Describe image frequency and why it can cause interference in a superheterodyne receiver.

• A. The image frequency is f_image = f_RF ± 2f_IF; an alternative frequency that also produces the same IF, causing potential image interference if not filtered.
• B. The image frequency is always equal to the LO frequency.
• C. The image frequency is the same as the RF frequency, causing no interference.
• D. The image frequency is twice the LO frequency and always filtered out.

Answer: (A)

In a superheterodyne receiver, the incoming RF is mixed with a local oscillator to produce a fixed intermediate frequency, determined by the difference between the LO and the RF. Because mixing depends only on the frequency difference, there is another RF frequency that can produce the same IF when combined with the same LO. This other frequency lies 2 f_IF away from the desired RF, so it is called the image frequency. If the LO is above the RF, the image appears at f_RF + 2 f_IF; if the LO is below, it appears at f_RF - 2 f_IF. Without proper RF filtering, a strong signal at this image frequency can downconvert to the same IF and interfere with the desired signal. That’s why the image frequency is described as an alternative frequency that also yields the same IF and can cause interference if not filtered out before mixing.