Question 6A467

In a superheterodyne receiver, the incoming Radio Frequency (RF) signal is mixed with a Local Oscillator (LO) signal to produce an Intermediate Frequency (IF). The receiver is tuned to the desired RF signal (F_RF) such that either F_LO - F_RF = F_IF or F_RF - F_LO = F_IF. An image frequency (F_IMAGE) is a different RF signal that, when mixed with the *same* LO frequency, *also* produces a signal at the IF. If F_LO - F_RF = F_IF (high-side injection), then F_RF = F_LO - F_IF. The image frequency would be F_IMAGE = F_LO + F_IF. The difference between the image and the desired frequency is F_IMAGE - F_RF = (F_LO + F_IF) - (F_LO - F_IF) = 2 * F_IF. Therefore, the incoming signal causing image interference is *twice* the IF above or below the frequency to which the receiver is tuned. This is a fundamental characteristic of superheterodyne receiver design.