Receiver performance linkage A receiver that exhibits poor intermediate-frequency (IF) selectivity will also suffer from which degradation most prominently, assuming other sections are typical of AM broadcast superheterodynes?

Difficulty: Medium

Correct Answer: Double spotting

Explanation:


Introduction / Context:
Selectivity determines how well a receiver separates a desired channel from adjacent or spurious responses. In a superheterodyne, IF selectivity strongly influences the shape of the passband and how unwanted signals near the IF are handled. One symptomatic issue is double spotting, wherein the same station is heard at two dial positions because of inadequate rejection of spurious responses near the IF path.


Given Data / Assumptions:

  • AM superheterodyne architecture.
  • IF selectivity is poorer than intended (wider or insufficient attenuation of adjacent responses).
  • Typical front end and local oscillator operation.


Concept / Approach:

Poor IF selectivity broadens the effective passband, allowing unintended responses within or near the IF to leak through. This can present as hearing the same station twice (double spotting), usually due to residual image or second-conversion-like responses being insufficiently attenuated by the IF filters.


Step-by-Step Solution:

Identify failure mode: inadequate IF filtering → broader passband.Result: spurious/adjacent signals are not sufficiently rejected.Symptom: receiver detects a station at two tuning points → double spotting.


Verification / Alternative check:

In practice, improving IF selectivity (sharper skirts, better filter alignment) removes secondary responses, reducing double spotting. Sensitivity is more tied to front-end noise figure and gain, and blocking mainly involves strong-signal handling and AGC linearity rather than IF bandwidth alone.


Why Other Options Are Wrong:

Sensitivity: Primarily governed by noise figure and gain, not IF bandwidth alone. Blocking: A strong adjacent signal desensitizing the receiver is more about dynamic range and AGC. Image rejection: Dominated by RF front-end selectivity and LO choice; IF selectivity has limited effect on true image rejection in single-conversion sets.


Common Pitfalls:

Assuming “all of the above” whenever performance degrades; conflating IF selectivity issues with RF image rejection; overlooking AGC and dynamic-range origins of blocking.


Final Answer:

Double spotting

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