C amplifier, employing air cooled tetrode type BEL 400 and drives P.A. stage. Screen supply is taken from plate supply. The output is a tuned circuit consists of a fixed capacitor C 29 (Value of C29 depends on the operating frequency) and coil L3. L3 is having a flipper, through it, fine tuning can be made.
This stage is modulated about 10 to 20%. A small secondary tap from the modulation transformer supplies the necessary audio and super-imposes on the DC Plate supply. When the triodes are anode modulated, the grid must be overdriven in the carrier condition in order that the drive level will be adequate to sustain the peak anode current at 100% modulation. Alternatively the drive must be modulated. Hence the 10 to 20% modulation. With tetrode the same effect is achieved by modulating the screen enabling the anode current peaks to be attained with the same drive level as that required for the carrier only condition. To some extent this ceases the grid dissipation limit.
Power Amplifier Stage:
This is a class - C power amplifier obtaining the required output by means of three parallel connected forced air cooled, directly heated triode tubes type BEL 3000. As a triode tube is used in this stage, neutralization technique is adopted to neutralize, the grid-plate capacitance. The output circuit is formed by PI () section and 'L' section made up of coils and condensers. There is a variable coil to tune the output. A second harmonic filter is connected at the output which attenuates the harmonics. This filter is a simple L C circuit tuned to the second harmonic frequency. The output circuit also matches the plate impedance of about 1100 ohms to the feeder impedance of 230 ohms, which is carried out at the time of installation of the transmitter using Impedance Bridge.
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. In addition, variable capacitor is used for varying the frequency of the crystal within a few cycles. The oscillator frequency is divided by 2, 4, or 8 which is selected by jumpering the appropriate terminals. The oscillator Unit gives 3 outputs, one each for RF output, RF Monitoring and RF output indication.
High Pass Filter:
The audio input from the speech rack is fed to active High Pass Filter. It cuts off all frequencies below 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:
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 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.
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 litres/min. 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 provide protection against excessive water temp. by tripping the transmitter up to stand-by if the temperature of the water exceeds 70o C.
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.