When you perform EMC tests, you often think of emissions measurements made with a spectrum analyzer. But, there are EMC applications for oscilloscopes. A relatively underutilized 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 ESD (electrostatic discharge), 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