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.
One of the following is unlikely to be used as a pulsed device. It is the
BWO
Three of the four are power devices used in radar transmitters, where pulsing is the norm; the backward-wave oscillator is not one of them — option 3.
| Device | Role | Pulsed? |
|---|---|---|
| TWT | Wideband power amplifier | ✓ Radar, ECM |
| CFA | High-efficiency power amplifier | ✓ Radar output stages |
| BWO | Low-power tunable oscillator | ✗ |
| Multicavity klystron | Very high power amplifier | ✓ The classic pulsed tube |
What the BWO is for. It is an oscillator in which the RF wave travels backward, against the electron beam — the reverse of the TWT's forward interaction, as the passage notes in calling it the TWT's oscillator counterpart. Its distinguishing property is that its frequency is set by the beam voltage, so it can be swept electronically over a very wide range, often an octave, simply by varying that voltage.
Why that role and pulsing do not fit together. A BWO is used as a swept signal source — in laboratory sweepers, spectroscopy, and as the local oscillator or jamming source in electronic-warfare receivers. These are continuous-wave, low-power applications, typically milliwatts to a watt. Pulsing is done to obtain very high peak power while keeping the average dissipation low, which is a power-amplifier concern; there is nothing to be gained by pulsing a milliwatt-level oscillator whose value lies in its tunability. The BWO's output power is in any case low and varies as it tunes.
Why the other three are pulsed. Radar wants a short, intense burst and a long listening interval: a 1 µs pulse at 0.001 duty gives megawatts of peak power for kilowatts of average. Klystrons, TWTs and cross-field amplifiers all deliver the necessary peak power, and CFAs additionally offer 40 to 70 % efficiency and a compact, low-voltage construction that suits airborne and shipborne transmitters.
The answer is flagged because BWOs have occasionally been pulsed in specialised work; the question asks what is unlikely, and among these four the BWO is unambiguously the odd one out.
Hence, the device unlikely to be pulsed is the BWO.
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
Indicate the false statement. Klystron amplifiers may use intermediate cavities to
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?
The primary reason behind identically zero magnetic field outside a coaxial cable is:
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One of the following microwave diodes is suitable for very low power oscillator only.
A uniformly spaced linear array of identical radiators having uniform amplitude of excitation and linear phase variation with non-zero gradient will produce
