VCO.V4

# VCO.V4 - L303.7.V4 Drexel University Electrical and...

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L303.7.V4 7-1 Drexel University Electrical and Computer Engr. Dept. Electrical Engineering Laboratory III, ECEL 303 E.L. Gerber WAVEFORM GENERATORS (VCO) Object The object of this experiment is to learn some of the basics of electronic waveform generation. Standard IC chips will be used to produce various waveforms as well as a frequency modulation circuit. Introduction One of the old standard integrated circuit voltage-controlled oscillator (VCO) devices is the LM566. It is capable of generating a predetermined frequency square wave and triangle wave. The frequency of these waves can also be varied via an external input voltage. This device has a maximum operating frequency of 1 MHz with a 10-to-1 range of frequency variation with a change in modulating input voltage. It is easy chip to use and only requires two external circuit elements to operate. Figure 1 depicts a basic block diagram of the LM566 VCO chip and the external timing circuit, R T and C T . Theory • Voltage-Controlled Oscillator Operation Fig. 1. Voltage Controlled Oscillator Block Diagram

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L303.7.V4 7-2 The chip has a basic oscillator circuit built in but requires external timing elements R T and C T . The chip provides a constant DC current source, which is reversed under internal chip command. The magnitude of the current, however, can be controlled externally via the timing resistor R T . The fundamental principle of the system is the current flow in a capacitor, i(t) = C(dv/dt). Since the current in the capacitor is constant the voltage across it (pins 7-1) will be a positive or negative ramp as seen in Fig. 2. Referring to Figure 1, the current source/sink circuit provides a constant charging or discharging current to the external timing capacitor C T . The current is controlled by the timing resistor, R T . Increasing the value of R T decreases the capacitor current. Control of this current is also possible by changing the voltage across the resistor via the modulating input. The voltage at pin 6 is normally maintained at the same voltage as pin 5. Thus, if the modulating voltage at pin 5 is increased, the voltage at pin 6 increases, resulting in less voltage across R T and, therefore, less charging current. All voltages must be positive. Refer to Fig. 1 again, the voltage developed on capacitor C T is applied to the Schmitt trigger circuit U 2 via the buffer amplifier U 1 . The output voltage swing on the Schmitt trigger goes from V CC to 0.5 V CC . Resistors R a and R b form a positive feedback loop from the output of U 2 to its non-inverting input. With equal dividing resistors R a and R b , the non-inverting input swing is from 0.5V
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