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Research Article | Open Access
Xiaodong Pan, Guanghui Wei, Xinfu Lu, Lisi Fan, Xing Zhou, "Research on Wideband Differential-Mode Current Injection Testing Technique Based on Directional Coupling Device", International Journal of Antennas and Propagation, vol. 2014, Article ID 143068, 13 pages, 2014. https://doi.org/10.1155/2014/143068
Research on Wideband Differential-Mode Current Injection Testing Technique Based on Directional Coupling Device
This paper presents a new kind of differential-mode current injection test method. The equal response voltage on the cable or the antenna port of the equipment under test (EUT) is regarded as equivalent principle for radiation and injection test. The injection and radiation response analysis model and the injection voltage source extrapolation model in high intensity radiated field are established. The conditions of using differential-mode current injection as a substitute for radiation are confirmed. On the basis of the theoretical analysis, the function and structure design scheme of the directional coupling device is proposed. The implementation techniques for the single differential-mode current injection method (SDMCI) and the double differential-mode current injection method (DDMCI) are discussed in detail. The typical nonlinear response interconnected systems are selected as the EUT. The test results verify the validity of the SDMCI and DDMCI test methods.
Bulk current injection (BCI) is a kind of traditional EMC test method. Essentially, the interference current is injected into the cable of the equipment to substitute for radiation susceptibility test [1, 2]. As a kind of complementary method, the core of the BCI research is how to keep the equivalence with the radiation test method in broader application range and higher precision [3–6]. Although the traditional BCI method has been proposed for nearly half a century, there are still insufficiencies to substitute for the high intensity radiated field (HIRF) effects test. First, the application frequency range is limited. When the test frequency becomes higher, the injection and monitoring currents change to be sensitive to the position of the cable because of the standing wave, and the performance of the ferrite current probe descends severely. These factors make the testing precision and injection efficiency decline obviously [7–9]. Numerous studies show that when the test frequency is higher than 400 MHz, the present BCI method cannot satisfy the practical requirements [10, 11]. Second, the BCI method cannot accurately substitute for HIRF radiation effect test for nonlinear systems. At present, the BCI method is effective when the relation between the radiated field intensity and the induced current on the cable of the equipment is linear. Thus, the injection current substituted for HIRF could be extrapolated according to the linear corresponding relation. But the majority of equipment under the condition of HIRF is nonlinear. If the same test method is applied for the nonlinear system, it may cause considerable error because of dissatisfying the extrapolation condition. Third, the BCI method is a common-mode current injection test method. It means that the interference signal injected by the current probe is a common-mode signal [12–14]. This method cannot simulate the effects caused by the differential-mode interference signal received from the antenna. The application range of the BCI method is limited.
In conclusion, HIRF in large-scale test space is very difficult to simulate under the condition of laboratory. Meanwhile, there are still many insufficiencies for the traditional BCI method to carry out the injection susceptibility tests. Hence, our research team proposes a new kind of wideband differential-mode current injection test method for system level EMC test.
2. Theoretical Analysis Model
The theoretical equivalent principle between the injection and radiation test method is the equal response of the equipment [15–17]. The engineering equivalent principle is the same effects caused by the two test methods. If the response voltage or the induced current on the cable port of the equipment could be ensured to be equal, the equivalence of the two test methods can be achieved [18, 19]. In this paper, the equal response voltage on the cable port of the equipment is selected as the equivalent principle of the two test methods finally.
2.1. Equivalence Analysis Model between the Injection and Radiation Response
In this paper, the typical interconnected system is composed of two types of equipment and the interconnected cable. It is shown in Figure 1. It is assumed that equipment B is the EUT and equipment A is either the interconnected equipment or the receiving antenna. In order to calculate the radiation response voltage of the equipment B, the interconnected system is divided into two parts at the position of A-A′ reference plane. A-A′ is located at the input port of the equipment B. The left branch of A-A′ can be equivalent to the Thevenin equivalent circuit. It is shown in Figure 2(a), where is the input impedance of the left branch of A-A′ and is the open-circuit voltage.
According to the transmission line theory, the input impedance can be calculated as follows: where is the reflection coefficient of the equipment A, is the characteristic impedance of the transmission line, is the transmission line length between A-A′ and the equipment A, and is the propagation constant.
The open-circuit voltage can be calculated with the BLT equation [20–22]. Assuming that A-A′ port is open, that is, , the open-circuit voltage caused by the transmission line coupling can be calculated as follows: where and are the source parameters in the BLT equation. If the equipment A is a receiving antenna, it can be regarded as a lumped voltage source . The open-circuit voltage caused by can be calculated as follows:
In (2) and (3), there is a linear relation between , , , and the radiated electric field intensity [20, 21]. We define that the linear transfer function between and is . Hence, the open-circuit voltage can be simplified as follows:
Therefore, in Figure 2(a), the radiation response on the impedance can be derived as follows: