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Intercom and GSM Buzz

Discussion in 'General Electronics Discussion' started by obo110x, Dec 9, 2010.

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  1. obo110x


    Dec 9, 2010
    I've developed an intercom system and it suffers quite badly with mobile phone noise. I've tried lots of things X2Y filter parts, metal enclosure, redraft of PCB, etc with limited success. I added a really simple RC LPF to the microphones and this improved things greatly but it's still there.

    I need a fresh pair of eyes - does anyone have any suggestions?

    Thanks in advance.
  2. davenn

    davenn Moderator

    Sep 5, 2009
    firstly determine where the intereference is getting into the system....\

    is it in the remote mike part ? the cabling ? or the main base station unit ?
    to be getting cellphone intereference you must be very close to a cell tower ???

    is you intercom wired or wireless ?? (keeping in mind my above comments)
    if its wireless between base and remote, what freq are you using ?

    some more info from you will be helpful... maybe cct diag's etc

  3. obo110x


    Dec 9, 2010
    I'm reasonably convinced that it is getting in through the remote mikes and the cabling, the system is wired and is in a vehicle. The presence of 2G phones inside the vehicle receiving/making calls and handshaking with cell towers/masts produces the noise so it is fairly constant.
    The mikes are wired with twisted pair with sheild connected at base unit via cap to PCB ground plane. The audio quality of this intercom is very good but the buzz is really bad.

    Schematic attached.

    Attached Files:

  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I presume that the interference only happens when there are phones in the car.

    (I know it may seem like a silly question, but...)
  5. obo110x


    Dec 9, 2010
    Yes only when 2G phones are in the car. either receiving a call or latching onto the next mast as you're travelling. iphone is absolutely the worst phone of all for this too.
  6. LTX71CM


    May 23, 2010
    I had a BlackBerry 8870 and it could take out the picture on a cable connected TV 30 ft. away, not rabbit ears (A specific TV in my home, not all TVs). Brain cancer concerns being true or not I preferred speaker phone when possible. :D

    Can you temporarily disconnect the microphones and then subject your project to a cell phone? Any change in behaviour? Have you tried moving the phone around in the car to see if a specific location makes the interference louder?
  7. shrtrnd


    Jan 15, 2010
    If it helps, we used to have a remote monitoring station close to a radio station transmitter. We put inductors on all the lines (believe it or not, inductors only), and cut down quite a bit on the interferrence. I don't know how well that'd work on a 2GHz line,
    but it's an idea to think about. At 2Gig, you may need something a little more complicated. Just my 2 cents.
  8. obo110x


    Dec 9, 2010
    The microphones are on 75mm leads with connectors that attach to the main cable, when these are disconnected the noise is still there, I put the base unit in a metal enclosure and that improved things.

    If you hold the phone near the microphones this is the worst case. I put simple RC filters at each microphone (50R + 33pF) and this has helped but still problematic. I'm thinking the next step is an active filter. I will try inductors in-line too. The 900MHz and 1800MHz is the issue, it just seems really difficult to determine where and how it's getting in.:mad:
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