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RF shielding and micros

Discussion in 'Electronic Design' started by hamilton, May 30, 2010.

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  1. hamilton

    hamilton Guest

    I have a project with a 27Mhz osc/amp and a microcontroller.

    The 27Mhz is about 200Watts.

    I have been seeing some of the RF in the micro boards power supplys.

    What is the proper way to isolate the RF out of the power supply ?

    Would putting all the wiring into a steel tube help shield everything ?

    My micro pcb is currently only 2 layer, the next one will be four layer
    with ground plains on the top and bottom layer.

    Does any one have any good references on RF shilding ?

    thanks

    hamilton
     
  2. Jamie

    Jamie Guest

    And what would this project be on 27Mhz at 200 watts?
     
  3. hamilton

    hamilton Guest

    Inductive heating
     
  4. Guest

    If you can see it, you have real problems.
    Shield. Filter what you can't shield.
    Why steel? Unless you need magnetic shielding (simple steel won't work),
    Aluminum is usually easier to work with. Copper or copper-clad FR4 is
    sometimes easier for a one-off, because it can be soldered easily. Copper
    tape is also very useful for prototypes.
    Why? This is almost always a *bad* plan. It gains nothing and is a royal
    PITA. Internal planes work (one ground, one power) and still allow access to
    the signals.
    It's pretty straight forward. Anything over 1/20 wavelength is too big. Are
    you looking for a commercial solution?
     
  5. If the power supply secondary is floating (no connection to case etc.)
    use a sufficiently thick twisted pair and run it through a ferrite
    (both wires) close to the micro board ?
    Does the power supply suffer from shutdown or erratic output voltages
    when the RF is present ? The RF is then entering through the DC output
    terminals and into the voltage sense feedback path, driving the
    differential amplifier into saturation etc. Some ferrites on the power
    supply side of the DC cable will usually help and some chip capacitor
    across the DC terminals as well. Of course the obvious place to filter
    would be in the differential amplifier inputs, but this may cause loop
    stability problems, unless you know what you are doing.

    In the presence of a strong altering magnetic field, you should avoid
    any large wire loops, since the induced current is proportional to the
    loop area. If you have a row of buttons on the front panel, do not run
    a common ground wire to all buttons and separate wires to the button
    hot side, since in the worst case, there will be a large loop, when
    the button is pressed at the opposite side to the common ground wire.
    Instead, run separate twisted pair wires to each button separately. A
    chip capacitor at the exact point where the twisted pair enters the
    PCB will also help.

    In general, use differential signaling (RS-422/485 or 20 mA current
    loop) when possible (instead of RS-232) etc. to simplify filtering.
    Optoisolation also helps in avoiding loops and antenna effects
    (capacitive coupling).
    More important is avoiding loops with large areas and reserving
    sufficient space for filtering components on the perimeter of the PCB
    to act as the first line of defense.

    The frequency is quite low (wavelength 11 m), so this should not be
    too hard to keep it out, but of course, you have to be careful about
    selecting a suitable ferrite material, in order to achieve a
    sufficiently large inductive reactance at that frequency.
     
  6. hamilton

    hamilton Guest

    With the Inductive heating coil in open air, there seemed to be no problem.

    When the coil was placed next to the material to be heated, the cpu resets.

    Looking at the +12v supply rail, (which +5V was derived) we saw 2.1
    Volts of 27 Mhz.

    We did add extras caps (.1uf) to the +12 and +5 sides of the 7805
    regulator and the resets stopped.

    But there is still .2 V of 27Mhz in there.

    I would guess that this will never pass EMI testing. :)

    Thanks for all the comments, I am getting a better idea what I need to
    do here.

    hamilton
     
  7. Check the reset circuit that it does not generate spurious resets due
    to RF rectification.

    Check that the MCU crystal oscillates at all times.
     
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