The Reflex Klystron makes use of velocity modulation to transform a continuous electron beam into microwave power. Electron emitted from the cathode are accelerated and passed through the positive resonator towards negative reflector, which retards and, finally, reflects the electron; and the electron turns back through the resonator. Suppose an hf-field exists between the resonator, the electron travelling forward will be accelerated or retarded, as the voltage at the resonator changes in amplitude. The accelerated electrons leave the resonator at an increased velocity and the retarded electrons leave at the reduced velocity. The electrons leaving the resonator will need different time to return, due to change in velocities. As a result, returning electrons group together in bunches. As the electron bunches pass through resonator, they interact with voltage at resonator grids. If the bunches pass the grid at such time that the electrons are slowed down by the voltage, energy will be delivered to the resonator; and Klystron will oscillate. Fig. 1 shows the schematic of a typical Klystron tube. Fig. 2 shows the relationship between output power, frequency and reflector voltage. The frequency is primarily determined by the dimension of resonant cavity. Hence, by changing the volume of resonator, mechanical tuning range of Klystron is possible. Also, a small frequency change can be obtained by adjusting the reflector voltage. This is called Electronic Tuning Range. The same result can be obtained, if the modulation voltage is applied on the reflector voltage VR as shown in the Fig

Block Diagram

There are two configurations for a low-powered klystron. One is a low-power microwave oscillator (Reflex Klystron) and the second is a low-power microwave amplifier (Two Cavity Klystron or Multi Cavity Klystron).