Cause of double spotting in superheterodyne receivers In a superheterodyne receiver, “double spotting” (hearing the same station at two nearby dial positions) most commonly results from:

Introduction / Context:
Double spotting is a classic symptom in superheterodyne radios where a strong station appears at two different tuning points. Recognizing the root cause helps diagnose front-end selectivity and alignment issues.

Given Data / Assumptions:

• Standard superhet architecture with RF stage(s), mixer/LO, and fixed IF.
• Observed double appearance of the same station.
• No special tracking filters beyond usual broadcast-band front end.

Concept / Approach:
The superhet converts the incoming signal by mixing with a local oscillator to a fixed intermediate frequency (IF). Signals at the image frequency (f_image = f_signal ± 2IF, depending on LO side) also mix to the same IF. If the RF front end does not sufficiently attenuate the image, both the desired and image signals pass to the mixer, causing “double spotting.”

Step-by-Step Solution:
Recall image relationship: f_image = f_signal ± 2IF.Insufficient preselector/RF filter rejection at f_image allows the image to reach the mixer.Mixer produces the same IF from both, so the same station is heard at two dial settings.Hence the principal cause is poor image rejection (inadequate RF selectivity).

Verification / Alternative check:
Improving or adding a tuned RF stage or a tracking preselector typically eliminates double spotting, confirming the diagnosis.

Why Other Options Are Wrong:
Poor sensitivity: Reduces signals, not duplications.

High IF gain: Can aggravate audibility of images but is not the core cause without poor RF selectivity.

Miscalibrated dial or LO drift: These cause frequency errors, not two valid conversion paths to the same IF.

Common Pitfalls:
Attempting to cure double spotting by only lowering IF gain; the correct fix is enhancing front-end filtering and alignment.

Final Answer:
Inadequate image-frequency rejection (insufficient RF selectivity)