What are the methods of oscilloscope testing for EMI radiated interference

The electromagnetic waves emitted by mobile phones, Bluetooth headsets, satellite radio, AM/FM radio, wireless Internet, radar and countless other potential electromagnetic interference sources are mixed in the real world. In order to ensure that the electronic components in the vehicle are still stable and effective, they need to be EMI tested in a controlled environment.

The radiation immunity room is an ideal fully sealed EMI test environment for conducting space, because it can completely control the frequency, direction and wavelength of the electromagnetic field generated in the space. In addition, because the electromagnetic field cannot enter the enclosed space, the vehicle components tested in the anti-interference room can receive accurate and highly controllable electromagnetic waves during the test process. At the same time, the electromagnetic wave cannot leave the interference chamber. The measuring instruments used for testing and the engineers controlling in the anti-interference room can avoid the damage of strong electromagnetic waves generated in the interference room.

Modern cars contain hundreds of electronic circuits to realize various functions related to safety, entertainment and comfort. These automotive electronic components, also known as electronic control units (ECUs), must comply with EMI interference standards.

Configuration of electromagnetic interference room

In the electromagnetic interference room, typical device-level anti-interference test settings include the measured electronic control unit (ECU), wire harness, and simulator, package or equivalent electronic load, and a series of peripheral devices to represent the interface of the automobile electronic control unit (ECU); The transmission and reception antennas are used to generate electromagnetic waves with high field strength; The mode tuner is placed in the interference chamber to change the geometry of the space to create the electromagnetic field effect required for the test. The vehicle electronic control unit (ECU) operates in preset mode and is exposed to electromagnetic interference field.

In the process of contacting the interference source, the vehicle electronic control unit (ECU) response is monitored to verify whether it exceeds the allowable capacity by monitoring the vehicle electronic control unit (ECU). For most people, RF needs to gradually adjust the range of interference sources to determine the anti-interference threshold of the equipment until the vehicle electronic control unit (ECU) determines the method of functional deviation.

The tested automobile electronic control unit (ECU) shall comply with the requirements of ISO (International Organization for Standardization) rules, automobile manufacturers and automobile electronic control unit (ECU) parts suppliers. Because of the slight difference in the anti-interference ability of each electronic component to the electromagnetic field, the performance deviation between the detection and the acceptable standard, and the determination of when these values exceed the tasks and responsibilities of the EMI test engineer in the test plan rules.

During the EMI test, the way to determine whether the vehicle electronic control unit (ECU) is still working properly is to let it output its working status through the ECU output port such as CAN bus. ECU output also includes analog sensor output and pulse width modulation output to drive actuator.

Field strength and consideration

The field intensity and frequency type used in the radiated (RF) immunity test described in ISO/IEC 61000-4-21 is a typical example. It uses a reverberation chamber containing a mechanical mode tuner. When sufficient tuner positions are obtained at a given test frequency, the available space of the reverberation chamber will generate a test frequency range of 0.4 to 3 GHz, and the field strength will be as high as 200 V/m (CM and AM) and 600 V/m (radar pulse) uniform fields.

As another example, in the anti-interference test of ISO11452-4RF, an embedded current injection probe is used to induce RF current to enter the DUT frequency range of 1-400MHz, and the level range is tens to hundreds of mA, which can create a strong enough site near the test platform and affect the operation of unshielded equipment. This test environment avoids the direct connection from the test instrument to the test setup.

One challenge is that the output data of the vehicle electronic control unit (ECU) comes from an enclosed space isolated from the test area. The test instruments and testers are located outside the enclosed space, so there must be a method to transmit the data generated in the enclosed space to the outside of the enclosed space for analysis. Because the traditional cable, such as BNC or SMA cable itself is conductive, it is easy to be affected by the interference of indoor electromagnetic waves. Therefore, it is necessary to transmit the signals sent by the optical transmitting and receiving unit and the optical fiber interference indoor ECU to the test equipment located outside the interference room. The optical fiber is non-conductive, so it will not be affected by the electromagnetic field in the interference room. In order to connect the cable from the interference chamber to the test equipment, the waveguide is used to output the optical signal at the boundary of the interference chamber, thus allowing the interference chamber to enter the ECU. During the signal output, it is still completely closed. The frequency of the optical waveguide is higher than the frequency range of the interference chamber test, so it will not interfere with the environment created in the interference chamber.

Setting of electromagnetic interference test equipment

Figure 1 below is a picture of the actual setting taken in the space of the closed interference room (when the transmission antenna is closed). The mode tuner is located on the right side of the interference room, and a CAN bus optical fiber transmitter is placed on the foam platform on the left side of the interference room, with a relative dielectric constant of 1.4. It is located in the available space of the reverberation room. The optical fiber transmitter will use the ECU to convert the output signal into light, enter the optical fiber, and leave the reverberation chamber through the waveguide without RF interference. The ECU used for testing, and the transmitting and receiving antennas are also located in the reverberation room, which are not shown in this figure.

Figure 1 Reverberation chamber with mode tuner (right) and optical fiber transmitter (left). Antenna and antenna ECU are not shown in the figure, but also exist.

The typical test method is to collect the signals reaching the reverberation room through the data acquisition equipment. The user needs to customize the software to determine whether the CAN bus signal, sensor signal, or PWM output from the ECU meets the specific requirements. The software development time and cost for describing all test requirements in the test plan will be very long and expensive, because there are many signals to be tested, and there are many test standards. The field of using oscilloscope EMI testing is a relatively less widely explored method. You can place an array oscilloscope outside the interference room and use multiple oscilloscopes for real-time analysis. Because the oscilloscope standard is equipped with template test and parameter gate limit test capabilities, it can directly perform many test requirements without spending a lot of software development time.

In Figure 2, the copper door to the EMC interference room is located on the right side of the test platform. On the left side, the optical signal in the orange optical fiber is converted into electrical signal through the function test result BNC input cable on the oscilloscope channel.

Figure 2 Dynamic analysis of anti-interference data in EMC oscilloscope array

The waveform template in the oscilloscope is used to analyze the waveform shape relative to the predefined consistency requirements. According to the functional standard of the measured signal, the size of the template can be automatically adjusted by the computer during the test.

In Figure 3, 4 and 5 below, an oscilloscope is used to monitor the analog ECU output. Because of the confidentiality of using analog data, it can be very close to the typical observation of ECU output. Channels 1 and 2 display the signals used by analog PWM to control the output drive actuator signal. The analog actuator signal is captured on channel 3, and the CAN captures the separation signal on channel 4.

EMC conformance test

Figure 3 below shows the data signals collected by the oscilloscope after the template is closed. The waveform shape of each signal can be clearly displayed and observed. The oscilloscope is triggered based on the edge of channel 2 and captures all four waveforms at the same time.

Figure 3 Simulated ECU output signal includes PWM signal of channel 1 and 2, executive driver output signal of channel 3, and CAN separation signal of channel 4

In Figure 4, the template test is opened. The shape of the template can be used to verify the high signal level, low signal level, frequency, duty cycle and other specifications and standards described in the test plan. The thickness of the template shows the specified capacity band near the nominal value. The template verifies whether each waveform collected deviates from the nominal value or the percentage of the nominal value. In this example, each waveform meets all test criteria. It is particularly important that the oscilloscope can use predefined template standards to continuously trigger edges and continuously monitor whether there are errors. The standard triggered by the oscilloscope appears at the edge of channel 2 and can be set to identify and archive errors.

Figure 4 Simulated ECU output signal, PWM signal displayed in channels 1 and 2, actuator drive output signal displayed in channel 3, and CAN separation signal displayed in channel 4 are all within the defined tolerance template and pass the template test standard

In Figure 5, the influence of EMI interference in the ECU leads to the failure of the template test of amplitude modulation, amplitude reduction, duty cycle and frequency change PWM output signal driven by the actuator. Unlike the other three signals, the CAN did not receive the EMI impact of the separated signal and continued to pass the test. This template testing method allows rapid testing of various standards at the same time.

Figure 5 When EMI is applied, the simulated ECU output PWM signal and actuator drive output signal cannot pass the template test, and the oscilloscope will prompt the operator that there is an error

In addition to the waveform template test, Pass/Fail limit test is also applicable to parameters to ensure whether the measured value results meet the specified value. As shown in Figure 5, the oscilloscope displays three failures using the red "Fail" message under the test standard. When the template test or parameter limit test fails, the oscilloscope can also automatically perform some actions, such as saving waveform data for direct comparison and archiving, saving screen images for archiving and evaluation, generating pulse signals for auxiliary automatic test, and sending a warning to the test operator.

conclusion

Although in the anti-interference test, the oscilloscope can be quickly executed to determine the EMC deviation parameter measurement, due to the lack of attention and sufficient number of oscilloscope channels in the past, the oscilloscope is often ignored in the anti-interference test. Generally, the analysis of parameter results requires the development of user-defined software, and may require users to design their own hardware - both of which are time-consuming and expensive. However, many oscilloscopes with pass/fail template and parameter limit test capability can be directly used to analyze the sensor output of each component.

In the anti-interference test, the oscilloscope array is the most cost-effective method to verify whether the sensor output meets the requirements, because most functions can use pass/fail to complete the template and parameter limit test functions in the oscilloscope. Compared with the cost of developing your own data acquisition software, the EMI deviation test is also strict. EMC engineers can save a lot of time and energy.