Selectivity: FM signals are typically modulated at very high frequencies, often in the range of 88 to 108 MHz. At these frequencies, it can be difficult to design a radio receiver that is able to select the desired signal while rejecting unwanted signals from adjacent channels. By converting the FM signal to a lower IF frequency, it becomes easier to design filters that can effectively separate the desired signal from the unwanted signals.
Gain: The process of converting the FM signal to an IF frequency can also provide some additional gain, which can be useful for weak signals. This is because the IF amplifier can be designed to have a higher gain than the RF amplifier, which is limited by the need to maintain a low noise figure.
Image rejection: When a radio receiver is tuned to a particular frequency, it can also receive a signal at a frequency that is twice the desired frequency. This is known as the image frequency, and it can interfere with the desired signal if it is not properly rejected. By converting the FM signal to an IF frequency, it is possible to design the IF amplifier to reject the image frequency more effectively.
AGC (Automatic Gain Control): The IF stage in a radio receiver can also be used for automatic gain control (AGC). AGC ensures that the output signal has a consistent level, even when the input signal varies. By using an IF amplifier with AGC, it is possible to maintain a constant level of output signal while rejecting unwanted signals.
In summary, converting an FM signal to an IF frequency allows for better selectivity, gain, image rejection, and AGC, which are all important factors in designing a radio receiver.