Question

The image channel rejection in superheterodyne receiver comes from

• A. IF stage only
• B. RF stage only
• C. Detector only
• D. RF and Detector stages

Hint:

Image frequency in superheterodyne receiver: \(f_{image} = f_{signal} \pm 2f_{IF}\) (sign depends on whether \(f_{LO}\) is above or below \(f_{signal}\)). If \(f_{LO} = f_{signal} + f_{IF}\), then \(f_{image} = f_{signal} + 2f_{IF}\).
Image rejection is primarily provided by the selectivity of the RF tuned circuits (preselector) before the mixer.
A higher IF frequency generally makes image rejection easier because the image frequency is further away from the desired signal frequency.

Answer 1

The Correct Option is B

In a superheterodyne receiver, the incoming RF signal (\(f_{RF}\)) is mixed with a local oscillator signal (\(f_{LO}\)) to produce an Intermediate Frequency (IF):

\(f_{IF} = |f_{RF} - f_{LO}|\) 

An image frequency (\(f_{image}\)) is an unwanted RF signal that, when mixed with the same \(f_{LO}\), also produces the same \(f_{IF}\).

If \(f_{LO} > f_{RF}\) (common case), then:

\(f_{IF} = f_{LO} - f_{RF}\)

The image frequency is:

\(f_{image} = f_{LO} + f_{IF}\)

Alternatively, if \(f_{LO} < f_{RF}\), then:

\(f_{IF} = f_{RF} - f_{LO}\)

The image frequency is:

\(f_{image} = f_{LO} - f_{IF}\) (which is \(f_{RF} - 2f_{IF}\)). More commonly stated as:

\(f_{image} = f_{RF} + 2f_{IF}\) if \(f_{LO} = f_{RF} + f_{IF}\) or \(f_{image} = f_{RF} - 2f_{IF}\) if \(f_{LO} = f_{RF} - f_{IF}\).

The primary rejection of the image frequency is achieved by the RF (Radio Frequency) stage, specifically the RF amplifier and the tuned circuits (preselector) before the mixer. These tuned circuits are designed to pass the desired \(f_{RF}\) and significantly attenuate signals at other frequencies, including the image frequency.

The selectivity of this RF stage determines the image rejection ratio.

The IF stage has high selectivity but is tuned to the \(f_{IF}\). If the image frequency signal manages to pass through the RF stage and get converted to \(f_{IF}\) by the mixer, the IF stage cannot distinguish it from the desired signal (as both are now at \(f_{IF}\)). The detector demodulates the \(f_{IF}\) signal.

Therefore, the image channel rejection primarily comes from the RF stage.

Final Answer:

RF stage only