How can you use a varactor diode in the generation of FM wave? Explain in detail.

Mumbai University > Electronics Engineering > Sem4 > Fundamentals of Communication Engineering

Marks: 10M

Year: May2014

  • Varactor diode modulator is the direct method of FM generation wherein the carrier frequency is directly varied by the modulating signal.
  • A varactor diode is a semiconductor diode whose junction capacitance varies linearly with applied voltage when the diode is reverse biased.
  • Varactor diodes are used along with reactance modulator to provide automatic frequency correction for an FM transmitter. The varactor diode modulator circuit is shown in Fig5. for generation of FM wave.

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Fig5. Varactor diode modulator

  • Varactor diode is arranged in reverse bias to offer junction capacitance effect. The modulating voltage which is in series with the varactor diode will vary the bias and hence the junction capacitance, resulting the oscillator frequency to change accordingly.
  • The external modulating AF voltage adds to and subtracts from the dc bias, which changes the capacitance of the diode and thus the frequency of oscillation.
  • Positive alternations of the modulating signal increase the reverse bias on the varactor diode, which decreases its capacitance and increases the frequency of oscillation.
  • Conversely, negative alternations of the modulating signal decrease the frequency of oscillation.
  • The RFC and capacitor C b act as a filter which transmits only the AF variations to the varactor diode and blocks high frequency RF voltage from reaching the AF stage.
  • The varactor diode FM modulators are widely accepted because they are simple to use, reliable and have the stability of a crystal oscillator.
  • This method of FM generation is direct because the oscillator frequency is varied directly by the modulating signal, and the magnitude of frequency change is proportional to the amplitude of the modulating signal voltage.
  • Varactor diode modulator is used for automatic frequency control and remote tuning.
  • The drawback of varactor diode modulator is that since it uses a crystal, the peak frequency deviation is limited to relatively small values. Thus they are used mostly for low index applications such as two way mobile radio. Also since they are a two terminal device, the applications are quite limited.
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