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Scope probe question...

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  • lcdpublishing
    While awaiting the arrival of my scope (should be here later today) I took the probes out of the package and looked them over. Nice little probes - good feel
    Message 1 of 3 , Aug 2, 2005
      While awaiting the arrival of my scope (should be here later today) I
      took the probes out of the package and looked them over. Nice little
      probes - good feel and they appear to be a quality unit.

      First question I have (of many I am sure) is that the ground wire is
      rather short. Is it okay to attach a additional lead to the ground
      wire to extend it? I am just guessing here, but I suspect that this
      would throw off the accuracy of the leads.

      Chris
    • Roy J. Tellason
      ... Good question! You may find, depending on what you re looking at, that this may matter a bunch, not very much, or not at all -- or that if your ground
      Message 2 of 3 , Aug 2, 2005
        On Tuesday 02 August 2005 12:11 pm, lcdpublishing wrote:
        > While awaiting the arrival of my scope (should be here later today) I
        > took the probes out of the package and looked them over. Nice little
        > probes - good feel and they appear to be a quality unit.
        >
        > First question I have (of many I am sure) is that the ground wire is
        > rather short. Is it okay to attach a additional lead to the ground
        > wire to extend it? I am just guessing here, but I suspect that this
        > would throw off the accuracy of the leads.

        Good question!

        You may find, depending on what you're looking at, that this may matter a
        bunch, not very much, or not at all -- or that if your ground wire isn't
        connected that might even still work.

        My Tek probes have ground wires that are detachable, and they came with two
        different length wires. The shorter one is a good idea for when you have
        fast rise times and higher frequency components involved, using a longer one
        tends to introduce some distortion into the resulting waveform, maybe making
        it ring like a bell (look for overshoot, etc.), so try different things and
        you'll get the idea.

        You can't hurt anything by adding some more wire here, but you may not be
        seeing what you're actually trying to monitor in the circuit, but instead
        may be looking at an artifact of your setup. Try stuff! Experiement. Soon
        you'll know when this matters and when it doesn't...
      • Stefan Trethan
        On Tue, 02 Aug 2005 18:11:33 +0200, lcdpublishing ... The ground wire is short on purpose. The higher the frequency you want to measure the shorter it needs to
        Message 3 of 3 , Aug 2, 2005
          On Tue, 02 Aug 2005 18:11:33 +0200, lcdpublishing
          <lcdpublishing@...> wrote:

          > While awaiting the arrival of my scope (should be here later today) I
          > took the probes out of the package and looked them over. Nice little
          > probes - good feel and they appear to be a quality unit.
          > First question I have (of many I am sure) is that the ground wire is
          > rather short. Is it okay to attach a additional lead to the ground
          > wire to extend it? I am just guessing here, but I suspect that this
          > would throw off the accuracy of the leads.
          > Chris


          The ground wire is short on purpose. The higher the frequency you want to
          measure the shorter it needs to be. For very high frequency you'd even
          change it for a spring you slide on the probe with the tip close to the
          probe tip.

          BUT this means for low frequency it can be much longer. For example i
          often remove the ground lead and connect the scope gnd via a 4mm (banana
          plug) lead to the circuit under test ground. My scope has a socket for
          that. so you have a fairly long loop there of several meters. Not an issue
          for low frequency work at all and it is nice to have the scope probe
          "free".

          if your scope has no 4mm socket you could simply add one by soldering a
          4mm socket to a BNC plug housing, and putting it on one of the rarely used
          inputs like ext. trigger. You can get BNC to 4mm adapters which do the
          same thing, just for both ground and center conductor.

          If you prefer say a wire with a alligator clip as probe you can connect
          that to the scope input, and for low frequency work it will work as fine
          as 1:1 probe.

          ST
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