e. The RF output depends on the amount
ELECTRON MODULATION CONCEPT
of energy that the electron beam gives to the output
cavity. The energy to the output cavity, in turn,
a. The graph in figure 48 is a simplified
depends on the density of the electrons as they
Applegate diagram. The vertical dimensions of the
approach the output cavity region. Electron velocity
graph represent the distance that the beam electrons
and drift time are two factors that contribute to the
travel through the drift tube.
beam's density. This means that the drift tube's
dimensions represents the time for the beam
length has an effect on the density of the beam.
electrons to travel the total length of the tube. Three
Although a longer drift tube allows better bunching,
sinusoidal waves represent the three cavities' RF
the electron transit time (between cathode and
voltages or voltage variation across the three gaps in
collector) increases as the drift tube becomes longer.
the tube. Each of the three diagonal lines represents
Odd as it may seem, this is a desirable feature.
the distance of travel per given time for the beam
You'll recall that long transit time causes ordinary
Each line, therefore, shows the
electron tubes to be less efficient at microwave
instantaneous velocity of a group of electrons.
frequencies. However, efficient operation of the
multicavity klystron largely depends on long transit
b. Any change in slope (or bend) of a line
time. For example, the electron transit time for a
indicates a change in velocity. For example, an
multicavity klystron may exceed the time for several
cycles of the klystron's cavity RF voltage as
downward bend indicates deceleration.
Figure 48. Electron beam transit time exceeds time for three RF cycles.