Short Questions and Answers on Radio Transmitters
Q.1. What is meant by low level modulation?
Ans. In low level modulation system, amplifier efficiency and bandwidth preservation are important factors since audio signal is having low power. This is especially used for laboratory purpose.
Q.2. Write the main requirements of AM broadcast transmitters.
Ans. The main requirements of AM broadcast transmitters may be listed as under:
- Must produce output within the limits of the 5 kHz audio bandwidth.
- Should have highest possible fidelity.
- The modulator circuits in the transmitter must produce a linear modulation function.
- Tuned class C amplifier must provide sufficient power gain to drive the final power amplifier.
- Antenna systems for AM transmitters must be located at some point remote from the studio operations.
Q.3. What are Forms of Amplitude Modulation?
- DSBFC (A3E): Double Side Band Full Carrier
- DSBSC: Double Side Band Suppressed Carrier
- SSBFC: Single Side Band Full Carrier
- SSBSC: Single Side Band Suppressed Carrier
- SSBRC: Single Side Band Reduced Carrier: An attenuated carrier is reinserted into the SSB signal, to facilitate receiver tuning and demodulation. This system is also referred to as pilot carrier system.
- ISB: Independent Sideband: Two independent sidebands can transmit different information. Carrier is suppressed.
- VSB: Vestigial Side Band: In this system, a vestige, i.e., a trace, of the unwanted sideband is transmitted, usually with a full carrier. It is used for TV transmissions all over the world.
Q.4. What is meant by Independent Sideband Systems?
Ans. For Single-sideband transmission, the carrier and one sideband are suppressed from the signal. It is possible to replace the suppressed sideband with another sideband of information created by modulating a different input signal on the same carrier, giving what is known as independent sideband (ISB) transmission.
Q.5. What is the difference between high and low level modulation?
Ans. In low level modulation, the modulation takes place at a low power level. The power level of modulated signal is then raised to the adequate value, using amplifiers. In high level modulation, the signals are amplified first and then modulation takes place at a high power level.
Q.6. What type of modulation is used in TV transmission?
Ans. Low level IF modulation.
Q.7. What type of amplifier is used for collector modulator?
Ans. Class C amplifier.
Q.8. Where is the operating point of class C amplifier located?
Ans. Below cut off.
Q.9. How much is the conduction angle of collector current of the class C amplifier?
Ans. Less than 180° or π radians. The collector current flows in the form of pulses.
Q.10. Why are the “linear amplifiers” used after modulator in low level AM transmitters?
Ans. To amplify the modulated signal without distorting its shape.
Q.11. Name different linear amplifiers.
Ans. Class A, B and AB are linear amplifiers.
Q.12. In collector modulation how do you obtain the AM wave across the tank circuit from unidirectional collector current pulses?
Ans. When current pulses are supplied to the tank circuit, it produces damped voltage oscillations. The amplitude of these voltage oscillations is proportional to the amplitude of current pulses supplied. This principle is used for producing AM in collector modulated circuit.
Q.13. Why is class C amplifier used in high level modulator?
Ans. Class C amplifier is used as the final modulator in high level modulation due to its high efficiency (greater than 95%).
Q.14. What type of modulation is used in TV for picture and sound transmission?
Ans. In TV, they use AM with VSB for picture and FM for sound.
Q.15. What type of propagation is used for MW band radio transmission?
Ans. Ground wave propagation.
Q.16. What type of propagation is used for SW band radio transmission?
Ans. Sky wave propagation.
Q.17. What type of propagation is used for TV transmission?
Ans. Space wave or line of sight propagation.
Q.18. What type of propagation is used for satellite communication?
Ans. Space wave propagation.
Q.19. Which vacuum tube is used for the plate modulated and grid modulated class C amplifier?
Q.20. Name the various terminals of a triode.
Ans. Plate (anode), grid and cathode. Grid is the controlling terminal.
Q.21. What are the advantages of grid modulator?
Ans. Low modulating power is required.
Q.22. State the disadvantages of grid modulator.
Ans. Low efficiency, low power output, harmonics are generated due to the nonlinear transfer characteristics.
Q.23. How is plate modulator superior to the grid modulator?
Ans. Because, it does not produce any distortion, it has higher efficiency and higher output power as compared to the grid modulator.
Q.24. Why is grid leak bias is used in plate modulated class C amplifier?
Ans. For controlling the grid current.