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A voltage controlled oscillator is a widely used electronic oscillator where the input tuning voltage determines the oscillation frequency. The output frequency of the voltage controlled oscillator is either sinusoidal or sawtooth. Voltage controlled oscillators are important components in many applications such as electronic jamming equipment, function generators and phase-locked loops.
The resonant frequency in the voltage controlled oscillator is controlled by input voltage. It produces a periodic signal, with the input voltage control related to the level of the frequency of the signal. A tuning element called a varactor diode is used in voltage controlled oscillators. With the help of clean direct current voltage applied to the varactor diode, the oscillator is tuned to vary the net capacitance provided to the circuit.
Depending on the waveform, voltage controlled oscillators are categorized into harmonic oscillators and relaxation oscillators. Harmonic or linear oscillators are comprised of a resonator and amplifier and produce a sinusoidal waveform. An LC-tank oscillator is a harmonic oscillator. Relaxation oscillators produce a sawtooth waveform and require only minimal external components to produce a wide range of operational frequencies. Harmonic oscillators show better frequency stability with respect to external factors like noise, temperature and power. There is better accuracy with respect to frequency control as well in harmonic oscillators. However, they are less suited for monolithic integrated circuit technology, but much better suited for relaxation oscillators. Moreover, relaxation oscillators can provide a wide range of operational frequencies.