Features of FM Receivers

FM receivers employ heterodyne principle. The FM broadcast signals lie in the frequency range between 88 MHz and 108 MHz. The intermediate frequency (IF) of a FM receiver is 10.7 MHz which is much higher than the IF value of 455 kHz in AM receivers.

Let’s consider the block diagram of an FM receiver below:

The FM Receiver is made up of the following components:

RF Tuner

The FM signals are usually in the range of 88 MHz to 108 MHz. The weak FM signal is picked up by the antenna and fed to the RF tuner. The RF tuner consists of the following components:

• RF amplifier
• Mixer
• Local oscillator

The RF amplifier amplifies the selected FM signal. The output from the RF amplifier is fed to the mixer stage where it is combined with the output signal from a local oscillator. The two frequencies beat together to produce an intermediate frequency (IF). The intermediate frequency is equal to the difference between the oscillator frequency and the RF frequency. The IF is always 10.7 MHz regardless of the frequency to which the FM receiver is tuned.

IF Amplifier Stage

The output from the mixer has a frequency of 10.7 MHz and is fed to the IF amplifiers where the IF signal is amplified.

Limiter stage

The output from the IF stage is fed to the limiter. This circuit is an IF amplifier that is tuned to 10.7 MHz but its main function is to remove AM interference from the FM signal. The input to the limiter is an FM signal, but it has different amplitude levels because AM interference has been added. Therefore, the limiter circuit keeps the output constant for different input levels.

FM detector

After the removal of amplitude modulation from the FM signal by the limiter, the IF signal, drives the input to the FM detector. FM detector converts frequency variations to amplitude variations. The FM detector is also termed to as discriminator because it can distinguish between different frequencies in the input and provide different output voltages.

The resultant amplitude modulated signal is then rectified and amplified before being fed to the speaker for sound reproduction.

Related articles:

• Types of AM Radio Receivers
• Demodulation in Telecommunication Systems
• Modulation in Communication Systems

The differences between FM and AM Receivers

Although both AM and FM receivers work on superheterodyne principle, they differ in the following ways:

• FM broadcast signals lies in the frequency range between 88 MHz and 108 MHz whereas AM broadcast signals lie in the frequency range between 540 kHz and 1600 kHz.
• An FM receiver has two additional stages i.e. limiter and discriminator, which are quite different from an AM receiver.
• FM receivers are free from interference and this implies that much weaker signals can be successfully handled.
• The intermediate frequency (IF) for FM receivers is 10.7 MHz whereas the IF for AM receivers is 455 kHz.
• FM bandwidth is about 200 kHz compared to 10 kHz for AM.