Sensing Device for In-Line EMI Checker of Small Electric Appliances

Toshiaki KOIZUMI  Kumio TAKAHASHI  Shun SUZUKI  Hideaki SONE  Yoshiaki NEMOTO  

IEICE TRANSACTIONS on Communications   Vol.E79-B    No.4    pp.509-514
Publication Date: 1996/04/25
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Issue on Discharge and Electromagnetic Interference)
electromagnetic interference inspection,  interference current sensor,  matching circuit,  noise amplitude distribution,  mass production line,  

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This paper discusses the design of a small sensing device for EMI measurement which has equivalent characteristics to the absorbing clamp method, and reports the results on evaluation of the device. The device can be applied to the inspection apparatus for products such as power tools to examine conformance to EMI regulations of electromagnetic radiation spectrum. For reducing the scale of the EMI inspection apparatus, new matching circuit being replaced with the absorbing clamp method is adopted in the sensing device. Length of the sensing device is smaller than one twelfth of a wavelength of the measuring frequency in order to regard the sensing device as a concentrated constant circuit. The matching circuit is a resonant circuit which consists of a coaxial coupled transformer and a variable capacitor, and the transformer is a spiral copper tube in which a pair of AC power line wires passes. Resonant frequency of the circuit is tuned to the measuring frequency by adjusting the variable capacitor so that the circuit would terminate the power line by impedance zero. Thus interference current propagating along the power line from a product is absorbed, and observed by means of a VHF current probe which is settled in the matching circuit. A simple circuit for measurement of noise amplitude distribution (NAD) of interference current was developed as well as an equation to estimate quasi-peak value from the NAD. Result of measurement by the sensing device and proposed procedure confirmed good correlation with the standard absorbing clamp method, and deviation was within 3dB. Measurement time was reduced to 25 s per product, and the in-line EMI checker with new sensing device can be employed in a mass production line.