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Tracking down ECU disturbances from EMI

Posted: 25 Nov 2015 ?? ?Print Version ?Bookmark and Share

Keywords:EMC? oscilloscope? electrostatic discharge? ESD? pulse-width modulation?

When you perform EMC tests, you often think of emissions measurements made with a spectrum analyser. But, there are EMC applications for oscilloscopes. A relatively underutilised use for oscilloscopes in EMC testing is for real-time functional performance evaluation, including deviation detection, of a device under test (DUT) during exposure to a disturbance. An oscilloscope can help you document how EMC affects your product's operation. We often use oscilloscopes but need to electrically isolate them from the EUT that's inside a chamber.

The term "deviation" refers to an EUT's response to a disturbance where one or more functions exceed allowable tolerances. These functions and tolerances are defined in an EMC test plan document, uniquely developed for the specific device, and approved by all concerned parties before testing commences.

Standard practice in the automotive industry has been to perform component-level tests to determine a device's immunity to disturbances such as electrostatic discharge (ESD), transients on power and I/O lines, conducted RF, and radiated magnetic and electric fields. These tests are conducted prior to full vehicle immunity testing. Acceptance criteria for immunity, such as RF field strength levels the DUT must endure, are defined in an OEM's engineering specifications while the procedures are typically performed to international standards.

The test setup that's common to most component-level immunity tests consists of a wire harness and a load simulator, which contains actual and/or electrically equivalent loads that represent the DUT's interface with the vehicle. The DUT is exercised in one or more modes of operation, defined in the test plan, and exposed to a disturbance. During exposure to the disturbance, the DUT functions are monitored for a response exceeding an allowable tolerance. Typical to RF immunity tests, detection of a deviation requires determination of the device's immunity threshold, a process where the magnitude of the disturbance is reduced significantly and increased in fine increments until the deviation recurs.

If the DUT has a CAN communication bus, then some information concerning its functional state can be sent over the bus. Unfortunately, other monitored functions details can't transfer over the bus. Examples include the analogue signals of a sensor or a pulse-width modulation (PWM) output to drive an actuator. We must measure these functions with an appropriate instrument.

RF immunity tests are typically performed in shielded chambers to prevent exposure of laboratory personnel to hazardous fields and to prevent malfunction of sensitive equipment. The conducted RF immunity test described in ISO 11452-4 utilises a clamp-on current injection probe to induce RF current into the EUT harness at frequencies from 1MHz to 400MHz at levels ranging from tens to hundreds of milliamps. Those currents create fields near the test bench at levels high enough to effect operation of unshielded equipment. The radiated RF immunity test described in ISO/IEC 61000-4-21 utilises a reverberant chamber containing a mechanical mode tuner which, 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. The test frequency range is 300MHz to 3GHz with field strengths as high as 200 V/m (CW and AM) and 600 V/m (radar pulses).

Maintaining the integrity of the shielded chamber prohibits directly connecting measurement instrumentation to the test setup over conductive cabling. Inside the shielded chamber, RF fields couple to the cable, which then acts as a radiating antenna outside the chamber. To avert that problem, we use isolated connections using RF hardened fibre-optic transmitter and receiver sets. The converted signals exit the chamber though non-conductive fibre-optic cables routed through waveguides having a lower cut-off frequency above the frequency range of the test. The optical signals are converted back electrical form by the receiver, which is connected to the measurement instrumentation.

In figure 1, the test setup (not shown) and RF hardened fibre optic transmitters are placed within the useable volume of the reverb chamber on a foam bench having a relative permittivity less than 1.4.

Figure 1: Reverb chamber equipped with a mode tuner (right). Transmit and receive antennas not pictured.

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