One really simple way to generate a pulse train of differring width when you already have an adjustable sinewave oscillator is simply to feed the sine through a clamped resistor, an attenuator, and into a schmidt-trigger gate, e.g. 74xx132. That should enable you to adjust from very short pulses to a 50% duty cycle if you have a relatively clean sine wave.

Think about that for a while and see if it won't do what you want.

RE