A DIY PWM to PPM converter using an Arduino Pro Mini from Al Prettybong. Oscar wrote a post on his blog detailing how Al made it:
For radio receiver, there are a few output signal formats. The traditional and also most common type of RX signal is the PWM and basically PWM requires 1 cable per channel. PPM is now getting more and more popular, because it can handle all 8 channels in 1 signal wire.