EMI TEST How to test EMC in semiconductors By Ding Ding Product Engineer Rohde-Schwarz China Ltd With the increasing complexity of ICs, test for electromagnetic radiation and susceptibility becomes an urgent requirement to meet new demands in design. Requirements for low EMI and strong anti-EMI capacity demand further improvements on electronic devices and systems to meet EMC standards. Moreover, product engineers must address EMC issues, particularly meeting threshold requirements and reduced EMI between the systems and other electronic devices. IC integration density doubles almost every year, making EMC a key priority in design. To address the issue, the International Electrotechnical Commission (IEC) released standards IEC 61967, for EME test of ICs with frequency from 150kHz to 1GHz, and IEC 62132 for EMS test of ICs with frequency from 150kHz to1GHz. Scheduled to be released soon is IEC 62215, a complementary standard to IEC 62132 that addresses EM disturbance on ICs. EME testing IEC 61967 requires the use of a transverse electromagnetic cell ( TEM cell) and field probe to measure electromagnetic emission (EME). It uses spectrometers or receivers with frequency range from 150kHz to 1GHz. Normally, the ICs to be tested are placed on one circuit board. The specification of the circuit board is shown in Figure 1. A power supply with low RF noise is needed for testing while the temperature should be at 23°C (±5°C). There can be cases of environmental noise as peripheral circuits are measured. If the background noise is lower than the threshold value by more than 6dB, a preamplifier is applied, if needed. This method measures radia- Figure 1: ICs to be tested using TEM cell method are placed on one circuit board. Figure 2: A flat chip as an inner conductor with a quadrate outer conductor connects to the receiver at one end and the matched load at the other end. Figure 3: The surface scan method tests the spatial distribution of the IC’s surface electric and magnetic fields. tion using a TEM cell, which is an irregular coaxial line. In the middle of the line, a flat chip such as an inner conductor with a quadrate outer conductor connects to the receiver at one end and the matched load at the other end (Figure 2). A quadrate vent at the top of the outer conductor is placed to fix the circuit board to be tested. Specifically, the IC will be installed inside the cell, with Electronic Engineering Times-Asia | February 16-28, 2007 | eetasia.com testing circuit boards recommended. By scanning the electronic magnetic fields over the IC surface, we can get information about the relative intensity of the EM radiation source. This method can accurately identify the area on the IC that has too much radiation. Partially-shielded electronic field probes and single-loop minielectronic field probes are recommended in this standard. Both probes can be made by 0.5mm semi-rigid coaxial cables. Figure 4: The 150Ω direct coupling method tests the disturbance voltage of single- or multi-output signal lines for impedance matching. the wiring side and the peripheral circuits facing outside. This way, the radiation emission will mainly come from the IC being tested. High-frequency current produced by the tested chip will flow along the wire, while wielding pins and package connections serve as the antenna of radiation emission. If the testing frequency is lower than the first-order high-mode frequency, only TEM as the major mode is transmitted. Note that Measuring instrument the power of the TEM cell’s port has a quantitative relation with the emission power of the disturbance source. Hence, the voltage value collected at a curtain area of the TEM cell can help evaluate the IC’s radiation emission. The TEM cell method implements surface scan by testing the spatial distribution of the IC’s surface electric field and magnetic field. In Figure 3, the electronic or the magnetic field probe scans over the IC surface. The frequency, emission value and spatial position are collected every time and processed by the software. Spatial frequency distribution of the field intensity is displayed in color figures. The result’s quality relates closely to the precision of the mechanical locating system and the size of the probes. Hence, this is applicable to general PCBs and may not necessarily use the standard Frequency band 150kHz to 30MHz 30MHz to 1GHz Spectrum analyzer (3dB) 10kHz 100kHz Receiver (6dB) 9kHz 120kHz Table 1: EMC testing also requires selection of resolution bandwidth for spectrum analyzers and receivers. EMI receiver or spectrum analyzer Signal source Signal SA DC F F Supply Output 100Ω Ferrite Input Electronic Engineering Times-Asia | February 16-28, 2007 | eetasia.com Auxiliary load Coax feed through Filter Supply source 50Ω F Figure 6: The magnetic probe method evaluates the radiation emission of ICs by testing the current through PCB wires. Radiation measurement The standard has three methods for radiation measurement: direct coupling, workbench faraday cage (WFC) and magnetic field probe. There are two measurement types for the direct coupling method: 1Ω and 15Ω. The 1Ω method tests the total disturbance current of grounded pins, while the 15Ω tests the disturbance voltage of output ports. After going through the chip, RF current will flow to the IC’s grounded pins, so the measurement against the ground RF current can reflect more precisely the IC electromagnetic disturbance. If you connect a 1Ω resistance in serial in the ground loop, you can obtain the RF current from the loop. Meanwhile, measure the impedance matching of the testing instruments and the grounding pin. The 150Ω method can be adopted to test the disturbance voltage of single- or multi-out- Figure 5: The IEC 61967 WFC method measures the conduction disturbance voltage of power lines and I/O signal lines. put signal lines (Figure 4). Note that 150Ω is the statistic average value of wire-harness common impedance. To match it with a 50Ω impedance testing system, apply a matching network. Meanwhile, the WFC method of the standard measures the conduction disturbance voltage of power lines and I/O signal lines. Place a standard or an application circuit board with the IC to be tested into the WFC. All the power and signal lines in and out of the WFC will be filtered. Connect the WFC testing ground port to testing instruments and the ground port to be tested to a 50Ω matching load. A well-shielded environment reduces the background noise. You can connect a 100Ω resistance in serial to match the impedance (Figure 5). Figure 7: The TEM cell method can also be used for susceptibility test. Figure 8: In the BCI method, interference power is injected into the single wire or wire harness of IC pins. Figure 9: DPI test injects capacitive coupling to measure susceptibility. Magnetic probe The magnetic probe method of the standard evaluates the radiation emission of ICs by testing the current through PCB wires (Figure 6). The pins of the chip are connected to the power source or periphery circuits via wires on the PCB; thus, the magnetic probe can obtain the RF current. Based on the law of electromagnetic induction, the voltage at the probe’s output end is proportional to the RF current of the wires. An easy and repeatable method is needed to evaluate the interference immunity of a chip, which is divided into two fields—radiation immunity and conduction immunity. This method collects the RF power if ICs fail to work. IEC 62132 divides the operation status of an IC into five grades; it will then detect the continuous and AM waves separately. Similar to ISO11452, peak-level constant AM with depth of 80 percent 1kHz is applied. This is different from IEC 61000-4-3. The IEC standard demands that the peak power of the modulation signal be 1.8x the carrier wave signal. However, when the peak level constant is under testing, the modulated signal power, with a modulation depth of 80 percent, is only 0.407x the carrier wave signal power. The TEM cell discussed in IEC 61967 is also suitable for radiated Electronic Engineering Times-Asia | February 16-28, 2007 | eetasia.com susceptibility measurement. One end of the cell is connected to a signal generator and PA, while the other end is connected to a proper matching load. The TEM waves—produced in the cell and similar to remote TEM waves—can be used to measure EMS. Other status monitoring instruments may be required to monitor the IC status in real-time. Figure 10: The WFC method in IEC 61967 can also test susceptibility, with the receiver replaced with signal resource and AP. Electronic Engineering Times-Asia | February 16-28, 2007 | eetasia.com Set of testing The standard uses three methods for the susceptibility test: bulk current injection (BCI), direct power injection (DPI) and WFC. In the BCI method, interference power is injected into the single wire or wire harness of IC pins. Because of inductive coupling, the cable tested produces the disturbance current that can be detected by another current probe. The method derives from the automobile electronic susceptibility test. Compared to BCI that injects inductive coupling, DPI injects capacitive coupling. RF signals are injected directly to a single pin or a group of pins of the chip. The coupling capacitance prevents DC from being directly placed on the output end of AP (Figure 9). The WFC method in IEC 61967 can also be applied for susceptibility test, but the receiver is replaced with signal resource and AP (Figure 10). The shielded structure and good filtering hold RF disturbance signals inside the cage, thus protecting operation personnel.