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 Multicavity Klystron
is not low-level amplifier because of noise
Option 1 is the only true statement of the four. A multicavity klystron is a power amplifier, and its noise figure — typically 15 to 25 dB — is far too high for it to serve as a low-level or first-stage amplifier.
Why the noise is high. The beam itself is noisy: electrons leave the cathode at random, producing shot noise, and the velocity spread that bunching depends on also randomises their arrival. Since a receiver's overall noise figure is set mainly by its first stage,
\(F=F_{1}+\dfrac{F_{2}-1}{G_{1}}+\cdots\)
putting a klystron first would ruin the system. Low-noise duties go to parametric amplifiers, masers or modern transistor LNAs, with the klystron used after them, or on the transmitting side where noise hardly matters.
Why each of the others fails.
Option 2 — repeller voltage. The multicavity klystron has no repeller at all. A repeller is the electrode that turns the beam back in a reflex klystron, the single-cavity oscillator described later in the passage. In a multicavity tube the beam travels straight through from cathode to collector.
Option 3 — pulsed operation. The opposite is true. Klystrons are the classic pulsed microwave source, delivering tens of megawatts in short pulses for radar transmitters and particle accelerators, where the duty cycle keeps the average dissipation manageable.
Option 4 — transit time in the buncher. Reversed again. The transit time across the buncher gap must be short compared with an RF period, so that each electron sees a nearly constant field while crossing and receives a clean velocity increment. If it were long the electron would see the field reverse mid-gap and the velocity modulation would be smeared out. What must be long is the transit through the drift space that follows, where the bunching develops.
What the extra cavities buy — the point of the "multicavity" name: each intermediate cavity re-bunches the beam, so gain multiplies from about 20 dB for a two-cavity tube to 40 or 60 dB for four or five cavities, and stagger-tuning them widens the bandwidth.
The answer is flagged because option 1's phrasing is loose, but its substance — a klystron is unsuitable as a low-level amplifier — is correct, and the other three are demonstrably false.
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
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
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?
The primary reason behind identically zero magnetic field outside a coaxial cable is:
Semiconductor diode used in switching circuits at Microwave range is
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
