What's an analog way to measure only the on-time of a servo PPM pulse?
You use an integrator to get the area of the pulse. Since they are all the same height, area is directly proportional to the width (on time) and therefore your voltage output is directly proportional to the pulse width.

You would have to reset the integrator though. Perhaps you can feed the integrator output into a S&amp;H amplifier triggered on falling edge then resetting the integrator.