Author: Fred German

This FLO/EMC validation note addresses the prediction of the radiated emissions from multiple modules attached to a backplane. Three different configurations of the geometry are presented here. Justification for a simplified model of the printed circuit board is discussed and then results are presented and compared to measured data. The motivation for this validation note, full details on the geometries, and the measurements used for comparison were originally published in [1]. More detailed information about the structures modeled and additional data is available in this reference.

Model Details

In a real electronic system employing a module in backplane type configuration, the geometry is quite complex involving many geometrically small details such as connectors (and pins), multiple circuit elements on the PCB's, multiple layers in the boards, etc. While in theory this level of detail can be incorporated into a FLO/EMC model for simulation, it would lead to large model preparation and simulation times. A natural question to ask is "How much can the actual structure be simplified for modeling purposes?" In [1], the authors compared measurements for the radiation impedance of a real, fully populated module connected to a backplane to the radiation impedance of a simplified module and backplane configuration. The measurements demonstrated a surprisingly good level of agreement between the two sets of measurements. The simplified geometry treated both the modules and the backplane as conducting plates connected with a wire which served as a connector pin with an attached source. Further justification and discussion of this simplified model, which has been used for the basis of the FLO/EMC simulations presented here, are available in the cited reference.

FLO/EMC Simulation Three configurations are presented in this validation note corresponding to one, two and three modules connected to the backplane. The geometric models used for the FLO/EMC simulations are shown in Figure 1.   In each case the source is modeled as a single pin connection at the center of the central module. For the multiple module cases the outer module(s) are physically connected to the backplane structure. The source attached to the pin is modeled as a wideband 1-mV lumped source with a zero ohm impedance. Since the FLO/EMC solution is done in the time domain, an impulsive (in time) source signal is used in order to yield wideband results with a single simulation. In practice, the response to any desired excitation can be quickly and easily obtained as a post-processing step in FLO/EMC once this impulse response has been computed. For example, an engineer may wish to examine the time domain current flowing in the source connection pin due to a digital pulse train of a given frequency and rise/fall time. This data is quickly extracted from the impulse response with just a couple of mouse clicks using FLO/EMC's powerful post-processing facilities.

Figure 1: One, Two and Three Module Models used for FLO/EMC simulations.

Figure 2 compares the radiated power spectrum as computed by FLO/EMC (blue solid line) and as measured (red dashed line) in [1] for each of the configurations. The level of agreement is quite good with all of the resonances and relative levels being predicted accurately by FLO/EMC. It was concluded in [1] that the slight shifts in frequency for the higher frequency peaks in the multiple module cases is likely due to the non-perfect parallel alignment in the measured configurations. Keeping in mind that the same exact source was applied to the central module in each case, it is interesting to note that the addition of the non-driven modules to the configuration has a very significant effect on the magnitude of the radiated emissions. One of the advantages of simulation for EMC design lies in the ability to use the computed electromagnetic field data to visualize the radiation physics. This allows the designer to further his intuitive understanding of the complex interactions taking place around the structure. FLO/EMC allows the three-dimensional radiated power patterns to be computed in both the near and far field of the structure.

Figure 2: Radiated emissions (dBuW) as a function of frequency for one, two and three module configurations

Figures 3 and 4 show (for the three module configuration and at each of the first three frequency peaks) the far-field spherical radiated power pattern (Fig. 3) and the near-field cylindrical power distribution on a cylindrical surface located 3 meters from the structure (Fig 4). This type of data permits the analyst to determine directions of maximum radiation.

Fields and surface currents in the vicinity of and on the structure are also available from FLO/EMC. As an example, Figure 5 shows the electric field magnitude on two orthogonal planes cutting through the structure and the surface currents flowing on the structure at the three peak frequencies for the three module configuration.

Reference
[1] K. Li, M. Tassoudji, S. Poh, M. Tsuk, R. Shin and J. Kong, "FD-TD Analysis of Electromagnetic Radiation from Modules-on-Backplane Configurations", IEEE Trans. Electromag. Comp., Vol. 37, No. 3, Aug 1995, pp. 326-332.