I was watching a video of Bob Pease discuss magnetic coupling of noise into the ground lead of an oscilloscope probe. See video at 5:40 https://www.youtube.com/watch?v=2vzvWUqUtb8

I noticed another person at the table say "you really need to worry about that when you do switchers because the noise comes in through the leads." "it acts like an antenna."

I don't understand what he is describing. So I started thinking...

  1. How exactly is noise magnetically coupled into a ground lead when the ground lead acts like an antenna?
  2. Are magnetic loop coupling, mutual inductance coupling, and magnetic "antenna" coupling different?

For my second question, take for example two parallel wires: a ground lead of a probe and a wire containing a large di/dt or large magnetic field around it.

There seem to be three or more ways noise can magnetically couple into the ground lead.

  1. Magnetic loop coupling. When a circuit is connected to an oscilloscope, a loop is formed. Any surrounding magnetic field can enter this loop and induce a current which affects the measurement. I believe I understand this alright.

  2. Mutual Inductance in parallel wires. Calculating Mutual Inductance in Parallel Wires. I don't understand the physics behind this.

  3. Magnetic "antenna" Coupling, discussed in the video. I don't understand the physics behind this.

Can someone explain these?

If the parallel wires are close together 1 and 2 seem to be valid. When the wires are separated or far apart, 3 seems valid. Are these forms of magnetic coupling the same or different?


1 Answer 1


A partial answer

All magnetic noise couplings are based on induction, one of the fundamental relations between electric and magnetic fields. Induction cannot be explained with any mechanical phenomenon, it only exists as one of the basic phenomenons in physics.

The Induction Law says that where ever and for whatever reason a magnetic field changes, there's also in the same space an electric field.

The induced electric field can easiest be detected by letting a wire be in a changing magnetic field. The electric field occurs as a voltage between the wire ends. That's utilized in electricity generators, electric motors and transformers. Every AC or pulse circuit with an inductor utilizes it. Harmfully the voltage occurs also in unwanted situations like in unshielded measuring or signal cables, no matter the noise cacthing wire happens to be the scope probe ground. The caught voltage is in series with the signal voltage.

Hopefully you see that no such mysterious thing exists that magnetic field somehow generates a voltage to a wire. The voltage is already there as a property of the electric field as soon as there's a changing magnetic field. The easy to move electrons in a metal wire make possible detect and measure it. Removing the wire doesn't remove the voltage if the changing magnetic field is still there.

A magnetic antenna is an artificial classification term for an antenna which is basically a small diameter wire loop, much smaller than 10% of the wavelength. It's called magnetic because its electromagnetic non-radiating nearfield in transmitting applications contains much more energy in the magnetic field than in the electric field. Respectively it's receiving performance can be approximated by assuming it's a wire loop in a changing magnetic field which induces a voltage to it. But that's not accurate as a general purpose method. Proper antenna analyses should take into the account exactly both the electric and magnetic field components of the electromagnetic wave and the fact that the wave propagates.

Unfortunately more in-depth no-nonsense qualitative explanations and all quantitative explanations of the said things must be based on Maxwell's equations. No exact antenna analysis happens from scratch without PhD level math. Using advanced numerical analysis software is marginally easier, but even it is out of the scope of this answer and beyond the capabilities of practical electricians like me. We can only browse handbooks and hope someone has derived useful formulas. Sorry.


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