Superheterodyne receiver symptoms and stages: match common issues/features to their likely causes or blocks. List I (Observation) A. IF noise B. Image frequency reception C. Station selector operation D. Excessive hum in audio List II (Likely block / cause) Ganged tuning (RF + local oscillator tracking) Spurious response due to image signal Leaky filter capacitor (power supply/AF stage) Mixer (frequency conversion) stage
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AA-3, B-2, C-1, D-4
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BA-2, B-3, C-1, D-4
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CA-1, B-2, C-3, D-4
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DA-4, B-2, C-1, D-3
Answer
Correct Answer: A-4, B-2, C-1, D-3
Explanation
Introduction / Context:In a superheterodyne radio, symptoms such as noise, images, tuning accuracy, and hum each point to characteristic stages. Recognizing these helps troubleshoot quickly during labs and service tasks.
Given Data / Assumptions:
- The receiver uses an RF stage, mixer, IF filters/amplifiers, detector, AF amplifier, and power supply filtering.
- “Image frequency” refers to the undesired frequency that also converts to the IF.
- “Ganged tuning” means mechanically linked variable capacitors for RF and LO tracking.
- Hum generally indicates power supply ripple or bad decoupling.
Concept / Approach:
Map each observation to the stage most directly associated with it: IF noise is strongly influenced by the mixer and early IF chain; images are spurious responses inherent to mixing; station selection relies on RF/LO tracking; hum relates to defective filtering capacitors in the audio/power supply chain.
Step-by-Step Solution:
IF noise → mixer stage (and initial IF gain) ⇒ A-4.Image reception → spurious response of heterodyning ⇒ B-2.Station selector → ganged tuning ensures proper tracking ⇒ C-1.Excessive hum → leaky/failed filter capacitor ⇒ D-3.Verification / Alternative check:
Block-level analysis: the mixer sets the noise figure heavily in many receivers; images appear at f_image = f_signal ± 2 f_IF; ganged tuning alignment restores selectivity; replacing a dried-out electrolytic in the supply typically removes hum, confirming the mapping.
Why Other Options Are Wrong:
- Blaming ganged tuning for hum misattributes an audio/power issue to RF mechanics.
- Connecting images to the audio stage ignores frequency-translation fundamentals.
Common Pitfalls:
Forgetting that front-end RF selectivity prior to the mixer is crucial for image rejection; if RF tracking is off, images worsen, but the phenomenon itself is due to mixing.
Final Answer:
A-4, B-2, C-1, D-3