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Test for Transfer Impedance (Shielding Effectiveness) of a Shielded Cable

IP.com Disclosure Number: IPCOM000049771D
Original Publication Date: 1982-Jul-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 1 page(s) / 11K

Publishing Venue

IBM

Related People

Smith, AA: AUTHOR

Abstract

A shielded cable is positioned parallel to a ground plane to establish characteristic impedance of a few hundred ohms for the shield and ground plane system. The shield is resistively loaded to have the same impedance. The cable is then tested by producing a known current at a known frequency on the shield and measuring the voltage in the signal conductor. The test gives relatively uniform value over a wide frequency spectrum because the matched shield impedance helps to reduce reflections.

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Test for Transfer Impedance (Shielding Effectiveness) of a Shielded Cable

A shielded cable is positioned parallel to a ground plane to establish characteristic impedance of a few hundred ohms for the shield and ground plane system. The shield is resistively loaded to have the same impedance. The cable is then tested by producing a known current at a known frequency on the shield and measuring the voltage in the signal conductor. The test gives relatively uniform value over a wide frequency spectrum because the matched shield impedance helps to reduce reflections.

A shielded cable is used when the signal conductor is to be isolated from sources of extraneous signals and/or to prevent the cable from radiating interference to other conductors. The transfer impedance is the ratio of signal conductor voltage to shield current and is a measure of the shielding ability of the shield. The test is usually made over a wide frequency spectrum.

Ordinarily there are wide differences in the transfer impedance that is measured at different frequencies. Much of this difference can be attributed to reflections that occur on the shield. The shielded cable is loaded with ferrite toroids to raise its resistance to a few hundred ohms and the cable is positioned above a ground plane at a height to give it a characteristic impedance which is approximately the same as the resistance introduced by the ferrite toroids.

A current probe is used to inject a current on the shield. The signal...