Electronics > Beginners

Why is my oscilloscope showing a signal?

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n_2:
My oscilloscope channel 2 is showing a signal (2.5V P/P, 88MHz, roughly sine wave) when :

channel 1 ground, channel 2 ground, and channel 2 probe are all connected to the same point in my test circuit
channel 1 probe is connected to a different point in my test circuit.

Channel 1 is showing the same shape with 4.5V P/P.

I don't understand why my channel 2 is showing a signal when its probe and ground are connected to the same point. What could I be doing wrong?

StillTrying:
Make sure both probes are on the same X1 or X10. At that high frequency the GND clips and their short length of wire make a lot of difference even if both probes are are connected to the same point.

n_2:

--- Quote from: StillTrying on November 17, 2019, 04:12:23 pm ---Make sure both probes are on the same X1 or X10. At that high frequency the GND clips and their short length of wire make a lot of difference even if both probes are are connected to the same point.

--- End quote ---

Thanks for the suggestion (and the explanation!).

Both probes are on X10 though.

StillTrying:
80MHz to 115MHz often comes from a nearby FM? radio station, but your up to 4.5Vpp seems very high, so maybe a nearby SMPS.
You could show a screen shot on about 250ns/Div. so we could see it.

German_EE:
Best guess, pickup through either a long ground lead on a scope probe or some sort of PCB loop acting as an antenna. You could also check that you don't have any math modes enabled on the scope that have the second channel as the output.

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