# How do I tell whether if it is my oscilloscope or circuit catching mains 50 Hz?

I see that my oscilloscope has some 50 Hz when the probe only is connected to the oscilloscope, which is normal. But I want to know if my circuit is catching the 50 Hz main from e.g. the power supply. Is there any good way of doing this?

I have a Siglent SDS 1052DL+ oscilloscope and I am measuring the output of a load cell connected to a board.

Edit: Measurements using Jerry Coffins and EforDogsFan methods.

Probe tip connected to probe GND: Showing no significant 50 Hz.

Probe tip connected to probe GND and PCB GND: Showing no significant 50 Hz. Rather some 330 kHz noise from a switching voltage regulator on the board.

Probe tip and probe GND not connected (open): Picking up significant 50 Hz noise, obviously changing in amplitude depending on how the probe wire is put on the table.

Measuring output of load cell: Showing significant 50 Hz noise compared to the DC voltage output of the load cell.

Measuring output of load cell with extra weight compared to previous measurement: Just to show the DC offset when adding significant weight.

• The probe tip connected to probe GND shows no 50 Hz noise. But having the probe tip and probe GND unconnected shows significant 50 Hz noise catched by the probe as it will vary in amplitude when stretching out the oscilloscope probe cable. Won't this actually be present when probing the output of the load cell on the PCB? Commented Dec 27, 2020 at 15:26

Connect whatever probe you're using to your scope.

Take the ground clip on the probe, and connect it directly to the probe tip.

Now look at the scope. It's now displaying whatever RFI/EMI the it's picking up between the probe tip and the display. Set the scope for roughly the vertical sensitivity you're using to measure your circuit. Or to make the test more stringent, turn the vertical sensitivity up a couple notches.

Chances are pretty good that even with the sensitivity turned up as high as possible, you won't see significant noise.

If you do see some noise, test with a different probe--a problem with the probe's shielding strikes me as a lot more likely than a problem with the scope itself.

But I doubt that'll happen. I have probes that have been used (and probably at least mildly abused at times) since decades before that 'scope came on the market, and most of them still work just fine (the one exception I can recall was pretty obvious too--it pretty much got ripped in half, so the damage was immediately obvious).

• you'll probably see a fair bit of noise, actually, if you're using the common ground leads that most scopes come with. That ground lead wrapping around to the probe tip makes for a pretty decent loop antenna. Commented Dec 23, 2020 at 2:27
• @Hearth: depends on frequency, of course--but the wavelength of 50 Hz works out to something like 6000 km, so a typical probe's ground wire falls just a tad short of a half wave... :-) Commented Dec 23, 2020 at 4:42
• Sure, but I speak from experience--you get a good bit of line noise pretty easily. It's far from an efficient antenna, but you can pick up a few mV! Commented Dec 23, 2020 at 14:01
• @JerryCoffin I have now done some measurements using your method. I cannot see any significant 50 Hz noise when connecting the probe tip with the probe GND. The probe is only picking up 50 Hz noise when not having them connected to anything except the oscilloscope input. Shouldn't this be considered as something that will be put into the measurements on the PCB? Or can I safely say that I have 50 Hz in my powering of my PCB? Commented Dec 27, 2020 at 15:21
• @RobinHellmers: well, we've shown that if there isn't a voltage at the probe, the scope shows essentially nothing. It seems to follow fairly directly that if the scope is showing something, it's because there is a voltage at the probe. If the probe is connected to your circuit, what it's measuring seems to be coming from your circuit. Commented Dec 27, 2020 at 22:06

To add one important step to Jeffry Coffin's suggestion, take that ground clip attached to scope tip setup and now touch that connected combination to the ground of the load cell PCB. NOW you are seeing the effects of the grounding setup you have between the load cell, its power supply, the scope, and its grounding to our AC mains. Hope this helps...

• Good point. [filler]. Commented Dec 23, 2020 at 2:06
• @EforDogsFan I have now done some measurements, see edited post and extra comment under Jerry Coffin's answer. Commented Dec 27, 2020 at 15:22