AM Radio Receiver

In order to reproduce the AM wave into sound waves, every radio receiver must perform the following functions.

1.      The receiving aerial must intercept a portion of the passing radio waves.

2.      The radio receiver must select the desired radio from a number of radio waves intercepted by the receiving aerial. For this purpose tuned parallel LC circuits must be used. These circuits will select only that radio frequency which is resonant with them.

3.      The selected radio wave must be amplified by the tuned frequency amplifiers.

4.      The audio signal must be recovered from the amplified radio wave.

5.      The audio signal must be amplified by suitable number of audio-amplifiers.

6.      The amplified audio signal should be fed to the speaker for sound reproduction.

Types of AM radio receivers

1.      Straight Radio receiver

2.      Superhetrodyne radio receiver

1. Straight Radio Receiver

• The Receiving antenna is receiving radio waves from different broadcasting stations. The desired radio wave is selected by the tuned RF amplifer which employs tuned parallel circuit. The selected radio wave is amplified by the rf amplifier.
• The amplified radio wave is fed to the detector circuit. This circuit extracts the audio signal from the radio wave. The output of the detector is the audio signal which is amplified by one or more stages of audio-amplifications. The amplified audio signal is fed the speaker for sound reproduction.

Limitations-

1. In straight radio receivers, tuned circuits are used. As it is necessary to change the value of a variable capacitors (gang capacitors) for tuning to the desired station, there is a considerable variation of Q between the closed and open positions of the variable capacitors. This changes the sensitivity and selectivity of the radio receivers.
2. There is too much interference of adjacent stations.

Superhetrodyne Receiver

Here the selected radio frequency is converted to a fixed lower value called intermediate frequency (IF). This is achieved by special electronic circuit called mixer circuit. The production of fixed intermediate frequency (455 KHz) is an important feature  of superhetrodyne circuit. At this fixed intermediate frequency, the amplifier circuit operates with maximum stability, selectivity and sensitivity.

The block diagram of superhetrodyne receiver is a shown in figure below.

                                                                       

Receiving antenna

1. RF amplifier stage- The RF amplifier stage uses a tuned parallel circuit L₁C₁ with a variable capacitor C₁. The radio waves from various broadcasting stations are intercepted by the receiving aerial and are coupled to this stage. This stage selects the desired radio wave and raises the strength of the wave to the desired level.
2. Mixer stage- The amplified output of RF amplifier is fed to the mixer stage where it is combined with the output of a local oscillator. The two frequencies  beat together and produce an intermediate frequency (IF).

IF= Oscillator frequency –radio frequency

The IF is always 455 KHz regardless of the frequency to which the receiver is tuned. The reason why the mixer will always produce 455KHz frequency above the radio frequency is that oscillator always produces a frequency 455KHz above the selected frequency. In practice, capacitance of C₃ is designed to tune the oscillator to a frequency higher than radio frequency by 455KHz.

3. IF amplifier stage- The output of mixer is always 455KHz and is fed to fixed tuned IF amplifiers. These amplifiers are tuned to one frequency (ie 455KHz).
4. Detector stage- The output from the last IF amplifier stage is coupled to the input of the detector stage. Here the audio signal is extracted from the IF output. Usually diode detector circuit is used because of its low distortion and excellent audio fidelity.
5. AF amplifier stage- The audio signal output of detector stage is fed to a multistage audio amplifier. Here the signal is amplified until it is sufficiently strong to drive the speaker. The speaker converts the audio signal into sound waves corresponding to the original sound at the broadcasting station.

Advantages of Superhetrodyne Circuit –

1. High RF amplification
2. Improved selectivity-losses in the tuned circuits are lower at intermediate frequency. Therefore the quality factor Q of the tuned circuits is increased. This makes amplifier circuits to operate with maximum selectivity.
3. Lower cost.