How to conduct EMI interference test in the anti-interference room

introduction
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 car are still robust and effective , They need to be tested for EMI interference in a controlled environment.
The radiation immunity room is a completely sealed conduction space, which is an ideal EMI test environment because it can fully control the frequency, direction and wavelength of the electromagnetic field generated in the space. Moreover, because the electromagnetic field cannot enter the confined space, the auto parts tested in the interference immunity room can receive and highly controllable electromagnetic waves during the test. At the same time, electromagnetic waves cannot leave the interference room, and the measuring instruments used for testing and engineers operating outside the interference-free room can be protected from the strong electromagnetic waves generated in the interference room.
Hyundai cars contain hundreds of electronic circuits to implement various functions related to safety, entertainment and comfort. These automotive electronic components, also known as electronic control units (ECU), must meet strict EMI interference standards.
electromagnetic interference room configuration
Inside the electromagnetic interference room, a typical device-level anti-interference test setup includes the electronic control unit (ECU) under test, wiring harness, and a simulator containing actual or equivalent electronic loads, as well as a series of peripherals to represent automotive electronic control The interface of the unit (ECU); the transmitting and receiving antennas are used to generate high-field electromagnetic waves; and the mode tuner is placed in the interference room to change the geometric size of the space to create the electromagnetic field effect required in the test. The automotive electronic control unit (ECU) operates in a preset mode and is exposed to electromagnetic interference fields.
In the process of being exposed to the interference source, the response of the vehicle electronic control unit (ECU) is monitored to verify whether it exceeds the allowable tolerance. For most RF interference tests, the deviation detection from the plan requires the determination of the anti-interference threshold of the device, which is determined by gradually adjusting the amplitude of the interference source until the function of the automotive electronic control unit (ECU) deviates.
The automotive electronic control unit (ECU) under test needs to comply with strict ISO (International Organization for Standardization) rules, as well as the requirements reached between the automotive manufacturer and the automotive electronic control unit (ECU) component supplier. Because each electronic component has a slight difference in the anti-interference ability of the electromagnetic field, it is the task and responsibility of the EMI test engineer to detect the performance deviation between the acceptable standard and determine when these values exceed the test plan rules.
The way to determine whether the automotive electronic control unit (ECU) is still working normally during the EMI test is to let it output its working status through the output port of the ECU, such as the CAN bus. Other ECU output also includes analog sensor output, and pulse width modulation output to drive the actuator.
field strength and consideration
The field strength and frequency type used in the radiated RF immunity test described in ISO/IEC61000-4-21 is a typical example. It uses a reverberation room containing a mechanical mode tuner. When in a given test When enough tuner positions are obtained under the frequency, the available space of the reverberation room produces a uniform test frequency range of 0.4~3GHz, field strength up to 200 V/m (CM and AM) and 600 V/m (radar pulse) field.
As another example, the conducted RF anti-interference test described in ISO 11452-4 uses a clamped current injection probe to induce RF current into the DUT harness. The frequency range is 1-400MHz, and the level range is tens to several. 100 mA, which can create a strong enough field near the test platform to affect the operation of unshielded equipment. Such a test environment avoids the direct connection of the test instrument to the test setup.
One challenge is that the output data of the automotive electronic control unit (ECU) comes from an enclosed space, which is isolated from the test area. The test instruments and test personnel are located outside the enclosed space. Therefore, there must be a way to generate the enclosed space. The data is transferred outside the enclosed space for analysis. Because traditional cables such as BNC or SMA cables are conductive and susceptible to electromagnetic waves from the interior of the interference room, the optical transmitting and receiving unit and optical fiber need to be used to transmit the signals from the ECU inside the interference room. Transmit to the test equipment located outside the interference room. The optical fiber is a non-conductor so it will not be affected by the electromagnetic field in the interference room. In order to connect the cables from the interior of the interference room to the test equipment, the waveguide at the boundary of the interference room is used to output optical signals, allowing the interference room to remain completely enclosed when outputting the ECU signal. The fiber optic waveguide has a high-pass cutoff frequency, which is higher than the frequency range tested in the interference room, so it will not interfere with the environment created in the interference room.
A picture taken in the space of a closed interference room (when the transmitting antenna is turned off) in the actual setting for offset detection in the anti-interference test. The mode tuner is located on the right side of the interference room, and there is a CAN bus fiber optic transmitter on the left side of the interference room, which is placed on the foam platform. The relative dielectric constant of the platform is <1.4 and is located in the available space of the reverberation room. The optical fiber transmitter converts the output signal of the ECU into light, enters the optical fiber protected from radio frequency interference, and leaves the reverberation room from a position close to the floor through a waveguide. The ECU used for testing, as well as the transmitting and receiving antennas are also located inside the reverberation chamber and are not shown in this figure.
A reverberation room equipped with a mode tuner (right) and a fiber optic transmitter (left). The antenna and ECU are not shown in the figure, but they do exist.
The typical test method is that the signal arriving outside the reverberation room is collected by the data acquisition device, and user-defined software is required to determine whether the CAN bus signal, sensor signal, or PWM output from the ECU meets the specific requirements. Because there are many signals to be tested, and there are many test standards, the software development time and cost to describe all the test requirements in the test plan will be very long and expensive. Using oscilloscopes in the field of EMI testing is a relatively unexplored method. This method can place an array of oscilloscopes outside the interference room and use multiple oscilloscopes for real-time analysis. Because the oscilloscope has standard mask test and parameter threshold test capabilities, it can directly perform many test requirements without spending a lot of software development time.
The copper-colored door leading to the outside of the EMC interference chamber is located on the right side of the test platform. On the left, the optical signal in the orange optical fiber carrying the functional test result is converted into an electrical signal and then input to the oscilloscope channel through the BNC cable.
The waveform template in the oscilloscope array oscilloscope used for dynamic analysis of anti-interference data outside EMC interference is used to analyze the waveform shape relative to the pre-defined conformance requirements. The size of the template depends on the functional standard of the signal under test, and can be automatically adjusted by a computer during the test process.
In the following figures 3, 4, and 5, an oscilloscope is being used to monitor the output of the simulated ECU. In view of the confidentiality reasons, the simulation data can be used to observe the output of a typical ECU very closely. Channel 1 and Channel 2 show the simulated PWM signal, which is used to control an output drive actuator signal. The simulated actuator signal is captured on channel 3, and the CAN separation signal is captured on channel 4.
EMC compliance test
Figure 3 below shows the data signal collected by the oscilloscope after closing the template. The waveform shape of each signal can be clearly displayed and observed. The oscilloscope is based on the edge trigger of channel 2, and all 4 waveforms are captured at the same time.

Figure 3: The simulated ECU output signal includes the PWM signals of channels 1 and 2, the actuator output signal of channel 3, and the CAN separation signal of channel 4 In Figure 4 below, the mask test is turned on. The shape of the template can be used to verify signal high level, signal low level, frequency, duty cycle, and other specifications described in the test plan. The thickness of the template shows the specified tolerance band around the nominal value. And the template verifies whether each acquired waveform deviates from the defined nominal value or a percentage of the nominal value. In this example, each waveform meets all test standards. It is especially important that the oscilloscope can use the pre-defined template standard to continuously trigger the edge and continuously monitor whether there are errors. The standard for oscilloscope triggering is to appear on an edge of channel 2. The oscilloscope can be set up to identify and archive errors every time.

Figure 4: The simulated ECU output signal, the PWM signal displayed on channels 1 and 2, the actuator drive output signal displayed on channel 3, and the CAN separation signal displayed on channel 4 are all within the defined tolerance template and pass the template test standard. Figure 5 In the simulation, the simulated ECU was affected by the EMI in the interference room, resulting in amplitude modulation, reduced amplitude, and changes in duty cycle and frequency, which made the template test of the PWM signal and the actuator drive output signal fail. Unlike the other three signals, the CAN separation signal is not affected by EMI and continues to pass the test. This type of mask testing method allows rapid testing of multiple standards at the same time.

Figure 5: When EMI is applied, neither the simulated ECU output PWM signal nor the actuator drive output signal can pass the mask test, and the oscilloscope will prompt the operator that there is an error. In addition to the waveform mask test, the Pass/Fail limit test also applies to parameters. It can be used to ensure whether the measured value result meets a specific specified value. As shown in the screen graph in Figure 5, the oscilloscope uses red "Fail" information below the test standard to indicate three failures. When a mask test or parameter limit test failure event occurs, 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, and generating a pulse signal for auxiliary automated testing. And a warning is issued to notify the test operator that there is a problem.
  in conclusion
Although in the anti-interference test, the oscilloscope can quickly perform the parameter measurement used to determine the EMC deviation, but 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. Typically, the analysis of parameter results requires the development of user-defined design software, and it is likely that users need to design their own hardware-both of which are time-consuming and expensive. However, the combination of multiple oscilloscopes with pass/fail templates and parameter limit test capabilities can be directly used to analyze the sensor output of each component.
In the anti-interference test, the oscilloscope array is used to verify whether the sensor output meets the requirements of the potential cost-effective method, because most of the functions can be completed using the pass/fail template and parameter limit test functions that are already available in the oscilloscope. At the cost of developing data acquisition software by ourselves to perform the same strict EMI deviation test, EMC engineers can save a lot of time and energy.