5V Square wave but not when polarity reversed

Discussion in 'General Electronics Chat' started by Jeebus9000, Jul 30, 2015.

  1. Jeebus9000

    Thread Starter New Member

    Jul 30, 2015
    So I'm generating a 5V square wave at 1000Hz through a 400ish ohm resistor. When I put the probe on, it showed up as a -5V square wave at 1000Hz. Ok, so swap the leads on the probe. Except when I do that, it has a terrible overshoot at the start of the cycle and a terribly slow decay for the second half of the cycle. I feel like this is not expected behavior.

    Why would a polarity change across a non-polar, linear element look like a completely different signal on my oscope? I included captures of the two signals. Thanks!

  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    If your powersupply and your Oscope share a common AC source voltage you might be grounding the signal output when you reverse the scope connections.
    Description of your power connections would let us know if this is happening.
  3. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    Also, what kind of scope is it?

  4. Reloadron

    Well-Known Member

    Jan 15, 2015
    Are you using an attenuator probe as in 1X or 10X selectable or a 10X probe. Does the probe allow for a compensation adjustment? Sounds to me and looks like a probe compensation issue.

    Normal Compensation:

    Normal Comp.png

    High Frequency Accentuation:

    Over Comp.png

    High Frequency Attenuation:

    Under Comp.png

    While the above images do not quite duplicate what you have you may want to check your probe compensation.

  5. crutschow


    Mar 14, 2008
    It's likely that the oscilloscope and the circuit have some sort of ground connection in common through the power line safety ground.
    Normally you do not connect the ground lead of a 'scope to the signal lead of a circuit.