RC PPM signal

RC Receiver Overview

A standard RC receiver is made of four main blocks :

  • Radio-frequency block (FM receiver + antenna) which converts through demodulation the signal into a form usable for the other blocks. This particular signal is a PPM (Pulse Position Modulation) coded signal.
  • PPM signal decoder which converts the pulse positions into PWM (Pulse Width Modulation) servo signals
  • Synchro blank detector block which detects PPM frames separation in order to re-initialize the PPM decoder.
  • Power supply block which supplies all blocks from external receiver supply (on one of the servo header)

standard_rc_receiver_block_diagram

Timing Diagram

RC receiver timing diagram

 

(1) PPM frame period duration is not critical and depends on RC Transmitter type & Manufacturer.

(2) Frame separation blank time depends on transmitter number of channel and vary according all channel durations (Tn)

(3) Blank detection time is not critical an depends on detection method

 

Previous timing diagram shows that:

  • PPM frame signal contains all channel informations
  • The number of channels of the receiver depends only on transmitter number of channel and receiver PPM decoder hardware (decoder output number and servo header number)

So if whe only take the PPM signal into account, the number of channels depends only on the transmitter (and not on the receiver).

For example, the oscilloscope screenshot below shows the PPM frame signal from a 4 channel receiver (Castle Creations Berg 4L) and 9 channel transmitter (Graupner MX22)

oscilloscope sreenshot of a 9 channels PPM frame

 

PPM Advantages

For custom RC small scale MCU based systems, a PPM signal is really advantageous:

  • easy to decode (input capture time measurement)
  • only 3 wires (+5v, Ground& PPM signal) between receiver and MCU board
  • small scale factor receiver (available receiver with the smallest number of channel can be used)

PPM signal divertion

Drawback of using PPM frame is that the receiver must be slightly modified:

  • An oscilloscope is needed to locate the PPM signal.
  • Soldering the wires on on such small board is not always easy.

modified_rc_receiver_block_diagram

As you can notice, this modification is not destructive (if you are clever with a soldering iron Content) and you can still use any servo output if needed.

To get more practical information on receiver modification procedure, please read RC Receiver Hacking Tutorial article.