In a superheterodyne radio, the local oscillator (LO) is commonly tuned above the incoming RF frequency. Why is the LO chosen higher in practice?

Introduction / Context:
Local oscillator (LO) injection can be above (high-side) or below (low-side) the desired RF. High-side injection is widely used because it eases several practical issues in consumer receivers and many communication sets.

Given Data / Assumptions:

• Fixed intermediate frequency (IF) is used.
• RF preselection has finite Q and limited attenuation far from the tuned channel.
• Receiver must track across a band with acceptable calibration and stability.

Concept / Approach:

With high-side LO, the image lies at f_image = f_RF + 2·IF, which is farther above the passband than the desired channel by twice the IF. This generally improves image rejection for the same RF filter selectivity. Designers also find dial tracking and band coverage convenient with high-side injection over common broadcast bands, though exact benefits depend on the band and front-end design.

Step-by-Step Solution:

Verification / Alternative check:

Receiver alignment guides and service literature frequently specify LO above RF for broadcast FM and AM designs, citing image and tracking considerations.

Why Other Options Are Wrong:

• “None of the above” contradicts standard practice.

Common Pitfalls:

Believing one LO side is universally superior; the optimal choice can vary with band, IF, and preselector design.

Final Answer:

All of the above