T4B12: Result of tuning an FM receiver above or below a signal’s frequency

Q: What is the result of tuning an FM receiver above or below a signal’s frequency?

The correct answer is D: Distortion of the signal’s audio. The correct answer is D: Distortion of the signal's audio. When you tune an FM receiver above or below a signal's frequency, the result is distortion of the signal's audio. FM receivers are designed to operate at the center of the signal's frequency, and tuning away from center causes the demodulated audio to be distorted. FM receivers use a discriminator or ratio detector that produces maximum output (and cleanest audio) when tuned to the exact center frequency of the signal. When you tune above or below the center frequency, the discriminator output becomes non-linear, causing audio distortion. This is different from AM receivers, where tuning off-frequency might just reduce volume. On FM, being off-frequency causes actual distortion of the audio quality.

Updated: Dec 9, 2025 | Source: 2022-2026 Question Pool | Topic: T4B

T4B12T4B

What is the result of tuning an FM receiver above or below a signal’s frequency?

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Deep Dive: T4B12

The correct answer is D: Distortion of the signal's audio. When you tune an FM receiver above or below a signal's frequency, the result is distortion of the signal's audio. FM receivers are designed to operate at the center of the signal's frequency, and tuning away from center causes the demodulated audio to be distorted. FM receivers use a discriminator or ratio detector that produces maximum output (and cleanest audio) when tuned to the exact center frequency of the signal. When you tune above or below the center frequency, the discriminator output becomes non-linear, causing audio distortion. This is different from AM receivers, where tuning off-frequency might just reduce volume. On FM, being off-frequency causes actual distortion of the audio quality.

Why Other Answers Are Wrong

Option A: Incorrect. FM doesn't change audio pitch when tuned off-frequency. The pitch is determined by the modulation, not receiver tuning. Option B: Incorrect. Sideband inversion is an SSB concept, not related to FM receiver tuning. Option C: Incorrect. Heterodyne tones are beat frequencies between two signals, not caused by tuning an FM receiver off-frequency.

Exam Tip

FM tuned off-frequency = Audio distortion. Remember: Tuning an FM receiver above or below the signal's frequency causes audio distortion, not pitch change or other effects.

Memory Aid

**F**M **O**ff-**F**requency = **F**uzzy **O**utput (think 'FOF = FOF' = Fuzzy Output, distorted audio)

Real-World Example

You're receiving an FM signal on 146.52 MHz, but your receiver is tuned to 146.525 MHz (slightly high). The audio sounds distorted - it's not clear, even though the signal strength is good. You retune to exactly 146.52 MHz, and the audio becomes clear. FM receivers must be tuned precisely to the center frequency for clean audio.

Source & Coverage

Question Pool: 2022-2026 Question Pool

Subelement: T4B

Reference: FCC Part 97.3

Key Concepts

FM reception Frequency tuning Audio distortion Discriminator

Verified Content

Question from the official FCC Technician Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the T4B topic.