I know this is an old thread but it is possible to get a 8bit PWM @ 980hz with repetier with using the TIMER0 ISR meant for the advance extruder interrupt. This can be achieved while maintaining all other aspects of the firmware. I did use this for a week until I decided I wanted to get quicker raster engraving. It is also possible to change the Timer0 prescaler to 8 and increase the PWM speed up to 7.8khz. I also removed the extruder and temperature controls completely from the firmware and used both TIMER0 interrupts, one for laser and one for fan control and other misc functions. In doing so I was able to increase printline cache up to 48.
So yes, raster engraving with repetier on RAMPS is possible without external hardware.
Comments
AFAIK, Repetier Firmware has a slow PWM output for the fan.
https://forum.repetier.com/discussion/comment/12840/#Comment_12840
16hz as Repetier says here.
It has an easy to use PWM based in timer 5 (for Arduino mega)
Just 1 line to change, works like a charm!