JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV MICROWAVE AND OPTICAL COMMUNICATIONS Previous Question Paper (February/March - 2022 - R18)

A Reflex Klystron is a low-power microwave oscillator that uses a single resonant cavity for both bunching and catching electrons. Electrons emitted from the cathode are accelerated by the grid towards the cavity. After passing through the cavity, they enter the retarding field region created by a negative repeller electrode. The repeller forces the electrons to reverse direction and return through the cavity. During this return journey, electrons that were accelerated earlier are slowed down, and those that were slowed down are accelerated, causing them to bunch together. These bunches pass through the cavity at the correct phase to transfer energy to the oscillating field.

The efficiency of a Reflex Klystron is defined as the ratio of the output microwave power to the input DC power. The electronic efficiency is given by:

Where I(1) is the fundamental component of the induced current, V(1) is the amplitude of the cavity voltage, I(0) is the DC beam current, and V(0) is the beam voltage. Using the Bessel function J(1) for the bunching parameter X, the maximum theoretical electronic efficiency is approximately 22.7%.

Velocity modulation is the process of varying the velocity of an electron beam by passing it through a time-varying electric field. Unlike amplitude modulation, where the density of the carrier is varied, velocity modulation changes the kinetic energy of electrons in a constant-density beam, which subsequently leads to density modulation as the faster electrons catch up with slower ones.

In a Klystron amplifier, velocity modulation is utilized through two cavities: the buncher and the catcher. The input signal is applied to the buncher cavity, creating an alternating electric field that accelerates some electrons and decelerates others. As the beam travels through the drift space, the velocity-modulated electrons form bunches. When these bunches reach the catcher cavity, they induce an amplified signal due to the periodic current flow, effectively converting the kinetic energy of the bunched electrons into high-frequency electromagnetic energy.