25-06-2013, 04:02 PM
VOLTAGE CONTROLLED OSCILLATOR
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INTRODUCTION
A voltage controlled oscillator is an oscillator circuit in which the frequency of oscillation can be controlled by an externally applied voltage.
The VCO provides the linear relationship between the applied voltage and the oscillation frequency.
Applied voltage is called control voltage
The control of frequency with the help of control voltage is also called voltage to frequency conversion.
Hence vco is also called voltage to frequency converter
Practically vco is used to produce square and triangular waveforms whose frequency is controlled by controlled voltage.
The commonly used VCO Ics are NE/SE 566, LM566 etc.
Manufacturer-signetics
It is 8 pin IC, which provides two output pins.its feature is that simultaneously IC provides square and triangular wave outputs which are the function of input voltage.
The input voltage is also called modulating input voltage
Operation
The op-amp A1 is used as a buffer. the op-amp A2 is used as a Schmitt trigger and the op-amp A3 is used as an inverter.
The voltage VC is applied to the modulation input pin ,which is controlled voltage.
The capacitor c1 is linearly charged or discharged by a constant current source.
The charging current can be controlled by controlling the voltage Vc at pin5 or by varying the resistance R1 which is external to the IC.
The charging and discharging levels are determined by Schmitt trigger
The output voltage of Schmitt trigger is designed to swing between +V and 0.5V .
For Ra=Rb, the voltage at non inverting terminal swings between 0.5(+V) to 0.25(+V)
Thus the triangular wave is generated due to alternate charging and discharging of the capacitor c1,in linear manner.
Features of 566 vco
Wide supply voltage range 10 V to 24 V.
Very linear modulation characteristics.
High temperature stability.
Excellent power supply rejection.
The frequency can be controlled by means of current,voltage,resistor,or capacitor.
Application of vco
The various application of VCO are.
FM modulation
Signal generation (triangular or square)
Function generation
Frequency shift keying i.e, FSK demodulator
In frequency multipliers
Converting low frequency signals such as EEG and EKG into audio frequency range signals
Tone generation