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This work analyzes the effect of conducting bands placed close to a common mode choke on the attenuation provided by an electromagnetic interference filter where the common mode choke is mounted. The main aim was to determine whether the response of the common mode choke can be modified and to clearly identify and quantify the physical mechanism responsible for the changes found in the attenuation of the common mode choke and the electromagnetic interference filter. The impact of grounded and ungrounded conducting surfaces was studied for the two more typical configurations of common mode chokes: vertically and horizontally mounted. This study has allowed us to devise an optimum shield for a common mode choke that can significantly improve the attenuation to both common mode and differential mode noise of an electromagnetic interference filter at high frequencies, where parasitic effects usually undermine the performance of the filter. Key features of the technique described here are that it can be applied easily and that it does not result in an increase of the weight, volume, or cost of the filter. [ABSTRACT FROM AUTHOR]