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FROM EDN EUROPE: EMC analyser points to filtering solutions

Using Laplace Instruments' ESA-2000 EMC
(electromagnetic-compatibility) analyser, you not only can measure the level of
conducted interference a unit under test generates, but also can obtain an
analysis of its common-mode and differential-mode components (Picture). These features provide an immediate pointer to the configuration of the most effective filtering approaches for suppressing the interference, says David Mawdsley, director of the product's designers.

The ESA-2000 covers 10 kHz to 1.1 GHz, with peak-, average-, and quasipeak-mode detectors. It has 9- and 120-kHz IF bandwidths, sensitivity as high as 0 dBmV, and a dynamic range of 70 dB. You can measure a range of signal levels with the aid of a 50-dB input attenuator that offers 10-dB steps. You control the analyser via a serial link from a Windows PC, using software that tabulates and graphs the measured results. The software includes an expert system that guides test setup and configuration.

Once you identify a conducted interference problem, you can apply four basic configurations of filtering components to the power lines: capacitors between the live and the neutral line; capacitors from each line to ground; common-mode chokes (inductors); and differential mode chokes, one in each line. The ESA-2000 uses a matching LISN (line-impedance-stabilisation network) that measures the interfering signal in each line separately and computes and graphically outputs the differential- and common-mode signal components; it also outputs the total interference signal and plots it as amplitude versus frequency. With this information, you can proceed with your design with less trial and error. For example, if you know of a common-mode interference problem, using line-to-line, or X, capacitors is ineffective, because the same voltage is present on both lines and cannot be shorted out.

Options for the ESA-2000 include an eight-section preselector for use with noisy broadband sources; the instrument costs £9000.

Laplace Instruments,
+44 1263 515160, www.laplace.co.uk.