Adv. Radio Sci., 14, 155–159, 2016www.adv-radio-sci.net/14/155/2016/doi:10.5194/ars-14-155-2016© Author(s) 2016. CC Attribution 3.0 License.

Characterization of DUT impedance in immunity test setupsSeyyed Ali Hassanpour Razavi and Stephan FreiOn-Board Systems Lab, TU Dortmund University, Dortmund, Germany

Correspondence to: S. A. Hassanpour Razavi (seyyed-ali.hassanpour-razavi@tu-dortmund.de)

Received: 22 January 2016 – Revised: 10 May 2016 – Accepted: 26 June 2016 – Published: 28 September 2016

Abstract. Several immunity test procedures for narrowbandradiated electromagnetic energy are available for automotivecomponents. The ISO 11452 series describes the most com-monly used test methods. The absorber line shielded enclo-sure (ALSE) is often considered as the most reliable method.However, testing with the bulk current injection (BCI) canbe done with less efforts and is often preferred. As the testsetup in both procedures is quite similar, there were severaltrials for finding appropriate modifications to the BCI in or-der to increase the matching to the ALSE. However, the lackof knowledge regarding the impedance of the tested compo-nent, makes it impossible to find the equivalent current to beinjected by the BCI and a good match cannot be achieved.In this paper, three approaches are proposed to estimate thetermination impedance indirectly by using different currentprobes.

1 Introduction

In automotive electronic system configurations often the wireharness is assumed to be the main coupling path. This istaken into account in the EMC test setups described in theISO 11452-2 (ALSE) and the ISO 11452-4 (BCI). In bothmethods, the electromagnetic energy is coupled to the wireharness. The ALSE method leads to a distributed electricand magnetic field coupling of the power along the entirelength the wire harness. However, during BCI tests, the cou-pling appears only at one point. Moreover, the radiated im-munity utilises the BCI for the lower frequency range from1 MHz to 400 MHz and the ALSE for the higher frequencyrange from 80 MHz to 18 GHz. In order to increase the per-formance of the BCI, different investigations are published inAdams (1992) and Pignari (1996). In Grassi (2008), a doubleBCI method is proposed to reproduce the currents injectedduring the system level testing via an antenna by enforcing

the equivalence between radiation and injection. Based on atheoretical investigation in Hassanpour (2014), it is shownthat the current flowing into a device under test (DUT) dur-ing the ALSE can be reproduced with an acceptable accuracyby means of a BCI probe. This is done by injecting an appro-priate power in to the BCI and an accurate positioning of theBCI probe along the wire harness, if the impedance of theDUT is available.

Apart from the coupling mechanism in the immunity teststructures, the only variable part of the test setups in bothALSE and BCI remains the impedance of the terminatingcircuitry. The termination impedances may be obtained ina direct measurement, nevertheless, disconnecting the wireharness from the terminating circuit means additional prepa-ration time and might cause problems to the proper systemfunction. Based on a detailed analysis on the impedance mea-surement with current transformers in Junge (2009), apply-ing a current transformer can be a convenient method for theindirect measurement of the DUT impedances in the immu-nity test setups. Undoubtedly, by obtaining this valuable in-formation about a DUT with appropriate current probes andby knowing the coupled current during the antenna testing asa reference, the correlation between the ALSE and the BCItest methods can be improved. The problem of concern in-cludes the estimation of the impedances terminating the wireharness ends with two different commercial current probes.

The number of wires involved in an immunity test setupis directly linked with the actual operation condition of theDUT. In case of a multi-wire transmission line, i.e. n con-ductors over ground structure, the effects of the terminal net-work impedances on the left- and right side of test harnessare contained in two n× n matrices. In general, these matri-ces are full, which means that there is cross coupling betweenall ports of the network. However, the most common case isthe terminal network configuration where these impedancematrices are diagonal and the coupling occurs only along the

Published by Copernicus Publications on behalf of the URSI Landesausschuss in der Bundesrepublik Deutschland e.V.

156 S. A. Hassanpour Razavi and S. Frei: Characterization of DUT impedance in immunity test setups

multi-wire transmission line (Paul, 1994). Therefore, mea-suring the termination impedances for each single wire sep-arately, provide the primary information to obtain the entireimpedance matrix. Furthermore, it is assumed that the ter-mination impedances consist of a passive RLC circuit. Theabove-mentioned issues are intended to assess the feasibility,strength and weakness of using current probes for measuringthe impedance indirectly with current probes and serve asa primary research, which should be extended to the practi-cally relevant case including a multi-wire harness and activecircuit components.

In Sect. 2, three approaches for characterizing the termina-tion impedances up to 500 MHz for a single wire over groundstructure are presented. In Sect. 3, the application of thesemethods is demonstrated for a measurement setup and theresults are compared. More discussions and the conclusionsare given in Sects. 4 and 5.

2 Impedance measurement

The test setup demanded in the ISO 11452, including a sin-gle wire and two unknown impedances ZL and ZR is illus-trated in Fig. 1. The short conductor clamped by the BCIprobe is considered as the secondary winding of the currenttransformer. In line with the idea of the indirect impedancemeasurement, the loop impedance can be considered as theentire impedance attached to the clamped conductor termi-nals. However, from another perspective, the loop impedanceat any point along the wire can be considered as the accu-mulated transformed impedance of the termination networks.The transformed impedances along the wire to the measure-ment point are represented by ZL

′ and ZR′. The first and the

second methods presented in this section, determine the loopimpedance as the primary value for the impedance measure-ment with different deembedding procedures. In contrast, thethird method measures the impedance at each wire end sepa-rately based on current measurement.

2.1 Single probe method (SPM)

The most straightforward method is to apply a reflectionmeasurement with a vector network analyzer (VNA) to de-termine the loop impedance. Measuring the input impedancewith an impedance analyzer is an appropriate alternative forthe reflection measurement with a VNA at this step, as themeasured data by each instrument can be mathematicallyconverted to each other. As depicted in Fig. 2 the BCI probe(FCC F-140) is connected to the VNA to measure the S-parameter of the structure. The reflection parameter is cor-rected with the full 1-port calibration before starting the mea-surement. In order to remove the frequency response of theBCI probe, the measured one-port S-parameter smeas is ex-panded to a two-port S-parameter matrix Sm for the measuredfrequencies with

Figure 1. The proposed measurement setup for the ALSE and theBCI based on the ISO 11452 standard.

Sm =

(smeas 0

0 −1

)(1)

The second port facilitates the mathematical operation onmatrices and has no influence on the electrical characteris-tic of the actual network. The scattering matrix Sm can nowbe converted to the T-parameter matrix Tm to simplify thedeembedding procedure. The T-parameter matrix can be splitand expressed as

Tm = TBCITloop (2)

where TBCI and Tloop denote the T-parameter matrix of theBCI probe and the loop network respectively. The matrixTBCI can be obtained directly with the VNA in the calibra-tion setup illustrated in Fig. 3 after a full 2-port calibration.With both matrices Tm and TBCI, the T-parameter matrix ofthe loop network is determined as

Tloop = T−1BCITm (3)

As described before, the estimated loop impedance can beconsidered as the sum of both transformed impedances alongthe wire. Assuming a characterized termination on the leftside and the BCI probe located as close as possible to the lefttermination, the extraction of the unknown impedance on theright side of the wire can be calculated using

ZR = Z0Zloop−ZL−Z0tanh(γ lR)Z0−

(Zloop−ZL

)tanh(γ lR)

(4)

where Z0 and γ indicate the characteristic impedance andthe propagation constant of the wire over ground structure

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S. A. Hassanpour Razavi and S. Frei: Characterization of DUT impedance in immunity test setups 157

Figure 2. Measuring the reflection from the test setup with a VNAto obtain the loop impedance after deembedding the probe’s fre-quency response.

respectively. The variable lR is the distance between the BCIprobe and the uncharacterized termination at the right side ofthe setup. In case of two unknown termination impedances,two measurements at different locations along the wire arerequired to estimate each impedance separately.

2.2 Double probe method (DPM)

In this method, two current probes (FCC F-140, FCC F-65) are employed to characterize the loop impedance usingtwo-port S-parameter measurement with a VNA. The mea-surement structure is illustrated in Fig. 4. The frequency re-sponses of both probes are deembedded using the calibra-tion procedure described in Liu et al. (2003). The proposedcalibration configuration is shown in Fig. 5. Two SMA ter-minations, short and 50 �, can be used in the calibrationprocedure to build the final equation calculating the loopimpedance. In order to simplify the complex final equation,the interim value S is defined as

S =s21

1− s11(5)

where s11 and s21 indicate the corresponding entries of a two-port S-parameter matrix. The loop impedance is then calcu-lated using

Zloop = 50�S50�

Sshort− S50�

(Sshort

SM− 1

)(6)

where Sshort and S50� are the calculated values of S, if thecalibration setup is terminated with the standard SMA ter-minations, short and 50 �, respectively. The value SM is thecalculated value of S for the two-port S-parameter measure-ment, if both probes are clamped around the wire in the actualtest setup.

Apart from the calibration process, the estimation ofimpedances in this method does not differ from the SPM.In order to build up an equation system in case of two

Figure 3. The proposed calibration setup to measure the probe’sfrequency response.

Figure 4. The measurement setup applying the DPM to calculatethe loop impedance.

unknown impedances, the measurement structure includingboth probes should be placed at two different locations.

2.3 Current distribution measurement (CDM)

This method employs two current probes to measure thecurrent distribution along the wire in order to extract theimpedance of each termination directly. Therefore, the BCIprobe (FCC F-140) is connected to the port 1 of the VNA toinject a certain interference level into the setup. The monitorcurrent probe (FCC F-65) is connected to the port 2 to mea-sure the transmission coefficient s21 in the specified distancesfrom the injection probe along the wire, as shown in Fig. 6.The number of measurements is directly linked with the mea-surement frequency range and the desired accuracy. In orderto increase the accuracy for frequencies up to 500 MHz, theS-parameter is measured with 3 cm step size. The measuredtransmission coefficient can be used to estimate the currentmagnitude I at a certain position using

I = s21

√PFwd · 50�ZT

(7)

where PFwd is the injected input forward power of the VNAand ZT is the transfer impedance of the monitor currentprobe. The ratio between the maximum and minimum mea-

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158 S. A. Hassanpour Razavi and S. Frei: Characterization of DUT impedance in immunity test setups

Figure 5. The calibration setup to deembed the frequency responseof both probes from the measurement.

sured current magnitude m can be expressed as

m=Imax

Imin(8)

By knowing the characteristic impedance and the propa-gation constant of the wire, the termination impedance Z canbe calculated as

Z = Z0 ·

1m− tanh(γ l1 min)

1− 1m

tanh(γ l1 min), (9)

where l1 min is the distance of the first minimum current mag-nitude from the end of wire. This method requires a wirelength of at least λ/4. In order to cover the lower frequen-cies, the length of the wire should be increased. The accuracyof the estimated results is extremely sensitive to the currentdistribution resolution. The main advantage of this method isthat the impedance for each termination can be extracted di-rectly. Additionally, the measurement can also be performeddirectly by using a signal generator and a spectrum analyzerinstead of a VNA, to measure the current magnitude alongthe wire.

3 Application and experimental results

In Sect. 2, three different methods for an estimation of thetermination impedances are presented. To investigate the reli-ability of each method, a setup based on ISO 11452 standardwith a single wire attached between two grounded metal fix-tures is prepared. The setup is terminated with a 50 � SMAresistor at the left side. The right side of the setup is termi-nated with two small PCBs including a 10 � and a 470 �SMD resistors successively, to perform two separate mea-surements. The characteristic impedance and the propaga-tion constant of the single wire over ground is characterizedin the frequency range from 1 to 500 MHz based on a two-port S-parameter measurement. The value of the terminationimpedance at the right side is assumed to be unknown. Forthe SPM and DPM, the current probes are located at the left

Figure 6. Appling the CDM along a wire to estimate the impedanceof terminations.

side of the setup close to the left fixture. All of three methodsare performed with a constant forward power of 10 dBm. Theimpedance of each PCB is measured directly in a separatemeasurement with the VNA and serves merely as a referencefor the comparison with the estimated results. The compari-son between the directly measured (gray) and the estimatedresults for the magnitude and the phase of the impedances areillustrated in Fig. 7. The results show a good correlation forthe SPM (blue) and DPM (green) up to 200 MHz. The CDM(red) shows a better accuracy at higher frequency.

4 Discussion

The reason for the deviation between the directly mea-sured impedance and the estimated results are mainlythe measurement- and deembedding errors. The actualimpedance at the end of wire cannot be measured explic-itly, due to the existence of the fixtures. The measuredimpedances show an inductive behavior for the PCB withthe low resistive component and a capacitive behavior forthe PCB with the high resistive component. The reason forthe inductive and capacitive behavior at the higher frequencyrange is the structure of the PCBs and the expected behav-ior of SMD resistors at higher frequencies. At the lower fre-quency range, the ability of the measurement instrument andthe characteristics of the ferrite core inside the current probeare the main restrictions for performing the impedance mea-surement. However, at higher frequencies, the measurementresults are very sensitive to the parasitic capacitive couplingbetween the conductive elements such as the probe’s metal-lic frame, the probe’s winding, the clamped conductor andthe ground plane. In addition, the quality of the datasets ob-tained from the calibration setups to remove the probes’ fre-quency response are significantly affected by the length ofthe clamped conductor and the added fixtures. Despite theexpected inductive behavior of the current probes, the capaci-tive coupling is dominant and reduces the local characteristicimpedance of the wire. Considering constant values for Z0and γ for the entire length of the wire, including the sectionclamped by the probe and the area near the fixtures, reduces

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S. A. Hassanpour Razavi and S. Frei: Characterization of DUT impedance in immunity test setups 159

Figure 7. The Comparison between the directly measured (grey) and the estimated termination impedance for a high resistive PCB and alow resistive PCB with the SPM (blue), the DPM (green) and the CDM (red).

the accuracy of all methods. The high attenuation and the lowdynamic of the current probes toward the loop impedance,due to the built-in resistive elements or the characteristics ofthe ferrite core limit the measurement bandwidth. Using cur-rent probes with smaller dimensions or assembling self-madecurrent transformers with appropriate ferrite core character-istics can improve the measurement results. In order to im-prove the measurement results by suppressing the common-mode currents on the coaxial cables to the VNA, differentferrite beads appropriate for the desired frequency range areused during the entire measurement.

5 Conclusions

Based on the demand to improve BCI testing, comfortableand accurate termination impedance measurement methodsare required. This paper presented three methods to estimatethe impedances of the termination circuits in the immunitytest setups using the clamp-on current probes. All of threemethods were compared and experimentally validated fordifferent resistive terminations. It is shown that the proposedmethods are applicable without any need to disconnect thewire harness from the DUT. In particular, it has been high-lighted that, the CDM estimates the impedance of both termi-nations with acceptable precision at the higher frequencies.However, the SPM and DPM can estimate the impedance ofthe termination up to 200 MHz. On the whole, the proposedmethods are aimed to highlight the potential of the indirectmeasurement of termination impedances and extending theapplication of current probes for impedance measurement.

6 Data availability

Part of this research was done within cooperation projectsand is subject to individual confidentiality agreements. Dataused for this publication cannot be disclosed.

Edited by: F. GronwaldReviewed by: three anonymous referees

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