Reflex Klystron Lab Practice: Why Use Square-Wave Modulation and Square-Law Detection? In a typical laboratory setup with a reflex klystron source and crystal detector, the output is examined while applying a square-wave modulation to the reflector voltage. Why is this practice used?

Introduction / Context:
Reflex klystrons are often characterized in the lab with simple amplitude-detection receivers using crystal diodes. A small modulation of the reflector voltage creates an amplitude variation that the detector converts to DC for display or measurement.

Given Data / Assumptions:

• Crystal (Schottky) detector used at low RF input power.
• Detector biased (or un-biased) so that it behaves in the square-law region.
• Square-wave modulation is applied at low frequency to sweep or dither the operating point.

Concept / Approach:

In the square-law region, detector output voltage V_out is approximately proportional to input RF power P_in (V_out ∝ P_in). Applying a low-rate square-wave modulation toggles the RF amplitude slightly, letting the detector deliver a corresponding square-wave DC variation that is easy to measure. This technique linearizes power readings and enables lock-in–style measurements without elaborate RF instrumentation.

Step-by-Step Solution:

Verification / Alternative check:

Detector transfer curves show a quadratic region at low powers; demodulated outputs track input power changes linearly in that region.

Why Other Options Are Wrong:

Ease of generation (A) is incidental; (C) is incorrect—frequency may still vary with reflector voltage; (D) oversimplifies; (E) is false—matching is still important.

Common Pitfalls:

Driving outside square-law region (saturation) breaks proportionality; using excessive modulation that distorts measurement.

Final Answer:

crystal diode operates in the square-law region of its i–v characteristic