For a reflex Klystron :
The loop gain is 1 and phase shift is multiple of 2π
A reflex klystron is an oscillator, so it must satisfy the Barkhausen condition: loop gain exactly 1 with a total phase shift that is a multiple of 2π — option 4.
\(|A\beta|=1,\qquad \angle A\beta=2n\pi\)
Why the two halves are both necessary.
The phase condition ensures the feedback is positive: the signal returning to the cavity must arrive in step with what is already there, reinforcing it. A multiple of \(2\pi\) means exactly that. A multiple of \(\pi\) — option 2 — includes the odd multiples, which is a 180° inversion, so the returning signal would cancel rather than reinforce and the oscillation would die.
The gain condition ensures the amplitude settles. If the loop gain were less than 1, as option 1 states, each round trip would return less than was sent and the oscillation would decay to nothing. If it were greater than 1, as option 3 states, the amplitude would grow until some non-linearity stopped it — which is exactly what happens at start-up, but the steady operating condition is the point where the growth stops, and that is where the loop gain has fallen to unity.
How the reflex klystron meets these conditions physically. A single cavity does both jobs. Electrons pass through the gap and are velocity modulated; the repeller, held at a negative potential, turns them back so they pass through the same gap a second time. If they return at the right moment they deliver energy to the cavity instead of taking it.
That moment is set by the transit time in the repeller space, and the requirement is
\(T=\left(n+\dfrac{3}{4}\right)\ \text{cycles}\)
The three-quarter cycle is what makes the returning bunch arrive in the retarding phase of the gap field, and the whole cycles n are the \(2n\pi\) of the phase condition. Each value of n is a distinct mode of the tube, reached by a different repeller voltage — which is why a reflex klystron's output power and frequency vary in discrete humps as the repeller voltage is swept, and why it is tuned electronically over a small range within each mode.
Hence, the loop gain is 1 and the phase shift a multiple of 2π.
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
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
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
