Traveling-Wave Tubes

the RF output may not perfectly follow the RF input, possibly resulting in the distortion increasing as the tube is driven closer to saturation on the peaks of the RF input signal. In general, klystron amplifiers should not be used to amplify amplitude-modulated signals if the RF output is driven higher than 70 percent of the saturation level. Considerable distortion may occur between 70 and 100 percent of saturation. A klystron amplifier generates a certain amount of white noise, just as in any other electron tube. White noise occurs usually because an electron beam is never perfectly homogeneous. The amount of elec- trons varies slightly with time, primarily due to shot noise at the cathode surface. This variation shows up as random noise in the RF output. A certain amount of noise may also be generated by electrons striking the drift tubes. Since klystron amplifiers are noisy, they are not usually used to amplify weak microwave signals, such as in a radar receiver RF amplifier. Traveling-Wave Tubes Traveling-wave tubes (TWTs) are high-gain, low- noise, wide-bandwidth microwave amplifiers, capable of gains of 40 dB or more, with bandwidths of over an octave. (A bandwidth of 1 octave is one in which the upper frequency is twice the lower frequency.) TWTs have been designed for frequencies as low as 300 MHz and as high as 50 GHz. The primary use for TWTs is voltage amplification (although high-power TWTs, with characteristics similar to those of a power klystron, have been developed). Their wide bandwidth and low-noise characteristics make them ideal for use as RF amplifiers. TWT OPERATION.— While the electron beam in a klystron travels primarily in regions free of RF electric fields, the beam in a TWT is continually inter- acting with an RF electric field propagating along an external circuit surrounding the beam. To obtain amplification, the TWT must propagate a wave whose phase velocity is nearly synchronous with the dc velocity of the electron beam. It is difficult to ac- celerate the beam to greater than approximately one- fifth the velocity of light. Therefore, the forward velocity of the RF field propagating along the helix must be reduced to nearly that of the beam. The phase velocity in a waveguide, which is uni- form in the direction of propagation, is always greater than the velocity of light. However, this velocity can be reduced below the velocity of light by introducing a periodic variation of the circuit in the direction of propagation. The simplest form of variation is ob- tained by wrapping the circuit in the form of a helix, whose pitch is equal to the desired slowing factor. TWT MIXER.— A TWT is also used as a micro- wave mixer. By virtue of its wide bandwidth, the TWT can accommodate the frequencies generated by the heterodyning process (provided that the frequen- cies have been chosen to be within the range of the tube). The desired frequency is selected by the use of a filter on the output of the helix. A TWT mixer has the added advantage of providing gain as well as simply acting as a mixer. TWT MODULATION.— A TWT can be modu- lated by applying the modulating signal to a modula- tor grid. The modulator grid can be used to turn the electron beam on and off, as in pulsed microwave applications, or to control the density of the beam and its ability to transfer energy to the traveling wave. Thus, the grid can be used to amplitude modulate the output . TWT OSCILLATOR.— A forward-wave TWT can be constructed to serve as a microwave oscillator. Physically, a TWT amplifier and an oscillator differ in two major ways. The helix of the oscillator is longer than that of the amplifier, and there is no input con- nection to the oscillator. TWT oscillators are often called backward-wave oscillators (BWOs) or carcino- trons. 2-19