The Impact of Card Grounding on EMC Emissions
A recent study by E-Mead Consulting Ltd. focused on paddle-board cards connected to the backplane of an equipment enclosure through connector pins. The study addressed modeling the effectiveness of the grounding pins using the concepts of partial inductance.
The FLO/EMC simulation software from Flomerics was used to investigate the impact of grounding, bonding and inductance in real-world designs that could not be easily be analyzed by "pure" mathematics. The basic arrangement of the study showed that simply changing from one to four grounding pins in a "coaxial" topography reduced emissions by more than 20 dB.
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Figure 1. In this example, changing from one to four grounding pins reduced emissions by more than 20 dB.
The results of this research study are equally applicable to daughterboards as paddle-boards. It is often necessary to pass a clock signal in the 10 to 50 MHz range to one of these boards, with one pin typically carrying the clock signal and the connector ground pins carrying the return. The L/di/dt voltage drop across the connector ground pins excites the paddle-board and associated cable. This assembly then acts like an aerial that can potentially radiate beyond FCC (Federal Communications Commission) Class A limits.
The study began by using the concepts of partial self- and mutual inductance to demonstrate the correlation between radiated emissions and the RF voltage drop across the ground pins. A typical daughter board with a 20 MHz clock routed from the motherboard was used as a reference. The partial self and partial mutual inductance of combinations of parallel conductors with the distance between them of less than their length and between ten and 100 times their radius was calculated. The partial inductance concept was then used to compute the "net inductance" of the ground wires or pins. (The "L" in L/di/dt).
FLO/EMC was then used to model the effectiveness of the grounding pins using the concepts of partial inductance. The advantages of FLO/EMC over the "first principles" approach are that it is capable of modeling geometries of nearly any complexity and determining the real world quantitative impact of grounding arrangements while shielding the user from the complex mathematical analysis involved. FLO/EMC makes it possible to identify EMC (Electro-Magnetic Compatibility) design issues early in the design cycle, well before physical prototypes are built. The simulation tool uses the TLM (Transmission Line Matrix) method for solving Maxwell's equations, solving for all frequencies of interest in a single calculation, therefore capturing the full broadband response of the system in one simulation cycle.
FLO/EMC was used to simulate five different grounding configurations: a reference design with a single ground pin, models 2 and 3 which both have two ground pins, model 4 which is a three ground pin arrangement and model 5 which has four ground pins. The results matched the theoretical calculations very closely. When comparing the different grounding configurations, the simulation showed major differences between them from an EMC standpoint. Models 2, 3, 4 and 5 generated 7 dB, 9 dB, 13 dB and 20 dB less emissions, respectively, than the reference design (see Figure 1).
This study demonstrates how EMC simulation can substantially reduce emissions in real-world applications by allowing engineers to investigate many more alternatives that would be practical with physical prototypes.
The plan is to build a hardware demonstrator to investigate more complex grounding arrangements, and to study the effect of paddle-board emissions with and without cables attached. FLO/EMC will be used to provide correlation with the laboratory measurements for the radiated emissions signature and to investigate the geometric dependencies of different grounding arrangements and PCB structure dimensions.
For more information about FLO/EMC, visit http://www.flomerics.com