Why does reversing the oscilloscope probes cause ringing in the signal

noiseoscilloscope

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?

Best Answer

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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