When the frequency of the carrier wave is changed in accordance with the intensity of the modulating signal, it is called frequency modulation. The amplitude and phase of the carrier signal remain constant, only change the frequency.
FM or frequency modulation has many advantages specially in mobile sector for radio applications. Frequency modulation is also widely used for VHF to provide a medium for high quality audio transmissions.
Advantages of frequency modulation are as given below:
- Mutual interference decline
- Removal of many effects of signal strength variations
- Transmitter amplifier efficiency
FM uses changes in frequency to carry the sound or other information that is required to be placed onto the carrier.
The figure shows the modulating signal voltage varies AF signal and carrier signal and modulated FM signal on the figure
Frequency modulator using varactor diode
Varactor FM Modulator, which is widely, used in transistorized circuitry uses a voltage-variable capacitor (VARACTOR).The varactor is simply a diode, or pn junction.
The capacitance of the circuit changes linearly with the change of special designed varactor diode. The varactor diode controls the thickness of the depletion region as the capacitance remain linear.
1. It uses the junction capacitance of a reverse biased varactor diode placed across the tuned circuit of an oscillator.
2. The reverse bias of the varactor diode in accordance with the modulating voltage placed in series with the bias.
3. The circuit frequently used for automatic frequency control and for remote tuning.
4. The varactor must not be forward biased because it cannot tolerate much current flow.
5. Proper circuit design prevents the application of forward bias.
6. The varactor diode depletion region is filled with dielectric.