Types of AM Radio Receivers

A radio receiver is a device which reproduces the modulated or radio waves into sound waves. The radio receivers used in reproducing amplitude modulated signals are called AM radio receivers.

So as to reproduce the AM wave into sound waves, every radio receiver must perform the following functions:

• The receiving aerial must intercept a portion of the passing radio waves.
• The radio receiver must select the desired radio wave from a number of radio waves intercepted by the receiving aerial. For this goal to be achieved tuned parallel LC circuit must be used. These circuits will select only that radio frequency which is in resonant with them.
• The selected radio wave must be amplified by the tuned frequency amplifiers.
• The audio signal must be recovered from the amplified radio wave.
• The audio signal must be amplified by suitable number of audio-amplifiers.
• The amplified audio signals must be fed to the speaker for the sound reproduction.

Related: General Principles of Radio Broadcasting, Transmission and Reception

Types of AM Radio Receivers

The AM radio receivers can be classified into two types:

1. Straight radio receivers
2. Superheterodyne radio receivers

The straight radio receiver was used in the early days of radio communication. Nevertheless, presently all radio receivers are of Superheterodyne type.

Straight Radio Receiver

The figure below shows a block diagram of a straight radio receiver:

The aerial is receiving radio waves from different broadcasting stations. The desired radio wave is chosen by the RF amplifier which employs a tuned parallel circuit. The selected radio wave is amplified by the tuned RF amplifiers. 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 to the speaker for sound reproduction.

The Limitations of Straight Radio Receiver

• In straight radio receivers, tuned circuits are used. Since it is necessary to change the value of the variable capacitors (gang capacitors) for tuning to the desired station, hence, 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.
• There is too much interference of adjacent stations.

Superheterodyne Radio Receiver

The shortcomings of straight radio receiver were overcome by the invention of Superheterodyne receiver by Major Edwin H. Armstrong during the 1^st world war. Presently, all modern receivers utilise the superheterodyne circuit. In this type of radio receiver, the selected radio frequency is converted to a fixed lower value, called intermediate frequency (IF). This is accomplished by a special electronic circuit called mixer circuit. There is a local oscillator in the radio receiver itself. This oscillator produces high frequency waves. The selected radio frequency is mixed with high frequency wave by the mixer circuit. In this process, beats are produced and the mixer produces a frequency equal to the difference between the local oscillator and radio wave frequency. The circuit is designed so that oscillation always produces a frequency of 455 kHz above the selected radio frequency. Hence, the mixer will always produce an intermediate frequency (IF) of 455 kHz regardless of the station to which the receiver is tuned. For example, if 700 kHz station is tuned, then the local oscillator will produce 1155 kHz. As a result the output from the mixer will have a frequency of 455 KHz.

From the block diagram in Figure (b) above, the selected radio frequency f₁ is mixed with a frequency f₂ from a local oscillator. The output from the mixer is a difference (f₂ – f₁) and is always 455 kHz regardless of the station to which the receiver is tuned. The production of fixed intermediate frequency (455 kHz) is the outstanding feature of Superheterodyne circuit. At this fixed intermediate frequency, the amplifier circuits operate with the maximum stability, selectivity and sensitivity. Since the conversion of incoming radio frequency to the intermediate frequency is achieved by heterodyning or beating the local oscillator against the radio frequency hence this circuit is called Superheterodyne circuit.

Stages of Superheterodyne Radio Receiver

Let’s consider the block diagram of a Superheterodyne receiver below:

The RF amplifier stage, mixer stager and oscillator stage are tuned parallel circuits with variable capacitors. These capacitors are ganged together as illustrated by the dotted interconnecting lines. The rotation of the common shaft simultaneously changes the capacitance of these tuned circuits.

RF Amplifier Stage

The RF amplifier stage employs 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.

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). The intermediate frequency is the difference between oscillator frequency and radio frequency, i.e.

IF = Oscillator frequency – Radio frequency

The IF is always 455 kHz regardless of the frequency to which the receiver is tuned. This is because the oscillator always produces 455 kHz above the selected radio frequency. This is realized by making C₃ smaller than C₁ and C₂. By making C₃ smaller, the oscillator will tune to a higher frequency. In practice capacitance of C₃ is designed to tune the oscillator to a frequency higher than the radio wave frequency by 455 kHz. This frequency difference (i.e. 455 kHz) will always be maintained because when C₁ and C₂ are varied C₃ will also vary proportionally. It can be noted, that in a mixer stage; the carrier frequency is reduced. The IF still contains the audio signal.

IF Amplifier stage

The output of the mixer is always 455 kHz and is fed to fixed tuned IF amplifier. These amplifiers are tuned to one frequency i.e. 455 kHz and provide a proper amplification.

Detector stage

The output from the last IF amplifier stage is coupled to the input of the detector stage; where the audio signal is separated from the IF signal. Normally a diode circuit is employed because of its low distortion and exceptional audio fidelity.

AF Amplifier stage

The audio signal output of a 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 of the broadcasting station.

The advantages of Superheterodyne Circuit

The Superheterodyne principle has the following advantages:

• High RF amplification –the Superheterodyne principle makes it possible to produce an intermediate frequency (i.e. 455 kHz) which is much less than the radio frequency. RF amplification at low frequencies is more stable since feedback through stray and interelectrode capacitance is reduced.
• Improved selectivity – losses in the tuned circuits are lower at immediate frequency. Hence, the quality factor Q of the tuned circuits is increased. This makes the amplifier circuits to operate with maximum selectivity.
• Lower cost – in Superheterodyne, a fixed intermediate frequency is obtained regardless of the radio wave chosen. This permits the use of RF amplifiers. The Superheterodyne receiver is therefore cheaper than other radio receivers.

Related: Demodulation in Telecommunication Systems