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filter like my finger

Discussion in 'General Electronics' started by ChronoFish, Feb 26, 2004.

  1. ChronoFish

    ChronoFish Guest

    Okay, here is another ultra-newby question. I have rigged up (somehow successfully) a series of amplifiers and "filters" that
    produce a pretty clear signal. However after fiddling with the circuit (physically touching it with my hands) I've moved parts
    around which has re-introduced a small amount of noise. If I touch the battery, the noise goes away.

    Other than the obvious - like "you have no clue what you're doing" - why does touching the battery (it's a 9V - and I just have to
    touch the casing - not the leads) filter out the noise? I've tried adding a capacitor across the leads but haven't been able to
    detect in significant change when doing this.

    -CF
     
  2. successfully) a series of amplifiers and "filters" that
    (physically touching it with my hands) I've moved parts
    does touching the battery (it's a 9V - and I just have to
    a capacitor across the leads but haven't been able to
    One obvious suggestion is that it is you that is making the difference. Try
    earthing yourself (not touching the battery, but an earth wire back to a
    radiator or something). If this has the same effect, you have identified the
    problem.

    Best Wishes
     
  3. I read in sci.electronics.design that ChronoFish <>
    You are acting as an antenna - picking up the noise and re-radiating it
    to sensitive bits of your circuit. When you 'ground' yourself to the
    battery, you become less efficient as an antenna. You could be replaced
    by a big sheet of tin or a tall column of salt water, with the same
    effect.
     
  4. Ban

    Ban Guest

    I bet your circuit is oscillating. Try the following: measure the current
    intake at the battery, and then come close to the circuit touching the
    ground line or supply and when the consumption suddenly goes down and the
    sound is clean- then it was oscillating at a very high frequency.
    capacitor across the supply as near to the opamp supply pins as possible, a
    small 100R in series with the output, there are more possibilities
    depending on the circuitry.
    good luck
    oscillators do'nt, amplifiers do
     
  5. You are sitting in a room filled with electric fields from the power
    wiring in the walls and from nearby radio stations. Your body has a
    complex voltage on its surface as a result of being exposed to these
    fields. Your circuit and its battery also have a voltage but since
    its size and location in space is a bit different, its voltage
    waveform is different from the one on your body. When you touch a
    signal node of your circuit, you pass a small AC current to that node
    as the voltage on your body equalizes with that of the circuit. But
    if you also touch a common point, like a battery terminal, most of
    that exchange current passes through that low impedance node, so much
    less passes through the other contact point, so there is less effect
    on the operation of the circuit from the total exchange current.

    To put this into a physical analogy, imagine that you are riding in a
    life raft on a rough sea (the waves represent the electrical fields
    surrounding you and your circuit). Someone else is riding nearby on
    another raft (that is your circuit). You reach out to them and try to
    pick something out of their eye (a sensitive contact point). You
    disturb the operation of that sensitive system because or your
    relative motion. Then you wrap your other arm around them and repeat
    the operation. This time it is more successful, because that other
    arm is coupling the two rafts together by transmitting most of the
    physical force between you two at a more rugged reference point, so
    the finger in the eye doesn't have to deal with as much relative
    motion.
     
  6. It sounds like an erratic oscillation. Putting your finger on the
    battery is probably stablizing the oscillation or injecting a local
    radio station signal.

    Check your feedback loops. If they're anything more then a few nearby
    passive components, they're causing too much delay. The amplifiers will
    oscillate because they're over-reacting to compensate for feedback that
    is arriving late. You can fix this using a small capacitor to provide a
    short feedback path for rapid signal changes.

    Here's a very simple example. The same holds true whether you're using
    op-amps or single transistors.


    complex
    +-feedback-+
    | filter |
    | |
    +----||----+ <- small feedback capacitor
    | | near amplifier
    | | \ |
    +--|- \ |
    | >---+---->
    | /
     
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