Effect of choosing a higher intermediate frequency (IF) in a superheterodyne receiver: which statement is correct?

Introduction / Context:
In a superheterodyne receiver, the choice of intermediate frequency (IF) strongly affects image rejection, selectivity, and tuning (tracking) between the RF front end and the local oscillator (LO). Designers balance these factors when selecting a practical IF for a given band and application.

Given Data / Assumptions:

• Higher IF increases frequency separation between RF and LO.
• IF selectivity depends on achievable Q and filter technology (bandwidth roughly scales with center frequency for a given Q).
• Adjacent-channel rejection depends on overall selectivity at IF.

Concept / Approach:

Raising IF improves image rejection because the image frequency moves farther from the desired RF, easing RF preselection. However, for a given practical Q, bandwidth BW ≈ f₀/Q increases with center frequency, so making IF higher can make narrow selectivity harder, potentially worsening adjacent-channel rejection unless higher-Q filters are used. Also, a larger separation between LO and RF increases tracking difficulty across the tuning range, since the LO and RF tuned circuits must maintain a precise frequency relationship across the band.

Step-by-Step Solution:

Verification / Alternative check:

Classic receiver design references recommend moderate IF choices (e.g., ~455 kHz AM broadcast) balancing image rejection and IF selectivity; higher IFs are used with crystal/ceramic/SAW filters to restore selectivity but otherwise complicate tracking.

Why Other Options Are Wrong:

• Improves selectivity: not generally true for given Q; selectivity typically worsens unless better filters are used.
• Decreases tracking problems: opposite of reality.
• Improves adjacent-channel rejection automatically: depends on selectivity; higher IF alone does not ensure it.
• Improves both selectivity and image rejection without trade-offs: unrealistic; there is always a trade-off.

Common Pitfalls:

Assuming one parameter (IF) can optimize everything simultaneously; in practice, designers use multiple conversion stages or high-Q filtering (e.g., crystal/SAW) to manage compromises.

Final Answer:

Increases tracking problems in the RF and local oscillator tuning