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 Image Frequency.In a superheterodyne receiver, the mixer converts the desired RF signal (\(f_{RF}\)) to the intermediate frequency (\(f_{IF}\)) using a local oscillator (\(f_{LO}\)). This relationship is defined as \(f_{IF} = |f_{LO} - f_{RF}|\). However, the image frequency (\(f_{image}\)) can also mix with the LO to produce the same \(f_{IF}\). If \(f_{LO}>f_{RF}\), then \(f_{image} = f_{LO} + f_{IF} = f_{RF} + 2f_{IF}\).
Step 2: Analyze Rejection Point.The IF amplifier is designed to only pass the IF (\(f_{IF}\)). It cannot differentiate between signals originating from \(f_{RF}\) or \(f_{image}\) because both are converted to \(f_{IF}\) before the IF stage. The detector, situated after the IF stage, also offers no rejection.Therefore, image frequency rejection must occur \textit{before} the mixer.
Step 3: Identify the Stage Responsible.The RF amplifier is the only stage preceding the mixer. This amplifier employs a tuned circuit (bandpass filter) centered on the desired RF frequency. Its purpose is to pass \(f_{RF}\) while attenuating other frequencies, including \(f_{image}\). The RF stage's selectivity determines the receiver's image frequency rejection ratio.