Looking at the diagram ... the pot over the left op amp is variable to adjust the frequency (gain)? But the center tapped resistor (supposing a trimpot) will adjust both the duty cycle and frequency ... what I am looking for is ...
1. adjust duty cycle between 10% and 90% depending on the needed supply
2. adjust the frequency between ~200Hz and ~3Hz
3. adjust duty cycle and frequency independently from each other (eg. one does not affect the other)
The diagram I have given (as I said before) will adjust the on and off times independently, thus I can adjust both to the specified requirements. Basically using a 1uF capacitor and 2 x 250k potentiometers, I can adjust the frequency between 720Hz and 2.88Hz, and can adjust the duty cycle between 0.398% and 99.602%. The problem is to adjust the frequency, I need to adjust both resistors in an equal manner.
What I am aiming for is to be able to drop the frequency down as low as it will go (~3Hz would be good), then adjust the duty cycle to get the required percentage of on versus off time, then turn the frequency up until operation speed is adequate. If I increase the frequency and the duty cycle changes as a result then it will not work ...