Near Field Probe for Detecting Resonances in EMC [627128]

Near Field Probe for Detecting Resonances in EMC
Application
Jiang Xiao#1, Dazhao Liu#2, David Pommerenke#3, Wei Huang#4, Peng Shao #5, Xiang Li#6, Jin Min*7,
Giorgi Muchaidze*8
# EMC Lab, Missouri University of Science and Technol ogy, Rolla, MO, 65401 USA
1 [anonimizat], 2 [anonimizat],3davidjp@ mst.edu, 4wh57f@ mst.edu, 5psrg8@ mst.edu, 6 xlcn3@ mst.edu
* Amber Precision Instruments, Santa Clara, CA 95054 USA
[anonimizat], [anonimizat]

Abstract — Resonances degrade the product’s EMI or
immunity performance at resonance frequencies. Near field
scanning techniques, like EMI scanning or susceptib ility
scanning determine the local behaviour, but fail to connect the
local behaviour to the system level behaviour. Reso nating
structures form part of the coupling paths, i.e., i dentifying
them will aid in understanding system level behavio ur of
products. In this article, a near field probe (pate nt pending) is
proposed to detecting the resonances frequencies, l ocations or
resonating structures and their Q-factors. The prob e is
suitable for integration into an automatic scanning system for
analysing resonances of PCBs, cables, structural el ements etc.
The mechanism of the probe has been verified with f ull wave
tools (CST MWS and Ansoft HFSS). Two samples of
application are presented.
I. INTRODUCTION
Resonances in products are caused by the circuit
topology and by the geometry of structural elements , cables,
etc. They can form lumped (L-C) element resonators or
they can be of distributed nature. Resonances may b e
caused by IC interconnect, traces, cable placement and
structural elements of a system [1][2]. It has been observed
that radiated immunity failures usually occur in re latively
narrow frequency bands. This cannot be explained wi th
broadband, simple (like inductive) coupling mechani sms.
Instead coupling from resonances must be considered . The
resonating structures can also form antennas and co uple to
the far field, consequently increase the EMI.
Some methods for detecting resonances are known.
Firstly, a S11 measurement method injects RF energy to a
probe and measures the signal reflected returning f rom the
probe. If the probe is able to couple to a resonati ng
structure, a dip in the S11 will be observed at the resonance
frequencies. This method has been known for at leas t 90
years (grid dipper). The depth of the dip is sensit ive to the
coupling to the resonating object, and making it di fficult to
implement this method in an automated scanner. The
second method uses two orthogonal probes. They are
decoupled from each other. A VNA (Vector Network
Analyzer) measures S21 which expresses the coupling
between the two probes [3]. The third method uses a far
field antenna driven by a VNA to illuminate the DUT
(Device Under Testing) and measures the excited loc al magnetic (or electric) field using probes [1][2]. T his
method will only excite the resonances that can be excited
by the far field. The near field excitation will al so excite
resonances that are locally excitable. Besides thes e three
methods, a newly designed resonance detection probe that
integrates an electrically small cone structure wit h a
shielded magnetic field loop is proposed in this pa per. Full
wave simulation based on CST MWS [6] and Ansoft HFS S
[7] and measurement results are shown and results o f
resonance scanning are presented.
II. DESCRIPTION OF THE PROBE GEOMETRY
Two probes have been designed that differ in the plane
the loop is mounted, shown in Figure 1 and Figure 2 .

Figure 1: Proposed structure 1 of the resonance pro be

Figure 2 : Proposed structure 2 of the resonance probe

2010 Asia-Pacific International Symposium on Electromagnetic Compatibility, April 12 – 16, 2010, Beijing, China
978-1-4244-5623-9/10/$26.00 ©2010 IEEE 243

Figure 1 shows the main parts of the probes: The sm all shielded horizontal loop and the cone. The coax cables are
connected to the VNA . Figure 2 shows the second de sign
and it uses a vertically mounted shielded loop.

Figure 3: Detailed structure of a real resonance pr obe (structure 1)

Details are shown in Figure 3. A semi-rigid cabl e forms the shielded loop. A gap in the shield allo ws the magnetic field to
couple. The inner conductor of the second coaxial c able connects to the lower part of the cone structu re. This excites the cone in
its inner side.
III. MECHANISM OF RESONANCE DETECTION
The loop forms an H-field sensor. The cone struc ture is
more complex. It is excited on its inside. To bette r
understand its functions, full wave simulation was used in
CST MWS. Field probes are placed underneath the con e,
while the cone is placed above a large ground plane . Figure
4 shows the geometry. Results of the E-field divide d by
377 and the magnetic field are shown in Figure 5. T he E-
field dominates the coupling.

Figure 4: Cone structure above PEC in CST

Figure 5: Comparison of the E-field (divided by 377 ) and the H field
underneath the cone
The resonance scanning probe contains both the c one and
the loop sensor. An S21 measurement is used to iden tify
resonances. If no resonating structure is present t he S21
value needs to be as low as possible, allowing the S21 to
rise if resonating structures are detected. Figure 6 shows
the full wave model in HFSS. Simulation and measure ment
results are compared in Figure 7. They match well.
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Figure 6: Full wave simulation model of the complet e probe
0.5 1 1.5 2 2.5 3-90-80-70-60-50-40-30-20-100
Frequency (GHz)dBS parameter of resonance probeD

Simualtion S11
Simualtion S21
Simualtion S22
Measurement S11
Measurement S21
Measurement S22

Figure 7: S parameter comparison
IV. SAMPLES OF RESONANCE DETECTION
Test samples have been scanned to identify reson ant
structures, frequencies and Q-factors. Two sample
structures are shown below. Sample 1 is a test stru cture
containing a ring structure with microstrip traces. Sample 2
is the commercial product.
Sample 1: Simple Resonance Structure

Figure 8: Ring structure with microstrip traces
Figure 8 shows the ring structure with microstri p traces.
In Figure 9, scan results are overlaid with a photo of the test sample. This data presentation does not distinguish between
different resonant frequencies. The color indicates the
magnitude S21 of the resonances. The resonance show s
strongly at the first resonance frequency is around 240
MHz.

Figure 9: Resonance scanning result of the first te st structure
The results show that the resonance is correctly de tected.
Also the probe doesn’t need to rotate by 90 degree to
measure the H-field over the DUT due to its symmetr y. One
disadvantage is that right over the middle of the t race the
field component detected (Hy) has a null. However t his
disadvantage can be solved by applying a deconvolut ion
algorithm in data post-processing.

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Sample2: Electronics product: computer

Figure 10: Resonance detection sample for the elect ronics product

Figure 11: Scan magnitude results for the DUT at 9 6.2, 109.5 and
123.2 MHz showing the resonance of the cable
The methodology was applied to scan the motherbo ard of
a computer, shown in Figure 10. It clearly reveals the cable
resonance around 109 MHz shown in Figure 11.
V. CONCLUSIONS
Resonances increase the coupling from the field to the
circuits, then cause decrease the immunity and enha nce the
emission of system. They are often the "missing lin k"
between system level performance and local level.
Therefore, identifying resonances is an important step in
understanding immunity sensitivities or emission ma xima.
This paper presents a method for detecting resonanc e in the
PCBs, ICs, and components. The product is scanned b y an
auto-scanning system (Smartscan) [8] and resonances are
detected via S21 measurement.
VI. REFERENCES
[1] D. Liu, J. Xiao, W. Huang, G. Muchaidze, J. Min , P. Shao, J.
Drewniak and D. Pommerenke, “Near field resonance sca nning as a
method for analyzing coupling for ESD and radiated immunity”.
EMC Compo 2009, France (accepted).
[2] G. Muchaidze, W. Huang, J. Min, P. Shao, J. Dr ewniak and D.
Pommerenke, “Automated Near-Field Scanning to Identif y
Resonances, ” Electromagnetic Compatibility – EMC Europe, 2008
International Symposium on, pp. 1-5, Sept. 2008.
[3] Tun Li, Yong Cheh Ho, David Pommerenke, “Orthog onal loops
probe design and characterization for near-field mea surement,” IEEE
EMC Symp. Detroit , PP:1 – 4, 18-22 Aug. 2008.
[4] J. Koo, Q. Cai, K. Wang, J. Mass, M. Hirata, A . Martwick, D.
Pommerenke “Correlation between EUT failure levels an d ESD
Generator paramters,” IEEE Trans. Electromagn. Compat ., vol.50,
no.4, pp. 794–801,Nov. 2008
[5] G. Muchaidze, J. Koo, Q. Cai, T. Li, L. Han, A . Martwick, K. Wang,
J. Min, J. Drewniak, D. Pommerenke, “Susceptibility S canning as a
Failure Analysis Tool for System-Level Electrostati c Discharge (ESD)
Problems,” IEEE Trans. Electromagn. Compat ., vol.50, no.2, pp.
268–281, May. 2008.
[6] CST MWS software. MWS created by CST Software. CST Software,
Darmstadt, Germany. [Online]. Available: www. CST.com
[7] HFSS software. HFSS created by Ansoft Software . Ansoft Software,
Pittsburgh, PA. [Online]. Available: www. An soft.com
[8]Smartscan www.amberpi.com . Amber Precision Instruments,
jinmin@amberpi.com

Cable Scanned region
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