Digital Data Modulation

Discussion in 'General Electronics Chat' started by wannaBinventor, Jul 14, 2010.

  1. wannaBinventor

    Thread Starter Member

    Apr 8, 2010
    I've managed, with the input of some of the always helpful members here, to get a PIC-based IR transmitter and receiver circuit going.

    Based on a button press, it sends a 38KHz carrier wave modulated at different lengths of time. For instance: a ~1.3 ms pulse = logic 1, a ~0.8 ms pulse = logic zero. If it matches the binary code that I programmed into the receiver circuit PIC, lights an LED.

    I'd ultimately like to change from IR to RF and I've looked at some of the product offerings that use things like ASK and FSK modulation.

    I've done a bit of reading on wikipedia on digital signal modulation, but I can't seem to figure out where my above method of modulating my IR carrier wave falls in the ASK, FSK, OOK, etc categories. Would it be PDM which I've read is the same as PWM. So in other words I'm using PWM to generate a carrier wave and then using a different PWM? Is this correct? If so, why would radio transmitters use things like FSK when the above method seems to work fine.

    Thanks for helping me understand.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    IR bursts are a lot like old Morse code transmission, which was basically on-off keying. Ignoring the one carrier (the IR light), the only thing that is sent are variable-length bursts of 38 KHz. The 38 KHz is not modulated. It's either there or not.

    For digital, a scheme based on QPSK might be superior -

    Notice the box on the right which gives the various analog and digital modulation schemes.