decelerated and give up energy to the cavity. For maximum transfer of energy, the bunches must arrive when the
cavity signal is positive.
c. The electrons that are outbound from the cathode take on most of their energy from the dc power
supply (Ea). The energy in the cavity (cavity signal) contributes very little to the energy the electrons take on.
This is how energy is taken from the dc source and transferred to the cavity so that the klystron can have a useful
ac output.
1-8. MODES OF OPERATION
a. Notice that during the first cycle of the cavity signal (voltage across cavity grids) the electrons are
accelerated on their way to the repeller plate. During the succeeding cycles of the cavity signal, the electrons are
bunched and decelerated. The cycle where the electrons are decelerated determines the mode of operation. When
the electrons are decelerated by the first positive signal after the initial acceleration, the paths of electrons are
operating in the first mode. When the electrons are decelerated by the second positive signal, the paths of
electrons are operating in the second mode. There are three or four modes in which it is possible for the reflex
klystron to oscillate.
b. The transit time determines the mode of operation, and the transit time is determined by the electron
velocity. The original velocity of the electron depends on Ea. The distance the electron travels before turning
back and the velocity with which it returns depend on the difference between Ea and Er. It is possible to adjust
the two voltages Ea and Er for any of the modes. The voltage Ea is usually fixed in magnitude, since varying it
produces greater initial velocity which, in turn, causes a farther excursion and a greater return velocity. Since it is
not feasible to make Ea variable, Er is variable. For operation in the first mode, the round trip must be completed
in the shortest time. This is accomplished by making the repeller plate most negative. For greater time in the
interelectrode space, the repeller is made less negative.
c. In figure 8 , showing power output and frequency of oscillations as functions of the repeller voltage
for three modes of operation, notice that the frequency at the point of maximum output is identical for all three
modes and is the resonant frequency of the cavity. In addition, note that the power output for the various modes
at the resonant frequency is not the same and that it is least in the highest mode.
1-9. REFLEX KLYSTRON TUNING
a. Within these modes it is possible to change the frequency of oscillation by changing the repeller plate
voltage. Thus it is possible to tune the oscillator by turning a dial which controls this voltage.
b. If the repeller voltage is greater than that required to bring the electrons back through the grids at the
instant of peak positive cavity signal, the electrons return too soon. The current between the grids then leads the
voltage, and the reactance is capacitive. This is equivalent to decreasing the capacitance between the grids. With
smaller capacitance, the circuit will be resonant at a higher frequency.
344 L1
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