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Test methods for TWC in ISO 11452-4:2011 (E)


introduction

In modern society, in addition to being used as a means of transportation, motor vehicles also integrate more and more electronic components to bring more comfortable and convenient driving experience to drivers and passengers. However, the reliability of electronic components of motor vehicles, especially how to ensure that the various functions integrated in the vehicle can maintain normal operation in the harsh electromagnetic interference, has become a problem to achieve electromagnetic compatibility. The importance of the EMC test of motor vehicle parts is highlighted.

In the EMC anti-interference test of motor vehicle parts, BCI (Big Current Injection) anti-interference test, as a relatively classic test method, has been widely adopted by major automobile companies as a standard. Its advantages lie in good test repeatability, relatively severe test intensity and the convenience of not needing to destroy the wiring harness structure.

However, as an injection device for BCI testing, the current injection clamp has defects. Because its electrical structure is inductive, it presents high impedance at high frequencies, which greatly reduces the injection efficiency and cannot be tested at higher frequencies. In the current international standards, the frequency of the BCI method generally does not exceed 400 MHz, and in the enterprise standards, there are only a few Japanese manufacturers' specifications, such as the Honda standard that uses more than 400 MHz, but the effect is not satisfactory due to various reasons ideal.

In the 2011 edition of ISO 11452-4 (E), a new test equipment - Tubular Wave Coupler (TWC), namely tubular wave coupler, was proposed. Due to the capacitive nature of TWC, it has a good response in the high frequency part. The TWC injection method simulates the coupling of higher frequency radio frequency signals to the wire harness of the test sample, which can be regarded as the high frequency extension of BCI injection. And this new test method successfully extended the upper limit of the test frequency to 3 GHz.

This article will analyze the structure of TWC, checksum usage in detail, and compare the coupling efficiency of this device with BCI injection devices.

1. The structure of TWC

A tubular wave coupler is a coaxial structural system with two input terminals and a tubular coupling device in the middle. It consists of two inner and outer tubular electrodes with the same axis, which can couple electromagnetic interference to the wire harness of the sample under test like a current loop, and retain the advantage of not needing to destroy the structure of the sample wire harness. The inner and outer electrodes are filled with hard insulating material. One of the two input terminals is connected to the power amplifier as a signal input, and the other is connected to a 50 Ω terminal impedance. The inner core of the coaxial input terminal is connected to the inner electrode, the outer electrode is connected to the shielding layer, and the knob in the middle is used to lock the coupler tightly.

According to the diameter and length of the inner and outer electrodes, the instrument manufacturer will divide them into different models to match various wire harnesses of different sizes. The structure diagram of TWC is shown in Fig. 1.

Figure 1 Structure of TWC

2. The working principle of TWC

When the sample is tested, the tubular coupler is closed, the tested wire harness passes through the inner conductor of the TWC, the interference signal is injected into the coaxial input terminal, and the generated transverse electromagnetic wave is coupled to the TWC through the distributed capacitance between the tubular inner electrode and the wire harness. on the wiring harness.

It can be seen that compared with the inductive coupling of the current injection ring, TWC adopts the capacitive coupling with better high-frequency response, and its equivalent circuit is shown in Figure 2.

 

Figure 2 Equivalent circuit of TWC

3. TWC checksum test

3.1 Verification of TWC coupling coefficient

      The verification method of the TWC coupling coefficient is similar to that of the BCI current injection loop (see Figure 3), the difference is that the impedance type used is 150 Ω instead of 50 Ω when the current loop is not verified. At present, each instrument manufacturer will equip the two terminals of the calibration fixture with 50/150 Ω impedance adapters, so that testers can calibrate the TWC with a 50 Ω signal generator and terminal impedance.


Figure 3 Schematic diagram of calibration of TWC coupling coefficient

      When calibrating, the terminal impedance and the power probe should have a sufficiently high rated power within the frequency range used. If necessary, an attenuator can be added in front of the power probe to protect the probe. First of all, whether it is the BCI method or the TWC method, the test is realized by the substitution method, that is to say, the forward power is used as a parameter for verification and testing. In ISO11452-4:2011 (E), the test level of TWC is in the unit of power dBm, and the target value of the actual test is the power value injected into the fixture. The relationship between the power value and TWC and the fixture is as follows: If the fixture The coupling coefficient is recorded as F, and the absolute value of the data obtained by verification is recorded as |S21|, then the actual insertion loss I L of TWC is:

I L |dB = | S21| - F|dB

When testing the DUT, the actual forward power P forward required is:

Pforward|dBm = Ptest|dBm + IL|dB

Among them, P test |dBm is the target test level.

3.2 TWC test

The sample to be tested should use a wire harness with a length of 1700 mm (0 ~ 300). And placed on a non-conductive medium with a height of 50 mm together with the sample. Refer to the test plan and the actual installation environment of the sample to consider whether the enclosure and auxiliary equipment are grounded. For samples with multiple bundles of wire harnesses or multiple interfaces, the distance between the wire harness not placed in the TWC and the wire harness under test shall be kept at least 100 mm. Different from the three test positions of BCI, TWC only needs to be placed at a distance of (100±10) mm from the DUT and kept isolated from the ground plane. The RF input end is connected to the terminal close to the sample under test, and the other end is connected to a 50 Ω terminal load. The terminal load should be at least 200 mm away from the test wiring harness and also isolated from the ground plane. The test arrangement can refer to ISO 11452-4:2011(E), as shown in Figure 4.

Figure 4 TWC test layout

      After the test layout is completed, according to the requirements of ISO 11452-4:2011 (E), select the required target level power, and compensate the TWC coupling coefficient obtained by the previous calibration in the test software, and then the normal substitution method test can be performed.

4. Comparison of interference effects between TWC and BCI

In order to intuitively express the respective frequency responses of the BCI and TWC methods, as well as the selectivity of which coupling method to use when testing the frequencies covered at the same time, you can use the BCI injection ring and TWC respectively on the same fixture for comparison .

Adjust the output power of the power amplifier to a constant value of 5 W in the frequency range of 100 MHz~2 GHz, then measure the actual effective coupling power on the fixture, and then compare the coupling effects of the two methods. The comparison results are shown in Figure 5.

Figure 5 Comparison of TWC and BCI

It can be seen that in the low frequency range, the actual interference effect of the BCI injection loop is better than that of the TWC. But above 400 MHz, especially above 900 MHz, the power coupling efficiency of TWC is relatively stable, and it is obviously better than BCI.

Of course, in actual use, due to the differences between different types of injection equipment and the transmission impedance of the tested sample, the injection effect may be nonlinear. But in general, for the good response of TWC at higher frequencies, testers will have a greater expansion in the selectivity of EMC tests.

5 Conclusion

In the TWC calibration method described in this article, the generator and power meter can also be replaced by a network analyzer. Two points should be paid attention to during calibration: First, the 50/150 Ω conversion factor of the TWC calibration fixture must be considered. Second, the terminal load and attenuator used in the test must meet the frequency range used and have sufficient rated power.

Since the wavelength is very short at high frequencies, the injected interference energy can easily form a radio frequency loop to the outside world through the tested wire harness and the device under test, thereby affecting the actual interference effect. Therefore, whether this method can replace the free field remains to be studied. However, the excellent high-frequency coupling efficiency of TWC makes EMC testing of motor vehicle parts above 1 GHz no longer rely solely on the anechoic chamber method.

The TWC test method was first proposed in DaimlerChrysler's corporate standard DC-11225 and Volkswagen's TL82166, but because this method is not covered by international standards, it is hardly used by various laboratories. Now that ISO 11452-4:2011(E) has included it, it is very likely that in the near future, TWC will become one of the mainstream test methods for EMC testing of motor vehicle parts like BCI.

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