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question re: Oscope — Parallax Forums

question re: Oscope

agfaagfa Posts: 295
edited 2008-08-12 18:46 in General Discussion
when using a scope to monitor·output activity on a prop, i get, what i believe is ringing·(a short oscillation after a·change of state).· what are the possible causes of this?· is it the scope, the probes or the layout of the circuit?· the circuit is just a resistor and LED.· when i push the speed limit of the scope it really distorts the signal.

Comments

  • FranklinFranklin Posts: 4,747
    edited 2008-08-12 15:00
    Have you tried contacting tech support.

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    - Stephen
  • jazzedjazzed Posts: 11,803
    edited 2008-08-12 15:06
    Ringing can be an artifact caused by the scope, probe, and probe ground. The shortest possible probe ground is necessary to get an accurate picture of a signal. I've never seen ringing on a propeller output pin with a proper probe ground.

    Assuming you really have this ringing and not just a scope artifact, it can be caused by a chip's output device. It can also be seen in situations where current on a wire is too high for the output device. Basically ringing is caused AC impedence problems on the little transmission line-oscillator-wire connecting two or more devices.

    One way to limit the effect is to put a small resistor in series with the output device and the next device connected. If you have wires with multiple termination points, the ringing can be hard to control and an end of line terminating circuit may be required. Having a 0.01 uf capacitor connected between a chip's power and ground can also help ringing. The best way to limit ringing with all else being normal is to have a good controlled slew-rate driver in your chip output device hardware.

    Ringing is mainly a design quality issue. On one hand, it can contribute to extraneous RF emissions ... think buzzing in your PC or car stereo speakers every time a cheap cellphone is in use. On another hand, it can damage the devices in the circuit over time because of overshoot or undershooting voltage limits.

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  • SteelSteel Posts: 313
    edited 2008-08-12 15:33
    can you capture an image to look at?
  • GICU812GICU812 Posts: 289
    edited 2008-08-12 18:46
    I dont know if this is what you are talking about, but I often find a 60khz signal sneaking into my scope whenever I am touching any part connected to it. In fact, if I leave the ground and touch the other probe, I get a clear, consistant, and rather strong 60hz signal.

    I can only assume from the frequency, this is somehow related to or sourced from the house power.
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