When testing electromagnetic compatibility (EMC), very often we already have a hunch what the sources of unwanted emissions are or where the vulnerable area might be. But you may find yourself testing for a friend or even be blindsided by something unexpected in your own work.
It happens to the best of us! In that event, you might consider near field probing with a magnetic or electric field probe.
Near field probing is a method of measuring the electromagnetic fields emitting from a product, close enough to the product that you can identify exact sources like PCB traces or even bond wires within an IC. A magnetic field probe uses a loop antenna of low impedance to check for unwanted magnetic emissions. An electric field probe has an open circuited plate that may be used for identifying the electric field sources.
This post will cover how to test EMC emissions and immunity with a near field probe and what to look for when you do. At the end, I’ll share a pro tip for what you can do if you need to complete near field testing in a pinch!
Examples of electric and magnetic field probes found in the EMC Guy's workshop
If the circuit is more likely high impedance, then consider an electric field for probing first, say for a clock or communication line. For low impedance circuitry, such as circuitry with high current switching, it’s best to start with a current probe to couple and sense the magnetic fields emanating.
Connect the probe close to a spectrum analyzer and set the center frequency to that of interest—i.e. one of your suspected frequencies of failure. Then, keeping the resolution bandwidth low for selectivity, sweep the probe over the board, homing in on the source. Note that the source may present as a point or, more likely, a standing wave maxima along a cable or trace.
As before, you’ll want to begin by focusing on the frequencies of failure, but bear in mind that it may not be the fundamental frequency that’s the most sensitive. The harmonics can often be sensitive, too, so sweep high in frequency.
A great advantage of probes is that you can inject radio frequency (RF) into many of them to highlight or reproduce system level immunity failure. However, it can be tricky deciding what level of RF to inject. Here’s what often works for me: First, go as high as your signal source provides to reproduce the failure. Then, with consecutive steps, reduce the power while maintaining the failure mode as you move closer to the most sensitive circuitry.
As engineers, we may be tempted to learn some lessons the hard way. Allow me to spare you that frustration by sharing a few things I’ve learned along the way about using near field probes.
Near field probes are great for design improvement comparisons, but they can require quite the investment in a scanner to easily translate near field data into far field results. If budget and lead times are an obstacle, some semi-ridged coax cables can be fashioned into probes.
To create a magnetic field probe, simply cut the coax and bend a single loop at the end, soldering only the exposed center conductor back to the exposed outer sheathing. To make an electric field probe, just expose the center conductor a little more than the coax sheath as an open circuited coax, job done.
If you are interested in exploring the utility of near field probes, it’s worth trying some simple homemade versions, which can deliver all the same advantages of commercial options. Some factors to consider, whether building or buying:
Finally, don't forget the flexible coax cabling, but be warned: Commercial probes are coated in plastic or rubber. If exploring with homemade probes, be mindful of either insulating the probes or otherwise preventing them from shorting circuitry.
How to create and verify homemade field probes could be an entire blog post unto itself. Readers, let me know in the comments if you'd find value in such a post!
