oscilloscope question

Discussion in 'General Electronics Chat' started by lokeycmos, Jan 7, 2012.

  1. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    quick question about osciloscope. on the front of my old school tektronix scope is a switch that lets me choose between "ac", "dc", and "ground".
    could someone please explain what this switch is for and when i would use each positon?. i attached a pic of my scope. i also connected a simple 555 circuit to my scope and took a pic of each waveform in each switch position. i get different results in each mode. thank you
     
  2. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    heres the 3 waveforms in each position. in order from left to right: ac,dc,ground
     
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    • DC.jpg
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    • Ground.jpg
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  3. Sparky49

    Active Member

    Jul 16, 2011
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    I believe that DC allows both DC and AC signals, whilst AC only allows AC signals.

    Posh name - AC and DC Coupling.

    EDIT: In some cases AC is used when you want to know where 0V is - it'll be in the centre of the waveform. But it's better to use DC for low-frequency (a few Hz).
     
  4. JMac3108

    Active Member

    Aug 16, 2010
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    This is the "coupling" for your input.

    (1) DC coupling shows the signal including any DC level.

    (2) AC coupling removes any DC level in your signal.

    (3) GND simply connects the input of the scope to ground.

    DC coupling is used the most. Use AC coupling when you need to look at a small AC signal on top of a large DC level. For example looking at 50mVAC of noise on a 12VDC power supply line.

    The ony thing I've ever used GND for is to locate my trace.
     
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  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The "AC" setting typically provides a capacitive input coupling. This eliminates DC, and also imposes an AC low-frequency cutoff, typically -3dB at 10Hz.

    If you look carefully at your waveform with the "AC" setting, you can see a tilt in the tops and bottoms of what should be a rectangular waveform. Essentially, what you are seeing are the beginnings of exponential capacitor discharge waveforms.
    If you looked a a lower frequency square wave, the tilt would become more pronounced.

    The "GND" setting can be useful to set the beam level to a graticule line, particularly when making DC measurements. This makes it easier to see what absolute voltages are displayed.
     
  6. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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  7. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    thank you all! very informative!
     
  8. Lundwall_Paul

    Member

    Oct 18, 2011
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    DC allows both DC and AC signals. You will see the AC riding the DC component of the signal.
     
  9. w2aew

    Member

    Jan 3, 2012
    219
    64
    Here is a link to a YouTube video that I did a few months ago on the AC and DC Coupling switch on the oscilloscope.
     
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