Powerline Modem Design

Discussion in 'The Projects Forum' started by Stroopy121, Apr 21, 2016.

  1. Stroopy121

    Thread Starter New Member

    Apr 20, 2016
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    Hello!

    I've had a look through the existing threads on the topic and am struggling to find info about my specific application.

    I need a pair of modems that'll allow me to plug in modbus over RS232, convert it to FSK, punt it up to 5km over a copper wire pair which will have up to 600V at 50Hz on it (but will be point to point so no intermediate transformers to worry about) and then at the other end, receive this and split it off into power and comms lines again, and convert back from FSK to 232.

    I had initially planned to use Bell 202 protocol, but I can't get an LCR filter that cuts enough of the 50Hz signal while still letting the 1200Hz component through. I want to use frequencies as low as possible (data rate isn't a concern, there's not much being passed back and forth).

    Given the long transmission distance, I'm assuming I'll have to put in a booster of some kind on the FSK when it arrives (and probably before it's coupled to the power line) but that's all by-the-by.

    As I understand it, the circuit will need to go as follows:
    RS232 in -> Convert to FSK -> analog signal boost (if used/needed) -> Line Coupling Transformer -> High Pass Filter -> High Voltage Line -> Line Coupling Transformer -> High Pass Filter -> analog signal boost (if used/needed) -> Convert to RS232 -> RS232 out

    There are lots of applications involving microcontrollers in the PLC coupling and I have no idea what they're used for and would rather avoid them if possible as it's just one more component to fail.

    Is there anything I'm missing here?
    If I just slap an HPF and a 1:1 transformer between the two then can I safely put FSK comms on a power line?
    Will there be a noticeable difference in the FSK signal if it gets passed through an HPF at BOTH ends of the power line?
    Will the FSK signal being on the power line noticeably affect the power quality?
    Should I add an LPF to the power output at the far end to block the FSK signal?
    If I do this, will the presence of the extra LCR components further affect and attenuate the FSK signal?
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,128
    266
    Most power line modems use much higher carrier frequencies, 1200 Hz is way to close to 50 hz.

    The ratio of power line voltage to signal voltage is going to be huge, you need to separate the frequencies much more to make filtering do-able.
     
  3. Stroopy121

    Thread Starter New Member

    Apr 20, 2016
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    I thought this myself - in the subsea industry, Bell 202 over powerline communication is pretty common, but I have absolutely no idea how it's achieved using these kinds of LCR filters!
     
  4. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,128
    266
  5. Stroopy121

    Thread Starter New Member

    Apr 20, 2016
    5
    0
  6. Papabravo

    Expert

    Feb 24, 2006
    10,138
    1,789
    What do the datasheets for various cables say for attenuation as a function of frequency and length? Don't guess -- verify!
     
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