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Oscilloscope (ground @ 0)

  1. Apr 14, 2013 #1
    I was instructed to display an AC signal on an oscilloscope with "ground at zero."
    Does anyone know what ground at zero is?
    The signal is basically a sine wave, BUT with the bottom cut off (when the sine wave is supposed to be negative, it is 0 instead).
  2. jcsd
  3. Apr 14, 2013 #2
    on the front of your oscilloscope you may have a switch marked DC/AC which relates to the input.
    Try changing this to see if it makes a difference. The AC position will place the trace in the middle of the screen
  4. Apr 14, 2013 #3
    This is what I thought of before and it might be the same thing as what you're suggestion. Is ground at 0, simply to put the flat part (where the sine wave signal is 0) at 0 mark on the oscillioscope display?
  5. Apr 14, 2013 #4
    I am not certain what you mean when you say....."basically a sine wave with the bottom cut off".......that sounds like half wave rectifiedvAC.
  6. Apr 14, 2013 #5
    yep, exactly.
  7. Apr 15, 2013 #6


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    See the first two explanations of how to use your oscilloscope to measure AC:
    http://www.wisc-online.com/Search.aspx?search=oscilloscope [Broken]
    Last edited by a moderator: May 6, 2017
  8. Apr 17, 2013 #7


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    Staff: Mentor

    Here we have an example of a phrase whose meaning is understood by the person who used it, but is clouded in ambiguity to those who haven't experienced it previously.

    My best guess is that it means with the AC/DC switch set to "DC". Let me explain why.

    When the switch is in the "DC" position, the CRO displays the full input waveform, the whole signal, all of it, with nothing missing. But there are times when you'd like to examine just the alternating part, to amplify it more, and on those occasions you can switch the AC/DC switch to "AC". In the AC position, any DC component is removed, and the zero volts line is moved to where the average value of your input signal lies. So if your waveform has a ripple or alternating part, the average of this is moved to the zero volts line. In this situation, the waveform has an area above the zero line equal to the area below that zero line. In effect, you lose track of where true zero is.

    To determine the average (i.e., by definition, the DC value) of any waveform, switch it between "AC" and "DC" and the amount of shift in the displayed waveform equals the average value of that signal.

    DSJI9.gif Remember, in position "DC" you always see the complete waveform, so you will usually want to start with that position to establish in your mind the full picture of the signal your CRO leads are connected to.
  9. Apr 17, 2013 #8

    jim hardy

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    i'll venture a guess

    by "zero" , instructor refers to some horizontal line on the scope's graticule. He didn't say which.

    Were I instructor i'd tell beginners:
    " We will for this exercise use center of screen as zero so that you can see both halves of your signal. Ground input and place trace on centerline.

    If I want you to use bottom of screen for zero, I will tell you to ground the input and move trace to bottom of screen.

    As you gain skill you will decide for yourself where to place zero. It is good housekeeping but not absolutely necessary to use either center or bottom line of graticule for zero.

    That seems a lot less ambiguous to me.
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