Development of Novel Methodologies For Measuring Digital Hardware for Immunity to Electromagnetic Fields

Executive Summary

The immunity of equipment to external radiofrequency interference (RFI) is measured using go/no-go criteria to determine failure. For analogue equipment degradation can usually be observed before the failure criterion is met thus giving an indication of the margin to failure. For digital equipment failure is usually abrupt, giving little or no prior indication.

If radiofrequency energy penetrates into a clocked digital circuit then cross modulation of the interfering signal and the operating signals will occur. This is manifested as a modification of the characteristic emission spectrum of the equipment. Measurement of the emissions spectrum under the influence of a threat field hence gives information about the penetration of the threat energy into the system.

Measurements were made on several systems to determine whether the re-radiated emissions method may be a useful tool in expanding the effectiveness of radiofrequency immunity measurements on digital systems.

The results show that for a single device switching, a marked change in re-emissions profile occurs as failure is approached.

For real products this change is more subtle because to initiate failure requires less disruption of the internal signals owing to more rigorous timing constraints.

The technique of measuring re-radiated emissions can be used to determine frequencies at which RFI can penetrate into a system, to estimate failure margins for a digital system and to aid in tracing the failure mechanism if an EMC test failure has occurred.

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