Assertion (A): A backward wave oscillator (BWO) can be used as a sweep generator. Reason (R): The oscillation frequency of a BWO can be varied by changing the beam voltage, which alters electron velocity and the interaction point. Choose the correct option about A and R.

Introduction:
Backward Wave Oscillators (BWOs) are vacuum electronic devices using slow-wave structures where the electromagnetic phase velocity is opposite to the electron beam direction. Frequency agility makes them suitable for sweep generation in radar and test equipment.

Given Data / Assumptions:

• Device: BWO with slow-wave structure (e.g., helix)
• Control variable: beam voltage that sets electron velocity
• Goal: judge truth of A and R

Concept / Approach:
Frequency in a BWO depends on the intersection of beam dispersion and slow-wave structure dispersion. Changing beam voltage shifts electron velocity, moving the interaction point and hence the oscillation frequency. This provides a continuous sweep over a designed range.

Step-by-Step Solution:

Verification / Alternative check:
Classic BWO characteristics show linear frequency–voltage behavior over segments, enabling calibrated sweeps.

Why Other Options Are Wrong:

• B: Explanation is directly causative, not incidental.
• C/D: Do not match device physics.

Common Pitfalls:
Confusing BWO with multi-cavity klystron (mechanical tuning) or assuming forward-wave operation; ignoring beam interception limits during tuning.

Final Answer:
Both A and R are correct and R is the correct explanation of A