Section I. VELOCITY MODULATION
1-1. MODULATION PRINCIPLES
a. A long transit time is used very effectively in special types of tubes. Tubes that use a long transit
time are commonly called velocity-modulated tubes. This is in contrast to the space-charge control of the
conventional electron tube.
b. In a conventional electron tube, operating in a conventional circuit, the electron beam is modulated by
varying the number of electrons. In a velocity-modulated tube, the electron beam is modulated by varying the
velocity of the electrons. The electron velocities are varied by causing some electrons to move slowly and others
to move rapidly through the inter-electrode space. When fast-moving electrons overtake a group of slower
moving electrons and two groups arrive at a designated point at the same instant, bunching occurs. In a velocity-
modulated tube, the electrons arrive at a designated point in bunches.
1-2. BUNCHING PRINCIPLES
a. Figure 2 shows the distance traveled by
the electrons as a function of time. The slower
moving or low-velocity electrons require a longer
period of time to cover the same distance than the
faster moving or high-velocity electrons. The point
where the lines for the different velocities intersect is
where bunching takes place. Therefore, if the
velocity of the electrons is controlled, bunching will
take place at a definite distance from the electron
Figure 2. Electron velocities.
b. The bunching of electrons is a necessary function in velocity-modulated tubes. When the electrons
are bunched, this group of electrons can then be accelerated or decelerated to the desired velocity. When
electrons change velocity, they change energy levels. Electrons that are accelerated take on energy, and electrons
that are decelerated give up energy.
Section II. FUNDAMENTAL KLYSTRONS
1-3. PRINCIPLES OF OPERATION
a. A common type of velocity-modulated tube is the klystron. A klystron consists of four parts: a beam
source, a velocity-modulating unit called a