Controlling PWM with an external oscillator and a JFET

Question

I'd like to control the pulse with modulation of a VCO with an external oscillator (triangle wave). The PWM circuit is a comparator fed with the triangle wave output of the VCO. The VCO schematic is a classic design taken from the datasheet.

I'm trying to control the pulse width using a JFET used as a variable resistor, but the output of the JFET the result is a strange behavior of the CV, a triangle wave followed by a continuous voltage like in the screenshot attached (in yellow the original CV signal and in light blue the output of the JFET).

I don't know how to go on... anyone could help me to resolve the problem? Thanks!

Answer 1

The goal of your circuit is to change the voltage on the comparator's inverting node. Consider using a summing amplifier to add the potentiometer offset to the CV signal instead of using a JFET to attenuate the potentiometer's wiper voltage.

This is how I do variable pulse width in my synthesizer:

The triangle input is 5Vpp. Nominal voltage to the comparator's noninverting input is 0V, giving 50% pulse width. It is variable from ~0% to ~100% (-2.5V to +2.5V) by both the potentiometer or external CV.

Answer 2

Not sure what you are trying to accomplish with the JFET. You can control the duty cycle simply by comparing the triangle wave to the control voltage, just feed each voltage into a comparator input.

Range will be the extents of the triangle wave, which will depend on Vcc (note that the output of the LM358 does not swing all that near to Vcc so it will be biased to the lower side of Vcc/2.