Voltage Controlled Oscillator

Posted on Feb 5, 2014

Voltage controlled oscillator is a type of oscillator where the frequency of the output oscillations can be varied by varying the amplitude of an input voltage signal. Voltage controlled oscillators are commonly used in frequency (FM), pulse (PM) modulators and phase locked loops (PLL). Another application of the voltage controlled oscillator is th

Voltage Controlled Oscillator
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e variable frequency signal generator itself. Block diagram of a typical voltage controlled oscillator is shown below. Voltage controlled oscillators can be broadly classified into linear voltage controlled oscillators and relaxation type voltage controlled oscillators. Linear voltage controlled oscillators are generally used to produce a sine wave. In such oscillators an LC tank circuit is used for producing oscillations. An active element like transistor is used for amplifying the output of the LC tank circuit, compensating the energy lost in the tank circuit and for establishing the necessary feedback conditions. Here a varactor (varicap) diode is used in place of the capacitor in the tank circuit. Varactor diode is type of semiconductor diode whose capacitance across the junction can be varied by varying the voltage across the junction. Thus by varying the voltage across the varicap diode in the tank circuit, the output frequency of the VCO can be varied. Relaxation type voltage controlled oscillators are used to produce a sawtooth or triangular waveform. This is achieved by the gradual charging and sudden discharge of a capacitor connected appropriately to an active element (UJT, PUT etc) or a monolitic IC (LM566 etc). Now a days relaxation type VCOs are generally realized using monolithic ICs. LM566 is a monolithic voltage controlled oscillator from National semiconductors. It can be used to generates square and...

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