Pendulum EMC near-field scanningEMI scannerpre-compliance testingpre-compliance

EMC/EMI Near-Field Scanning and Pre-Compliance Testing

The test house only tells you which frequency failed. A near-field scan tells you which component and which trace on the board it is coming from.

Engineer operating a Detectus near-field scanning system

In short

When a product fails in the test house chamber, the report lists the frequencies and amplitudes over the limit, not the location of the source. Near-field scanning brings the measurement back to the board surface. Pendulum SCN series scanners step at 0.1 mm with probe sets to 10 GHz on four scanner sizes from 200×100 mm to 600×400×300 mm, logging near-field strength point by point and overlaying it on an image of the device under test. Where adjacent components or the inside of an IC have to be resolved, the HRE series offers 25 μm steps and ±0.02 mm positioning accuracy. 4D models rotate the probe at every point to find the worst-case angle, and scans before and after a revision can be subtracted directly. This article covers how a scanner and a spectrum analyzer form a system, the difference between 2D and 4D, how to size the scanner, and why fixing the source beats adding shielding afterwards.

Far field tells you how much over; near field tells you where

Near-field scan result overlaid on a circuit board photo — Near-field emission map

A radiated emissions test at a test house measures field strength 3 m or 10 m from the product. The result is a spectrum plot and a few frequencies above the limit. It proves whether the product passes, but it cannot answer the question the engineer actually has: is the energy coming from a switching component, a trace, or a break in the ground plane? Every return trip to the chamber has to be scheduled and paid for, and fixing one spot often raises another.

Near-field scanning places a probe a few millimeters from the device under test, measures point by point along a fixed path, and combines the spectrum at each position with its X, Y and Z coordinates into a color heat map overlaid on a photo of the board. It does not replace compliance measurement, since there is no simple conversion between near-field strength and the limit at 10 m, but it shows you where the problem is before you submit, and it lets you see the effect of a change in your own lab.

What a system consists of

SCN-500 series near-field scanner — SCN scanner

A complete scanning system is four things: the scanner mechanics, near-field probes with a preamplifier, a spectrum analyzer, and a PC running Detectus Scanning Software. The scanner moves the probe, the probe output feeds the spectrum analyzer, and the software reads back the spectrum at each point over a VISA interface (USB, LAN, GPIB). Most users already have a spectrum analyzer, and Pendulum's driver library covers hundreds of models from the major vendors, so the scanner itself does not have to be bought again when the analyzer changes.

SCN series scanners ship with a 3 GHz or 6 GHz probe set: an E-field probe, a vertical and a horizontal H-field probe (from 30 MHz), a low-frequency H-field probe for 100 kHz to 50 MHz, and a 30 dB preamplifier; Option S310 adds a 1 to 10 GHz probe set. When choosing the upper frequency, look at the harmonics of the operating frequency rather than the fundamental: the 30th harmonic of a 100 MHz clock already sits on the 3 GHz boundary, and if the measurement range excludes the harmonics, the band you most need to see falls just outside it. The optional SPCK stripline calibration kit uses a spectrum analyzer with a tracking generator to derive probe correction factors from 10 MHz to 6 GHz, so readings from different probes and cables can be compared with each other.

2D and 4D: height variation and polarization

A 2D scan sweeps the probe across a plane at a fixed height, which is adequate for bare boards with little height variation. Real boards carry electrolytic capacitors and heatsinks, and a probe set high enough to clear the tallest part is too far from the small components beside it to measure them properly. A 3D scan moves the probe at a constant distance along the surface of the device under test; height data can be entered manually, imported as an STL model, or built automatically by the Option SLDM laser distance meter. It also takes measurement beyond the board surface: the inside of an enclosure, the gaps between boards stacked in a rack, and cables and connectors all fall within the scan volume.

4D adds rotation of the probe about its own axis. Near-field probes are not equally sensitive at every angle, and a probe held in one orientation can miss radiation in a given direction. The 4D models of both the SCN and the HRE rotate the probe at every measurement point with 1° resolution, find the worst-case angle and record the amplitude. The angle itself carries information: the difference between readings at the same point in different probe orientations reveals the polarization of the field, and polarization often points to the real coupling path, letting you follow it along a trace, a cable, or even the bond wires inside an IC.

Will it fit on the scanner: dividing the work between SCN and HRE

The first constraint in selection is not resolution but size. The SCN-522 is a 200×100 mm 2D model; the SCN-524, 534 and 564 are 4D with scan ranges of 200×100×100, 300×200×100 and 600×400×300 mm respectively. The largest accepts a device 770 mm wide and 300 mm high, which means a fully assembled product rather than just a daughterboard. Many coupling problems, such as cable routing, enclosure seams and board-to-board connectors, appear only in the assembled state and cannot be seen by scanning boards separately. The 0.1 mm step of the SCN is sufficient for board-level localization.

Once the problem has been narrowed to an area and you need to tell two adjacent components apart, or see which group of pins inside an IC package is radiating, the HRE series is required: minimum step 25 μm, positioning accuracy ±0.02 mm, scan area 280×180×85 mm (HRE-02/42) or 390×290×130 mm (HRE-03/43), with 0° to 360° probe rotation on the 4D models. The standard probe set reaches 10 GHz, and user-supplied higher-frequency probes extend this to 70 GHz provided the spectrum analyzer covers the band. Both series run the same software, so scan results and comparison workflows are interchangeable. Locating the area with the SCN and then the component with the HRE is a common division of labor.

Before and after comparison: turning improvement into numbers

Near-field scan comparison before and after a revision — Before and after comparison

The greatest value of near-field scanning lies not in a single heat map but in the difference between two. Because the probe follows a repeatable fixed path, a scan before and after a revision under the same conditions lets the software subtract one result from the other, leaving only what actually changed as a result of the layout or component change. Option SBCN for the SCN places positioning beacons on the scanner bed and calibrates probe position automatically before each scan, so the two heat maps overlay exactly; the HRE has the same beacon calibration built in. Comparing six decoupling capacitor configurations is six scans and six maps side by side.

A power module manufacturer's case illustrates the approach. Their GaN isolated AC/DC converter switches above 1 MHz, the choice of decoupling capacitor directly determines EMI performance, and the cost of simulation and repeated test-house visits mounted quickly. After switching to 3D near-field scanning with a real-time spectrum analyzer, the time to scan the whole board at 1 mm spatial resolution dropped to an acceptable level, because the real-time analyzer still captures the spectrum at each point quickly across a wide span, without sacrificing resolution bandwidth to shorten the scan. They could see the emission distribution around the inductor and verify the effect of each change immediately.

Fix the source, or add shielding

The most common response to a failed compliance test is to add a shield can, a filter or a ferrite. They work, but each one is material cost, assembly time and mechanical space, paid on every unit for the life of the product. The purpose of near-field scanning is to avoid that route: find the source at the design stage and reroute a trace, change a return path or move a decoupling capacitor, at a cost that is incurred once. The same equipment serves in production for incoming material comparison: when an IC or module changes supplier, a scan under identical conditions against the baseline map catches a lot with a shifted spectrum before it ships.

Evencat is the authorized distributor for Pendulum in Taiwan. We help match the size of your device under test to the SCN and HRE models, confirm the probe set frequency limit and driver support for your existing spectrum analyzer, and provide equipment calibration documentation along with installation, probe calibration and other technical support in use.