In a classic superheterodyne receiver, what is the image frequency?

• A. The RF frequency that, when mixed, also converts to the same intermediate frequency.
• B. The LO frequency.
• C. The baseband frequency.
• D. The demodulated audio frequency.

Answer: (A)

In a superheterodyne receiver, the mixer produces the intermediate frequency from the difference between the local oscillator and the incoming RF signal. Because that difference can be achieved by two different RF frequencies for a fixed LO, there exists an unwanted RF frequency—the image frequency—that would also convert to the same IF when mixed. This is why front‑end filtering is used to reject the image and keep only the desired signal. The image frequency is not the LO itself, nor the baseband, nor the demodulated audio; it is a separate RF frequency that, for a given LO and IF, would yield the same IF after mixing (f_image = f_LO ± f_IF).