The first section is the RF section, which is a tunable circuit connected to the antenna terminals. It is used to select only the desired RF signal out off a number of frequencies to the receiver. The RF amplifier is a tuned voltage amplifier and it contains a parallel LC tunes circuit. This tuned circuit selects the desired RF signal from a number of frequencies to the receiver.
In the mixer the incoming signal frequency is mixed with the frequency generated by a local oscillator to convert it into a lower fixed frequency called intermediate frequency. It is 10.7 MHz in FM receivers. The local oscillator will be a high frequency oscillator. The RF amplifiers and the local oscillator are tunes together, so that difference frequency at the output of the mixer will be equal to the intermediate frequency. The local oscillator frequency always kept above the signal frequency by an amount equal to IF.
The output of the mixer is applied to the IF amplifier stages. The intermediate frequency and the bandwidth required in FM are higher than that in AM receivers. Typical bandwidth for a receiver operating in 88-108 MHz and IF of 10.7 MHz is 200 KHz. Two IF amplifier stages are often provided.
FM demodulation is totally different from AM demodulation. Balanced slope detector, Foster-Seeley discriminator and Ratio detectors are common types of demodulators used for FM detection. De-emphasis circuit is used to attenuate the high frequencies in order to compensate the boosting at the transmitter.
The amplitude of the FM signal remains constant. But by traveling from the transmitter to the receiver antenna, external sources produce unwanted variations in the signal amplitudes. These variations are easy to detect because the amplitude of the original FM signal remains constant. The limiter is a form of clipping device that does not produce an output, when the positive or negative amplitude of the FM signal exceeds a pre-determined level. So FM receivers can be integrated with amplitude limiters to take away the amplitude variation caused by noise. Hence FM reception is more immune to noise than AM reception.
There are different methods of obtain AGC in an FM receiver. The limiter user has leak type bias; this bias voltage changes proportional to the input voltage and is thus used for Automatic Gain Control. Occasionally a further Automatic Gain Control detector is used which takes positive output of the final IF amplifier and it rectifies and filters in the common manner.
