Question

The image channel rejection in a superheterodyne receiver comes from

• A. detector and RF stages only
• B. detector, RF and IF stages
• C. IF stages only
• D. RF stages only

Hint:

Image frequency rejection is the primary purpose of the RF stage (preselector) in a superheterodyne receiver. Once a signal is converted to IF, it's too late to reject the image.

Answer 1

The Correct Option is D

Step 1: Define the Image Frequency. In a superheterodyne receiver, the mixer down-converts the desired RF signal (\(f_{RF}\)) to the intermediate frequency (\(f_{IF}\)) using a local oscillator (\(f_{LO}\)). The relationship is \(f_{IF} = |f_{LO} - f_{RF}|\). However, another frequency, the image frequency (\(f_{image}\)), can also mix with the LO to produce the same IF. If \(f_{LO}>f_{RF}\), then \(f_{image} = f_{LO} + f_{IF} = f_{RF} + 2f_{IF}\).
Step 2: Analyze where rejection occurs. The IF amplifier is designed to pass only the IF (\(f_{IF}\)). It cannot distinguish between a signal that was originally at \(f_{RF}\) and one that was at \(f_{image}\) because both have been converted to the same intermediate frequency before reaching the IF stage. The detector operates after the IF stage, so it also cannot provide any rejection. Therefore, the rejection of the image frequency must happen \textit{before} the mixer.
Step 3: Identify the relevant stage. The only stage before the mixer is the RF amplifier. The RF amplifier is a tuned circuit (a bandpass filter) centered around the desired RF frequency. It is this stage's job to pass \(f_{RF}\) while attenuating all other frequencies, including the image frequency. The selectivity of the RF stage determines the image frequency rejection ratio of the receiver.