Voltage Controlled Duty Cycle Sawtooth Circuit

Posted on Feb 6, 2014

There was a discussion on synth-diy a while ago about the problems of building a voltage controlled oscillator with a sawtooth wave output whose duty cycle (the ratio of ramp-up time to ramp-down time) is adjustable with a seperate control voltage. [Don: When you get around to it, talk about the musical advantages of such a thing, morphing, the ha

Voltage Controlled Duty Cycle Sawtooth Circuit
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rmonic spectra, the way to build a potentiometer controlled variable duty cycle oscillator, why you can`t just integrate a PWM square wave, and so forth. ] In the following schematic, the input on the left is an audio frequency rising sawtooth wave going from 0. 0V to +5. 0V. The input on the upper left is a duty cycle control voltage going from -5. 0V (falling sawtooth) through 0. 0V (triangle wave) to +5. 0V (rising sawtooth). The power supply is +/- 15. 0V. A1 and A2 are CA3280 OTAs. A3 and A4 are LF412 op amps. The transistors are reasonable high beta NPNs and PNPs. The values for the trimming circuitry is intentionally left off. The basic idea is to take our rising sawtooth source and it`s inverse, a falling sawtooth from +5. 0V to 0. 0V, adjust the relative gains of those two, and use the arithmetic minimum of those two signals as the output. The gains would ideally go from 1. 0 to infinity, but for practical reasons I`ve limited the range to 1. 11 through 10. If the two gains are equal to 2. 0 the output will be a triangle wave. To keep the output amplitude constant the two gains should be such that the sum of the recipricals of the gains is constant. And if the output amplitude is going to be the same as the input amplitude, that constant value needs to be unity. For this application, x is proportional to the duty cycle control voltage, centered around zero for a 50% duty cycle. 1+ex is proportional to one of the...

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