FM Transmitter Block Diagram with Explanation of Each Block
Block diagram of a low level FM broadcast transmitter is shown in figure. The master oscillator generates the RF signal (carrier) required for modulation. Master oscillator is generally a well defined LC oscillator. The buffer amplifier is used to make the oscillator frequency free from the loading of the next stages.
The frequency modulation is achieved by the reactance modulator. In reactance modulator, a voltage variable reactance is placed across the tank circuit of the master oscillator. It can be a varactor diode, FET, BJT or vacuum tube. The tank is tuned so that the oscillating frequency is equal to the desired carrier frequency. When we apply the modulating voltage, the reactance of the voltage variable reactance changes and hence the tank circuit reactance also varies. This changes the frequency of oscillation of the oscillator. When the modulating voltage is increasing positively, the oscillator frequency is also increasing. When the modulating voltage is zero, there is no reactance oscillation and the frequency of oscillation remains unchanged. When the modulating voltage is increasing negatively, the oscillator frequency will be decreasing. Thus the frequency modulation is obtained.
Since the reactance modulator operates on the tank circuit of an LC oscillator, the master oscillator cannot be crystal controlled. But it must have the stability of a crystal oscillator since it is the part of a commercial transmitter. If the frequency of the master oscillator shifts, the output frequency of the whole system must drift equally. Hence automatic frequency control (AFC) must be employed.
In AFC circuit, the master oscillator frequency is mixed with the frequency obtained from a crystal oscillator. The crystal oscillator will be tuned such that the resulting difference frequency of the output of the mixer will have usually about 1/20th of the master oscillator frequency This intermediate frequency (IF) signal from the output of the mixer is amplified by the IF amplifier and fed to the input of a phase discriminator. The discriminator output will be zero when the master oscillator frequency is equal to the carrier frequency (Centre frequency).
When the master oscillator frequency increases the discriminator produces a positive DC voltage. This voltage is fed in series with the reactance modulator. Then the master oscillator frequency decreases correspondingly. If the master oscillator frequency decreases, the discriminator output will be a negative DC voltage and the master oscillator frequency increases correspondingly. The phase discriminator will not react to normal frequency changes due to frequency modulation, since they are very small frequency changes. Discriminator will react only to slow changes in the incoming frequency. The RF power amplifier raises the power of the frequency modulated signal to a required level for transmission through the antenna. It will be a class C amplifier. The transmitting antenna radiates the RF power in to space.