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DC pulses on AC-coupled oscilloscope

Discussion in 'Electronic Basics' started by John Marsh, May 28, 2004.

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  1. John Marsh

    John Marsh Guest

    Hello all,
    I have an old, AC-coupled oscilloscope which has no
    calibrated V/cm Y-axis. I want to use a simple 555 pulse generator,
    whose peak pulsed DC output is 9V. With this signal into the
    oscilloscope, what exactly am I seeing on the screen? @Scope is
    Heathkit/Daystrom 10-12U.
    Viva valves,
    John Marsh.
  2. Bob Masta

    Bob Masta Guest

    What you are seeing is an AC-coupled pulse, which means
    that "it depends". Depends upon the time constant of the
    input coupling and the pulse parameters. For example,
    if you make your pulse into a 50% duty cycle square wave
    and run the frequency up, you'll get a decent looking square
    wave on the scope. As any portion (high or low) of the wave
    gets too long for the time constant, you'll see the wave peak
    and then decay away with a standard RC time constant decay
    until the next transition. Note that this applies to the "off"
    state as well as the "on" state of the pulse, so you see an
    "up decay" after an off-transition. Once you get familiar
    with what you are looking at, you can probably make pretty
    decent measurements just by looking at the peaks.

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  3. John Marsh

    John Marsh Guest

    Dear Bob,
    Thanks for your help. So:-I need to lower the frequency of
    the pulses and check that there is no decrease in indicated amplitude.
    If there is, I need a higher frequency for my work.

    I think I learn more using cheap gear that needs a little effort.
    £7.50 well spent!
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