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PB2010.06 Probe Characterization and Data Process for Current Reconstruction by Near Field Scanning Method

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Abstract- previously a measurement technique for ESD current spreading on a PCB using near field scanning was developed in order to connect the local ESD sensitivity to system level ESD failures in time and spatial domain. The concept of such scanning methodology is proved and several scanning results were processed. However the validation, precision and weakness of such methodology need to be further investigated before the application of such scanning methodology on complex circuit or system.

This article investigates the current reconstruction by near field scanning technique and methodology. It studies the probe factors including coupling frequency response characterization and deconvolution method, spatial resolution for scanning and orthogonal-scan data combine process.

I. INTRODUCTION

This article is a research continuation of a previous paper “A Measurement Technique for ESD Current Spreading on A PCB using Near Field Scanning”. The ESD injection setup and current return paths are shown in Fig. 1 and Fig.2

II. PROBE CHARACTERISTICS

To successfully recover both the magnitude and direction of injected surface current or coupled trace current from probe’s near field induced voltage signal, the understanding of probe characteristics is of great significance. A probe’s coupling frequency response; loss, spatial resolution and directional response need to be evaluated to estimate how well the injected surface current or coupled trace current can be recovered. Then the reverse process from local induced probe signals to desired surface or trace current vectors can be constructed based on the probes characteristics.

A. Frequency Response and Deconvolution Method

For using a magnetic probe to capture a transient magnetic field or current and recover it numerically, the probe’s magnetic coupling frequency response is one of the most important factors to construct the reverse process, or probe frequency response deconvolution.

The probe should have enough inductive coupling bandwidth to cover the main spectrum of the transient magnetic field or current. Then a compensation function of the coupling frequency response can be calculated to recover the transient magnetic field or current from the induced probe voltage signal. In addition, the experimental measurement including the probe should have enough signal noise ratio and dynamic range of H-field or current coupling to get recoverable measurement results.

A probe’s coupling factor for surface current or trace current can be measured by a terminated TEM cell or trace as Fig. 3 shows:

The coupling mechanism between the probe and the excitation can be modelled as Fig 4.

From the model, the frequency response is inductive coupling dominated and the excitation recovery process can be achieved with a process as Fig 5 shows:

A compensation function of the probe’s frequency coupling response and loss with filters is then created with steps as Fig 6 shows:

Finally the desired field or current data can be rescaled from the recovered excitation_ An example of probe deconvolution is followed_ Fig_ 6 is the measured probe voltage signal from TEM Cell excitation. After the deconvolution process a comparison of transient H-field from processed probe signal and directly measured H-field are shown in Fig 8:

Some of the high frequency and low frequency components are lost due to the filters integrated into the compensation function, but they are important to reject low frequency noise and high frequency resonance. Overall the method works well and the recovery result matches direct measurement.

B. Spatial Resolution

Spatial resolution of a probe reflects how well a magnetic field probe can resolve the field strength variation or a trace current probe can resolve the trace current underneath during the probe’s spatial offset.

A shielded single loop probe is usually good for magnetic field measurement since it captures the magnetic flux crossing the loop and the loop size can be reduced to increase spatial resolution.

A shielded trace current probe against plane current coupling was design with complex structure for ESD current reconstruction scanning_ Its spatial resolution is measured and simulated over a 2.8 mm wide trace with sideway offsets as Fig 9 shows:

The probe frequency coupling responses of all the sideway offsets are obtained as Fig 10 shows:

Then the responses at the same frequency (in the inductive coupling frequency range) for al1 the sideway offsets are plotted in Fig.ll :

For a trace current probe, the spatial resolution indicates the scanning resolution should be much smaller than the 6 dB width to get the maximum coupling position scanned_ And the side peaks coupling should be reduced as good as possible in order to avoid “fake trace current” from scanning result.

C. Directional Response and Orthogonal Data Combine

The directional response of a probe reflects how it couples to field or current during rotation, usual1y from the maximum to the minimum coupling direction.

A shielded single loop probe usual1y has good directional response that fol1ows cosine drop from the maximum to minimum coupling direction. If a probe’s directional response follows cosine drop, the orthogonal measurement signals such as V x and Vy can be separately processed and directly mapped to vectors Hx and Hy, or Jx and Jy.

A shielded trace current probe, depending on the coupling mechanism, may not have good directional response that fol1ows cosine drop. Thus the process of orthogonal scanning data needs to compensate for its directional response.

The directional response of a probe is only compensable in its linear dynamic range, or inductive coupling range. Fig 12 shows a trace current probe with deficient directional response. The linear region is from 50 MHz to 1 GHz.

The normalized directional response of the probe is shown in Fig 13. The 90 degree rejection is less than 20 dB, which suggests certain common mode coupling exists.

The coupling frequency responses of a probe with better directional response are shown in Fig 14. The probe has wider inductive coupling range and dynamic range (from 5MHz to 3GHz). In addition, its normalized directional responses in this range are shown in Fig 15.

The orthogonal scanning data can be compensated if its directional response is symmetry between [0, 90] and [0, -90], and also monotone decreasing in [0, 90], like Fig 15. Suppose an orthogonal data set of V x and Vy is measured from scanning using this probe as Fig 16 shows.

The trace current under the probe is 8 degree off the X direction and 90-8 degree off the Y direction, The process to calculate I and 8 from V x and Vy is to map the V xlVy in the interpolated V xlVy curve as the Fig, 17 shows.

Since the probe’s normalized directional response is monotone decreasing, the VxlVy curve is also monotone decreasing and there is a one-to·one correspondence relation between the V xIV y and 8. Then the maximum coupling response V can be calculated from the probe’s normalized directional response as Fig 18 shows, Finally, the trace current vector I can be recovered from previous deconvolution process.

In this way, the current vector I can be recovered precisely as long as the probe’s normalized directional response is monotone decreasing, even it doesn’t follow cosine drop. The common mode h-field coupling maximized at 90 degree direction won’t be added into the recovered vector 1.

Ill. CONCLUSIONS

In this paper, several important probe characteristics for recovering surface current density J or trace current I by near field scanning method are analysed, The probe’s coupling frequency response characterization and compensation methodology are described; probe’s spatial resolution and relation to minimum scanning resolution are illustrated; and probe’s directional response and compensation method for orthogonal scanning data are explained.

In the next step, probe design and optimization for current reconstruction by near field scanning should be continued for better frequency response, spatial resolution, unwanted coupling rejection, sensitivity and directional response. Secondly how a probe or structure can influence the near-field around the measurement target should be studied, especially when the measurement interest is the coupled signal on the target, not the driven signal on the target.

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PB2010.06 Near Field Probe for Detecting Resonances in EMC Application

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Abstract— Resonances degrade the product’s EMI or immunity performance at resonance frequencies. Near field scanning techniques, like EMI scanning or susceptibility scanning determine the local behaviour, but fail to connect the local behaviour to the system level behaviour. Resonating structures form part of the coupling paths, i.e., identifying them will aid in understanding system level behaviour of products. In this article, a near field probe (patent pending) is proposed to detecting the resonances frequencies, locations or resonating structures and their Q-factors. The probe is suitable for integration into an automatic scanning system for analysing resonances of PCBs, cables, structural elements etc. The mechanism of the probe has been verified with full 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 be 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 relatively narrow frequency bands. This cannot be explained with broadband, simple (like inductive) coupling mechanisms. Instead coupling from resonances must be considered. The resonating structures can also form antennas and couple 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 from the probe. If the probe is able to couple to a resonating structure, a dip in the S11 will be observed at the resonance frequencies. This method has been known for at least 90 years (grid dipper). The depth of the dip is sensitive to the coupling to the resonating object, and making it difficult 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 local magnetic (or electric) field using probes [1][2]. This method will only excite the resonances that can be excited by the far field. The near field excitation will also excite resonances that are locally excitable. Besides these three methods, a newly designed resonance detection probe that integrates an electrically small cone structure with a shielded magnetic field loop is proposed in this paper. Full wave simulation based on CST MWS [6] and Ansoft HFSS [7] and measurement results are shown and results of 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 shows the main parts of the probes: The small shielded horizontal loop and the cone. The coax cables are connected to the VNA . Figure 2 shows the second design and it uses a vertically mounted shielded loop.

Details are shown in Figure 3. A semi-rigid cable forms the shielded loop. A gap in the shield allows the magnetic field to couple. The inner conductor of the second coaxial cable connects to the lower part of the cone structure. This excites the cone in its inner side.

III.MECHANISM OF RESONANCE DETECTION

The loop forms an H-field sensor. The cone structure is more complex. It is excited on its inside. To better understand its functions, full wave simulation was used in CST MWS. Field probes are placed underneath the cone, while the cone is placed above a large ground plane. Figure 4 shows the geometry. Results of the E-field divided by 377 and the magnetic field are shown in Figure 5. The Efield dominates the coupling.

The resonance scanning probe contains both the cone and the loop sensor. An S21 measurement is used to identify resonances. If no resonating structure is present the 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 measurement results are compared in Figure 7. They match well.

IV.SAMPLES OF RESONANCE DETECTION

Test samples have been scanned to identify resonant structures, frequencies and Q-factors. Two sample structures are shown below. Sample 1 is a test structure containing a ring structure with microstrip traces. Sample 2 is the commercial product.

Figure 8 shows the ring structure with microstrip 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 shows strongly at the first resonance frequency is around 240 MHz.

Resonances increase the coupling from the field to the circuits, then cause decrease the immunity and enhance the emission of system. They are often the “missing link” between system level performance and local level. Therefore, identifying resonances is an important step in understanding immunity sensitivities or emission maxima. This paper presents a method for detecting resonance in the PCBs, ICs, and components. The product is scanned by an auto-scanning system (Smartscan) [8] and resonances are detected via S21 measurement.

The methodology was applied to scan the motherboard 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 enhance the emission of system. They are often the “missing link” between system level performance and local level. Therefore, identifying resonances is an important step in understanding immunity sensitivities or emission maxima. This paper presents a method for detecting resonance in the PCBs, ICs, and components. The product is scanned by an auto-scanning system (Smartscan) [8] and resonances are detected via S21 measurement.