What do you need to pay attention to in EMC testing in the early stage of product design?

With the improvement of product complexity and density and the continuous shortening of design cycle, the problem that EMC can be solved at the later stage of design cycle (EMC) becomes more and more impractical. At higher frequencies, you usually use it to calculate EMC's rules of thumb no longer apply. You may easily misuse these rules of thumb. As a result, 70%~90% of new designs fail to pass the first EMC test, making the cost of later redesign very high. If the manufacturer delays the delivery date, the lost cost of sales will be even greater. In order to identify and solve problems at a lower cost, designers should consider adopting collaborative style EMC simulation based on conceptual analysis as early as possible in the design process.

Higher clock speed will increase the difficulty of meeting the requirements of electromagnetic compatibility. In the field of gigahertz, increasing the number of shell resonance will enhance the electromagnetic radiation, making the aperture and gap become a problem; The special integrated circuit (ASIC) radiator will also increase electromagnetic radiation. In addition, regulatory agencies are formulating rules to ensure compliance at an increasing frequency. In addition, when engineers plan to design radiators in the system, the trend of integrated wireless functions (such as Wi-Fi, Bluetooth, WiMax, UWB) poses further challenges.

Traditional EMC design method

Under normal circumstances, electrical hardware designers and mechanical designers act independently when considering electromagnetic compatibility issues, and there is no or little communication between them. They often use experience rules in the design process, hoping that these rules are sufficient to meet the equipment requirements they design. When the design reaches high frequency, leading to test failure, many EMC design rules have become outdated.

At the end of the design phase, the designer makes a prototype and tests the electromagnetic compatibility. When electromagnetic compatibility is considered too late in design, various EMC problems often occur in this process. Expensive design fixes are often the only viable option. When the design is transferred from the system conceptual design to the specific design to the verification stage, the design modification usually increases by more than one order of magnitude. Therefore, it only costs $100 to modify the design in the conceptual design phase, and it may cost hundreds of thousands of dollars in the test phase, not to mention the negative impact on the market time.

Challenges of EMC simulation

In order to pass the EMC test in the laboratory and ensure the timely delivery within the budget, it is necessary to take EMC design as an integral part of the product production cycle. Designers can use Maxwell's 3D solution to achieve this goal. Maxwell equation is a simple mathematical expression of electromagnetic interaction. However, electromagnetic compatibility simulation is not a common problem in other fields of computational electromagnetics.

Typical EMC issues are related to enclosures, and the impact of enclosures on EMC is greater than that of slots, holes, and cables where EMC is important for performance. Accurate modeling requires that the model contain both large and small details. This requirement leads to a large vertical and horizontal ratio (ratio of maximum feature size to minimum feature size), which requires the use of fine mesh to analyze the most detailed details. Compression model technology allows you to include large and small structures in the simulation without too much simulation time.

Another issue is that you must complete EMC features over a very wide frequency range. The time required to calculate the electromagnetic field at each sampling frequency may be daunting. For example, the transmission method (TLM) time domain method can use broadband excitation to calculate the electromagnetic field in the time domain, so as to obtain the data of the entire frequency band in the simulation process. The space is divided into elements modeled at the intersection of orthogonal transmission lines. Voltage pulses are emitted and scattered in each cell. You can calculate the electric field and magnetic field according to the voltage and current on the transmission line.

EMC simulation can get accurate results. Figure 1 compares the calculated radiation power (red) and measured radiation power (blue) of the three modules installed on a backplane (block) (Reference 1). The calculated value of radiation power is 1nw as the reference, and the unit is dB. You can attribute the small difference in the resonant peak position of multiple module configurations to the difficulty in accurately aligning multiple modules during the measurement process. It is worth noting that since the input power of the three configurations is the same, the difference in the resonance peak and amplitude of the radiated power is only caused by different system layouts.

Potential application areas

EMC simulation can be used to detect the influence of radiation distribution of components and subsystems, such as radiator grounding, on frequency characteristics, or evaluate the influence of grounding technology, radiator shape and other factors. In addition, you can also compare the shielding effect of different vent sizes and shapes and metal thickness. In the latest application in this field, one research work is to evaluate the shielding effect by placing two plates with small back-to-back spacing.

In order to calculate the broadband shielding effect and broadband electromagnetic radiation optimization, EMC calculates the broadband shielding effect, broadband electromagnetic radiation, 33-D far-field radiation map, which is used to simulate the turntable measurement and visualization of cylindrical near-field electromagnetic radiation, and is helpful to determine the current and electromagnetic field distribution at the electromagnetic and hot spots. Typical system-level EMC applications include: shell design to ensure maximum shielding effect, distribution position of components inside the shell, effect evaluation and calculation of cable coupling inside and outside the EMC system, and detection of cable radiation effect. EMC simulation also helps to find the mechanism of harmful electromagnetic waves in the enclosure and subsystem, such as cavity resonance, perforation, electromagnetic radiation at slots, connectors and other seat openings, conducting radiation through cables, coupling with radiators and other elements, optical elements and displays, LED intrinsic parasitic waveguides and other elements installed on the base.

Influence of joint type on EMC

You can use a simple and fast shell model to design connector configurations. Figure 2 evaluates the radiation generated by butt joints and the radiation generated by overlapping shell joints. Through the relative shielding level, engineers can evaluate EMC's budget and the cost of implementing a specific design configuration based on the shell. Adding internal components in the simulation process has a small impact on the simulation time, so designers can easily evaluate the shielding effect of connectors, resulting in slot resonant coupling, cavity mode and interaction with internal structures. The design rules of slot leakage do not apply to the above factors, which will lead to costly over-design and arrears design.