Radio transmission - AM, FM transmitter and AF, RF section

Radio transmission

After modulation, the radio waves are transmitted over long distances with the help of electronic circuits called transmitters. The simplest form of transmitter consists of an oscillator, generating a high frequency wave connected to an antenna. In this section, we shall discuss the amplitude and frequency modulated transmitters.

Amplitude modulated (AM) transmitter

Fig  gives the block diagram of amplitude modulated radio transmitter. It consists of two sections (i) Audio frequency (AF) section and (ii) Radio frequency (RF) section.

AF section

The AF section of the transmitter generates the modulating wave (signal). The conversion of sound energy into electrical energy is performed by the microphone.

The electrical energy available from the microphone is very low. Hence, it is amplified through an amplifier. The output from the AF amplifier is fed to the AF power amplifier. The power amplifier provides the required audio frequency power. The output of the AF power amplifier is given to the modulator. A modulator is an electronic circuit with transistor and passive components, which performs the process of modulation.

RF section

In the RF section, the high frequency carrier wave is generated by a crystal controlled oscillator. The output of the crystal controlled oscillator is power amplified by RF power amplifier. The buffer* isolates the RF power amplifier from the oscillator. This arrangement keeps the frequency of the crystal controlled oscillator as a constant. In the modulator the RF wave and modulating AF signal are mixed to produce the amplitude modulated wave. The output of this section is fed to the antenna for transmission.

Frequency modulated (FM) transmitter

Frequency modulated systems are operated usually at a frequency above 40 MHz. Frequency modulated broadcasting is done in television sound, mobile radio etc. The functional block diagram of a FM transmitter employing phase modulation is shown in Fig . The phase modulation is essentially a frequency modulation.

It consists of a crystal oscillator, which produces the carrier wave and the output of this is fed into the phase modulator. The buffer is a low frequency amplifier which isolates the crystal oscillator from the phase modulator.

The modulating signal is produced from a microphone. Since this AF modulating signal has uneven power, it is fed into a network called pre-emphasis network, where all the frequencies in the modulating signal are made to have equal power. The output of the pre-emphasis network is then amplified and sent for phase modulation. The modulated output is then power amplified using a power amplifier and then fed into the transmitting antenna for transmission.