Difficulty: Medium
Correct Answer: Varying the voltage that controls the electron beam velocity
Explanation:
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:
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:
Electron beam velocity v ≈ √(2eVb / me), where Vb is beam voltage.Phase synchronism requires ω / β ≈ v / η (η is structure constant), linking frequency ω to Vb.Therefore, sweeping Vb forces ω to shift to maintain synchronism, providing practical frequency tuning.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:
Beam current alone: alters amplitude/efficiency, minimal frequency shift.Thermionic emission or collector potential: secondary effects, not the principal tuning handle.Combined minor tweaks still rely primarily on beam-voltage control.Common Pitfalls:
Confusing BWO with reflex klystron repeller-voltage tuning; both change electron transit but in BWO the primary, smooth tuning is via beam voltage.Final Answer:
Varying the voltage that controls the electron beam velocity
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