Microwave Engineering deals with the design of microwave circuits, systems & components. Various fundamental principle are used for the analysis and design purpose with the application of field measurement techniques. Reflex klystron is a microwave device that works on principle of velocity modulation in addition to current modulation. Reflex klystron is also known as a ‘Sutton’ klystron. This name is given by the name of its inventor Robert Sutton, It allow the passage of electron beam through a single resonant cavity. Now understand the concept of Reflex klystron in microwave engineering in details through following questions and answers.
Question and Answers on Reflex Klystron
1. Why can’t conventional tubes be used at microwave frequencies?
Ans. Conventional tubes can’t be used at microwave frequencies because of transit time effect. Lead inductance and inter electrode capacitance of the devices will finally limit the output which may even be zero.
2. What is transit time?
Ans. It is the time taken by the electrons to travel from cathode to anode.
3. What is the operating principle of reflex klystron?
Ans. It works on the principle of velocity modulation and current modulation. The operating principle of reflex klystron is clearly explained from the following figure:
4. What is velocity and current modulation in a reflex klystron?
Ans. Variation in velocities of the electrons in the electron beam is called velocity modulation variation in the current density of the electron beam is referred to as current modulation.
5. How does bunching occur in a reflex klystron?
Ans. A reference electron passing the gap when the gap voltage is zero travels with no change in velocity. An electron leaving the gap earlier during slightly positive voltage would travel further into repeller space and hence would take longer time then the reference e to return to the gap. An electron leaving the gap later will face slightly negative voltage & gets retarded. So it returns back after a shorter travel in the repeller space. Thus all the electrons would arrive back to the gap in bunches. Bunching around reference electron takes place once per cycle of RF oscillations.
6. Why do different modes of operation exist for a reflex
ans. There are several combinations of repeller voltage and anode voltage that provide favourable conditions for bunching. Accordingly there may exist several modes of operation, expressed by N + ¾ where N is an integer.
7. What modes are generally used in a reflex klystron?
Ans. 1 ¾ and 2 ¾ are the most commonly used modes in a practical reflex klystron.
8. What is the operating frequency and power output of a reflex klystron?
Ans. Frequency range: 4GHz – 200GHz Output power: maximum 3W in X-band to 10mW at 220GHz.
9. What is the maximum theoretical efficiency of the reflex klystron?
Ans. 22.7% for 1 ¾ mode.
10. What is mechanical tuning and electronic tuning in a reflex klystron?
Ans. Variation in frequency of resonance of cavity by varying its dimension by a mechanical
method like adjusting screws is called as mechanical tuning. Variation of frequency by the method of adjusting repeller voltage is called electronic tuning.
11. What is the range of frequency variation of reflex klystron in the X-band by the above two methods?
12. What are applications of reflex klystron?
Ans. They are used as
1) signal source in microwave generators.
2) Local oscillators in receivers.
3) Pumposcillators in parametric amplifiers.
13. Leaving reflex klystron, what other devices are used as microwave signal sources?
Ans. 1) Magnetron oscillator
2) Backward wave oscillator
3) Gunn oscillator
4) IMPATT diode oscillator
5)TRAPPAT diode oscillator
14. Is Gunn diode a p-n junction?
Ans. No. It isn’t.
15. What is Gunn Effect?
Ans. When the voltage gradient across a slice of GaAs exceeds about 3300V/cm, negative
resistance will develop and oscillations occur if the slice is connected to a suitable tank circuit.
16. What are the materials used in the construction of Gunn diode?
Ans. GaAs (Gallium Arsenide), InP (Indium Phosphate) doped in Si.
17. What is negative resistance?
Ans. A device is said to have negative resistance characteristics if, during its operation an increase in the voltage across it leads to decrease in its current.
18. Can any negative resistance device be used for amplification?
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