Prediction of Pcb Radiated Emissions (Emc Symposium Zurich 1998)


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The paper shows the experimental validation of predictive results of radiated emissions of a multilayer pcb. The radiated field is calculated from simulated results of pcb signals obtained from DWN analysis of interconnects.

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Prediction of Pcb Radiated Emissions (Emc Symposium Zurich 1998)

  1. 1. THE USE OF AN ACTIVE TEST BOARD TO VALIDATE A METHOD THAT PREDICTS DIFFERENTIAL MODE RADIATED EMISSIONS FROM PRINTED CIRCUIT BOARDS Emmanuel Leroux, Ronald De Smedt, Paolo Fogliati, Bernard Demoulin Andrea Giuliano, Jan De Moerloose, Piero Belforte, Carla Giachino Willem Temmerman High Design Technology Alcatel Bell Telephone CSELT University of Lille Corso Trapani 16 Francis Wellesplein 1 Via Reiss Romoli 274 59655 Villeneuve d’ascq 10139 Torino Italy B-2018 Antwerpen Belgium 10148 Torino Italy France Abstract - The design of new Printed Circuit Boards As it will be explained in the part III, the active test (PCBs) could be conveniently supported by the use of a board used to validate the method is made of microstrip fast and reliable estimations tool of the Radiated structures. The proposed method is now explained as Emissions (RE). In this paper an active test board is applied to an embedded microstrip made of only one used to validate a method aimed at predicting RE due to rectilinear trace. If more rectilinear traces are present, differential mode currents on PCB traces. The method the result is made of the vectorial summation of the takes into account effects of dielectric layer in the field contributions of each trace. calculation and does not need to discretise traces in As dielectric layers can severely modify radiation short segments. It can simulate all traces of a PCB in a patterns [1], it is important to take into account for each reasonable computation time. The simulation results are microstrip structure the actual medium existing between compared with measurements in a semi-anechoic the metal plane and the air. The presented method is chamber in far field. Radiation from the routed power able to calculate the RE of any microstrip structure, as net, on board batteries and batteries holders is put in shown in figure 1, which shows two dielectric layers but evidence though measurement in near field. The paper the presented method can take into account an arbitrary shows the importance of modeling of components and of number of layers. taking into account all couplings in the measurement set- z up when comparing RE results. The method validated in observation point this paper is included in the Telecom Hardware P Robustness Inspection System (THRIS) for evaluation of telecom hardware. R L I. INTRODUCTION y The introduction of severe international P' standards in the field of Electromagnetic Compatibility w (EMC) in the last years had as result an increasing interest for computing the Radiated Emissions (RE) from electronic products. In fact the manufacturers of x electronic systems are interested in optimising from Figure 1: Representation of an embedded microstrip EMC point of view their products before arriving to the structure, L: trace length, R: ”antenna” position where compliance tests in order to reduce time to market and the electric field is computed contain the costs. In most of cases electronic products are composed of Printed Circuit Boards (PCBs). The A way to take into account dielectric layers in the RE possibility of having a fast and reliable estimation of the calculation is to use the dyadic Green's function of the radiated field of PCB at design level is so highly actual layered structure [2]. Then Sommerfeld integrals desirable. have to be solved in a numerical way and this leads to a In this paper an active test board is used to validate a long computation time. The presented method uses some method aimed at predicting RE due to differential mode technics to give an analytical solution to the problem. currents on PCB traces. The starting point is the determination of the actual current distribution along each trace. The method just II. MODELING OF PHYSICAL PHENOMENA needs the knowledge of voltage and current on one of the two extremities of each rectilinear trace. This information is given in Time Domain by PRESTO [3] 1
  2. 2. (Post-layout Rapid Exhaustive Simulation and Test of obtained for any rectilinear radiating trace as shown in Operation) environment. It is a post-layout quality check Figure 1. software that performs electrical simulations of PCBs to evaluate Signal Integrity (SI). A Fast Fourier Transform   Zo    ER j e jko R e jkoh cos Pex , x Pey , y Pez , z (2) (FFT) is then performed to obtain this information in 2R 0 Frequency Domain. Then the current waveform at any abscissa x on the trace is determined by means of the where: Transmission Line Theory (TLT) assuming that only the o quasi-Transverse Electric Magnetic (TEM) mode is Zo = = wave impedance in the air present along the trace. Then, the radiated o ElectroMagnetic (EM) field can be calculated using 2 dyadic Green’s function. Ko = = propagation constant The electric field radiated from a surface current o distribution is obtained by means of the Green Dyadic = wave length in the air o    h = substrate thickness G( r r ) of the layered structure, which can be interpreted as a transfer function between the surface Pex ( , ) , P ( , ) and P ( , ey ez ) are essentially  plane-wave transfer functions of the dielectric layered current distribution J and the electric field as shown medium [2], that combine TE and TM plane-wave in (1): modes. They depend on:     the spherical coordinates of the measuring antenna     E(r ) j o G( r r ). J ( r )d ( r ) (1) position in the local reference system placed on the trace. where: the spatial Fourier Transform of the current density on the trace  - r is the coordinate of the point where the electric The obtained expression of the Electric field in far field field is computed;  conditions is analytical, does not need to discretize the - r is the coordinate of a point placed on the trace and takes into account dielectric layers in the field rectilinear trace.    calculation. In general, G( r r ) does not admit a close form expression. However, with the assumption of being in III. EXPERIMENTAL VALIDATION far field conditions, the Green Dyadic can be substantially simplified. III.1 Description of the active test boards For Information Technology Equipment (ITE) the values of limits for RE in the frequency band 30 MHz to To simplify the comparison between simulations and 1 GHz are given by EN 55022 [4] standard. The far measurements of the radiation of a PCB, a dedicated set field condition applies approximately in the whole of active test boards have been developed [6]. They are frequency range described in the EN 55022 standard for constructed using a standard four layers technology, RE, which justifies the use of far field Green’s function. with a full power and full ground plane at the inside. Being difficult to directly calculate the electric field due The component side contains all the components and to the current density of a segment buried in dielectric signal lines. The solder side only contains the layers, the far field method applies the same current decoupling capacitors, the batteries holders with source on the observation point where the EM field has batteries and a routed net for power distribution. A large to be calculated and exploits the theory of reciprocity and a small version exists with respective dimensions: [5] to transform the radiated emission calculation in a 295mm x 240mm and 195mm x 160mm. The layout of problem of the induction of a plane wave on the the large board is shown in figure 2, that of the small stratified structure. board is similar. The active test board contains an Then the method assumes that the field arriving at the oscillator (U6) with interchangeable clock frequency air/dielectric interface is also locally a plane wave which (e.g. 5MHz, 16 MHz, 18MHz, 50MHz, 75MHz, with can be divided into two components, the transverse the restriction that the digital logic circuitry must be able electric (TE) and transverse magnetic (TM) modes. By to handle such frequencies). The oscillator drives three the use of modal analogy [2], Transmission Line Theory 4-bit counters (U1, U3, U4). The output of these (TLT) can be then applied to the propagation of these counters is connected to large busses (varying from two modes in the embedded microstrip structure to 150mm to 250mm on the large board, 55mm to 150mm produce two transfer functions for the actual medium on the small board). between the metal plane and the air. The following expression of the Electric field in far field conditions is 2
  3. 3. driver receiver Zs 75 75 50 100 pF driver receiver Zs 75 50 50 100 pF Figure 3: Configuration of most of the nets, SLT and Figure 2: Layout of the large test board PLT are present Before to compare simulation and measurement Two busses are terminated with inverters (U2, U5) of for RE it is important to verify the quality of the which the output is connected to loads. The third bus simulated signals on PCB traces and so the quality of ends with a symmetric line driver (U7) of which each models for active components. The board has been symmetrical signal (pair of lines) passes through a simulated with behavioural models which include for common mode filter (which can be shunted to inhibit its drivers the measured unloaded output voltage. The functioning) and is directed towards a connector. All figure 4 shows the comparison between the spectrum, lines have controlled impedance (75 Ohms). Several obtained by the means of a Fast Fourier Transform termination schemes can be implemented: SLT (Series (FFT), of the measured and simulated voltage at the Line Termination) at the begin, AC PLT (Parallel Line output of (U6) component. Termination) at the end. All four combinations of 20 dBV termination have been examined (SLT present or not and PLT present or not). Several versions of the test 0 FFT on the board exist, which are equipped with different, but measured voltage (standard) pin-compatible logic (LS-TTL, HCMOS, -20 ACT). To simplify measurements and simulations, the board is able operate without any cabling. The power -40 can be supplied by unshielded rechargeable batteries -60 fixed on the solder side of the board by batteries 10MHz 20 100MHz Frequency 1GHz dBV holders. They are connected to the power and ground FFT on the plane by a power net and shunted by a decoupling 0 simulated voltage capacitor. -20 III.2 Measurement and simulation of the active test boards -40 For the results reported in this paper only the large -60 10MHz 100MHz Frequency 1GHz board has been considered and equipped with a 16MHz Figure 4: FFT on measured and simulated voltage at the clock, with ACT technology where SLT and PLT are output of (U6) present. The simulation needs technological, The agreement is quite good until 300 MHz. geometrical and physical data which are extracted from The radiated fields of the active test boards have the layout tool used to design the board. It presents 177 been measured extensively. The measurements have components and 107 nets which are microstrip been performed in the semi-anechoic rooms at Alcatel structures. Most of nets are based on the same Bell, Antwerpen (Belgium), and at CSELT, Turin (Italy) configuration which is described in the figure 3 (SLT which are normally used for EMC normative tests. and PLT are present). Several different positions of the board on the table have been considered. The measurements have been performed on the large and small test boards, with the 3 logic families and with various clock frequencies (as long as supported by the logic). Several combinations of line termination have been considered as well. 3
  4. 4. The comparisons reported on this paper are relative to the measurements made in CSELT for which the setup is shown in figures 5 and 6. front side of the board on board batteries h=2 m H=1.01 m L=10 m Metal floor of semi-anechoic chamber Figure 5: Measurement set-up in CSELT Figure 7: Back of the board, presence of battery holders without e.m. shield The envelope of measured and simulated maximum electric field are shown in figure 8. The measurement in vertical polarisation has been obtained with a 120 kHz bandwidth spectrum analyser. 40 measurement 35 Electric field in dB V/m 30 25 20 15 Simulation 10 5 0 8 9 10 Frequency in Hz 10 Figure 6: Far field measurement in CSELT The test board is placed on a wooden table in vertical position, at a fixed height of 1.01 m above the ground plane of the room. At 10 m distance the receiving antenna is placed at a fixed height of 2 m. The board is supplied by 4 nickel cadmium batteries placed on the solder side of the board itself. No shielding structures (for example boxes, conducting ribbon, etc.) have been Figure 8: Comparison between the envelope of used to limit the radiation of batteries as it is shown in maximum measured and simulated electric field the figure 7. The envelopes of simulated and measured radiated emissions are similar and show a minimum at around 400 MHz. But between 100 MHz and 350 MHz, the simulation underestimates the measurement results of 4
  5. 5. some 15 dB. In order to investigate this difference, some This range is exactly the same where the prediction measurements in near field have been performed. The underestimates the far field measurements. As no near test bench shown in figure 9 has been used to measure field to far field interpolation can be practically used, we the magnetic field radiated at a distance of 1cm from the can not make any quantitative explanation but we can back of the board. It uses a spectrum analyser HP reasonably justify the impact of the batteries emission 8595EM, a near field probe HP 11940A and a three-axis on the far field measurement, looking at its spectral table for scanning the test board. distribution in near field. On the first designed board, due to the absence of accessible ground plane, it was impossible to introduce a shield. So a new version of the board has been produced with an “EMI approach”. On the back (solder side) of the new board a ground metallization has been brought to the surface through several plated-through holes as it is shown in the figure 11 and the batteries have been mounted on it. By this way a flexible EMI shield has been soldered as shown in the figure 12: it covers all the batteries and cuts the spurious radiation from the back of the board. Figure 9: Three-axis scanning set-up for near field RE By this way the radiated field from the new test board measurements, the top layer of the board is shown can be reasonably supposed coming only from the front of the board (signal traces side). The figure 13 shows the In figure 10 the magnetic field measured at 1 comparison between the envelop of maximum measured cm from the back side of the board near the batteries is radiated emissions by the original board and the new shown. one. 70 Hfield [dB a/m] 60 50 40 . 30 20 2 3 10 10 Frequency [MHz] Figure 10: Measured magnetic field at 1 cm from the back side of the board near the batteries It is important to notice that the same measurement made far away from the batteries on the back side of the board shows emissions due to edges effects that are much lower than the ones of the batteries themselves. Figure 11: Top and bottom view of the new board The radiation measured from the batteries is not produced by the batteries themselves but by the power and ground noise ( I noise) which is re-injected from the active components over the power and ground planes into the batteries, batteries connections and the power routed net. These measurements show that the radiation of the signal nets is present along the whole frequency range, whereas the radiation due to the batteries is restricted to the 100 MHz, 350 MHz frequency range. 5
  6. 6. Electric field in dB V/m Measurement Simulation Figure 12: EMI shield soldered to the ground Frequency in Hz metallization on the back of the board Figure 14: comparison between measurement and 40 simulation of the envelope of maximum radiated 35 Original board emissions by the new board Electric field in dB V/m 30 The envelope of measured radiation spectrum is well 25 reproduced by the simulation and the gap between 20 measurement and simulation is less than 4 dB until 300 New board MHz. After this frequency the gap increases. It can be 15 partially due to the validity limit of models used for 10 active components as an example has been shown in 5 figure 4. This limit could be extended by the means of Time Domain Reflectometer measurements made on a 0 10 8 10 9 sample of the devices. Such measurements permit to Frequency in Hz replace the value of the output capacitance of a driver that often suffers from uncertainty due to the dispersion Original board of this capacitance value by a Scattering parameter in Electric field in dB V/m time domain. This topic has been described in [7]. It is important to notice that only 15 minutes on a SUN New board ULTRA 1 workstation were required to simulate the currents and RE of all the 107 nets of the board that presents 177 components. IV. CONCLUSIONS Frequency in Hz Figure 13: Comparison between the envelopes of An analytical method to predict RE due to maximum measured radiated emissions by the original differential mode currents on PCB traces has been board and the new one presented, it takes into account effects of dielectric layer in the field calculation and does not need to discretise A comparison between the radiation spectrum of the old traces in short segments. It can simulate all traces of a and new implementation shows a significant difference PCB in a reasonable computation time. The comparison of emission levels in the frequency range between 70 between measurements and simulations made on an MHz and 350 MHz where the radiation of the batteries, active test board shows that all couplings involved in the batteries connections and the power routed net of the measurement set-up must be well understood to be able original board plays its role. This confirms the to explain the results of the comparison. Measurements hypothesis made above. in near field permitted to show the radiation of the We can now compare the simulation results already batteries, batteries connections and the power routed net obtained above to the measurement of radiated of the board. A new design of the board was emissions due to the new board as shown in figure 14: implemented to cut this spurious effect and permitted to validate the method used to predict the differential RE from PCB traces. But this experience also shows how much can be important to include the radiation from the power/ground distribution. The 4-ports models for 6
  7. 7. digital drivers used in PRESTO post-layout analysis tool allows the simulation of Simultaneous Switching Noise on power and ground pins of active components taking into account the actual loads. It is then possible to predict I noise on routed traces and on planes by the means the Transmission Line Method (TLM) . These features are being used through a european ESPRIT ESD project in order to predict the RE from the power distribution network of a board. A good modelling of components which can avoid uncertainty due to the dispersion of their electrical parameters is also necessary for reliable simulations, especially in the higher frequency ranges. The simulation software used for signal integrity and e.m. predictions as well the near field equipment used for measurements are included in the THRIS [8] environment. On the basis of results coming from this kind of experimental validation, further improvements of radiated field prediction will be included in the future release of this tool. REFERENCES [1] E. Leroux, F. Canavero, G. Vecchi, "Prediction of radiated electromagnetic emissions from PCB tracks based on Green dyadics," Proc. EURO- DAC, Brighton (UK), Sept. 18-22, 1995, pp. 354- 359. [2] C. Felsen, N. Marcaviz, Radiation and scattering of waves, Chp. 5, Prentice - Hall, Eaglewood Cliffs, 1973 [3] S. Forno, S. Rochel, "Advanced simulation and modeling techniques for hardware quality verification of digital systems," Proc. EURO-DAC, Grenoble (F), 1994 [4] EN55022, “Limits and methods of measurement of radio interference characteristics of information technology equipment”, 1985 [5] Monteath, Applications of the Electromagnetic Reciprocity Principle, Pergamon Press, 1973. [6] R. De Smedt, W. Temmerman, “Using an Active Test Board for Evaluation of EMC Tools on PCB Level”, COST Action 243 EMC Workshop, Paderborn (Germany), 7-8 April 1997 [7] E. Leroux “Conception et validation d’une méthode numérique hybride appliquée à la prédiction du rayonnement d’une carte électronique connectée à son câblage”, PhD thesis, University of Lille, june 1998 [8] P.Belforte, G.Guaschino, F.Maggioni, “A new system for the evaluation of telecom hardware robustness”, EMC’96 Roma 7