Most 72 MHz RC radios used with model airplanes output a PPM (Pulse-Position Modulation) signal from their trainer port. This is often used to allow another model airplane pilot to help you learn to fly your airplane or for interfacing your RC radio with a computer flight simulator.
When my Rigol oscilloscope arrived the first electrical signal my brother & I tested was a PPM signal. Watching a PPM signal update in real-time on an oscilloscope provides a much better understanding of how it works.
Knowing how a PPM signal works is important if you have an interest in trouble shooting problems with your electronics or if you want to build your own microcontroller powered robotic devices.
A PPM signal can come in one of two formats; Negative Shift and Positive Shift. This defines whether the positive or negative part of the signal carries the PPM timing pulse. The duration of the pulse defines the analog servo position and the number of pulses defines how many channels of control are available. The average PPM radio signal updates at approximately 50 Hz (times per second). My Hitec Optic 6 RC radio updates at approximately 45 Hz and my Walkera radio updates at 54 Hz. Each PPM channel on my Hitec radio has a duration ranging from 0.67 ms(full left stick position) to 1.5 ms (full right stick position). The neutral stick position value is approximately 1 ms. The “flat notch” delay between each channel pulse is about 400 µs.
On my 6 channel Hitec RC radio after the last channel is output there is a blanking period of 10 to 12 ms before the next update. On a 9 channel RC radio the blanking period would be shorter due to the addition of the extra channels. The Hitec radio trainer port outputs a PPM signal with a range of 9.5 to 10 Volts VPP depending on the internal battery level. The PPM signal output from the trainer port on my Walkera brand radio (included with the Twister Co-Axial RC Helicopter kit) was 4.8 Volts VPP when I measured it.
On my Hitec Optic 6 radio the throttle stick has 38 throttle detent positions with a spacing of 20 µs per notch. The smallest manual stick movement is approx 8 µs which equates to about 95 positions on the throttle / rudder / aileron channels. A single click of the trim button on my Hitec radio adds or subtracts 0.004 ms ( 4 µs) from the channel duration.
Here is an oscilloscope view of the output from my Hitec Optic 6 RC radio:
Sample PPM Signal
If you want to hear what a raw PPM signal sounds like here is a sample MP3 recording of the waveform captured using the sound card on my desktop computer.
Note: The raw PPM sound is loud and fairly harsh on the ears.
You can also download the sample .MP3 recording of a PPM data stream. If you zoom in on the recording using an audio editing program you can view the individual PPM pulses.