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Oscope measurement and probe grounding

Discussion in 'Electronic Basics' started by jw, Aug 28, 2005.

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  1. jw

    jw Guest


    I have just set up my ERA-1SM amplifier, like it says to do here (P. 2) :

    You can clearly see the output point.

    My question involves my Oscope measurement technique.

    What am I measuring if I just connect the probe to the output point without
    grounding it? That is, just attach the probe point to the output and do not
    clip the probe ground to circuit ground. I am getting values, but I'm not
    sure what it represents?

    I am using a Pico 212 scope that is attached to my laptop using the RS-232

    Further I am using probe that is rated to 100MHz but I believe the circuit
    is operating more toward 1 GHz - so do I need to get a "faster" probe to
    meaure this circuit? I don't need any very precise results, I just want to
    see if I have the amplifier set up correctly to work.

    --TIA, jw
  2. Ralph Mowery

    Ralph Mowery Guest

    It is no real way of knowing what you are measuring without the ground
    connected. Many times the scope will just pick up 60 hz powerline noise
    (assuming you are in an area near powerlines and they are 60 hz.

    Also you need to provide an input to the amp. Without that you should not
    see much at the output.
  3. At best you are measuring the circuit you are probing with a ground
    that is referenced to your scope ~ So you measurement is off by the
    voltage drop induced by the probe cable. At worst your probe is just an
    antenna and your picking up a lot of additional noise.

    How ever because you probe does not even cover the frequency being
    measured it may look better then it really is. You need a probe that
    works over the frequency range being measured, and a probe which is
    grounded as near the probe point as possible.
  4. ehsjr

    ehsjr Guest

    Probe ground has to be connected. Whether you
    will see anything worthwhile at 1 GHz with a 100
    MHz probe and a 25 - 50 MHz scope I can't say.

  5. mike

    mike Guest

    Probe ground is essential. Furthermore, the ground wire needs to be WAY
    shorter than you probably think. Zero is a good value.

    One way to get a feel for the problem is to consider the probe input
    capacitance and the inductance of the ground lead as a series resonant
    circuit. At the resonant frequency, the probe puts a dead short across
    your circuit. The measurement becomes useless at much lower frequencies.

    So, what's the inductance of the probe wire, the scope, the line cord,
    the wall plug another line cord another power supply and the wires to your
    test circuit?

    Hook up the ground with as close to zero length as you can get.
    And all this assumes the bandwidth of the probe and scope are up to the

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  6. jw

    jw Guest

    Looks like I have to rent a fast scope. --jw
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