Superheterodyne AM Receiver

In superheterodyne radio receivers, the incoming radio signals arc intercepted by the antenna arid converted into the corresponding currents and voltages. In the receiver, the incoming signal frequency is mixed with a locally generated frequency. The output of the mixer consists of the sum and difference of the two frequencies. The mixing of the two frequencies is termed heterodyning. Out of the two resultant components of the mixer, the sum component is rejected and the difference component is selected. The value of the difference frequency component varies with the incoming frequencies, if the frequency of the local oscillator is kept constant. It is possible to keep the frequency of the difference components constant by varying the frequency of the local oscillator according to the incoming signal frequency. In this case, the process is called Superheterodyne and the receiver is known as a superheterodyne radio receiver.

Superheterodyne AM Receiver
Superheterodyne AM Receiver Block Diagram

In Figure the receiving antenna intercepts the radio signals and feeds the RF amplifier, The RF amplifier selects the desired signal frequency and amplifies its voltage, The RF' amplifier is a small-signal voltage amplifier that operates in the RF range. This amplifier is tuned to the desired signal frequency by using capacitive tuning.

After suitable amplification of the RF signal it is fed to the mixer. The mixer takes another input from a local oscillator, which generates a frequency according to the frequency of the selected signal so that the difference equals. a predetermined value. The mixer consists of a non-linear device, such as a transistor. Due to the non-linearity, the mixer output consists of a number of frequency components. It provides sum and difference frequency components along with their higher harmonics. A tuned circuit at the output of the mixer selects only the difference component while rejecting all other components. The difference component is called the intermediate frequency or IF the value of IF frequency is always constant and is equal to 455 KHz.

For a constant IF frequency for all incoming signals, the frequency of the local oscillator is adjusted using capacitive tuning. The incoming signal is also selected using capacitive tuning. The two capacitors used to select the incoming signal and the oscillator frequency is ganged together so that the tuning of both the RF amplifier and the local oscillator circuits is done simultaneously. This arrangement ensures that the local oscillator has the correct frequency to generate constant IF frequencies. The mixer stage is also tuned to IF frequency using capacitive tuning. The tuning capacitor is also ganged with the RF amplifier and the local oscillator. Thus all the three stages are tuned at the same time to the required frequency through the ganged Capacitor, which consists of the three tuning capacitors.

The IF signal is fed to an IF amplifier with two amplifier stages. This provides enough signal amplification so that the signal is properly detected. The amplified IF signal is fed to the linear diode detector, which demodulates the received AM signal. The output of the detector stage is the original modulating signal. This signal is given to the audio driver stage, which amplifies its voltage to drive the power amplifier, which is the last stage of the receiver.

The power of the modulating signal and finally is passed to the power amplifier amplifies the speaker. The speaker converts the audio currents into sound energy.