半導体技術の飛躍的な進歩に伴いICの高速・高集積化が達成された結果，高性能かつ高機能化された様々な電子機器が市場に供給されている。しかしながら，これらの電子機器の電磁雑音に対する耐性の低下が問題となっており，特に，電した人体によって引き起こされる静電気放電（ESD:Electrostatic discharge）は，広帯域の過渡電磁雑音を含むため，ハイテク機器ほど深刻な誤動作を引き起こす。このような背景から，ESDに関する電子機器の耐性（イミュニテイ）試験法が国際電気標準会議（IEC:International Electro-teChnical Commission）で既に標準化されている。%International Electrotechnical Commission (IEC) has prescribed an immunity test (IEC61000-4-2) of electronic equipment against electrostatic discharges (ESDs), in which a discharge current to be injected onto equipment under test is being specified by a Heidler formula. As for the waveform, however, not the whole-waveform but only the rise time, the first peak, and the current amplitudes at 30 ns and 60 ns are being specified in the time domain together with their uncertainty so that a commercially available ESD generator (ESD-gun) shall be checked by injecting a current onto an IEC recommended calibration target before testing. In this study, to clarify the discrepancy between measured discharge currents and the above-mentioned calculated current in the frequency domain, we measured discharge currents for contact discharges of an ESD gun onto the calibration target with respect to various arrangements of the ESD gun and its earth return cable, and obtained the ratio of their current frequency spectra to the calculated current spectrum from the Heidler formula. As a result, we found that according to the inclination of the ESD gun and arrangement of the earth return cable current spectra vary by -10dB in the frequency range from 10 MHz o 200 MHz and by 15dB at frequencies over 300 MHz. This finding suggests that the arrangements of an ESD gun and its earth return cable should affect immunity testing results.
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