All the diodes exhibit small junction capacitance in the reverse biased condition. The varactor diodes are specially designed to optimize this characteristic. The junction capacitance of the varactor diode changes as the reverse bias across it is varied. The variations in capacitance of this diose are aide and linear. The varactor diodes provide the junction capacitance in the range of 1 to 200 pF.
Fig shows how varactor diode can be used to generate FM. L1 and C1 form the tank circuit of the carrier oscillator. The capacitance of the varactor diode depends upon the fixed bias set by R1 and R2 and the AF modulating signal. Either R1 or R2 is made variable so that the center carrier frequency can be adjusted over a narrow range. The Radio Frequency Choke (RFC) has high reactance at the carrier frequency to prevent the carrier signal from getting into the modulating signal circuits.
At positive going modulating signal adds to the reverse bias applied to the varactor diode D, which decreases its capacitance and increases the carrier frequency. A negative going modulating signal subtracts from the bias, increasing the capacitance, which decreases the carrier frequency.
The frequency of the LC oscillator changes due to temperature effects. Hence crystals are used in FM generators to provide frequency stability.
Fig shows the circuit diagram of indirect FM generation. It consists of the varactor diode D1 in series with tuned L1 R1 network. The complete series and parallel network is in series with crystal oscillator. The modulating signal is applied to varactor diode. The capacitance of varactor diode is changed by modulating signal. This changes phase angle of the complete network. This creates phase shift in the carrier signal from crystal oscillator.
The instantaneous phase shift is directly proportional to instantaneous amplitude of modulating signal.
The crystal oscillator is isolated from modulator. Hence frequency stability is more.
- Capacitance versus voltage characteristic of varactor diode is nonlinear. This results in distortion in the modulated waveform.
- Amplitude of modulating signal should be kept small to avoid distortion.
Indirect FM is used for narrowband low index FM generation.