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I have a portable oscilloscope, and when I connect just one probe to it, sometimes, moving the probe into the air without connecting it to anything, then I can see (in some of the places of my house) a very stable 50 Hz sine wave of around 200 mV appear on the screen.

Its intensity varies as I walk (from 0 to almost 200 mV), being stronger when I am near an LCD screen, a wall switch, or next to the refrigerator. Ir sometimes also works a little when I am not close to anything, apparently.

Sometimes it varies also if I move my body and the scope and the probe stays in the same position.

As I don't believe in spirits, I suspect that it may be some kind of 'induction' from the installation of the alternate current (50 Hz, in Spain) in my house that transmits itself 'into the air' (as my probe is never touching anything). Could it be?

If that is true, how does it work?

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    \$\begingroup\$ Which portable oscilloscope it is and what kind of probes it has? Like multimeter probes with banana plugs or like real oscilloscope probes with BNC connector? Likely just capacitive coupling picked up by the scope. \$\endgroup\$
    – Justme
    Commented Dec 25, 2022 at 22:57
  • \$\begingroup\$ oh, but what if you did believe in spirits? you could have such fun ideas about what is happening \$\endgroup\$
    – amara
    Commented Dec 26, 2022 at 19:10

3 Answers 3

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The probe may be characterized by a capacitive coupling. This make the probe acting like an antenna receiving 50 Hz as power lines radiations.

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    \$\begingroup\$ The antenna is also the guy holding the oscilloscope. \$\endgroup\$
    – Grabul
    Commented Dec 26, 2022 at 2:00
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    \$\begingroup\$ Sure! A simple check by touching/connecting to a grounded conductor \$\endgroup\$
    – Alesandro Giordano
    Commented Dec 26, 2022 at 2:11
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    \$\begingroup\$ This is a great object lesson introducing what might be the greatest nightmare in the microelectronic world - capacitive coupling, better known as crosstalk. \$\endgroup\$
    – JBH
    Commented Dec 26, 2022 at 23:03
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It gets a lot worse than that. We regularly see people read 50-100 volts from that effect, and we're in 120V country!

Get an long extension cord, preferably 3-wire. Also get a spool of hookup wire. Run the wire alongside the extension cord, e.g. tape them to each other with tape so they are very close. Don't twist. Set your scope in 400V range and put it on the extra wire. Surprise! You will see dozens if not hundreds of volts there.

That is capacitive coupling. It drives electricians (people who wire buildings) crazy, especially when guiding novices using high-impedance DVMs, which are very sensitive to it.

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Welcome to the grid hum.

The source is your power grid. Your probe may be coupled capacitively or in rare cases, inductively (near big transformers or motors). It may as well happen because of a ground loop inside the scope or between the scope and the circuit you are measuring.

Since you can control the coupling by moving yourself, it is most likely a capacitive coupling.

The phenomenon is not limited to oscilloscopes, it is a real pain in audio engineering and wherever a low-noise signal is dealt with.

One usually doesn't get a pure sine wave, because some of the coupling mechanisms are non-linear (e.g. corona discharge) and both the inductive and the capacitive coupling promote higher frequencies.

Ground everything and hope for the best.

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    \$\begingroup\$ And grid hum is like a fingerprint in time. youtu.be/e0elNU0iOMY \$\endgroup\$
    – chicks
    Commented Dec 29, 2022 at 21:33

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