In a superheterodyne receiver, if the signal frequency is fs and the intermediate frequency is fi, what is the formula for the image frequency fsi that must be rejected by the RF front end?

Introduction / Context:
The image frequency is a well-known challenge in superheterodyne receivers. It arises because both the desired frequency and another undesired frequency can produce the same intermediate frequency (IF) after mixing. To avoid distortion, image frequency must be rejected by RF filters.

Given Data / Assumptions:

• Superheterodyne receiver with IF = fi.
• Signal frequency = fs.
• Local oscillator frequency = fs + fi (high-side injection assumed).

Concept / Approach:
The image frequency is defined as the frequency that is separated from fs by twice the IF. Mathematically, fsi = fs + 2fi for high-side injection. For low-side injection, fsi = fs − 2fi.

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