Local oscillator setting in radio receivers In a standard superheterodyne receiver using high-side injection, how is the local oscillator (LO) frequency chosen relative to the incoming RF signal frequency?

Introduction / Context:
Superheterodyne receivers translate the desired RF signal to a fixed intermediate frequency (IF) for consistent amplification and filtering. The local oscillator sets the difference frequency and thus determines the IF outcome and potential image frequency location.

Given Data / Assumptions:

• Conventional high-side injection is assumed (common in broadcast receivers).
• IF is fixed (e.g., 455 kHz for AM broadcast).
• Mixer produces sum and difference frequencies.

Concept / Approach:

With high-side injection, the LO is set above the RF such that IF = fLO − fS. This places the image at fIM = fLO + IF, separated from fS by 2*IF, which can simplify front-end rejection. Although low-side injection is possible, most broadcast designs use high-side for practical tracking and image considerations.

Step-by-Step Solution:

Verification / Alternative check:

Service manuals and receiver schematics show oscillator tracking curves where LO runs above the RF across the tuning range.

Why Other Options Are Wrong:

(b) describes low-side injection, which is not the standard assumption here; (c) would give IF = 0; (d) is incorrect because LO is precisely chosen; (e) is unrelated to superhet translation.

Common Pitfalls:

Confusing image location and injection side; forgetting that either injection side can be used depending on design.

Final Answer:

LO frequency is higher than the incoming signal frequency