DesignCon 2019 A Fast and Simple RFI Mitigation Method without Compromising Signal Integrity

In modern wireless consumer electronic devices, there is an increasing need for smaller, compact, and denser design. This often requires wireless components like transceiver, front-end and antenna to be placed very close to noise sources like memory, power supply, and main processor in the device. Electromagnetic noise from noise sources interferes with wireless receiver components causing radio frequency interference (RFI) issues in the device. As a result, wireless performance metrics like range and throughput is degraded, which impacts the user experience. In this paper, a popular consumer electronic device is studied. The device has many complex subsystems like CPU, DDR memory, and power supply co-located with Wi-Fi circuitry. Due to small size, the device has RFI issues from CPU memory interface, which affects the Wi-Fi range. Typically, RFI problem can be mitigated from a mechanical design perspective by adding shield can, or from a signal integrity perspective by modifying clock and slew rate of the highspeed signals. In this paper, a novel RFI mitigation method is proposed. Through nearfield scanning, an equivalent dipole moment of the noise source (CPU and DDR3) is reconstructed, and the near-field components of the victim (Wi-Fi antenna) are measured. By determining relationship between dipole moment and antenna near field, the noise source is rotated by a certain angle to reduce RFI. Rotating the source to reduce RFI is implemented in such a way that it doesn’t compromise signal integrity, and it doesn’t require an additional shield can. New boards with the suggested changes are fabricated and the measured results show a good RFI reduction (up to 8 dB) compared to original boards. Author(s) Biography Qiaolei Huang received the B.E. degree in Electrical and Computer Engineering from Huazhong University of Science and Technology, Wuhan, China, in 2013, and the M.S. degree in electrical engineering from EMC Laboratory, Missouri University of Science and Technology, Rolla, MO, USA, in 2016. He is currently working toward the Ph.D. degree in electrical engineering at EMC Laboratory, Missouri University of Science and Technology, Rolla, MO, USA. His research interests include radio frequency interference, radiated emission modeling, and EMC measurement method. Ling Zhang received the B.S. degree in electrical engineering from Huazhong University of Science and Technology, Wuhan, China in June 2015, and MS degree from Missouri S&T in Dec 2017. He worked in Cisco as a student intern from Aug 2016 to Aug 2017. He is now pursuing his PhD degree in EMC lab, Missouri S&T. His research interests include machine learning applications in PI/SI/EMC, RFI source reconstruction and coupling paths analysis, and Emission Source Microscopy (ESM) technique for EMI source localization. Yang Zhong received the B.S. degree in Optoelectronics Science and Engineering from Huazhong University of Science and Technology, Wuhan, China, in 2017. He is currently a MS student in electrical engineering at EMC Laboratory, Missouri University of Science and Technology, Rolla, MO, USA. His research interest is electromagnetic interference. Jagan Rajagopalan is a Senior RF System Engineer in Wireless Technology Group of Amazon Lab126. Deepak Pai is an RF System Engineer in Wireless Technology Group of Amazon Lab126. Chen Chen is an RF System manager in Wireless Technology Group of Amazon Lab126. Amit Gaikwad is a Senior RF System manager in Wireless Technology Group of Amazon Lab126. Chulsoon Hwang is with EMC laboratory at Missouri S&T (Formerly University of Missouri−Rolla) as an assistant professor. He received his PhD degree at KAIST in 2012 and worked at Samsung Electronics as a senior engineer from 2012 to 2015. He has authored or co-authored 50+ IEEE journal/conference papers and received a best paper award at AP-EMC 2017, DesignCon 2018 and an outstanding young scientist award at AP-EMC 2018. His research area includes RF desense, signal/power integrity, and electromagnetic interference. Jun Fan is currently the Director of the Industry-University Cooperative Research Center (IUCRC) for EMC, supported by the National Science Foundation (NSF) and led by the Missouri S&T EMC Laboratory where he is also the Director. Dr. Fan received his B.S. and M.S. degrees in Electrical Engineering from Tsinghua University, Beijing, China, in 1994 and 1997, respectively. He received his Ph.D. degree in Electrical Engineering from the University of Missouri-Rolla in 2000. From 2000 to 2007, he worked for NCR Corporation, San Diego, CA, as a Consultant Engineer. In July 2007, he joined the Missouri University of Science and Technology (formerly University of Missouri-Rolla), and is currently an Associate Professor with the Missouri S&T EMC Laboratory. His research interests include signal integrity and EMI designs in high-speed digital systems, dc power-bus modeling, intra-system EMI and RF interference, PCB noise reduction, differential signaling, and cable/connector designs. Dr. Fan served as the Chair of the IEEE EMC Society TC-9 Computational Electromagnetics Committee from 2006 to 2008, and was a Distinguished Lecturer of the IEEE EMC Society in 2007 and 2008. He currently serves as the Vice Chair of the Technical Advisory Committee of the IEEE EMC Society, and is an associate editor for the IEEE Transactions on Electromagnetic Compatibility and EMC Magazine. Dr. Fan received an IEEE EMC Society Technical Achievement Award in August 2009, and various Best Paper Awards from several international conferences. Dr. Jun Fan is a Fellow of the IEEE. Introduction With the recent progress in Internet of Things (IoT), several consumer electronic devices around the world are now connected to the Internet. These devices range from a refrigerator to a small car key fob, which is connected wirelessly to the internet access point through Wi-Fi. Although wireless technology in IoT devices provides great convenience, it also brings a lot of design challenges to RF integration engineers. As the complexity of digital subsystem increases, potential noise sources in devices such as microcontroller unit (MCU), System On Chip (SoC), high-speed traces, flexible cables, and power converters couples to a co-located wireless antenna. This unintended electromagnetic noise, called RF interference (or simply RFI) interferes the functionality of wireless radio and reduces the usable wireless range of the device. There are a few conventional ways to mitigate RFI. For example, designers use shield can and grounding schemes to suppress noise radiation by modifying the coupling path. However, in small form-factor products, it is difficult to use a shield can due to mechanical (product design) fabrication and cost limitations. Another approach is to modify the noise source itself. Clock or slew rate can be lowered to reduce the power spectral density (PSD) of noise source in the frequency where RFI issues occur. However, modifying signal quality may reduce the signal integrity margin. A third approach is to route high-speed signals in a stripline fashion, instead of a microstrip. A stripline structure can provide better shielding compared to a microstrip structure. However, the choice of using a stripline routing or a microstrip routing can increase the PCB layer count, affect signal integrity, and also add complexity to the overall board layout. Therefore, it is important to explore alternate ways to mitigate RFI without compromising signal integrity or adding cost to the product. Recently, fast and efficient methods to predict RFI in a real product have been introduced in [1] and [2]. Reciprocity theorem is used to estimate RFI by decomposing the overall problem into two steps forward problem and reverse problem. In the forward problem, the noise source’s near-field radiation is studied while the victim antenna is terminated. The forward problem can reconstruct noise source into an equivalent dipole moment model. In the reverse problem, the antenna is excited and the noise source is turned off. Using the equivalent dipole moment and the reverse field, RFI from noise source to antenna can be obtained. However, there has been little effort to conduct a systematic way to mitigate RFI. In this paper, a popular consumer electronic device is studied. This small form-factor device has many complex digital sub-systems, including CPU, DDR memory, power supply, and two Wi-Fi antennas (for MIMO functionality) as shown in Fig. 1. Through measurements, it is determined that high speed DDR3 traces on the main logic board is the dominant noise source for RFI issue on the Wi-Fi antennas. Having identified the noise, the below outlined steps are followed to measure, reconstruct, and mitigate RFI issue. Fig. 1. DUT illustration. (could add more description here) First, near-field measurements of source and antenna are performed using electromagnetic (EM) scanner to understand the RFI radiation and coupling physics. Then, dipole moment based reciprocity method is utilized to propose RFI mitigation methods like movement and/or rotation of noise source. The theoretical derivations and steps to maximize RFI reductions for two antennas are also provided. Finally, the results of the final modified board with reduced RFI are presented. Comparisons of measured RFI reduction and theoretical RFI reduction for both antennas are also presented. Noise Source and Antenna Characterization EM scanner with spectrum analyzer is a fast and convenient tool to measure nearfield emissions from any radiating source. Using EM scanner, the near field distribution of noise sources (i.e., SoC and DDR3 interface) is measured. The measurement setup and a simple illustration are shown in Fig. 2. During the measurement, the device was configured in stress mode to emulate high-speed data transfer between CPU and DDR3. The resonant-type H field probe at 2.45 GHz is used and scanning is done at channel 6 of Wi-Fi. The resoluti