The modulation process is an important part of the communication system, superimpose high frequency with our main signal is the process that happened at the modulator device.

The modulator has applications at every communication system, if it is the audio signal or video signal, even at our satellite communication, modulation circuits had a great role to play.

In this post we are going to explaining about varactor diode modulator, before going to the varactor modulator, we need to know something about modulation type.

When we consider the modulation process, it is in two type’s direct and indirect methods, the varactor diode modulator includes the direct method, so we include two varactor diode modulator circuits using one is using RLC tank circuit and crystal oscillator circuit.

## Varactor diode

The varactor diode is the main component on this modulator circuit, so it is a semiconductor device they have capacitance properties at the junction, the capacitance of the varactor diode varies with the applied voltage.

## Varactor diode symbol

This figure has shown the symbol of the varactor diode, the capacitor symbol indication at the symbol shows the capacitance property of the varactor diode.

## Varactor diode capacitance formula

Cd = Co/(1 + V/ø )a

Co = Capacitance at zero bias

Φ = Constant potential of junction

a = constant depending on type of junction (0.5 to 0.33)

## Varactor diode modulator for FM generation

The FM or frequency modulation is one of the main modulation types, varactor diode-based modulator circuits have applications at almost all modulator types, here at frequency modulator or FM, the capacitance of the varactor variation result change in the frequency of the circuit.

## Varactor diode modulator circuit

The varactor modulator circuits we include are in two types first is LC oscillator-based varactor diode modulator and crystal oscillator-based varactor diode.

1. ### Varactor diode modulator using LC oscillator

The circuit shows the varactor diode modulator circuit, which is a frequency modulator type. The oscillator circuit is built with an LC oscillator circuit that is a combination of inductor and capacitor.

### Varactor diode modulator working

An important property of the varactor diode that operates in reverse biased condition, and the RFC choke is a radio frequency choke whose reactance is high at the carrier frequency to prevent the carrier signal from getting into the modulator signal circuit.

We know the capacitance property of the varactor diode varies with applied voltage, so when modulator signal voltage (Vm) from the transformer is getting into varactor diode.

But the important thing is that the modulator signal have +VE and –VE half cycle, at the time +VE half-cycle reaches, reverse bias voltage increase and capacitance decreases, and frequency increases (Vm (+VE) = RB↑ – C↓ – F↑)

At the same time –VE half-cycle reaches, reverse bias voltage decrease that result increase in capacitances and frequency decreases (Vm (-VE) = RB↓ – C↑ – F↓)

The combination of both these results is the process that happened at the frequency modulator using varactor diode.

1. ### Varactor diode modulator using a crystal oscillator

In this varactor diode modulator circuit, crystal is used as the oscillating component.

### Varactor diode modulator working

The working of the crystal oscillator-based varactor diode modulator circuit is almost similar to the LC modulator, the frequency of a crystal oscillator can be varied by changing the value of capacitance in series or parallel with the crystal.

The modulating signal is applied to the varactor diode which changes the oscillator frequency, the circuit derivate the frequency of the crystal oscillator using the varactor diode.

The crystal oscillator means that the output waveform is stable and accurate.

## Applications of varactor diode modulator

• Automatic frequency control(AFC)
• Remote turning