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Cavity Resonators Questions and Answers

Direction for [ Question No: 1 To 3 ] :

Read the paragraph and answer the questions :

Microwave tubes are used as microwave amplifiers and oscillators. Three general type of microwave tubes in which third type tubes are important because in these tubes there is an interaction between an electron and an RF field is continuous. The Travelling Wave Tube (TWT) is the prime example of this interaction. It is an amplifier, whose oscillator counter part is called Backward Wave Oscillator (BWO). The second sub-group consists of tubes in which a magnetic field ensures a constant electron beam - RF field interaction, and this is complemented by the Cross-Field Amplifier (CFA). Multicavity Klystron is used as high and very high power amplifiers in the UHF and microwave ranges. The frequency range covered is from about 250 MHz to over 95 GHz. The reflex-Klystron is a low power microwave oscillator. It is assumed that oscillations are started by switching transients. For oscillations to be maintained the transient time in the repeller space cycle is given as \(T = \left(n + \frac{3}{4}\right)\) where n is an integer, each value of n is said to correspond to different reflex klystron mode. Reflex Klystrons with integral cavities are available in the frequency range 4 to 200 GHz.

1.

One of the reasons why vacuum tubes eventually fail at microwave frequencies in that their

Exam Year(s)

UGC NET - 2014,

Answer: D

The answer is that the noise figure increases. At microwave frequencies conventional tubes fail because of transit time (which becomes comparable to the period, not too short), inter-electrode capacitance and lead inductance, and because their noise figure increases.

Option A says the transit time becomes too short, but it is too long. Option B says the shunt capacitive reactance becomes too large, but at high frequency it becomes small and shunts the signal away. Option C says the series inductive reactance becomes too small, but it grows with frequency.

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2.

The Multicavity Klystron

Exam Year(s)

UGC NET - 2014,

Answer: A

The multicavity klystron is not suited as a low-level amplifier because of its noise. Noise from the beam, which is velocity-modulated and bunched, becomes large relative to a small input, so the klystron is used as a high-power amplifier.

The rapid-transit-time statement is wrong because a reflex klystron's repeller voltage sets the transit time of the repeller space and the multicavity klystron has no repeller. A multicavity klystron can be pulsed, and a long transit time through the buncher cavity would lower, not ensure, the current modulation.

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3.

Indicate the false statement. Klystron amplifiers may use intermediate cavities to

Exam Year(s)

UGC NET - 2014,

Answer: A

The false statement is the first one. Intermediate cavities in a multicavity klystron do not prevent the oscillations of two-cavity klystrons. They are added to increase the gain, the bandwidth and the efficiency, because they bunch the beam more strongly, and they are tuned slightly apart (stagger tuning) to widen the band.

The other three are therefore true: more cavities improve the power gain, increase the bandwidth and raise the efficiency of the klystron.

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Direction for [ Question No: 4 To 4 ] :

Directions : Q. Nos. 61 to 70, the following items consist of two statements, one labelled the "Assertion (A)" and the other labelled the "Reason (R)". You are to examine these two statements carefully and decide if the Assertion (A) and the Reason (R) are individually true and if so, whether the Reason (R) is a correct explanation of the Assertion (A). Select your answers to these items using the codes given below and mark your answer accordingly.

4.

Assertion (A) : Magnetron are not cross field devices.

Reason (R) : They make use of electric and magnetic fields simultaneously. The fields are perpendicular to each other.

Exam Year(s)

UGC NET - 2013,

Answer: D

The assertion is false and the reason is true, the fourth option. Magnetrons are cross field devices, since the electric field between the cathode and anode is perpendicular to the magnetic field of the magnet, so the assertion that they are not is false.

The reason states that they use electric and magnetic fields together, at right angles to each other, which is just what makes them cross field devices, so it is true. The options calling the assertion true are wrong.

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Direction for [ Question No: 5 To 5 ] :

Directions : The following items consist of two statements, one labelled the "Assertion" (A) and other labelled the "Reason" (R). You are to examine these two statements carefully and decide if the Assertion (A) and the Reason (R) are individually true and if so, whether the Reason is a correct explanation of the Assertion. Select your answers to these items using the codes given below and mark your answer accordingly.

5.

Assertion (A) : A reflex klystron makes use of a single cavity.

Reason (R) : The single cavity performs the functions of buncher as well as catcher cavity.

Exam Year(s)

UGC NET - 2013,

Answer: A

Both statements are true and the reason explains the assertion, which is option (A). A reflex klystron has one resonant cavity. The beam passes through it, is reflected by the repeller and returns through the same cavity, so the same cavity first bunches the electrons and later extracts energy from the bunched beam, acting as buncher and catcher. Options (B), (C) and (D) deny either the assertion or the explanation, but the single-cavity operation is exactly the reason for the single cavity.

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6.

Match the following lists :

List - IList - II
(a) Magnetron(i) Detector
(b) Pin diode(ii) Bunching
(c) Klystron(iii) Low Power Oscillator
(d) Gunn diode(iv) π-mode

The correct match is :
(a) (b) (c) (d)

Exam Year(s)

UGC NET - 2013,

Answer: D

The correct match is a-iv, b-i, c-ii, d-iii, the fourth option. A magnetron oscillates in the \(\pi\)-mode, a PIN diode is used as a detector, a klystron works by velocity modulation and bunching of the electron beam, and a Gunn diode is a low power oscillator.

The other options pair the magnetron with bunching or with a detector, or give the klystron the detector, none of which fits these devices.

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7.

The correct sequence of sub-systems of Klystron amplifiers as they appear in the direction of flow of electron beam is

Exam Year(s)

UGC NET - 2014,

Answer: C

The sequence is cathode, buncher cavity, catcher cavity, collector. In a klystron amplifier the electron gun at the cathode launches the beam. The beam first passes the buncher cavity, where the RF input velocity-modulates it, then drifts and forms bunches, which give up energy to the catcher cavity, and the spent beam ends at the collector.

The options that start with the buncher before the cathode, or place the collector before the catcher, reverse the direction of the beam.

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