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.
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