FM demodulation comparison: Which statement is NOT a true advantage of a phase discriminator over a slope detector?

Introduction / Context:
FM demodulators convert frequency variations to voltage. The slope detector is a simple, single-tuned AM detector biased to operate on the slope of a resonance curve, whereas a phase discriminator (e.g., Foster–Seeley/ratio-type architectures) uses phase relationships with typically two resonant circuits to obtain a more linear and stable demodulation characteristic.

Given Data / Assumptions:

• Slope detector employs one tuned circuit and is sensitive to amplitude changes.
• Phase discriminator commonly employs double-tuned circuits and is paired with limiter stages for AM rejection.
• We compare typical advantages in broadcast/communication receivers.

Concept / Approach:

Slope detectors are simple but have poor linearity, high amplitude sensitivity, and narrow effective deviation range. Phase discriminators improve linearity and, with limiters, greatly reduce amplitude noise. However, they do not use fewer tuned circuits; in fact, they generally use more (a key trade-off for performance).

Step-by-Step Solution:

Verification / Alternative check:

Receiver design texts show Foster–Seeley and ratio detectors using a double-tuned transformer or equivalent, confirming increased circuit complexity compared with a single tuned slope detector.

Why Other Options Are Wrong (i.e., they are true advantages):

• Better linearity: phase detection around quadrature is inherently more linear than operating on a single slope.
• Greater immunity to AM (with limiter): limiters suppress amplitude noise before the discriminator.
• Alignment more robust than a slope point: not confined to a tiny slope region; discriminator balance can be centered.
• Wider dynamic range of deviation: less distortion over practical FM deviations than slope detectors.

Common Pitfalls:

Confusing phase discriminators with ratio detectors (a specific AM-insensitive variant) or assuming simplicity equals better performance.

Final Answer:

Fewer tuned circuits are required