Comparison between EMC Measurements in Frequency and Time Domain

The subject of the paper is the comparision between EMC measurements in frequency and time domain. In practice, a replacement or supplement of frequency domain measurements by faster measurements in time domain can save time and cost. To investigate the correllations between EMC measurements in frequency and time domain, it is useful to divide the field of EMC measuring techniques in two parts: non-standardized measurements and susceptibility tests according to national or international standards. Non standardized EMC-measurements means basic emission and susceptibility tests to get an information about the EMC performance of the test object. For this kind of susceptibility measurements, in most cases the wiring harness is responsible for the disturbance signal amplitudes coupled in to the electronic devices. Therefore several measurements with different wiring harnesses and electronic devices have been performed in frequency as well as in time domain. The correspondance between these two measurement principles is very good as long as non-linear components are not involved. In addition the influence of non-linear components has been investigated to compare the characteristics of both principles. Emission measurements in a TEM-cell can be performed by using modern digital sampling oscilloscopes. An estimation of possible measuring uncertainities ensures, that the electronic device conforms with the requirements of emission standards. For special susceptibility tests according to international standards, it is very difficult to reach exactly the same severity level by measurements in frequency and time domain. It is very difficult to define equivalent susceptibility tests because this equivalence depends on the EUT. Some examples for electronic devices which are tested whith the same severity level in frequency and time domain are presented. The definition of equivalent severity levels for complex electronic devices i.e. microprocessor systems is very difficult. Therefore in practice, a replacement of frequency domain measurements by time domain measurements with exactly the same severity level is often impossible.