APPLICATION NOTE Electromagnetic Compliance: Troubleshooting with NearField and Current Probes INTRODUCTION: Electromagnetic interference (EMI) can cause a host of problems, especially when developing a product or attempting to pass mandatory electromagnetic compliance (EMC) tests. Garbled displays, bad data, or complete malfunctions can occur when a design is effected by EMI. To minimize the effects of interference, government agencies like the Federal Communications Commission (FCC) in North America create and enforce standards that set limits on the EM output of a product type. Testing to the specifications is commonly referred to as Electromagnetic Compliance (EMC) testing. Many EMC test failures stem from the interaction of unintentional radio frequency (RF) emissions with a circuit or element within the design itself. The electric and magnetic fields that cause this interference are not visible to the unaided eye, which can present complications when trying to isolate the root cause and minimize the effects of the EMI.  What is causing the issue? Where is the source of the signal or energy causing the radiation? Fortunately, there are simple tools and techniques that can help identify the sources of EMI. Once you can identify the source, you can begin to build up a list of solutions to the problems. These techniques are not part of the mandatory compliance tests required to pass EMC testing. Rather, these are pre-compliance test techniques that help identify potential areas of EMI as quickly as possible without the burden of expensive test equipment and setups. In this application note, we are going to introduce some common pre-compliance test techniques for identifying potential problematic EMI sources using near-field and current probes. These techniques can save you time and money by isolating problem areas quickly, and with a little fixturing, you can create repeatable test stations to help correlate data. This knowledge can then be used to “design for EMC” in your future prod

NOTE: Pre-compliance tests are designed to help identify and resolve issues that may hinder passing full compliance tests. Pre-compliance testing is not a replacement for full compliance testing at a certified lab. ELECTROMAGNETIC RADIATION BASICS: In electronics, EM radiation is most commonly caused by a current flow or voltage buildup along or through a conductor. This includes traces on a PC board, discrete wires, component leads/pins, connectors, or any other metal, including the chassis, rack, or product enclosure. Recall that EM radiation is actually a combination of electric and magnetic...

Visualizing the magnetic field can be a bit easier if you go back to your Physics texts. Recall that the magnetic field of an infinitely long straight wire can be calculated by applying Ampere’s law: For a circular path centered on the wire, the summation becomes:  0 is the permeability of free space (a constant)  is the current is the distance from the center of the conductor Figure 2 is a physical representation of this relationship. Note, this is also described by the “right-hand-rule” wherein if you were to point the thumb of your right hand in the direction of the current flow, then the...

for EM radiation on surfaces like heatsinks or metal enclosures. The effects of the electric field also tend to dominate further away from the source (far-field). Far-field measurements are more susceptible to error due to environmental factors like radio stations, WiFi, and intentional RF. Far-field measurements, like those performed during radiated emissions portion of a compliance test, require more setup, equipment, and expertise than near-field. By measuring the amplitude and frequency of the magnetic and electric fields that are generated by elements of a product, we can identify the areas...

PROBES: Since EMI cannot directly observed by the human eye, we need some tools to help. Recall that moving charges in a conductor produce magnetic and electric fields that radiate throughout space from the conductor. We can use these fields to induce a voltage in a circuit. Then, measure that induced voltage and therefore indirectly measuring the strength of the original field. The two most common types of probes used in EMI troubleshooting are near-field probes and current clamps. Magnetic field probes and current clamps operate on a similar principle. The magnetic field that flows through...

Figure 4: Magnetic field probe orientation and position affect measurement amplitude. Figure 5: SIGLENT SRF5030 near-field probe kit. Figure 6: Probing a PCB using a SIGLENT SSA3X and SRF5030 probe.

Cables and interconnects can make very nice (and unintentional) antennas if they are not shielded/grounded correctly. Small currents flowing on the outside of the conductor can easily cause radiated emissions that can exceed the set EMC limits. A current clamp can be used with a spectrum analyzer to provide insight into the cause of radiating cables/interconnects. Current clamps operate on the same principle as magnetic loop probes. They can be purchased or made by wrapping a few rounds of wire around a ferrite clamp and epoxy a BNC connector as shown in figure 7. Simply attach the clamp to the...

Figure 8: Measuring the RFI of a USB cable connected to a scope. Current clamps, especially handmade, are susceptible to picking up environmental RF that can skew or overwhelm the signals that you wish to measure. Connect and arrange all cables, probes, etc.. and then measure the environmental RF by simply keeping the DUT powered OFF. Then, compare it to measurements made with the DUT ON. It may also be a good idea to retest periodically to account for any environmental changes. Figure 9: Traces of the environmental pickup from a current clamp (Yellow) and with the DUT powered ON (Pink). If you...

SCANS AND EVALUATION: It is highly unlikely that data collected during probing will directly correlate to radiated emissions test performance. But, by observing the RF output of cables, switching power supplies, displays, and cutouts, you can have information that can lead to faster troubleshooting if you do happed to fail. Here are optional techniques that can help provide more insight: 1. Most spectrum analyzers do not have pre-selection filters. If you are using a spectrum analyzer without pre-selection filters, the peaks you observe may not be real. Analyzers without pre-selection filters...