In reflex Klystron following statements are given : (a) It is used as local oscillator. (b) Its efficiency can be calculated by \(\dfrac{P_o}{V_aI_a}\). (c) Its efficiency can be calculated by \(\dfrac{P_{ac}}{P_{dc}}\) (d) Its output depend on repeller and beam voltage. Out of the above following is correct :
(a), (b) and (d)
What a reflex klystron is. A single-cavity microwave oscillator: the electron beam passes through the cavity gap, is velocity-modulated, then is turned back by a negative repeller electrode and returns through the same gap. If it re-enters at the right instant it gives energy to the cavity and oscillation is sustained.
(a) Used as a local oscillator ✓. Its output is modest (tens to hundreds of milliwatts) and its efficiency low, but it is easily and rapidly tuned by voltage, which is exactly what a superheterodyne receiver's local oscillator needs. It has long been the standard LO in microwave receivers and radar test sets.
(b) Efficiency = Po/(VaIa) ✓. Efficiency is the microwave power delivered divided by the d.c. power drawn by the beam, and the d.c. input is the beam (anode) voltage times the beam current. Theoretical maximum efficiency is only about 22.7 %, and in practice 10–20 % — a consequence of the fixed bunching parameter and of the fact that only the electrons in the favourable phase give up energy.
(d) Output depends on repeller and beam voltage ✓. The transit time in the repeller space must satisfy
\(T=\left(n+\dfrac{3}{4}\right)\ \text{cycles}\)
for the returning bunch to arrive in the retarding phase. Since that transit time is set by the repeller and beam voltages, changing either moves the device between "modes" — and within a mode the frequency shifts, which is precisely what makes electronic tuning and frequency modulation possible.
(c) writes the same efficiency as Pac/Pdc — physically the same ratio in more generic symbols — but the keyed set takes (b) as the intended form, since it names the actual beam quantities Va and Ia that a klystron datasheet quotes. The keyed answer is therefore (a), (b) and (d).
Hence, the correct answer is (a), (b) and (d).
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.
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