Voltage drop interference issues with a communication system

Discussion in 'The Projects Forum' started by Boo27, Mar 5, 2015.

  1. Boo27

    Thread Starter New Member

    Mar 5, 2015
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    Hi folks,

    First post here.

    I'm currently working on a little project of my own. It's a communications system,

    I have a Canford audio comms box linked to a baofeng 888s two way radio. By turning on the
    VOX system on the radio and connecting the output side of the cans headset, via a noise gate
    to the input side of the radio, I have managed to create a rudimentary (half duplex) wireless comms system.
    The whole system is boxed up neatly and the radio is run from the 20v power source through this:

    http://www.amazon.co.uk/DC--DC-Conv...=1425573897&sr=8-4&keywords=dc+buck+converter

    Set to 3.5v

    As I said, the whole function works rather well, the radio talks to the comms system without a problem,
    the issue I have is that when I talk to the radio from the comms, Although I can hear voice I'm getting lots of 'pulsating/modulating' interference.

    I have solved the issue part way by turning off the display on the buck converter. When this is turned off
    a lot of the noise goes away, However there is still some about and I can't locate it. It becomes an issue
    because once it starts making noise, not only is the output signal modulated, but some kind of feedback loop
    is created and it locks the noise gate open meaning the VOX on the radio doesn't stop transmitting.

    I have tried resistors and capacitors and diodes of various values in series and parallel with the output signal path (to the gate) and nothing seems to help. Everything is grounded properly and if I pull power from
    the buck and use the cans box on it's own the noise is completely gone so I'm pretty sure it's interference from the buck.

    I've had a look around the net and can't really find anything specific enough for my needs
    (now someone will post a generic link identifying my problem and a simple way to fix it)

    Please help...

    Thanks in advance!
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Welcome to AAC.

    Is buck converter working properly ?
     
  3. Boo27

    Thread Starter New Member

    Mar 5, 2015
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    I assume so, it runs the radio no problem, getting 20+V in and 3.5V out, I've metered it and it seems to be fine.

    Oh I should also mention I don't have an oscilloscope. I have a very good multi meter though!
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    With and without the Display.?
     
  5. Boo27

    Thread Starter New Member

    Mar 5, 2015
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    Yep, display is just a meter, it can be switched to show input, out put or switched off.
    When switched off the radio still runs and I've metered the out put and it's fine.
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    so it does it when transmitting? move your antenna further awaay from the audio box and shield wires. it sounds like your getting rf feedback.
     
  7. Boo27

    Thread Starter New Member

    Mar 5, 2015
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    I'll try that and let you know.
     
  8. Boo27

    Thread Starter New Member

    Mar 5, 2015
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    Just realised, I'm using foil shielded audio cable and I've just plugged in now and turned up my volumes on my devices and I'm getting the hum even when not transmitting. It Just seems to be amplified by transmitting.
     
  9. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    then you have a ground loop causing hum. also, rf feedback usually causes a hum.
     
  10. Boo27

    Thread Starter New Member

    Mar 5, 2015
    11
    0
    Probably a bit Billy basic, but what would be the best way of abolishing a ground loop.

    Everything is mounted in a plastic box and the only earth to speak of is to the shroud on the xlr sockets.

    Edit: grounded via capacitors.
     
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