AM SUPERHETERODYNE RECEIVER

• AM broadcast system covers the frequency range from 535 kHz to 1605 kHz.It consists of 106 channels each with a bandwidth of 10 kHz.

• Therefore,all AM receivers must be sensitive enough,to select only one desired frequency from among the signals received.

• The signal received by the antenna is coupled into a tunable-circuit RF section.

• The RF amplifier selects the tuned frequency and rejects all others.

• The signal is then amplified by RF amplifier.

• In the mixer stage,mixing(heterodyning) of the received RF signal and the local oscillator signal occurs.

• The RF,mixer and local oscillator stages are ganged for simultaneous tuning.

• The output of the mixer stage is the difference frequency between the RF signal and the local oscillator signal frequencies.

• The difference frequency is a constant value which lies between the original RF carrier and AF modulating frequency.

• Therefore it is termed as intermediate frequency(IF).

• For AM,IF is fixed as 455 kHz.In the block diagram,since RF signal can vary between 535 to 1605 kHz,local oscillator frequency should be between 535+455 to 1605+455=900 to 2605 kHz.

• IF section is designed for optimum results at the single,fixed frequency.It can contain any number of amplifier circuits.

• The amplified IF signal is coupled to the detector where the original modulating information is recovered.

• The detected audio signal is coupled to suitable voltage and power amplifiers,and finally to the loudspeaker.