How do PPM FM channels work?
PPM = Pulse Position Modulation. Multiple servo channels are sent on the same frequency using a form of Time Division Multiplexing. The PPM modulator first sends a start pulse, then sends a pulse for each channel (making a total of 10 pulses for 9 channels). The time between leading or trailing edges represents the servo PWM pulse width (typically 1~2ms). Finally a gap significantly greater than the maximum servo pulse width is required so the receiver can synchronize to the start pulse. This whole sequence is called a frame.

If all 9 channels are at their maximum width then the minimum time required to transmit 1 frame is 9*2ms + >2ms = >20ms (or even longer if the maximum servo pulse width is greater than 2ms). A frame period of 22ms is commonly used, corresponding to a repetition rate of ~45Hz.    

   

Many R/C transmitters have a 'trainer' port which outputs a PPM pulse stream. Pulse polarity and amplitude vary depending on the brand and model of transmitter. The pulses may be AC or DC coupled, positive or negative going, and anywhere from 1V to 12V in amplitude. You will need to identify what signal levels are present, and if necessary convert them to the levels required by your FM transmitter.

A positive going PPM signal may either increase or decrease the transmission frequency of your FM transmitter, depending on what FM modulator circuit it uses. At the receiving end this may cause the pulses to come out either positive or negative going. Some receivers have switchable or automatic polarity detection, others are fixed and need to have the correct modulation 'shift' in order to decode the signal properly. If the 'shift' is wrong you just need to invert the PPM signal sent to the FM transmitter.