In superheterodyne and communication receivers, what is the purpose of inserting a narrow 'notch filter' in the signal path?

Introduction / Context:
A notch filter (also called a band-stop or interference reject filter) is a very narrow filter used in radio and communication receivers to suppress a single unwanted tone or narrowband interferer without disturbing the surrounding spectrum. This question tests practical receiver architecture and interference-mitigation knowledge.

Given Data / Assumptions:

• The receiver is otherwise properly aligned and operating at its intended intermediate frequency (IF) or audio frequency (AF) stage.
• There is a persistent single-frequency interference (heterodyne whistle, carrier leak, hum tone, or birdie).
• We want to reduce that tone while preserving nearby wanted signals.

Concept / Approach:

A notch filter provides very high attenuation at one precisely tuned frequency f0 and minimal attenuation at frequencies near f0. In receivers, this is useful for eliminating heterodyne whistles, beat notes, or strong narrow carriers that ride on top of the desired signal.

Step-by-Step Solution:

Verification / Alternative check:

If the notch is correctly adjusted, the specific tone disappears in the demodulated audio or the IF spectrum while speech/music intelligibility or data throughput remain unaffected.

Why Other Options Are Wrong:

• Increase gain at one frequency: that is the opposite of a notch (a peaking filter does that).
• Make selectivity uniformly sharper: that is a job for narrow IF roofing/ceramic/SAW filters, not a single-frequency notch.
• Spread bandwidth: a notch does not broaden bandwidth; it removes a very narrow slice.
• Convert AM to FM: filtering does not change the modulation type.

Common Pitfalls:

• Mistaking a notch (narrow stop) for a band-pass peak.
• Trying to use a notch to solve wideband noise problems; it only helps for narrow single-tone issues.

Final Answer:

Reduce the receiver gain at one specific interfering frequency (create a deep, narrow attenuation notch)