Effect of choosing too high an intermediate frequency (IF) in a superheterodyne radio receiver: Which statement is correct?

Introduction / Context:
The selection of intermediate frequency (IF) in superheterodyne receivers is a key design compromise. Higher IF values impact selectivity, tracking, and adjacent-channel behavior. Understanding these trade-offs is essential in communication system design.

Given Data / Assumptions:

• Receiver uses a single, fixed IF.
• We are evaluating the effects of choosing an excessively high IF.
• Quality factor (Q) of tuned circuits and practical filter limitations apply.

Concept / Approach:

Bandwidth BW for a tuned circuit is approximately f_center / Q. For a given practical Q, a higher center frequency (higher IF) increases bandwidth, which reduces selectivity and worsens adjacent-channel rejection. Tracking—the ability of RF and LO tuned circuits to follow each other across the band—tends to be easier with higher IF because frequency offsets are proportionally larger and front-end alignment becomes less critical.

Step-by-Step Solution:

Verification / Alternative check:

Standard receiver design texts note that increasing IF improves image rejection but degrades selectivity and adjacent-channel performance unless very high-Q filters (or crystal/SAW filters) are used.

Why Other Options Are Wrong:

• (b) alone is not the best single answer; although often true, it does not address the main trade-off asked here.
• (c) is incorrect because poorer selectivity causes worse, not better, adjacent-channel rejection.
• (d) All of the above is false since (c) is false.
• (e) None of the above is false because (a) is true.

Common Pitfalls:

• Confusing image-frequency rejection (which improves with higher IF) with adjacent-channel rejection (which degrades without higher-Q filters).

Final Answer:

Selectivity will be poor