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.
A parametric amplifier has an input and output frequency of 2.25 GHz and is pumped at 4.5 GHz. It is a :
Degenerate amplifier
Check the arithmetic first — the numbers decide the classification.
\(\dfrac{f_{p}}{f_{s}}=\dfrac{4.5}{2.25}=2\qquad\Rightarrow\qquad f_{p}=2f_{s}\)
In any parametric amplifier the pump, signal and idler are related by
\(f_{i}=f_{p}-f_{s}\)
so here
\(f_{i}=4.5-2.25=2.25\ \text{GHz}=f_{s}\)
The idler has fallen exactly on the signal frequency. That is the defining condition of the degenerate parametric amplifier — option 2 — and it is consistent with the question's statement that input and output are at the same frequency.
| Type | Condition | Output at |
|---|---|---|
| Degenerate | \(f_{p}=2f_{s}\), so \(f_{i}=f_{s}\) | The signal frequency |
| Non-degenerate | \(f_{p}\ne2f_{s}\) | Signal frequency, idler separate |
| Up converter | Output taken at \(f_{i}=f_{p}+f_{s}\) | A higher frequency |
| Down converter | Output at \(f_{p}-f_{s}\), below fs | A lower frequency |
Why the other options are ruled out. Both up-converter options require the output frequency to differ from the input, which contradicts the question's premise that both are 2.25 GHz. A travelling-wave amplifier is a distributed structure altogether — a helix interacting continuously with an electron beam, as the passage describes for the TWT — and not a pumped-reactance device at all.
The characteristic property of the degenerate amplifier is that it is phase sensitive. Because signal and idler coincide, the gain depends on the phase of the signal relative to the pump: a component in phase with the pump is amplified while the quadrature component is attenuated. This makes it useful as a phase-sensitive detector and, in modern optics, as a squeezed-light generator — but awkward as a general amplifier, since the input phase must be controlled.
The Manley-Rowe relations govern the power flow in all such devices and explain why the up-converter has gain at all: power is exchanged between pump, signal and idler in proportion to their frequencies, so shifting a signal upward in frequency necessarily amplifies it.
Hence, the amplifier is a degenerate amplifier.
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
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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
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