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What Are the Functions of a Reactance Modulator?

A reactance modulator is an electronic circuit typically found in FM radio transmitters. It exploits a property called reactance in which a capacitor responds more favorably to high frequencies than low ones. An active component such as a transistor dynamically "tunes" the capacitor's reactance. Using this principle, a radio transmitter modulates a high-frequency carrier wave with audio-frequency signals.

Capacitive Reactance
- In alternating current circuits, capacitors exhibit reactance, a property similar to direct-current resistance. At a frequency of 0 Hertz, a capacitor has infinite reactance and permits no current flow; at very high frequencies, current is maximized. Audio, radio and other AC circuits use capacitors to perform time- and frequency-dependent functions in oscillators, filters and other equipment.
Reactance Modulator
- Graphed as a function of frequency, an individual capacitor has a single, distinct reactance curve that depends on its capacitance value in farads. Although this is useful, being able to change, or modulate, the reactance opens the door to more sophisticated applications. Vintage radios employ mechanically variable air capacitors -- a set of spaced metal plates that, when rotated, changes the device's capacitance. On the other hand, a transistorized reactance modulator circuit acts as a variable capacitor controlled by a voltage, varying its reactance rapidly, accurately and automatically with no mechanical parts.
FM Radio
- In FM radio, the frequency of a radio signal carries audio information. The standard frequency of FM stations fall in the range of 88 to 108 MHz, but the actual frequency at any given moment is slightly higher or lower because the audio program modulates the frequency. To accomplish this, the radio transmitter circuit uses a reactance modulator. The circuit, of which the reactance modulator is a part, generates the high radio frequency. The AC voltage of an audio signal controls the reactance of the circuit, changing the frequency rapidly up and down. With no audio input, the reactance is nominal and the circuit produces the standard frequency. With audio, reactance changes, varying the frequency with it. The FM transmitter's antenna broadcasts this frequency as radio waves. An FM receiver picking up this signal decodes the small variations in frequency, turning them back into audio.
VCOs
- A reactance modulator is part of a general class of circuits called voltage-controlled oscillators. The frequency of a VCO changes when you apply an external voltage to its control input -- the bigger the voltage, the greater the change. In addition to FM transmitters, VCOs see use as motor speed controls, FM tuners and musical instruments, where the control voltage from a keyboard determines the audio frequency of the oscillator.

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