Superheterodyne FM Receiver
The block diagram of an FM receiver is illustrated in Figure (a). The RF amplifier amplifies the received signal intercepted by the antenna. The amplified signal is then applied to the mixer stage. The second input of the mixer comes from the local oscillator. The two input frequencies of the mixer generate an IF signal of 10.7 MHz. This signal is then amplified by the IF amplifier. Figure (a) shows the block diagram of an FM receiver.
The output of the IF amplifier is applied to the limiter circuit. The limiter removes the noise in the received signal and gives a constant amplitude signal. This circuit is required when a phase discriminator is used to demodulate an FM signal.
The output of the limiter is now applied to the FM discriminator, which recovers the modulating signal. However, this signal is still not the original modulating signal. Before applying it to the audio amplifier stages, it is de-emphasized. De-emphasizing attenuates the higher frequencies to bring them back to their original amplitudes as these are boosted or emphasized before transmission. The output of the de-emphasized stage is the audio signal, which is then applied to the audio stages and finally to the speaker.
It should be noted that a limiter circuit is required with the FM discriminators. If the demodulator stage uses a ratio detector instead of the discriminator, then a limiter is not required. This is because the ratio detector limits the amplitude of the received signal. In Figure (a) a dotted block that covers the limiter and the discriminator is marked as the ratio detector.
In FM receivers, generally, AGC is not required because the amplitude of the carrier is kept constant by the limiter circuit. Therefore, the input to the audio stages controls amplitudes and there are no erratic changes the volume level. However, AGC may be provided using an AGC detector. This generates a dc voltage to control the gains of the RF and IF amplifier.
RF Amplifier Using FET
The RF amplifier in FM receivers uses FETs as the amplifying device. A bipolar junction transistor can also be used for the purpose, but an FET has certain advantages over BJT. These are explained below:
- An FET follows the square law for its operation, the characteristics; curves of an FET have non-linear regions. Due to the non-linearity, higher harmonics of the signal frequency are generated in the output. The major advantage of an FET is that it generates only the second harmonic components of the signal. This is known as the square law. Harmonics higher than the second harmonic is nearly absent in the output of an FET amplifier. The higher harmonics produce harmonic distortions and arc undesirable. In FETs, as only the second harmonics are present; it is easy to filter these out by using the tuned circuits. BJTs also generate higher harmonics, but they do not follow the square law. Therefore, they provide more harmonic distortion than FETs. Thus, FETs are always preferred in the RF amplifier of an FM receiver.
- In BJT amplifiers, cross-modulation occurs if a strong signal of an adjacent channel gets through the tuned circuits in the presence of a weak desired signal. The adjacent channel will generate higher harmonics, which may come within the pass-band of the desired signal. This will produce noise and distortions at the output. On the other hand, The effect of cross-modulation is minimized in FET amplifiers, as the unwanted adjacent channel will also produce only its second harmonic components, which may not fall into the pass-band of the desired channel and thus are easily filtered out.
- The input impedance of an FET becomes small due to the small input capacitive reactance of FET at very high FM frequencies. This makes it easy to match the small impedance of the antenna, typically 100 ohms, with the small input impedance of PET. This is not possible with BJTs.
Limiter circuit is used in FM receiver to remove the noise present in the peaks of the received signal and to remove any amplitude variation in the received signal; the output of the limiter has constant amplitude. This is very in important in FM receivers because at amplitude variation in the received carrier will result in unfaithful reproduction of the audio signals. Figure (b) shows the typical circuit diagram of a limiter circuit used in an FM receiver.
A typical circuit diagram of a limiter using FET is illustrated in figure (b). This circuit has a leak-type bias at the gate, through R. and C, The source resistance is RS and the source bypass capacitor is C, The capacitor CN provides the neutralization of the signal passing through the internal capacitance between the gate and the drain. The limiting action is provided by the gate and drain circuits.
Gate Limiting Action
If the input voltage increases, then the gate bias of an FET accordingly increases. The increase in the negative bias at the gate will reduce the gain of the amplifier. This will reduce the output of the circuit so a constant amplitude signal will be applied to the discriminator. It should be noted that for small input voltages, the limiting action will not take place as there will be no appreciable change in the gate biasing voltage. The limiting action only takes place for large input signals.
Drain Limiting Action
The limiting action for low amplitude variations is achieved by using the drain circuit. The drain DC supply is kept at half the normal DC drain voltage through the dropping resistance Rd. With this arrangement, low input voltages result in the saturation of the output current. This action limits the amplitude of the output signal. Under this condition, it may be possible that the gate-drain section forward-biased. If this happens, then the input and output will be short-circuited. To avoid this undesirable situation, a small resistance of a few hundred ohms, R, is placed in between the drain and the tank circuit, as shown in figure (a).