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How to use ferrite beads to attenuate high-freq noise?

Discussion in 'Electronic Basics' started by Dummy, Sep 16, 2003.

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  1. Dummy

    Dummy Guest

    I have four radios with different transmission frequencies and there's
    a power supply and voltage regulator nearby. I put some ferrite beads
    on the power supply cable, hoping that it will attenuate the high
    frequency signal from radios. Somehow, sometimes I found that the
    interference became slightly significant or did not reduced after
    introducing the ferrite bead. Any effects to put several ferrite beads
    in series along the power supply cable? Is the location of the ferrite
    beads at the cable important?
  2. Yes, very important! Place it as close to your radio as possible.

    But usually using beads is not enough or even the wrong solution. What
    they do can be two things:
    - dissipate hf by eddy cirrents, but you need special beads with loss
    for this!
    - make the hf impedance of the cable part after the bead higher so if
    <IF!> your radio has a low impedance there, some attenuation will be
    Putting beads in series increases the impedance, so the cut-off
    frequency will get lower.

    A good radio is not so sentitive to its power lines. So i doubt the
    usefullness of the beads.

    I personally think that your should not only look at the radio's but
    go to the source of the problems: the power supply and voltage
    regulator. Are they analog or switching?

    Also look at the wiring, at loops etc. Are the radio's grounded?


    Pieter Hoeben
  3. Dummy

    Dummy Guest

    Guess my question was a bit vague.

    I have four voltage regulators which supply voltages for 4 radios
    respectively. The radios sit very close (about 2 inches) to the
    regulators. My aim is to eliminate the noises that appear at the
    output of the voltage regulators. The noise amplitudes were getting
    very very high ( up to 13.0V peak to peak! ) when 4 radios were keyed
    up simultaneously. I am not sure whether it's the RF frequencies that
    emitted by antennas cause non-linearity to the voltage regulators and
    distorted the output voltage.

    The voltages were supplied to the radios through a battery connector.
    But the battery connector supply was not tapped directly from the
    output pin of the regulator. Regulator voltage would travel through
    and PCB trace, go through a banana jack and power supply cable before
    reaching battery connector. In order to eliminated the annoying
    noises, I have:

    -Put ferrite beads at the battery connector supply line. It didn't
    seem to be working fine. The regulator circuits were covered by metal
    plates which has the common ground as the circuit. The weird thing was
    when the plate was shorted to main supply ground, the noise reduced
    significantly, from about 13.0V pkpk to 3.5V pkpk. A factor of about
    3.7! Nevertheless, this method had to be used along with the ferrite
    bead at the battery connector supply line. If the ferrite bead was
    removed, noise amplitudes would stay at the same level.

    -Put a big cap (4700uF) at the output of the regulator. Not good
    though. By observing the noise through oscilloscope, the noise
    frequencies were below 100Hz. Hmmmm...noises only appeared when radios
    keyed up.

    -Wrapped the regulator with aluminium foil. However, noise amplitude
    still wouldn't go down.

    -Now I'm planning to put two bypass caps 0.1uF and 680nF as close as
    possible to the output regulator on the PCB. I doubt whether that
    would work.

    I really don't have any ideas why this would happen. Ok, I know by
    separating the radios some distance away from the regulators would
    help to reduce the noise. Any other suggestions other than this?
  4. Robert Baer

    Robert Baer Guest

    You have not stated if the regulators are linear or switching.
    I assume a single supply feeds the regulators, and there is one
    regulator per radio.
    So far, so good - but that means a common lead/wire is used for the
    negative line.
    That trace shoult "wye" out from the supply to each regulator, and
    each regulator output + and - line should go "directly" to a given radio
    via a simple wire pair.
    If the regulators are 3-terminal linears, use 0.22uF bypass caps right
    by each one.
    Now as long as the radios themselves are not connected to each other
    by *ANY* other means, it would
    make no difference if one radio has a - common chassis and another has a
    + common chassis.
    Switching regulators should have bypassing according to manufacturer
    specs; no more and no less.
    Using a large electrolytic cannot help RF bazzfazzz, they aer not
    designed for high frequency (say above 1MHz) work.
    If the above does not help, you may have something connected
    incorrectly; good power supplies have noise in the low millivolt region
    at worst.
    The fact that connecting a bunch of radios to the power system is a
    very good sign of a serious interconnection problem.
    You can temporarily add a 50 ohm resistor in series on the + supply
    line from each supply to its radio (10 radios, 10 resistors,
    independently connected).
    Connect up one radio at a time, then power that radio on, and look at
    the noise.
    Is the scope grounded? Is the supply grounded? Is it the same ground?
    If so, you have created a ground loop which can *create* problems!
  5. Dummy

    Dummy Guest

    Main power supply, scope's probe and regulator have common ground.
    That means each ground is connected to the same point. I'm using 3
    terminal linear regulator. Recommended capacitors have been put at the
    input and output of the regulator. Before keying up the four radios,
    regulated voltage was smooth with pkpk noise 120mV. Not quite bad

    The four regulators are connected in such a way that they are sharing
    unregulated voltages from one main power supply. 10V to 12V
    unregulated to 8V regulated.

    I've added two ceramic caps at the regulator's output on the PCB. It
    didn't work though. Since the noise frequency that I've seen in scope
    was quite low (100Hz or below). It wasn't RF frequecy. I guess no
    point to add those two bypass caps.

    Meanwhile, I will try to add the 50ohm res to the regulator's supply
    line. By the way, what's the purpose of the 50ohm resistor? Matching
  6. Robert Baer

    Robert Baer Guest

    1) So the scope ground line connects to the ground of the supply. Is
    power ground crom the supply power cord connected to the supply output
    ground? If so, you caneasily have a ground loop - look for 60Hz and
    120Hz "ripple".
    Use a handheld (battery powered) DVM in AC mode (or VOM using blocking
    capacitor) to make similar checks.
    2) The idea for 50 ohm resistors on the output of each regulator is to
    partly absorb Rf, and partly to isolate the load from the regulator.
    Perhaps the problem will not appear, or will show up with 5 radio
    loads as compared with 4 radio loads if no resistors were there.
    Or maybe the problem only shows with a given radio (other 4 connected,
    a specific one as the 5th doing the deed).
    It is a means of helping to find where the cause may be....
  7. Basically you've done the right thing. The ceramic caps are necessary for
    the regulators, so keep them connected. The noise (100Hz) looks to me like
    being caused by an insufficient PS transformer voltage. Make sure you
    _really_ get 10 to 12V from the PS. Maybe add several 4700 microF caps
    before the regulators, however this will not help if the transformer
    voltage is too low. Remember that a linear regulator is likely to need
    about at least 2V between IN and OUT to operate properly. There will be no
    8V at OUT if there are just 8.5V at IN. If there is insufficient IN
    voltage, it is very likely to leave the rectifier 100Hz noise pass through.
    DO NOT connect the 4700s to the output of the regulators as you have done,
    conect them to their INPUT. The strength of the noise looks to me like
    there is either no cap at the input at all or there is a very small one or
    the transformer cannot deliver the required power because of internal
    resistance. Don't use a 50Ohm res. Caps should suffice and if the trafo is
    wrong, it won't help either. 100Hz noise looks like a wrong voltage
    somewhere, a res won't kill it.

  8. Robert Baer

    Robert Baer Guest

    I hope the OP appreciates your comment.
    I *should* have thought of that!
  9. Dummy

    Dummy Guest

    I don't even know that scope setting would affect the voltage and
    noise level reading. Let's say the noise level is 4V pkpk. There are
    some BW limit and ch1 50 Ohm setting. Turning ON BW limit option, you
    wouldn't see much noise. Something like 5x reduction. Turning ON ch1
    50 Ohm option, the voltage would drop from let's say 8V to 2.5V.
    However, noise level still remained at 4V pkpk. I'm confused. Who
    knows I've been measuring the wrong thing all this while. Maybe the
    'real' noise isn't that bad at all.
  10. JeffM

    JeffM Guest

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