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Near-Field Magnetic Probes

Written by Robert Lacoste

Low-Cost EMI Detection

Robert Lacoste’s very first topic covered in his Circuit Cellar column back in 2007 was electromagnetic interference (EMI). Coming full circle, this month he examines EMI once again, but this time from a different standpoint. Robert explains what near-field magnetic probes are, and how they can help you in your designs. More importantly, he shows how to build one at virtually no cost.

Welcome back to “The Darker Side.” Twelve years ago, I started this column with an article on electromagnetic interference (EMI) (“Let’s play with EMI!” Circuit Cellar 205, August 2007). That subject was clearly written in the spirit of Steve Ciarcia (Circuit Cellar’s founder) and I wanted to focus “The Darker Side” on “…some of the lesser-known, more obscure aspects of electronic design.” As a regular reader, you now know that nothing is black magic. Some subjects may seem scarier than others at the beginning, but all are understandable with some basic knowledge. And, after all, playing with them is a lot of fun!

This month, I will once again write about EMI, but from a different perspective. I’ll explain what a near-field magnetic probe is, and how it can help you in your designs. More importantly, I will show you how to build one at virtually no cost. So, take a seat and enjoy!

EMI?
Let’s start with a refresher on EMI. EMI appears when a disturbance generated by a given device affects another device, either through conducted or radiated coupling. It is not a new topic. According to Wikipedia, the first international commission on EMI was launched in 1933. CISPR (French acronym for “Comité International Spécial des Perturbations Radioélectriques”) was born, and is still an active IEC commission [1].

EMI generated by a badly designed circuit can easily affect radio communications, including critical ones. However, it can also jeopardize the behavior of other electronic circuits, especially if they are not well protected against EMI. If you want to learn more about some real-life incidents or even accidents caused by EMI, I strongly recommend the NASA document titled, “Electronic Systems Failures and Anomalies Attributed to Electromagnetic Interference” [2]. It is 30 pages long and includes, as an example, an EMI-linked accident in 1967 on the ship USS Forrestal that caused the deaths of 134 sailors.

Even if you are not designing weapon-control systems, EMI can still be a concern in your projects. Have you ever gotten more noise than expected on sensor circuits? Maybe it was due to EMI conducted or radiated by power circuits or digital circuits nearby? Have you ever seen a microcontroller (MCU) behaving erratically from time to time? Maybe this happened because some EMI inadvertently triggered its reset or interrupt inputs? Last but not least, EMI is now controlled by regulations such as CE marking in Europe and the FCC in the US. If your product is not adequately designed in terms of EMI, then you simply will not be able to sell it to anyone or even to have the right to switch it on outside of your own lab. For more on CE, see “The Darker Side – CE Marking in a Nutshell,” Circuit Cellar 257, October 2011).

MAGNETIC & ELECTRIC FIELDS
As noted earlier, EMI can be either conducted (transmitted between two circuits through a wire), or radiated (transmitted wirelessly). I will now focus on the latter. You know that a wireless transmission is done using a propagating electromagnetic field. Such a field is a combination of an electric field and a magnetic field—interacting together as defined by Maxwell’s equations. Don’t be afraid. I won’t do any math here. I simply want to point out that these two fields are jointly propagating in a well-defined manner.

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Now, how is an electromagnetic field created? Either by creating an electric field or a magnetic field. An electric field is produced by stationary charges (electrostatic sources), whereas a magnetic field is created by moving charges (current sources). So, the field around an EMI source is mainly either electric or magnetic, depending on the physical phenomenon that generates this EMI. This area close to the source is called the “near-field zone.” Further from the source, Maxwell’s equations transform this field into a gentle electromagnetic field—the “far-field zone.”

Now think about an average electronic design, with some parts soldered on a PCB and interconnecting wires to the outer world. Do you think you are more likely to have magnetic or electric EMI sources? Magnetic is often the good answer because such designs more often have high currents and low voltages, rather than the opposite.

NEAR-FIELD PROBES
Imagine that you have a prototype that generates too much EMI, and you need to improve it. You first need to know where this EMI came from. Who is the culprit? Why not install a radio receiver (or a spectrum analyzer) a couple of meters in front on your product, meaning in far-field conditions? Usually this is done in an anechoic chamber to avoid external perturbations. You will then measure the frequency spectrum of the EMI, and this may help a lot. For example, if EMI is mainly at frequency multiples of 24 MHz—and if your CPU clock is 24 MHz—then you have some clues.

A complementary method is investigating far closer to the device, using so called “near-field probes” (also called “close-field probes”). Such probes are intended to help locate the exact source of emission on a circuit. Just connect it to a measuring instrument—using a spectrum analyzer, or even an oscilloscope, as I will show you—and drag it over the board. The stronger the signal, the closer the source. The signal level will vary a lot when moving the probe, but qualitatively it is quite easy to find the hottest spots. So, near-field probes are mainly used for qualitative and relative measurements.

Because an electromagnetic field can be generated either by an electric or a magnetic source, near-field probes exist in two variants: electric probes (E-probes) and magnetic probes (H-probes). Because magnetic EMI sources are more common than electric sources, a magnetic probe is the first to get in your lab. By the way, another advantage of an H-probe is that a magnetic field fades faster than an electric field as the distance increases, and this helps to locate the source.

MAGNETIC PROBE
Now let’s look at how to make a magnetic probe. Some theory may help here. You may have learned that any current circulating in a conductor generates a magnetic field, which is calculated using the Biot-Savart law (Figure 1, left). If the conductor is a straight wire, then the magnetic field will turn around the wire as illustrated. Its direction can be determined using the so-called “right-hand law.” Put your right hand around the cable, with the thumb in the direction of the current. Your fingers are in the direction of the magnetic field.

FIGURE 1 – Any current circulating in a wire creates a magnetic field, and a loop positioned in a magnetic field generates an electromotive force proportional to the time rate of change of the flux of this field through the loop.

But there is also the Faraday law (Figure 1, right). This one states that a current is induced in any conducting loop put into a magnetic field. This current—or more exactly the electromagnetic force (EMF)—is not proportional to the strength of the magnetic field, but rather, to the rate of change of the magnetic flux through the loop. This means that no current will be generated in a fixed loop put in a fixed magnetic field. However, there will be a current if the magnetic field is changing, and this is the case when the current generating this magnetic field is AC. This is the basic principle of transformers. There will also be a current if the magnetic field is fixed, but the wire loop is moved, and this is the basic principle of electric generators.

A magnetic probe is very easy to build. It is simply a wire loop (Figure 2). The smaller the loop, the more precise is the localization of the source, but the lower is the induced current. This induced current can be increased by using several wire turns, as in a transformer, but this reduces the bandwidth of the probe. Ok, that’s simple, but are there some issues to take care of? You bet there are.

FIGURE 2 – A close-field magnetic probe is nothing more than a small loop. It could be unshielded (left) or shielded (middle and right).

First, a loop like this is generating a current and not a voltage. Therefore, it should be terminated on a resistor, which will transform the current into a voltage through Ohm’s law. Usually this is done by connecting the probe to the 50 Ω input of a spectrum analyzer, which makes the measuring resistance. If you connect it to an oscilloscope, then either switch the scope input to 50 Ω, or don’t forget to add a resistor in parallel to the input.

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Second, as noted earlier, a loop is generating a current only if the magnetic field is AC. Therefore, such a probe can’t be used to measure DC currents. Another kind of current probe—such as a Hall-effect probe—is needed for that purpose. That isn’t relevant for EMI, but it should be mentioned. For more along those lines, see “The Darker Side – The Art of Current Probing,” Circuit Cellar 345, April 2019.

Third, a wire loop will not grab just the magnetic field. It will also act as an electric antenna, and will be sensitive to electric fields nearby. That’s why it is a good idea to shield the wire loop—to protect it from electric fields. The theory states that such a shield will not change the magnetic coupling of the measuring loop, as long as the shield is cut somewhere. Figure 2 illustrates two possibilities for such a cut shield—either in the middle of the loop (center) or at the end (right). Just don’t be confused. The inner wire is still connected to the shield at the end of the loop, since the braid of the coaxial cable is used for return connection.

LETS BUILD ONE
Enough theory. I am sure you are anxiously waiting to know how to build such a probe yourself. It’s very easy and absolutely inexpensive. There are plenty of variants, but look at Figure 3 for the parts of my last home-built, near-field magnetic probe. The main part is a small length of 50 Ω flexible coaxial cable, already soldered on an SMA connector on one of its ends. A BNC connector will do the trick, too. A small ferrite bead is also useful to reduce the so-called hand effect on the cable, meaning to reduce the influence of your hand handling the cable when making the measurement. In fact, this ferrite will replace a balanced-to-unbalanced (balun) transformer between the loop and the cable. Finally, I used some heat-shrinkable sleeves and two coffee stirrers!

FIGURE 3 – Shown here, are the materials needed to build your own probe.

How to proceed with that? First, slide the ferrite bead onto the cable, and insert the two stirrers between the ferrite and the coaxial cable. They will help to make the probe more rigid. You can forget the stirrers, if your cable is rigid enough. Protect this section with a small length of heat-shrinkable sleeve, taking care that the full assembly is still sliding on the cable (Figure 4).

FIGURE 4 – A small length of heat-shrinkable sleeve protects a section of the cable. Don’t forget to insert the ferrite on the cable first!

Now we move on to the making of the loop itself. First decide what diameter you want for the probe. In my case, I wanted a diameter of 15 mm. Then calculate the perimeter of the loop: 15 mm × 3.14 = 47 mm. Starting from the open end of the coax, draw a mark at 47 mm and another at 47/2 = 23.5 mm (the middle of the loop). This is where you’ll need some dexterity (Figure 5). At the 47 mm mark, remove a ring of the outer plastic jacket, but don’t cut the shield. At the middle mark, remove both the plastic jacket and a full ring of the woven shield, but don’t cut the inner insulator. Finally, strip the end of the coax cable.

FIGURE 5 – Two rings of the outer plastic jacket must be removed from the coaxial cable. However, the woven shield must be cut only at the middle ring.

At that stage, it may be wise to check with an ohmmeter that the shield is indeed well cut, but that the inner wire is intact. Then insert a small length of sleeve to protect the middle ring, and bend the coax to form the loop (Figure 6). You then simply solder both the ending inner cable and shield to the shield at the 47 mm position (Figure 7). That’s it! Add some more heat-shrinkable sleeve, if you want, and you have your probe, as illustrated in Figure 8. It will surely be as good as hundred-dollar professional probes.

FIGURE 6 – A heat-shrinkable sleeve protects and insulates the middle ring, while the wire is gently bent in its final form.
FIGURE 7 – Both shield and inner core wire are soldered to the outer shield.
FIGURE 8 – Some more heat-shrinkable sleeve and a hot-air iron, and the probe is finished.

AN EXPERIMENT
Because I wanted to show you what this probe can help to localize, I opened my drawer and found a now obsolete Raspberry Pi 1 processor board. This version was known to be a significant EMI emitter, so I thought it could be a good candidate for testing. I switched it on. Instead of a spectrum analyzer, I also switched on one of my company’s oscilloscopes—a Tektronix MSO70404C. This baby, with 4 GHz bandwidth and 62 Mpts memory depth, is largely overkill for the job, but I had it on hand (Figure 9).

FIGURE 9 – Here the probe is moved across an old Raspberry Pi 1 computer module, while a Tektronix MSO 70404C oscilloscope displays the measured signal and its frequency spectrum.

I connected the probe to one of its 50 Ω inputs, configured the scope to calculate the frequency spectrum of the input (FFT function), and dragged the probe over the Raspberry Pi board. In minutes, I picked up a strong signal close to the processor chip. The measured signal was clearly at 4.43 MHz and its harmonics, as illustrated on the scope screen capture (Figure 10). Doesn’t 4.43 MHz sound familiar? Yes, that’s the carrier frequency of PAL video signals, so it’s likely that the video output is generating some EMI.

FIGURE 10 – The measured electromagnetic noise is mainly a 4.43 MHz carrier and its harmonics, which is the PAL video carrier.

You now understand that such simple probes enable you to localize quite precisely the source and type of a perturbance. In real life, you will probably start with a large probe, and sniff around the product. You will easily locate the major noise source and its frequencies. Then you’ll switch to a smaller probe to locate exactly where the noise is coming from.

And don’t forget to turn the probe around its axis. This type of probe grabs the most current when its plane is parallel to the trace or cable carrying the EMI current. This will be especially helpful to determine which track is the noise source in multi-layer designs, where the tracks are often orthogonal between layers. If you want to learn more about these investigation techniques, there is a nice article published by Holland Shielding Systems BV on its website: “EMI troubleshooting, step-by-step” [3].

AUTOMATED MEASUREMENTS
Manual analysis of the board is usually more than enough to locate an EMI source. It is also possible to make an automated measurement of the entire board, using a “near-field scanner.” Basically, the concept is either to move a probe through the board, using a motorized arm, and plot the map of the measured signals, or to include hundreds of probes under a fixed table and electronically scan it. Such scanners are produced by companies including EM-Scan, Detectus and Amber Precision Instruments and are not actually accessible to hobbyists or occasional users. However, nothing prevents you from building your own electromagnetic scanner. This is indeed quite simple, as long as you have some spare time and don’t need high performance.

In fact, my company did it with the help of three successive trainees (my thanks to them!). Our first version was based on an old pen plotter. We just replaced the pen with a sensor probe similar to the one described in this article, and put the board under test upside down above the plotter. The mechanical section was rather clumsy and soon died, so we rebuilt one using a nice Chinese-made laser-engraving X-Y table. This table was bought on eBay for about $200, including the stepper motors and a USB-driven controller (Figure 11).

FIGURE 11 – This is our automated X-Y close-field measurement table, built using a laser engraver kit and a low-cost RF Explorer spectrum analyzer.

To have an autonomous system, we added a low-cost USB-driven spectrum analyzer from RF Explorer. Such a low-cost instrument doesn’t provide the performance of standard equipment, but is usually enough for EMI measurements. Last but not least, we developed PC-based software to drive the measurement. It does the configuration of the spectrum analyzer, configuration of the corners of the board and move/measure/store result sequencing. Developing and debugging this software was the longest job. We used National Instruments’ LabView, but you can use the programming environment of your choice.

As an example, I put the Raspberry Pi 1 on our scanner and launched the software. The result, with the near-field measurement superimposed on the board image, is shown in Figure 12. As suspected, there seems to be a PCB trace going from the processor to the yellow video connector, which is radiating significantly more than the other sections. With an analysis like this, the board designer can take counter-measures, such as shielding this PCB trace, checking the line terminations or adding some current-limiting resistors somewhere. Nice, isn’t it?

FIGURE 12 – The close-field analysis of the Raspberry Pi 1 clearly shows that the culprit is the trace between the system-on-chip and the RCA video connector.

WRAPPING UP
Here we are at the end. I hope that you have already switched on your soldering iron and plan to make your own probe in the next 10 minutes. Grab a board and try to find EMI sources. The motherboard of your PC may be an interesting target. If you already have a spectrum analyzer, just use it. If not, switch on your oscilloscope and use its “FFT” function. And if you don’t have a scope at all—or if it’s too old to have an FFT—then look for one on eBay.

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You can buy an RTL-SDR USB dongle for $10 to $20. These modules are based on DVB-T receiver chips, and can be used as a poor man’s spectrum analyzer, with some of the available open-source software. Another solution, if you’re a ham-radio person, is to connect the probe to the antenna port of your shortwave radio receiver. That should work! Anyway, try to experiment. And have fun! 

For detailed article references and additional resources go to:
www.circuitcellar.com/article-materials
References [1] through [3] as marked in the article can be found there.

RESOURCES
National Instruments | www.ni.com
Tektronix | www.tektronix.com
RF Explorer | http://j3.rf-explorer.com/ (company home pagee)
RF Explorer | www.rfexplorer.com (USA site)

PUBLISHED IN CIRCUIT CELLAR MAGAZINE • DECEMBER 2019 #353 – Get a PDF of the issue


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Founder at Alciom | + posts

Robert Lacoste, France (Founder, Alciom; Columnist, Circuit Cellar)