Introduction
In order to ensure that electronic components used within automobiles are robust and will continue to function correctly in a real-world environment increasingly filled with electromagnetic (EM) waves originating from cell phones, Bluetooth headsets, satellite radio, AM/FM radio, wireless internet, RADAR, and countless other potential sources of electromagnetic interference (EMI), they must first be tested for EMI immunity within a controlled environment.
A radiated immunity chamber is a completely sealed conductive room which provides an ideal environment for EMI testing, because it allows for complete control over the frequency, direction, and strength of electromagnetic fields generated within the room. Since EM fields cannot enter the sealed chamber, the automotive components tested in the chamber receive precise and highly controlled EM waves during testing. Also, since electromagnetic waves cannot leave the chamber either, the measurement instruments used during testing and the engineers operating the test equipment outside of the chamber are safely protected from the strong EM waves occurring inside of the chamber.
Modern automobiles contain hundreds of electronic circuits which perform various functions related to safety, entertainment, and comfort. These automotive electronic components, also known as electronic control units (ECUs) must meet rigorous standards for EMI immunity.
EMI Chamber Configuration
Inside of the EMI chamber, a typical component-level immunity test setup consists of the ECU under test, a wire harness, and a load simulator containing actual or electrically equivalent loads, as well as other periphery required to represent the ECU’s interface with the vehicle. Transmit and receive antennas are used to generate standing waves having high field strengths, and a mode tuner is also placed into the chamber to alter the geometry of the room in order to create EM field effects during testing. The ECU is then exercised in predetermined modes of operation and exposed to the EM disturbance fields.
During exposure to the disturbance, the ECU's functions are monitored for responses exceeding allowable tolerances. For most RF immunity tests, detection of a deviation from plan requires the determination of the device’s immunity threshold, a process by which the magnitude of the disturbance is adjusted in fine increments until a deviation in the ECU function occurs.
ECUs under test typically must comply both with strict ISO (International Organization for Standardization) guidelines, and additionally with requirements agreed upon between the automobile manufacturer and the ECU component supplier. Because each electronic part varies slightly in its immunity to electromagnetic fields, the task of detecting performance deviations from accepted criteria, and determining when those values fall outside of the test plan limits becomes the responsibility of the test engineers performing the EMI tests.
The method to determine whether an ECU is still functioning correctly during EMI testing is for the ECU to output its functional state through an output port on the ECU, such as via a CAN bus output. Other example ECU outputs include analog sensor outputs, and pulse width modulated (PWM) outputs which drive an actuator.
Field Strengths and Considerations
As an example of the types of frequencies and field strengths typically used during testing, consider the radiated RF immunity test described in ISO/IEC 61000-4-21, which utilizes a reverberant chamber containing a mechanical mode tuner that, when a sufficient number of tuner positions have been obtained at a given test frequency, produces a statistically uniform field within the useable volume of the chamber with test frequencies ranging from 0.4 to 3 GHz and field strengths as high as 200 V/m (CW and AM) and 600 V/m (radar pulses).
As another example, the conducted RF immunity test described in ISO 11452-4 utilizes a clamp-on current injection probe to induce RF current into the DUT harness in the frequency range of 1 - 400 MHz at levels ranging from tens to hundreds of mA, creating fields near the test bench at levels high enough to effect operation of non-shielded equipment. Test environments such as these prohibit direct connection of measurement instrumentation to the test setup.
Given the challenge that the ECU output data is being generated from within a sealed chamber that is isolated from the test area, and measurement instruments and test personnel are located outside of the sealed chamber, there must be a means to transmit the data from inside the chamber where it is generated, to outside of the chamber where it can be analyzed. Since traditional cables such as BNC or SMA cables are themselves conductive and susceptible to interference from the EM waves generated inside the chamber, fiber optic transmitter and receiver units, and fiber optic cables are needed to transport the signal from the ECU inside of the chamber to the test equipment outside of the chamber. Fiber optic cables are not conductive and are not subject to the EM fields in the chamber. In order to route the cables outside of the chamber to the test equipment, waveguides are used at the boundaries of the chamber to transport out the optical signals, allowing for the chamber to remain completely sealed while still enabling the ECU outputs to leave the chamber. The fiber optic waveguides have a high pass cutoff frequency which is above the frequency range being tested in the chamber, and and therefore are immune to interference created in the chamber.
EMI Test Equipment Setup
Figure 1 shows a real configuration for deviation detection in immunity testing, photographed from inside the sealed chamber (while the transmitting antenna was powered off). Note that the mode tuner is shown to the right of the chamber. The left side of the chamber has CAN bus fiber optic transmitters placed on a foam bench having a relative permittivity < 1.4 and located within the usable volume of the reverberant chamber. The fiber optic transmitters optically convert the output signals from the ECU under test and the signals are transported through the chamber with RF-hardened fiber optic cables which exit the chamber near the floorboard via waveguides. The ECU under test, and the transmit and receive antennas are also present in the chamber but are not shown in the figure.
Once the signal arrives outside of the chamber, a typical method for testing it involves data acquisition equipment which requires custom software to be written to qualify the result to determine whether the CAN bus output, sensor signals, or PWM outputs from the ECU meet the specified requirements or not. Since there are many signals to test, and many test criteria, often the software development time and costs are high to address all of the needed test requirements described in the test plan. A relatively unexplored use for oscilloscopes within this field of EMI testing is to place an array of oscilloscopes outside of the chamber and use multiple scopes for real-time analysis. Since oscilloscopes are already equipped with mask testing and parameter limit test ability, they are able to perform many, if not all, of the test requirements directly, without any significant amount of software development time needed.
In Figure 2, the copper-colored exterior door (open doorway) to the EMC chamber is shown to the right of the test bench. On the left side, the orange optical cables which carry component functional test results are converted from optical to electrical signals and routed to the oscilloscope channels as BNC cable inputs.
Waveform masks in the oscilloscope are used to analyze the wave shape relative to pre-defined compliance requirements. The dimensions of the mask depends on the functional criteria being tested, and will be actively modified via automated computer control throughout the test period.
In Figures 3, 4, and 5 below, an oscilloscope is monitoring the output of a simulated ECU. For confidentiality reasons, simulated data is used which closely approximates what would be observed from a typical ECU output. Channels 1 and 2 show simulated PWM signals which control an output driver actuator signal. The simulated actuator signal is captured on Channel 3, and a CAN split signal is acquired on Channel 4.
EMI Compliance Testing
Figure 3 shows the acquisition with mask testing turned off, the wave shape of each signal is observed. The oscilloscope is Edge-triggered on Channel 2, and all four waveforms are captured synchronously.
In Figure 4, mask testing is enabled. The mask shape verifies that the signal high level, signal low level, frequency, duty cycle, and other criteria fit within tolerance limits described in the test plan. The mask thickness forms the specified tolerance band around a defined nominal value, and the mask verifies that each acquired waveform does not deviate more than a specified percentage beyond the defined nominal value. In this example, the waveforms each meet all of the specified test criteria. Note that the oscilloscope continues edge triggering, continuously monitoring for failures using the predefined mask criteria. The criteria for triggering the scope is an edge occurring on Channel 2, and the scope is configured to identify and document each of the failures as they occur.
In Figure 5, the simulated ECU is subject to EMI within the chamber, resulting in amplitude modulation, reduced amplitude, and changes in duty cycle and frequency which subsequently fail the mask test criteria for both of the PWM signals and the Actuator Driver Output signal. Unlike the other three signals, the CAN Split signal is not affected by the EMI and continues to produce a passing result. This type of mask testing allows for multiple criteria to be rapidly tested in real time.
In addition to waveform mask testing, pass/fail limits are also applied to the parametric data to ensure that the numerical measurement results also comply to specified limits. Note that on the screen image in Figure 5, the scope has indicated the three failures which have occurred denoted with the red "Fail" message on the screen under the test criteria. In the event of a mask failure or parameter limit failure, an oscilloscope can also automatically execute actions, such as saving the waveform data to be used for direct comparison and documentation, saving a screen image to be used for documentation and evaluation, generating a pulse out of the oscilloscope to assist with test automation, and sounding an alarm to inform the test operator when a problem occurs.
Conclusion
Although oscilloscopes are well equipped to perform the rapid parametric measurements required for determining EMC deviation in immunity testing, they have been often overlooked in the past mainly due to lack of awareness and lack of sufficient oscilloscope channel count. Typically, analysis of the parametric results requires custom-designed software to be developed, and possibly custom-designed hardware to be produced as well -- both of which are time consuming and costly to develop. However, many oscilloscopes are equipped with built-in pass/fail mask and parametric limit testing capabilities which can be directly applied to analyze the component sensor outputs.
Using an array of oscilloscopes is potentially the most efficient and cost effective method to qualify component sensor outputs during immunity test, since most of the functionality using pass/fail mask and parameter limit testing has already been implemented, saving EMC test engineers significant cost and effort for immunity functional testing compared with the costly software development time needed to implement data acquisition software to perform the same rigorous test requirements of EMI deviation determination testing.
