Microwave Engineering

Viva Questions:

Q1) What is Transit Time ?

Ans: It is the time taken by the electrons to travel from cathode to anode.

Q2) 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 device will finally limit the output which may even be zero.

Q3) What is the operating principle of Reflex Klystron ?

Ans: It works on the principle of velocity modulation and current modulation.

Q4) What is Velocity Modulation ?

Ans: Variations in velocities of electrons in the electron beam is called velocity modulation.

Q5) What is Current Modulation ?

Ans:  Variation in the current density of electron beam is called current modulation.

Q6) How does bunching occur in reflex klystron ?

Ans: A reflex electron while passing the gap when the gap voltage is zero passes 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 than the reference electron to return to gap. An electron leaving the gap late will face slightly negative voltage and gets retarded. So it returns back after a shorter travel in repeller space. Thus all the electrons will arrive in bunch. This is Bunching effect and this is how bunching takes place in reflex klystron.

Q7) What modes are generally used in Reflex Klystron ?

Ans: 1suffix(3/4) and 2suffix(3/4) are the mostly used modes in reflex klystron. Its modes are generally Nsuffix(3/4) where N is any Integer.

Q8) What is the operating frequency and power output of reflex klystron ?

Ans: Frequency Range: 4GHz to 200GHz 
        Output Power: maximum 3W in X Band to 10mW at 200GHz

Q9) What is the maximum theoretical efficiency by reflex klystron ?

Ans: 22.7% for 1suffix(3/4) mode

Q10) What is Mechanical Tuning and Electronic Tuning in reflex klystron ?

Ans: Variation in frequency of resonance of cavity by varying its dimensions by mechanical methods like adjusting screws is called Mechanical tuning. And Variation of frequency by adjusting repeller voltage is called Electronic tuning.