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 microwave amplifier must be cooled :
to improve the noise performance
The parametric amplifier exists for one reason — extremely low noise — and cooling is what pushes that advantage further.
Why it is quiet in the first place. A parametric amplifier gains its energy not from a DC supply through a resistive device, but from a pump oscillator varying a reactance — the voltage-dependent junction capacitance of a varactor diode. A pure reactance dissipates no power, so it generates no thermal (Johnson) noise. Almost the only noise left is that of the small series resistance of the diode, and of the surrounding circuitry.
What cooling does to that residue. Thermal noise power is
\(P_{n}=kTB\)
and equivalent noise temperature adds directly to the system:
\(T_{e}=(F-1)T_{0}\)
Cooling the diode and its circuit reduces T, and hence the noise contribution, in direct proportion. Liquid nitrogen at 77 K cuts the physical temperature by a factor of nearly four compared with 290 K, and liquid helium at 4 K by a factor of seventy — taking a good uncooled parametric amplifier from a noise temperature of perhaps 150 K down to 20 K or less. That is option 4.
Why the other options fail. Option 1 has the causality backwards — the amplifier handles very small signals and dissipates almost nothing, so there is no heat problem to solve. Option 2 is wrong because bandwidth is set by the circuit's Q and the pump arrangement, not by temperature; if anything, parametric amplifiers are narrowband. Option 3 is simply untrue: uncooled parametric amplifiers work perfectly well and are common — cooling is an improvement, not a necessity.
| Amplifier | Typical noise temperature |
|---|---|
| Maser (cooled) | 4 – 20 K |
| Parametric, cooled | 20 – 50 K |
| Parametric, uncooled | 100 – 200 K |
| Travelling wave tube | > 500 K |
Where this matters : in satellite earth stations and radio telescopes, the received signal is so weak that the receiver's own noise, not the transmitter's power, sets the limit. Every kelvin removed from the front end is worth as much as extra transmitter power, which is why the trouble of cryogenic cooling is accepted at all.
Hence, a parametric amplifier is cooled to improve the noise performance.
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