Effect of low AC load impedance in a diode envelope detector In an AM diode detector, what is the principal distortion observed when the AC load impedance is too low (i.e., the RC time constant is too small)?

Introduction / Context:
AM envelope detection depends on an RC network that follows the modulation envelope while rejecting the carrier frequency. Incorrect selection of the RC time constant leads to distortion. The classic failure for too small an RC is diagonal clipping, where the recovered envelope cuts across the modulation valleys.

Given Data / Assumptions:

• Simple diode detector with capacitor and load resistor.
• Modulation envelope varying much slower than the carrier.
• AC load impedance too low → RC too small.

Concept / Approach:

When RC is too small, the capacitor discharges too quickly between carrier peaks, so the output cannot faithfully track the envelope; instead, it forms straight-line segments connecting peaks—“diagonal clipping.” If RC is too large, high-frequency audio components are lost (muffled sound).

Step-by-Step Solution:

Verification / Alternative check:

Textbook plots of diode detector waveforms explicitly label this effect as diagonal clipping for undersized RC time constants.

Why Other Options Are Wrong:

“Negative peak clipping” refers to overdrive or AGC bias issues; “poor AF response” is broader and not the specific distortion mechanism here; “poor AGC operation” is secondary; “crossover distortion” is a push-pull audio amplifier issue, not an AM detector artifact.

Common Pitfalls:

Trying to fix diagonal clipping by raising RF gain instead of choosing a correct RC that balances distortion and audio bandwidth.

Final Answer:

Diagonal clipping