Symmetry Feedback Maximizes Function Generators Linear Sweep Range

Posted on Feb 5, 2014

This basic VCO/waveform generator IC circuit includes a voltage follower loop (formed with IC1) and symmetry feedback loop (IC3) to eliminate an asymmetric duty cycle, which would cause distortion at low frequencies. Fig 2. The VCO used in this circuit incorporates two npn/pnp transistor pairs that buffer the control voltage entering at pin

Symmetry Feedback Maximizes Function Generators Linear Sweep Range
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8 so this voltage appears across R2 and R3 (connected to pins 5 and 4, respectively) with little change. Fig 3. Experimental results show that the VCO circuit using the symmetry feedback loop experienced low distortion over a sweep of more than 1000:1, even at low frequencies. This design describes a symmetry feedback loop  that maximizes the linear sweep range of a voltage-controlled oscillator (VCO). The circuit uses an ICL8038 precision waveform generator/VCO, 1 but the design could be applied to other VCOs. The ICL8038 is a popular function generator that provides sine, square, and triangular waveforms simultaneously over a wide frequency range ”1000:1. However, this range has been impossible to achieve without an asymmetric duty cycle and, therefore, higher distortion at lower frequencies. 2 The symmetry feedback loop maintains the duty cycle close to 50% over a frequency sweep of greater than 1000:1, providing a sine wave with low distortion. IC2, the ICL8038, forms the core of the circuit ( Fig. 1 ). The control voltage, fed into pin 8, is buffered by two independent internal npn/pnp transistor combinations and appears at pins 4 and 5 with little change ( Fig. 2 ). The voltages across R2 and R3 control two current sources in such a way that capacitor C1 is charged continuously by the current through R2 and discharged, through a switch, by twice the current through R3, based on an internal flip-flop switched...

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