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.
In the parametric amplifiers, idler frequency is :
double the signal frequency
Double the signal frequency — option 1, as recorded in the official key.
What the idler is. A parametric amplifier has no amplifying resistance. Gain comes from a pump source that varies a reactance — usually a varactor's junction capacitance — at a high frequency. Mixing the pump against the signal in that non-linear reactance produces a third component, the idler, and the three frequencies are locked together by the Manley-Rowe power relations :
\(f_{p}=f_{s}+f_{i}\qquad\Longrightarrow\qquad f_{i}=f_{p}-f_{s}\)
The idler is not an output anyone wants; it is the auxiliary component through which pump power is transferred into the signal. An idler circuit must nevertheless be provided, because without a path at \(f_{i}\) the energy exchange cannot take place and there is no gain at all.
Reading the keyed option against that relation. Setting \(f_{i}=2f_{s}\) in \(f_{p}=f_{s}+f_{i}\) gives a pump at \(f_{p}=3f_{s}\), which is the arrangement the key's wording describes — the idler placed at twice the signal frequency, well separated from it so that the two can be filtered apart.
| Arrangement | Pump | Idler | Comment |
|---|---|---|---|
| Degenerate | \(f_{p}=2f_{s}\) | \(f_{i}=f_{s}\) | Idler coincides with the signal, so they cannot be separated; gain becomes phase-sensitive |
| Non-degenerate | \(f_{p}>2f_{s}\) | \(f_{i}\neq f_{s}\) | The usual case; signal and idler are separable by filtering |
| The keyed case | \(f_{p}=3f_{s}\) | \(f_{i}=2f_{s}\) | A well-separated non-degenerate design |
A note on the option set. The four choices are loosely worded — options 3 and 4 constrain the pump rather than the idler, and option 2 gives an inequality without a reference. Working from \(f_{i}=f_{p}-f_{s}\) is the only reliable way through such an item; the relation itself is what is being examined.
Why parametric amplifiers mattered. Because gain comes from a nearly lossless reactance rather than from a resistance, very little thermal noise is added, and cooled parametric amplifiers reached noise temperatures of a few tens of kelvin. They were the standard front end for satellite ground stations and radio astronomy before GaAs FETs and HEMTs displaced them.
Hence, the answer recorded is double the signal frequency.
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
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