A traveling-wave tube (TWT) is a specialized vacuum tube used in wireless communications, especially in satellite systems. The TWT can amplify or generate microwave signals. Two common types of TWT include the Klystron and the magnetron. In the Klystron, a negatively charged cathode emits a beam of high-speed, high-energy electron s that travel through the cylindrical tube in straight lines to a positively charged anode. A coil is wound around the tube. When the coil is energized with a radio-frequency (RF) signal, the electrons in the beam alternately bunch up and spread out. In the magnetron, the electrons move in circles rather than in straight lines. The circular motion, produced by magnets at either end of the tube, allows the electrons to pick up energy over a greater distance. Inside the TWT, the regions of high and low electron concentration move along or around the tube in waves. When the tube is properly operating, some of the energy from the electrons is imparted to the signal in the coil. The result is amplification of the signal. A TWT can be made to function as an oscillator by coupling some of the output back into the input. This configuration is called a backward-wave oscillator, because the feedback is applied opposite to the direction of movement of the electrons inside the tube. Such an oscillator can generate up to approximately 0.1 watt of signal power in the microwave range. A parametric amplifier is a TWT amplifier that operates from a high-frequency alternating current (AC) power source, rather than the usual direct current (DC) source. Some characteristic of the circuit, such as its impedance, is made to vary with time at the power-supply frequency. Parametric amplifiers are useful because they generate very little internal noise. This makes it possible to obtain excellent sensitivity in receiving systems, minimizing data-transfer errors