Time-Domain Approach Benefits EMI Testing

By using a time-domain approach for CISPR 16 compliant EMI testing, measurement time can be greatly reduced without degrading accuracy.

To perform electromagnetic-interference (EMI) measurements using the stepped-frequency method, a significant amount of time is needed—even when time-saving procedures are applied. EMI measurement times, however, can be significantly reduced by using time-domain methods based on the Fast Fourier Transform (FFT) technique to identify interference. In fact, time can be reduced by a factor ranging to 1000. Yet care must be taken to ensure that testing is carried out in accordance with the CISPR 16 standard. In an eight-page white paper titled “Time-Domain Scan Increases Speed of CISPR 16 Compliant EMI Measurements,” Rohde & Schwarz describes the fundamentals of a time-domain EMI measurement system.

To measure unknown disturbance quantities in the frequency domain, the test receiver or analyzer must sweep through a frequency range. Ideally, the sweep time is as short as possible. However, additional factors must also be taken into account, which add to the measurement time. Using a spectrum analyzer instead of a test receiver does not solve the problem, as it requires the time of a single sweep to be long enough for at least one disturbance pulse to fall into the resolution bandwidth at each frequency. In addition, spectrum analyzers are limited by their reduced number of sweep points in comparison with test receivers.

The time-domain technique has proven to be effective in reducing EMI measurement time without degrading accuracy. Conventional EMI measurement systems allow only the signal within the resolution bandwidth to be measured in a given measurement time. FFT based time-domain measurements, in contrast, allow a much wider part of the spectrum to be simultaneously analyzed. When using the time-domain technique, special measures must be taken to ensure correct detection of all possible signals that can appear in a disturbance spectrum. The windowing process is described, which allows only a certain number of discrete signals in the time-domain to be used to calculate the frequency spectra.

The document includes a technical implementation using the ESU EMI test receiver from Rohde & Schwarz. This instrument combines both frequency- and time-domain capabilities. The accuracy of the time-domain technique was verified by comparing results from the time-domain scan with the conventional stepped-frequency scan method. The results from the two scans showed a negligible difference. In addition, the time-domain scan reduced measurement time by as much as 1000 times in comparison with the stepped-frequency scan method.

Rohde & Schwarz, Mühldorfstrasse 15, 81671 München, Germany; +49 89 41 29 0.

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