5.3 Sspd_chapter 3.2.7.1.. application of hyperabrupt junction

3.2.7.1. Application of Hyperabrupt Junction Diode as Varactor Diode in Voltage-controlled Oscillator(VCO).

In Equation (3.2.7.2) we saw that in hyperabrupt Junction diode, junction capacitance (C _j ) varies inversely as the square of the Reverse Junction Bias Voltage (V _j ).

Hence:

If the diode with this unique power law is utilized in a parallel resonance circuit then the resonance frequency ω ₀ becomes directly proportional to the diode voltage as shown in subsequent paragraphs.In this set up hyper-abrupt junction diode acts a variable reactance device hence it is called Varactor Diode and application of Varactor Diode enables the synthesis of Voltage Controlled Oscillator(VCO) which is one of the most popular method of Frequency Modulation of a Carrier Signal in FM Radio Broadcast System.

In Figure 3.17, VCO using a Varactor Diode is illustrated.

Theoretical Formulation of VCO.

In the Hartley Oscillator shown in Figure 3.17., the frequency of oscillation is as follows:

Therefore:

Equation (3.2.7.4) gives the theoretical basis of VCO. The voltage generated from the microphone is pre-amplified and then coupled to the reverse bias loop of the varactor diode. The varactor diode junction capacitance forms a part of the parallel resonance circuit and the audio voltage generated from the microphone directly controls the carrier frequency generated by the Hartley Oscilator. Thus we have achieved a VCO where audio signal of the Radio Broadcast is carrying out the Frequency Modulation of the carrier frequency generated by the Hartley Oscillator.