The microstrip line method shown in Fig. (3) is developed by TDK for shielding effectiveness measurement on electromagnetic shielding material [9 TDK Corporation. TDK Flexield-EMI test method, BSF-B05CC, 2012.]. The method is to place the material under test (MUT) on top of 50Ω microstrip line. Since the shielding material on top of microstrip line will affect line s impedance, it will thus provide high impedance and suppress the high frequency components of signals and further achieve the filtering function. In this paper, we will illustrate the measurement of two different shielding materials (Graphene and magnetic material FAM 5(0.2 mm)), and also show the effect of different stack-up arrangement on shielding or filtering effectiveness. The comparison of the various measurement results is shown in Fig. (4).

Fig. (4) Measurement results with microstrip line method.

Fig. (5) Schematic of IEC 61967-6 magnetic probe method.

Fig. (6) Simulation configuration and results of material coupling effect on trace with magnetic probe method.

The magnetic probe method [10 Integrated Circuits-Measurement of Electromagnetic Emissions, Part 6: Measurement of Radiated Emissions –Magnetic Probe Method, IEC 61967-6, Pre-release of official standard, 2002. Available at: https://webstore.iec.ch/publication/6193] illustrated in this paper is utilizing the magnetic probe described in IEC 61967-6 for chip level EMI measurement from trace as shown in Fig. (5) The probe is designed here to measure the magnetic flux generated by 50Ω microstrip line. With the attenuation or detour of magnetic flux by shielding material, the shielding effectiveness can be obtained with received flux or induced voltage via magnetic probe. In this study, we first use copper for electromagnetic simulation and testing experiment to investigate the coupling effect between microstrip line and material under test to identify the real shielding mechanism (impedance mismatching, reflection, or absorption). The simulation configuration and results are illustrated as shown in Fig. (6). It shows that the impedance of signal trace is severely affected by the proximity of shielding material, and thus impairs the signal integrity. To avoid the measurement problem and error invoked by this proximity effect, we further increase the separation between the microstrip line and material under test to assure the resultant shielding effectiveness obtained indeed the desired energy attenuation mainly by reflection or absorption against filtering mentioned in previous test configuration.

Fig. (7) Test setup of magnetic probe method for shielding effectiveness.

Fig. (8) Measured results with 1mm spacing as specified by IEC 61967-6 (with proximity effect).

The test setup of magnetic probe method is shown in Fig (7) and the measured results with 1mm spacing as required by IEC 61967-6 are shown in Fig. (8). As to avoid proximity coupling effect influcing the measuring accuracy, we increase the separate between the microstrip line and material under test and not fix at 1mm height as specified in IEC 61967-6. The measured results with larger separation to avoid proximity effect are shown in Fig. (9).

Fig. (9) The test illustration and measured results with larger separation to avoid proximity effect.

Fig. (10) Measurement principle and setup of coaxial cell method.

The coaxial cell method utilizes the coaxial transmission line configuration described in ASTM D 4935 [11ASTM International. Standard Test Method for Measuring the Electromagnetic Shielding Effectiveness of Planar Materials. ASTM D 4935-10, West Conshohocken: PA 2010. ] and is illustrated in Fig. (10). The coaxial transmission line (CTL) is the most widely used measurement system in transmission-reflection (TR) methods. CTL systems have been used to measure EM shielding effectiveness (SE) of planar shields [12Nicolson AM, Ross GF. Measurement of the intrinsic properties of materials by time-domain techniques. IEEE Trans Instrum Meas Nov. 1970; 19(4): 377-82.
[http://dx.doi.org/10.1109/TIM.1970.4313932] -14Vasquez H, Espinoza L, Lozano K, Foltz H, Yang S. Simple Device for Electromagnetic Interference Shielding Effectiveness Measurement, IEEE EMC Society Newsletter 2009; (220): 62-8.] the experiment setup for measuring the scattering parameters of the single-layered RF absorber, which includes a vector network analyzer and a 50-Ohm coaxial sample holder. The coaxial sample holder, being an enlarged coaxial line to hold an MUT, is designed to have an inner radius (b) of 38 mm for the outer conductor and a radius (a) of 16 mm for the inner conductor. The characteristic impedance (Z_C) and the upper frequency limit for pure transverse EM (TEM) mode operation can be computed as , respectively [6Lin H-N, Hsieh D, Huang W-H, Chou Y-C. Effect on RF receiving sensitivity and near field EMC analysis of solid state drive (SSD), In: 2013 Asia-Pacific Radio Science Conference, EDBK-4. Taipei: Taiwan 2013: p. 9.], where ε^r is the dielectric constant of the material between the inner and outer conductors, which for air (without filling material) is equal to 1, and v is the speed of light equal to 3 x 10⁸m/s . We investigate two different shielding materials (Graphene and magnetic material FAM5 (0.2 mm)), and also analyze the effect of different stack-up arrangement on shielding effectiveness. The measured results for different material stack-up combinations with coaxial cell method are shown in Fig. (11).

Fig. (11) Measured results with coaxial cell method.

Surface scan method referred to IEC 61967-3 is originally developed to identify the EMI noise sources and measure electromagnetic field distribution across the chip level [15Integrated Circuits-Measurement of Electromagnetic Emissions, Part 3: Measurement of Radiated Emissions – Surface Scan Method, IEC 61967-3 2005. Available at: https://webstore.iec.ch/publication/6186]. The test mode is to activate the integrated circuit (IC) under test and empower auxiliary circuits on the PCB test board for normal operation, and then measure the near-field emitted electromagnetic energy for all dominant EMI frequencies. The test method utilizes the miniaturized E-probe, H-probe, or electromagnetic probe to scan the whole surface of IC die or package in the near-field region for each dominant EMI frequencies. With the multi-layer scanning of near-field emission, we can also reconstruct the EMI radiation of the noise source inside the chip under test. The positioning system of field probe and controlling software provides the accurate spatial step and orientation of measurement probe, and thus assures the repeatable and reproducible measured results. The test setup for near-field surface scan method is shown in Fig. (12).

Fig. (12) Illustration of near-field scan method.

To illustrate the application of shielding material for wireless device, we use an Android wireless (WiFi and Bluetooth) micro-projector as a demonstration as shown in Fig. (13). We analyze the data rate reduction of WiFi connection by measuring the antenna port noise level to quantify the signal-to-noise ratio (SNR) first, and then utilize the near-field scan method to electromagnetically scan the complete area of projector to identify and characterize the EMI noise source. The measured result is illustrated with hot-spot graph as shown in Fig. (14), and the red area indicates the most serious and noisy EMI region. We then further locate the specific scanning area on dominant noisy IC for analysis, and finally apply the shielding material on top of the noisy IC to improve the wireless communications performance. The measured EMI results of dominantly noisy IC without and with shielding material are shown in Figs. (15-17) respectively, and also show the EMI level reduction with the shielding materials.

Fig. (13) Wireless micro-projector with wifi and bluetooth RF modules.

Fig. (14) Near-field surface scanned result of wireless micro-projector (before applying shielding material).

Fig. (15) Near-field surface scanned result of dominant EMI IC with parallel (left) and perpendicular (right) probe orientation (before applying shielding material).

Fig. (16) Near-field surface scanned result of dominant EMI IC with parallel (left) and perpendicular (right) probe orientation (applying graphene).

Fig. (17) Near-field surface scanned result of dominant EMI IC with parallel (left) and perpendicular (right) probe orientation (applying FAM 5).

To quantify the improvement of shielding material on wireless communications performance, we measure the antenna port noise level to show the increase of signal-to-noise ratio (SNR) by reducing the EMI level with shielding material. The platform noise under investigation is analyzed by noise floor measurement system. The complete PNS (platform noise measuring system) is composed of shielded box, pre-amplifier, spectrum analyzer, and EUT (Micro-projector). The noise level measuring system and setup for EUT in frequency domain is shown in Fig. (18).

The test configuration of EUT and results for platform noise measurement to verify SNR improvement without/with shielding material is shown in Fig. (19). The measured results clearly finds the increasing SNR by reducing the EMI level of noisy IC around 2.4 GHz WiFi frequency band with shielding material, and thus improve the performance of wireless communications.

Fig. (18) Setup for antenna port noise level measurement.

Fig. (19) Test configuration and noise level for antenna port noise measurement.