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Why is RF circuit shielded

Discussion in 'Electronic Basics' started by Jack// ani, Nov 4, 2005.

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  1. Jack// ani

    Jack// ani Guest

    Hi all,

    Why is RF circuit shielded inside a metal case...like a cell phone,
    where whole of the pcb and its components are shielded.

    Thanks
     
  2. Howard Long

    Howard Long Guest

    Five reasons I can think of:

    1. To stop unwanted radiation exiting the device, possibly interfering with
    other devices (eg, local oscillators, unwanted mixing products, harmonics);

    2. To prevent unwanted radiation from external devices affecting the device
    (eg mixing products and image rejection, and for devices using external
    antennas almost any digital device, and some analogue ones, in your
    domicile);

    3. To stop subsytems within the device interfering with each other (eg,
    self-oscillation, unwanted mixing products).

    4. Increased surface area decreasing the resistance due to skin effect.

    5. Maintaining correct impedance in transmission lines.

    Cheers, Howard
     
  3. Bob Agnew

    Bob Agnew Guest

    one more that I can think of :

    In order to comply with FCC regulations Part 15 and other regualtions.
     
  4. Joerg

    Joerg Guest

    Hello Bob,
    That is usually the reason.

    Regards, Joerg
     
  5. Jack// ani

    Jack// ani Guest

    Thanks all. I got it, but why only RF circuit, why don't they do the
    same at low frequency? Do HF devices produces more noise compared to LF
    devices, thats why or something else??

    Thanks
     
  6. Howard Long

    Howard Long Guest

    I think you'll find that Part 15 is only a US regulation. The rest of the
    world operate under different regulations. My previous answer (1)
    essentially covers this, but there was no indication that the OP was in the
    US. FWIW I had contemplated citing various regulatory requirements but I
    thought the OP was after practical reasons.

    Kind Regards, Howard
     
  7. Jack// ani

    Jack// ani Guest

    I had contemplated citing various regulatory requirements but I
    Hi Howard,

    Yep, you are very true...here. I did wanted to know the practical
    reasons.
     
  8. Joerg

    Joerg Guest

    Hello Jack,
    In other countries you have regulations similar to what the FCC does in
    the US. They are just called differently, for example in Germany the
    agency's name would be RegTP. EMI limits aren't very different. That's
    the practical reason.

    As for your question why not LF: LF circuits usually do not radiate
    higher frequency harmonics. The ones over 30MHz are the main concern
    here. Even a regular digital circuit can generate enough trouble up
    there to require at least some shielding.

    Regards, Joerg
     
  9. Rich Grise

    Rich Grise Guest

    So that it doesn't radiate extraneous signals and interfere with
    other equipment. At line and audio frequencies, this is not so
    much of a problem because of the physical characteristics of the
    radiated electromagnetic field.

    Hope This Helps!
    Rich
     
  10. Andy

    Andy Guest

    Andy writes:

    Let me add #6 to Howard"s post


    6. To minimize interference which external radiation sources might
    cause to the circuit inside the shield can.......

    You gotta worry about bad stuff getting in, as well as good
    stuff getting out......

    Think of an RF shield as a metallic condom..... Lots of bad
    stuff
    out there that can mess up your life......

    Andy
     
  11. Jack// ani

    Jack// ani Guest

    Thanks all.

    Andy

    hehe...thats very funny.
     
  12. Howard Long

    Howard Long Guest

    Hi Andy
    That's what I was aluding to in:

    2. To prevent unwanted radiation from external devices affecting the device

    Cheers, Howard
     
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