What does electrical noise that is mitigated by a human hand tell us about the noise itself?

Question

I'm sorry if this is too vague, I'm not thinking of a specific instance when this happened. There have been at least several times when I've been debugging the source of some unwanted electrical noise that is either mitigated or eliminated entirely when I put my hand close to the wire or trace that the noise is on, and I was suddenly curious about it again this morning.

I understand that me being close to the wire capacitively couples me with the wire, and I understand how this capacitance could introduce to some low-pass filter behavior into the circuit. I'm confused why only the noise (not the intended signal) is affected by my hand. Does it have to do with the relative drive strength of the signals?

What does electrical noise that is mitigated by a human hand tell us about the noise itself? Are there common fixes for noise that is mitigated by proximity to a hand/human?

Answer 1

What does electrical noise that is mitigated by a human hand tell us about the noise itself? Are there common fixes for noise that is mitigated by proximity to a hand/human?

Chances are that the "mechanism" is capacitive coupling and, of course, the hand can act as quite a good "earth barrier" in these circumstances. But, like you said the hand (as a capacitor) can also slightly slow down a signal on a PCB trace to enough of an extent that the problem is fixed; not because of interference but because the signal edges were slightly slowed down.

I'm confused why only the noise (not the intended signal) is affected by my hand.

Because the hand-to-ground capacitance is going to be in the region of several hundred pF whereas the noise-to-wanted-signal capacitance is only going to be a few pF. Using the hand as a barrier represents a big attenuation to the potential noise.

There is less chance of it being magnetic coupling because the human hand would not have anything like the effect you are seeing.

As for fixes, try using a bit of aluminium foil between likely noise source and victim signal to see if it helps when the aluminium is close to the victim or close to the possible perpetrator. If close to the possible perpetrator then it's likely interference. If close to the victim then there's more chance that adding a small capacitor to ground will slow the signal down enough that it doesn't cause a problem.

You have to be logical about this of course and also try some ferrite shielding to see if that has an effect.