FM Receiver

An FM (frequency modulation) receiver is a type of radio receiver that is designed to receive and process FM signals. Here are the basic sections and operation of an FM receiver:

1. Antenna: The FM antenna captures the incoming FM signals and feeds them into the receiver.

2. RF Amplifier: The RF amplifier amplifies the weak incoming signals captured by the antenna to a level that can be processed by the rest of the receiver.

3. Mixer: The mixer section combines the amplified signal from the RF amplifier with a local oscillator (LO) signal to produce an intermediate frequency (IF) signal. The IF signal is typically at a fixed frequency, such as 10.7 MHz.

4. IF Amplifier: The IF amplifier amplifies the IF signal produced by the mixer to a level that can be further processed by the demodulator section.

 5. Demodulator: The demodulator section is responsible for separating the audio signal from the carrier wave. In an FM receiver, the demodulator typically uses a frequency discriminator or phase-locked loop (PLL) to extract the audio signal.

6. Audio Amplifier: The audio amplifier section amplifies the demodulated audio signal to a level that can be heard through a speaker or headphones.

7. Control Section: The control section of the receiver is responsible for managing the operation of the receiver, including tuning to the correct FM frequency and providing user interface controls.

Overall, the main advantage of FM receivers is their ability to provide high-quality audio with low noise and interference. FM receivers are commonly used in radio broadcasting, two-way radio communication, and music playback systems.

Block diagram of FM receiver

FM receiver is a Superheterodyne receiver and the basic block diagram is same as AMreceiver. The basic differences are as follows.
1. Generally much higher operating frequencies in FM
2. Need for limiting and de-emphasis in FM.
3. Totally different methods of demodulation.
4. Different methods of obtaining AGC.

Comparison with AM Receiver

 Different stages of FM receiver are explained below.

RF Amplifier

Since FM signal has a larger bandwidth it is likely to encounter more noise. Hence to
reduce the noise figure of the receiver, an RF amplifier is used. The RF amplifier stage matches the antenna to the receiver. For this purpose and to avoid neutralization, grounded-base or grounded gate circuits are employed for this stage. Both circuits have low input impedance, suitable for matching with antenna impedance and nether require neutralization. 

Grounded-gate FET RF amplifier

In figure, since gate terminal is grounded, the input and output sides are isolated for RF purposes. There is no feedback and hence no instability. Therefore circuit does not require neutralization. The low impedance of the FET amplifier is matched to antenna through a tuned RF transformer. Both the input and output tank circuits are tuned to carrier frequency.

Oscillators and Mixers

The oscillator circuit may be Clapp and Colpitts which is suited in VHF operation
.Tracking is not normally much of a problem in FM broadcast receivers. This is because the tuning frequency is only 1.25:1.much less than in AM broadcasting.
The mixer stage uses a tuned circuit as its load. The circuit is tuned to Intermediate
frequency of 10.7 MHz and hence selects the difference between incoming carrier frequency and locally generated oscillator frequency.

Intermediate Frequency and IF amplifier

The types and operation do not differ much from their AM counterparts. But the
intermediate frequency and bandwidth required are far higher than in AM broadcast receivers. For receivers operating in the 88 to 108 MHz band is an IF of 10.7 MHz and a bandwidth of 200 KHz. Due to large bandwidth, gain per stage may be low. Two IF amplifier stages are often provided, in which case the shrinkage of bandwidth as stages are cascaded must be taken into account.

Amplitude Limiter:

Frequency modulation is developed to provide a communication system which is less
noise sensitive than AM system.The most of the nose accompanying the desired signal
accompanies it as AM. Thus the intelligence is contained in the frequency variation of signal. So at the receiver remove all the amplitude variations without loss of the information content of the desired signal. To remove the amplitude variations of the signal is the main function of the limiter. At the output of the limiter stage, a constant amplitude signal is obtained even though the amplitude of input signal may be varying.

Amplitude Limiter

Limiter is basically a clipper circuit which clips off the undesired amplitude variations of the input signal. The input signal provides the bias for the FET circuit. Negative bias increases as input increases and hence it lowers the gain of the amplifier for high amplitude of the input signal and output voltage remains constant.
The basic function of the limiter is flat topping(Squaring off) the upper and lower
extremities of the signal.
Although the signal is distorted, it makes no difference as far as FM is considered, since the information is contained in frequency variation and not in amplitude variation.

Use of AGC and Double Limiting

Sometimes it is quite practicable that average input signal amplitude may lie outside the limiting range. As a result, further limiting becomes necessary. The solution for this is the use of double limiter consisting two amplitude limiters in cascade. This gives satisfactory limiting range.
An alternative to the used of second limiter is automatic gain control. The AGC ensures, by reducing the gain for higher signal strengths, that the signal applied to the limiter is within the limiting range of the limiter. This also prevents the overloading of the last IF amplifier stage.