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 \(T=\left(n+\dfrac{3}{4}\right)\) 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.
Indicate the false statement. Klystron amplifiers may use intermediate cavities to
prevent the oscillations that occur in two cavity Klystrons
Three of the statements are genuine benefits of adding intermediate cavities; the first is not, and is the false statement asked for — option 1.
Why option 1 is false. A two-cavity klystron is an amplifier and does not inherently oscillate; there is no such defect for intermediate cavities to cure. If anything the influence runs the other way — adding more resonant cavities to a high-gain chain increases the risk of unwanted feedback and self-oscillation, which is why practical tubes need careful cavity isolation and sometimes deliberate loading. Cavities are added to improve performance, never as a cure for oscillation.
| Statement | Verdict |
|---|---|
| Prevent oscillations in two-cavity klystrons | ✗ FALSE — the answer |
| Increase the bandwidth | ✓ By stagger tuning |
| Improve the power gain | ✓ The principal reason |
| Increase the efficiency | ✓ Through tighter bunching |
Gain — the main purpose. Each intermediate cavity is excited by the partially bunched beam passing through it, and the voltage it develops re-modulates the beam more strongly than the input signal alone could. The effect compounds along the tube: about 20 dB for two cavities, 40 dB for three, and 50 to 60 dB for four or five — enough that a few watts of drive produce megawatts of output.
Bandwidth — by stagger tuning. All cavities tuned to the same frequency would give maximum gain over a very narrow band. Detuning them slightly from one another — stagger tuning — makes their individual responses overlap, so the overall response is broader and flatter. Gain is traded for bandwidth, exactly as in a staggered IF strip.
Efficiency — through better bunching. Stronger, tighter bunches mean a larger fraction of the beam's electrons arrive at the catcher gap in the retarding phase and give up energy, rather than arriving out of phase and taking energy back. Efficiency accordingly rises from perhaps 15 % in a two-cavity tube to 40 % or more in a multicavity design.
The answer is flagged because the question's own wording is awkward, but the reasoning is clear: three statements are standard textbook benefits, and only the first describes a purpose intermediate cavities do not serve.
Hence, the false statement is that they prevent the oscillations that occur in two cavity Klystrons.
Similar Questions
There are three general purpose microwave tubes. The first is ordinary gridded tube, having electrodes like vacuum tube diode and triodes. The second type are those in which interaction between the electron beam and RF field takes place. The klystron is the example of the second type of microwave tubes. The third category of the device is one in which interaction between an RF field and electron beam is continuous. TWT (Travelling Wave Tube) is the example of this category.
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 \(T=\left(n+\dfrac{3}{4}\right)\) 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.
The correct sequence of sub-systems of Klystron amplifiers as they appear in the direction of flow of electron beam is
Assertion (A) : TWTA is a narrow band device and has a helical structure and it is used as amplifier and oscillator.
Reason (R) : In TWT, the electron beam travels through a slow-wave structure and speed of electron beam is \(\dfrac{1}{10}\) of the velocity of light.
The primary function of the helix in a travelling wave tube is to
The Multicavity Klystron
One of the reasons why vacuum tubes eventually fail at microwave frequencies is that their
One of the following is unlikely to be used as a pulsed device. It is the
In a two cavity Klystron the secondary cavity is called
For a reflex Klystron :
Important Questions from Microwave Engineering
Which of the following frequency bands fall under microwave frequency?
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