In a backward-wave oscillator (BWO), which adjustment most effectively tunes the oscillation frequency?

Introduction / Context:
A backward-wave oscillator (BWO) is a vacuum-electron microwave source using a slow-wave structure that supports a backward wave (negative group velocity). Frequency control is crucial for agile sources and swept measurements.

Given Data / Assumptions:

• BWO oscillation relies on interaction between the electron beam and a backward wave on a slow-wave structure (often a helix or coupled cavities).
• Beam voltage sets electron velocity; beam current primarily influences output power and starting conditions.

Concept / Approach:
The synchronism condition determining the oscillation frequency depends on matching the electron beam's axial velocity with the slow-wave phase velocity. Changing the beam voltage changes the electron velocity and thus shifts the phase synchronism point, directly tuning the oscillation frequency. Beam current affects gain and saturation but has far less direct leverage on frequency.

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer data for BWOs show wideband tuning via anode (beam) voltage sweeping; the beam current control is mainly used to set output power and avoid over-loading, not to tune frequency precisely.

Why Other Options Are Wrong:

Common Pitfalls:

Final Answer: