I've built a pwm using this design. It generates a variable sawtooth wave when you replace R4 with a potentiometer.
(https://overunityarchives.com/proxy.php?request=http%3A%2F%2Fstudents.umw.edu%2F%7Ejsera9mu%2Fwfc%2520site%2Findex.16.gif&hash=406f850978ae9137698e948e68863c6530918916)
The question now is, can I pass this signal into another set of pins on the LM324 quad-op-amp IC and modulate the duty cycle?
I think this board has potential, it uses fewer components and no decade switching. A 500k potentiometer should be able to increase frequency even higher. Check out 'My Experiments' at the bottom of this pagehttp://students.umw.edu/~jsera9mu/wfc%20site/ (http://students.umw.edu/~jsera9mu/wfc%20site/)
I'm not sure what the min max frequency are but will know soon as I get a better scope.
I got the idea from a video posted by Hydroman here on this forum, I will link when I find it
Thanks
Car that runs on water