Receiver performance terminology In broadcast and communication receivers, “fidelity” refers to which characteristic measured across the modulation-frequency range at a resistive load?

Introduction / Context:
Receiver specifications use distinct terms: sensitivity, selectivity, fidelity, and signal-to-noise ratio. Candidates often confuse these metrics. Fidelity specifically addresses how accurately the receiver reproduces the audio (or baseband) spectrum without amplitude distortion across the intended frequency range.

Given Data / Assumptions:

• Baseband (audio) output is measured across a resistive load.
• We consider small-signal linear operation without intentional compression.
• The receiver processes amplitude-modulated or frequency-modulated signals and presents an audio-frequency spectrum.

Concept / Approach:
Fidelity is the flatness of the amplitude-versus-frequency response at baseband. A receiver with high fidelity delivers an output whose amplitude does not unduly rise or fall with modulation frequency within the specified audio band (for example, 50 Hz–5 kHz in AM broadcast). This is distinct from selectivity (channel discrimination) and sensitivity (minimum discernible signal).

Step-by-Step Solution:
Define fidelity → faithfulness of the output waveform spectrum to the input modulation.Measurement → sweep modulation frequency at constant modulation index and note output amplitude across a resistive load.Interpretation → smaller variation indicates better fidelity.

Verification / Alternative check:
Receiver datasheets specify audio frequency response (e.g., ±3 dB from 100 Hz to 3 kHz). This response curve is a direct indicator of fidelity.

Why Other Options Are Wrong:

• Sensitivity: relates to minimum input for specified output-noise ratio, not spectral flatness.
• Selectivity: measures discrimination of nearby channels and adjacent frequencies, not audio response.
• AGC action per dB: pertains to dynamic range control, not fidelity.
• “None of the above” is incorrect because a precise definition is given in option (c).

Common Pitfalls:

• Equating fidelity with distortion alone; although connected, fidelity is commonly quantified via frequency response flatness.
• Confusing baseband frequency response with IF/RF filter bandwidth specifications.

Final Answer:
Variation of output amplitude with modulation frequency when the output impedance is resistive