Here's a schematic for what is probably a fairly standard way to make a saw/sine to pulse circuit with PWM.

Below is the circuit I used in a module I built. If I was going to build a new one though I would use something like the circuit above.

A completed "Triple Sine to Pulse Converter". If mini pots and smaller knobs were used, I might have been able to squeeze in another one. Oh well. The module face is a Frac Rack panel.