The answer is option 'A' - The Preselector and RF stages.

Explanation:

In a superheterodyne AM receiver, the image channel selectivity is determined by the Preselector and RF stages. Let's understand why.

What is a superheterodyne AM receiver?

A superheterodyne AM receiver is a type of radio receiver that uses frequency mixing to convert the incoming radio frequency (RF) signal to a lower intermediate frequency (IF) signal. It consists of several stages, including the antenna, preselector, RF amplifier, mixer, IF amplifier, demodulator, and audio amplifier.

Understanding image channel selectivity:

In a superheterodyne receiver, the image channel refers to a condition where two different radio frequencies can produce the same intermediate frequency. This occurs when the sum or difference of the RF frequency and the intermediate frequency is equal to the local oscillator frequency.

For example, let's consider a superheterodyne receiver with an RF frequency of 1000 kHz, IF frequency of 455 kHz, and a local oscillator frequency of 1455 kHz. In this case, both an RF frequency of 1000 kHz and an RF frequency of 1910 kHz (1000 kHz + 910 kHz) will produce the same intermediate frequency of 455 kHz.

The role of the Preselector and RF stages:

The preselector and RF stages play a crucial role in preventing the image channel from causing interference in the receiver. Here's why:

1. Preselector: The preselector is a bandpass filter that is placed before the RF amplifier stage. Its purpose is to select the desired RF frequency and reject unwanted frequencies outside the desired frequency range. It helps in reducing the possibility of image channel interference by attenuating the signals that may produce the same intermediate frequency when mixed with the local oscillator frequency.

2. RF stages: The RF stages, including the RF amplifier, further amplify the selected RF signal from the preselector. They also provide additional selectivity by filtering out unwanted signals that may still be present after the preselector. The RF stages help in improving the signal-to-noise ratio and reducing the possibility of image channel interference.

Conclusion:

In a superheterodyne AM receiver, the image channel selectivity is primarily determined by the preselector and RF stages. These stages help in rejecting unwanted frequencies and attenuating signals that may produce the same intermediate frequency as the desired signal. By effectively filtering out interference, the preselector and RF stages ensure that only the desired RF frequency is mixed with the local oscillator frequency to produce the desired intermediate frequency.