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Analyzing RC Radio PPM Signals

The 72MHz FM Hitec Optic 6 RC radio provides excellent long-range control for model airplanes and features a handy trainer port.

The 72MHz FM Hitec Optic 6 RC radio provides excellent long-range control for model airplanes and features a handy trainer port.

This photo shows the reverse side of a Hitec Optic 6 RC radio.

This photo shows the reverse side of a Hitec Optic 6 RC radio. The Hitec trainer port uses a round DIN connector to provide PPM output. The pinout for the trainer port is the same as Futaba’s older DIN connector.

In this photo I am analyzing the PPM signals from a Hitec Optic 6 RC radio on a Rigol DS1052E digital oscilloscope.

In this photo I am analyzing the PPM signals from a Hitec Optic 6 RC radio on a Rigol DS1052E digital oscilloscope.

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:

This  animation shows the changing PPM signal on a Hitec Optic 6 channel RC radio. In the animation I am wiggling the stick controlling channel 3 on the radio from full left to full right.

This animation shows the changing PPM signal on a Hitec Optic 6 channel RC radio. In the animation I am wiggling the stick controlling channel 3 on the radio from full left to full right.

Sample PPM Signal

Here I have zoomed in on the analog waveform of the PPM recording listed below using an audio editing program called Amadeus Pro.

Here I have zoomed in on the analog waveform of the PPM recording listed below using an audio editing program called Amadeus Pro.

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.

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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.

 

8 comments
  1. Hi, I worked on a project a while ago and we used the recorder PWM ( old analogue set) on a MP3 player then, used a modified receiver to control servos, its a good way to programme robots for repetitive tasks, without having to go down the U controller route.
    Later we used a compact flash on a pic, but it was a lot more involved but same result.
    Nice post, just got back into radio control, learning rapidly that I’m years out. ESC’S BEC agghhh so much to master. Wish this stuff had been around when I was doing animatronincs.

  2. Hi Mark.

    Thanks for the suggestion about replaying an RC Control PWM signal using an MP3 player.

    I was actually thinking about doing the same thing by feeding a prerecorded PWM audio signal into the trainer port on my Hitec radio but I never got around to assembling the correct DIN cable to inject the signal.

    Regards,
    Andrew

  3. PPM level question. I want to use my various brand TXs as trainer boxes. Some like JR and Spektrum are 3vptp PPM and my Turnigy 9x wants 5V ptp input.
    I want to build a level and polarity converter inline ext box that allows level and polarity changing so that any tx can be used with any tx as trainer or master. Any ideas on a circuit? I’ve failed so far.
    Thanks. Glen

  4. Turnigy 9x reflash to er9x = in software positive / negative selection, frame length from 12.5 msec to 32.5 msec, selection from 4 to 16 ch, pulse delay from 100 to 800 uSec, ppm in from 2.5 vpp

  5. Hi Andrew
    I want to add the FrSky module to my Walkera Devo 7 in the cleanest way possible. Nobody says where is the PPM port or how to get it to put it or solder the yellow cable of FrSky to the radio. Do you have an idea what can I do?

  6. Hi Oscar.

    I don’t own an FrSky or Walkera Devo RC radio so I don’t know anything about the PPM signal wiring on the Walkera Devo radio.

    If you know someone with an oscilloscope you can use an oscilloscope probe to test the various pins on the radio to look for the PPM signal.

    Regards,
    Andrew

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