Near-field scanners let you see EMI

-June 23, 2016

I love near field probes because they let me "see" magnetic and electric fields with an oscilloscope or with a spectrum analyzer. They let locate sources of emissions in board, cables, and systems. Near-field scanners also let you see emissions, particularly all over a board. That's hard to do with a single probe.

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. I use it in my lab. Here's an example of how I used it to evaluate a decoupling network for a 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 by A. Mediano.

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 s.

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 +5 V 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 50 Ω 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 going 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.

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