Transfer functions and current distribution algorithm for the calculation of radiated emissions of automotive components

Radiated emission testing of electric components, modules and systems is mandatory. Such tests are performed in absorber-lined shielded enclosures during the development process. Due to strict time schedules, limited access to EMC measurement facilities and cost reasons, alternative methods are needed. This contribution presents an approach for a pre-compliance test method for radiated emissions of automotive components. It is adapted to gain measurement results according to the CISPR 25 standard. The later operation site of this pre-compliance test method is the developer's laboratory setup. For obtaining the radiated field strength of an tested device common mode currents on the cable harness are measured. As the current distribution along a harness is needed for accurate calculation in higher frequency ranges, an algorithm is presented retrieving the current distribution out of current probe measurements. Those current distributions in combination with multiple transfer functions lead to a calculation of the electrical field strength. Transfer functions represent the correlation between common mode current in the defined test setup environment and their electric field strengths. The methodology of transfer functions using a vector network analyzer will be explained. The transfer functions for a single segment and multiple segments will be verified, compared, and applied to a realistic test setup.