2.5 Modeling of the common mode coupling from ICs to the cavity field

An emission simulation of a device has also to consider ICs and active components. This requires IC emission models which can be efficiently integrated into the emission modeling technique of the device. The evolution of IC standard modeling methods for RF emission in Table 2.2 was presented by [3] in May 2008.

Table 2.2: Evolution of IC standard modelling methods for RF emission [3].

Bandwidth	Type	2005	2010	2015	2020
Below 3 GHz	Conducted	Industrial use (ICEM)
	Radiated	Solution exists (ICEM dipole)	Industrial use
3-10 GHz	Conducted	NOT known	Solution exists	Industrial use
	Radiated	NOT known	Solution exists	Industrial use
10-40 GHz	Conducted	NOT known	NOT known	Solution exists	Industrial use
	Radiated	NOT known	NOT known	Solution exists	Industrial use

According to Table 2.2, there are currently no IC standard modeling methods available for the frequency range above 3GHz.
The table indicates that a solution for radiated emission IC modeling exists below 3GHz. This method models the radiated field of an IC by dipoles placed along the interconnects of the lead frame. The dipole moments are modeled by the interconnect currents from a network simulation with an ICEM model [51]. Therefore, the modeling method requires only simulations, but no measurements. Although a comparison of results from this method with three-dimensional full wave simulation demonstrated good agreements for canonical structures, a comparison of IC model results with measurements showed some significant deviations of about 6dB [52]. The main reasons for the deviations were reported by [52] to be inaccuracies of the utilized geometrical package model and uncertainties of the currents on the package. There are currently no results with increased accuracy from this modeling approach in the literature.
Another approach for modeling the near field of an IC has been presented by [6]. This method models the package with the three-dimensional full wave simulation program HFSS^® and introduces excitation ports at the chip side and on the PCB side of the package. The ports are excited by frequency domain excitations, obtained by FFT of time domain network simulations with an ICEM model. A good agreement of simulation results to measurement results has been achieved by this method on a 16bit microcontroller with a 144pin TQFP package.
Both methods of [6] and [52] require three-dimensional full wave simulation for the consideration of an enclosure. They do not provide an explicit relation from the IC model sources to the common mode coupling from the IC to the enclosure. This makes optimization inefficient and prevents an integration into the cavity device model developed in the course of this dissertation and predictive simulations of mTEM measurement results.
The common mode coupling is also the coupling mechanism from an IC to a mTEM cell, which is evident, because the magnetic and the electric common mode coupling moments of an IC can be obtained by mTEM measurements [53]. Main standardized EMC measurements for ICs are based on mTEM cells [54]. This indicates the significance of the common mode coupling. The coupling of the IC to the mTEM cell is also modeled in [6]. However, the results of this modeling show deviations from the measurement results above 300MHz and the modeling is carried out by using lumped coupling capacitors which have no relation to the previously mentioned near field model.
This work models the common mode coupling from a trace on a PCB to the parallel-plane cavity field between the PCB ground plane and a metallic enclosure cover by an analytical formulation. Only the vertical current segments of the trace couple to the cavity. Therefore, the coupling can be described by the currents on the two trace ends (source and load positions), which are obtained from a network simulation of the trace, the load, and the source. The common mode coupling of an IC can be modeled by the same approach. The currents on the IC package can be obtained by network simulation with an ICEM model, as already presented by [6] and [52]. With these currents on the vertical elements of the package, the common mode coupling can be modeled accurately up to high cavity resonance frequencies in the GHz range. The model provides explicit information about the influence of every individual geometric package part on the overall common mode coupling of the device. This information enables efficient EMC optimization of both the package geometry and the part placement inside an enclosure.

C. Poschalko: The Simulation of Emission from Printed Circuit Boards under a Metallic Cover