Which statement about image interference and filter placement is correct?

• A. An image interference occurs when the image frequency is not properly filtered before mixing, causing the same IF as the desired signal.
• B. Image interference is caused by LO drift and cannot be mitigated by preselection.
• C. Image interference is only a concern in digital receivers.
• D. Image interference is mitigated by decreasing the IF frequency.

Answer: (A)

In a superheterodyne receiver, there is an image frequency that, when mixed with the local oscillator, also produces the same intermediate frequency as the desired signal. If that image is not filtered out before the mixer, it will fold into the IF and interfere with the wanted signal. A preselection filter at the front end is used to reject this image frequency, passing only the desired RF channel and thus preventing the unwanted signal from reaching the mixer. This filtering is what stops image interference from corrupting reception.

LO drift is a separate issue; preselection helps with image rejection but doesn’t fix all drift-related problems. Image interference isn’t limited to digital receivers, and decreasing the IF frequency typically makes image interference harder to filter, not easier, since the image lies closer to the desired signal.