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I am measuring a PWM signal in an oscilloscope using probes with a 1X and 10X switch (active?).

I'm measuring across a resistor which is in series with an LED which is driven by an NPN 2N3904 transistor. The signal is about 2.3V.

One way around (with the ground croc clip attached to the higher voltage side and the probe hook/clamp on the lower voltage side) I get a clean PWM signal. The other way around though I get distinct ringing on the 0V portion of the PWM signal.

The frequency is about 47kHz. This happens when I measure other parts of the circuit e.g. across the LED and the effect is far stronger, almost completely obscuring the signal.

Why would the oscilloscope have this effect and how can I avoid it?

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Pictures and circuits would help. –  Andy aka Jun 13 at 13:03

2 Answers 2

up vote 8 down vote accepted

Your probe ground clip really is ground i.e. tied to safety in the plug.

Here is snip the Tektronix website:

Traditional Oscilloscopes

Most traditional oscilloscopes have the “signal reference” terminal connected to the protective grounding system, commonly referred to as “earth” ground or just “ground”. This is done so that all signals applied to, or supplied from, the oscilloscope have a common connection point.

This common connection point is usually the oscilloscope chassis and is held at (or near) zero volts by virtue of the third- wire ground in the power cord for AC-powered equipment. This means each input channel reference is tied to a single ground reference. A traditional passive probe should not be used to directly make floating measurements on a ground referenced oscilloscope. Depending on the amount of current flowing through the reference lead, it can begin to get hot or, if the current is high enough, it will melt open similar to the way a fuse operates.

Don't do that.

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Ah OK, I didn't realise that the oscilloscope ground is basically actual ground and will pull the signal down. I've been used to using voltmeters which are relatively non-intrusive - I've only recently invested in a scope and I'm just getting used to using it! –  Brendan Jun 13 at 18:47
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A common misunderstanding, To measure differentially you need to buy a differential probe (v expensive) or for your low speed case, you take two probes, clip their ground leads together, and then use the two probes and the (A-B) setting of the 'scope. –  placeholder Jun 13 at 20:56

Your scope probe is an unbalanced input. This means that the impedance presented to the target system (with respect to earth) is not the same for probe tip and croc clip. The probe tip might have 10 Mohm resistance to earth and the croc clip connection might have a few hundered ohms to earth. There might be 5pF capacitance between probe and earth whilst there might be several to hundreds of nF from croc clip to earth.

If your target system were battery powered and very small (so that it's capacitance to earth was really tiny) then it wouldn't matter which way round you connected stuff but, it's more likely that you are powering it from something that does have a significantly low earth impedance and therefore you should try to connect the croc clip to the power (preferably the one that is most likely to route to ground) on the circuit rather than on a signal.

If you connect the croc clip to a signal it's like hanging a few hundred ohms and several nanofarad between that signal and ground and, it probably wouldn't surprise you that if you did that with proper components the signal would become distorted and misshaped.

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