Oscilloscope probe resonantly picking up 50Hz

[H_man]

Hi all,

I've noted for years that oscilloscope probes tend to pick up noise from AC power cables, but just recently I tried connecting an antenna (piece of wire) to the probe and placing it near some power cables and voltage recorded was off the scale.

I have calculated that the power radiated from a few meters of normal everyday cable with say an amp or two flowing at 50 Hz should be of the order of 10^-10 Watts, so what is the probe picking up? After all, if I'm not mistaken to build a resonant circuit at 50 Hz you either need a lot capacitance or a lot of inductance and I don't think the probe is big enough to contain either relatively large components.

Anyone dealt with this before in any detail?

Thanks
Harry

 

[uart]

Hi Hman, any time you're looking at 50/60 Hz pickup you're going to be much closer than a wavelength so it's pretty much always near field coupling (mutual inductance in the case of the H field and capacitive coupling in the case of the E field).

The scope probe is like a little dipole, so you're looking at near field E coupling here - basically just capacitive coupling.

Say for example you had just 0.5pF coupling from the power line to the probe, then impedance at 50Hz is about 6300 Meg. Your using 50Hz so I'll assume 240 volts, the scope input is typically about 10M, so the coupled voltage is about 240*10/6300 = 380 mV. That's about 1 volt peak to peak already. Add some wire to increase the capacitance and you'll get even more.

 

[H_man]

Thanks uart, this is now clear to me :-)