PPM Signal Format

The signal that the transmitter sends out of the trainer jack is in a form called PPM.

PPM:

Pulse Position Modulation. A method of encoding information in a signal by varying the position of pulses. The unmodulated signal consists of a continuous train of pulses of constant frequency, duration, and amplitude. During modulation the pulse positions are changed to reflect the information being encoded.

The transmitter generates a pulse for each function or channel. The width of the pulses varies between 0.9 – 2.1 msec. For example if the aileron stick is fully deflected to the left the pulse width will be around 1.0 msec. If left trim is added to the full left stick then the pulse width will be around 0.9 msec. If the stick is deflected to the right the pulse width will be about 2.0 msec. The pulse width of the stick at center is generally about 1.5 msec.

The transmitter will generate a variable pulse width for each function, example, aileron, throttle, elevator, rudder, etc. It sends these variable pulses in sequence followed by a sync pulse. A complete series of pulses is called a frame. A Frame is sent about every 20 msec. A typical frame of data is shown below.

So the PPM signal works by sending pulses of constant height and width (about 350 usec). The position of the pulses though is varied according to the pulse width of the channel being sent. The time between the pulses is the channel pulse width. They are sent in sequence. The first pulse indicates the start of a frame. The next pulse is indicating the end of the first channel pulse width as well as the start of the next channel pulse width. When all the channel pulse widths are sent there is an idle time which is usually called a sync period. This is to indicate to the receiving circuits to get ready for a new frame of data.

Unfortunately not all radio manufacturers send the pulses in the same order. See table below for channel order.

Manufacturer	Ch1	Ch2	Ch3	Ch4	Ch5
Futaba	Aileron	Elevator	Throttle	Rudder	Gear
JR	Throttle	Aileron	Elevator	Rudder	Gear
Airtronics	Elevator	Aileron	Throttle	Rudder	Gear

To make matters even more complicated not all manufacturers have the same format of PPM signal. Futaba uses baseline high with pulses that go down to 0 volts. JR also uses baseline high but the pulses are much smaller and only drop about 3 volts. Airtronics uses a completely reversed scheme where the baseline is low with large positive pulses.

So what our circuits need to do is wait for the sync period. Then we know that the next pulse will be the start of a new frame. Then start timing the period between each pulse. Keep doing this until we detect the sync period and then do it all over again.