Reflex klystron tuning principle: Is the oscillation frequency determined primarily by the resonant frequency of the cavity, or by another adjustable parameter?

Introduction / Context:
A reflex klystron is a single-cavity microwave oscillator in which electron bunching is controlled by a repeller electrode. Although the cavity sets a central resonance, practical frequency control is achieved by adjusting the repeller voltage, which changes transit time and the phase with which bunched electrons return to the gap.

Given Data / Assumptions:

• Single resonant cavity with quality factor Q.
• Repeller electrode establishes the electron turnaround region.
• Small-signal operation within a tuning lobe.

Concept / Approach:

The oscillation condition requires the electron bunches to deliver energy in phase with the RF field. The electron transit time between the cavity gap and the repeller depends on repeller voltage. Changing that voltage shifts the phase condition and hence the oscillation frequency, while the cavity determines the baseline frequency and mode lobes.

Step-by-Step Solution:

Verification / Alternative check:

Typical datasheets plot frequency versus repeller voltage with distinct mode lobes. The cavity shape fixes the approximate band, but the knob for frequency is the repeller voltage.

Why Other Options Are Wrong:

• Asserting cavity dominance ignores the essential transit-time control in reflex designs.
• Beam current, cathode material, or Q values influence efficiency and stability but are not the primary tuning parameter.

Common Pitfalls:

Confusing reflex klystrons with multicavity amplifiers where cavities are tuned stages. In reflex oscillators, transit-time tuning via the repeller is central.

Final Answer:

False — it is determined mainly by the repeller voltage (transit time)