Near-field scanners let you see EMI
There are several EMI/EMC scanners on the market today such as those from EMSCAN, DETECTUS, and Api, and others. A scanner is essentially a series of near-field probes placed in a grid. Thus, it can produce an image of a board’s emissions that’s more consistent and repetitive than you can get by manually scanning a board with probes.
The EMxpert scanner from EMSCAN is one such scanner, which I use in my lab. Here’s an example of how I used it to evaluate a decoupling network for a training course. Figure 1 shows a scanner with a scan area 21.8 cm x 31.6 cm scanning a PCB under test.
Figure 1. A PCB under test on top of the near field scanner. (Photo; author)
The scanner consists of thousands of loops spaced so that it provides resolution of less than 1 mm. Frequency range goes from 50 kHz to 8 GHz, depending on the model. The loop antennas are sensitive down to -135 dBm and a high-speed electronic switching system provides real-time analysis in less than 1 sec.
EMI scanners let you quickly analyze and compare design iterations and optimize hardware design. I use them for troubleshooting and for teaching. Here, I’ll use it to demonstrate how a decoupling network can reduce emissions from a board.
Consider, for example, a typical circuit with a 24 MHz clock (Figure 2). The board containing this circuit also has a decoupling circuit. The +5V power comes from a USB connector. The board includes an SMD fuse, a small LED for visual feedback, and a couple of decoupling capacitors. Load for the clock is a 50Ω resistor.
Figure 2. Basic schematic for the decoupling network of the IC clock.
A transient current (is) is required from the power supply to operate the IC. Usually, the high frequency content of that current (harmonics) is the source of many conducted and radiated EMI problems.
A decoupling network (usually surface-mount capacitors and ferrites) is used to minimize the high-frequency components passing through the power-supply. If the decoupling circuit is working as expected, current iPSU will be reduced to DC because transients will take the path through the decoupling capacitor (iC) to power return. With two jumpers, we can enable/disable the decoupling network and evaluate its effectiveness. In Figure 3, the VCC trace is on the top layer. GND trace (no ground plane) is on the bottom layer.
Figure 3. General view of PCB for our decoupling example.
Spectral and spatial scans
A spectral scan lets us identify signals from the board, which often come from oscillators and clocks. Signals may be parasitic oscillations or ringing, which are difficult to prevent. With the spectral scan, we can measure any signal from the board.
The spectral scan in Figure 4 lets you identify the harmonics of the 24 MHz clock’s transient currents and some EMI from the environment, including FM broadcasting signals (88 MHz to 108 MHz).
Figure 4. A spectral scan clearly shows emissions from the PCB.
With a spectral scan and spatial scan, you can identify the current path for that signal, critical information if you want to minimize EMI/EMC and SI problems. Figure 5 shows the emissions from the board when the decoupling network is not part of the circuit. That is, the switch across the ferrite bead is closed and the switch in series with the capacitor is open. The result: a big loop that produces emissions over much of the board. The big loop can create distortion for the clock signal, radiated emissions of the high-frequency harmonics, crosstalk with other boards or cables, and injected noise in the power supply or cables.
Figure 5. Spectral and spatial scan without decoupling network. The path for current is clearly identified in a big loop (maximum levels are read as dBµV in red colour).
With the decoupling network enabled, the loop size is much smaller (transients take the path of the decoupling capacitor) and EMI currents are contained in the area closer to the clock IC (Figure 6). Note maximum levels in red colour are now more than 16 dB below the previous measurement (with the same scale, this new plot would be basically blue).
Figure 6. With the decoupling circuit enabled, emissions are greatly reduced.
A typical question when doing the review of a product is: “Did you use a decoupling capacitor?” Usually, the response is something like: “Of course, I have a 100 nF capacitor.” That’s not enough.
Sometimes, you have a capacitor (or decoupling circuit) in your system but there is no effective decoupling because terminal impedances don’t match the topology you have chosen, the capacitor technology/value isn’t correct, or parasitic effects in the layout and package are dominant. With a near field scan, you can detect how your decoupling system is really working.
About the author
Arturo Mediano received his M.Sc. (1990) and his Ph. D. (1997) in Electrical Engineering from University of Zaragoza (Spain), where he has held a teaching professorship in EMI/EMC/RF/SI from 1992. From 1990, he has been involved in projects in EMI/EMC/SI/RF fields for communications, industry and scientific/medical applications with a solid experience in training, consultancy and troubleshooting for companies in Spain, USA, Switzerland, France, UK, Italy, Belgium, Germany, The Netherlands, and Malaysia. He is the founder of The HF-Magic Lab, a specialized laboratory for design, diagnostic, troubleshooting, and training in the EMI/EMC/SI and RF fields. Since 2011, he is instructor for Besser Associates (CA, USA) offering public and on site courses in EMI/EMC/SI/RF subjects through the USA, especially in Silicon Valley/San Francisco Bay Area.