RF design for circuit

Discussion in 'The Projects Forum' started by david1234, Sep 18, 2014.

  1. david1234

    Thread Starter Member

    Nov 27, 2013
    81
    1
    Hello
    wanted to know
    I have circuit that act strange near RF of 400Mhz
    when I use the circuit near a device that transmit on freq 400Mhz It start to act crazy
    how difficult it will be to built to it a "RF guard " that will cancel the noise from this freq?
     
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  2. sirch2

    Well-Known Member

    Jan 21, 2013
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    What is the device? What is the source of the 400MHz RF? What does "act crazy" mean?

    Wrap it in tinfoil and connect it to a good ground.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
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    If you can post more information about the device that is made to misbehave with the 400 MHz signal, and describer the nature of the misbehavior, somebody here might be able to make specific suggestions as to how to improve the RF immunity of your device.
     
  4. david1234

    Thread Starter Member

    Nov 27, 2013
    81
    1
    I will explain
    I have a pir sensor with a circuit that give me "1"\"0" in it's output -
    so when the sensor detect a movement I'm getting "0".
    I also I have a device that transmit 2-7 dBm in freq between 400-500Mhz
    when the sensor is near it ~2-3 meter I get "false alarm" - the sensor showing "0" all the time - like he detect a movement.
    when I close the RF - it works just fine.

    I have try to use tinfoil and rap the sensor and also the circuit - but still getting "0" all the time.
    this is the sensor I'm using
    http://www.kube.ch/downloads/pdf/kube_pyroelectric_c172.pdf

    the image is the circuit I have built
    sensor_circuit.PNG

    20140921_120600.jpg sensor_circuit.PNG
     
  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287
    Hey, Nandu! Good to see you on AAC! Looks like quite a few "Zedders" are finding their way here! :)
     
  6. david1234

    Thread Starter Member

    Nov 27, 2013
    81
    1
    how can I ground it?
    if I take a metal box and close it with the PCB inside ,then connect the box to my wooden desk
    will it do the job?

    also you think the problem is in the PCB and not in the PIR sensor?

    Thanks ,
     
  7. DickCappels

    Moderator

    Aug 21, 2008
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    You would ground the shield by connecting the shield to the (common) ground connection on your circuit.

    For shielding the PIR detector itself, you might want to use brass or copper screening -it would let the IR through but stop the RF.

    As v2unan pointed out, the long leads are quite likely a problem. If at all possible, put the PIR sensor on the circuit board.
    Aluminum foil is usually a poor choice for shielding because it is difficult to make a good electrical connection to the aluminum through the oxide film on the surfaces.

    Low value (hundreds of pf or lower) high quality capacitors to decouple various points in the circuit to the circuit's ground might solve the problem. I would start with the input circuit by putting 470 to 1000 pf capacitor across C1 and C3. A couple cm in placement and lead length can make the difference between the decoupling working or not.

    Those are some ideas that might take you in the right direction.
     
  8. david1234

    Thread Starter Member

    Nov 27, 2013
    81
    1
    just to see I understand you correctly :
    1. I'm going to use less then 1 cm cable between the PIR and the circuit .
    2. I tool a metal box and connect it to the GND of the circuit (Then circuit is in it ) and the PIR sensor is outside of the box.
    3. I will take 470 pf (this is what I have here right now) and connect it in parallel to C1 and C3 , right? or in line?
    4. also I going to take a very thin copper something like paper width and put it on the sensor ,right?
     
  9. sirch2

    Well-Known Member

    Jan 21, 2013
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    When DickCappels says "screening" I think he means mesh rather than foil. Copper/brass foil is not going to let the IR through
     
  10. david1234

    Thread Starter Member

    Nov 27, 2013
    81
    1
    did all of it , didn't help
    any more ideas?
     
  11. DickCappels

    Moderator

    Aug 21, 2008
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    Maybe its time to do some troubleshooting on the circuit while operating. Using whatever test equipment you have on hand, try find the earliest part in the circuit that is adversely affected by the RF. That would be where to start applying additional countermeasures.
     
  12. sirch2

    Well-Known Member

    Jan 21, 2013
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    Check the cabling to the PIR sensor, is that screened? is the screen gounded? You could consider putting a choke on the cable close to the PIR.
     
  13. david1234

    Thread Starter Member

    Nov 27, 2013
    81
    1
    you think the problem is the PIR sensor ?
    I will try to cover as much as I can the pins of it
     
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