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VCO which is often referred to as voltage-controlled oscillator which is intended to be controlled in oscillation frequency by a voltage input. DC voltage is applied to vary the frequency of oscillation. In addition to this, modulating signals may also be provided into the voltage controlled oscillator to root frequency modulation (FM) or phase modulation (PM).
VCOs are also categorized into two groups, namely harmonic oscillators and relaxation oscillators on the basis of waveform created. Brief introduction to the types of VCOs are enlisted herewith:-
• Harmonic oscillators: These types of Oscillators help to generate sinusoidal waveforms. Harmonic oscillators consist of amplifier that delivers adequate gain and a resonant circuit that supply a back signal to the input. Oscillation happens at the resonant frequency where positive gain takes place around the loop. Some of the examples of harmonic oscillators include crystal oscillators and LC-tank oscillators.
• Relaxation Oscillators: Relaxation oscillators produce triangular waveform that is usually used in monolithic integrated circuits. If you are looking forward to get wide range of operational frequencies with minimal number of external components, then going for relaxation oscillators is the best option. Relaxation oscillators can have three topologies, namely grounded-capacitor VCOs, emitter-coupled VCOs and delay-based ring VCOs. Out of these three topologies, the first two has the same functionality. The amount of time in the first two topologies depends on the time required by a current to charge or discharge a capacitor whereas delay-based ring VCO functions differently. In delay-based ring VCO, the gain takes place in a ring.
Therefore, the output frequency is then a function of the delay in every stage.

However, harmonic oscillator experience extra advantage over relaxation oscillators with respect to several factors such as
• Accuracy for frequency control
• Frequency stability with regard to temperature, noise and power suply
But in case you want to implement monolithic ICs, relaxation oscillators take an edge over harmonic oscillators.

About the author
Fox Electronics is a leading global supplier of standard and custom frequency control products, including the industry’s broadest line of quartz crystal, oscillators, Crystal oscillator, TCXOs.