Simple and cost-effective method for improving the high frequency performance of surface-mount shunt capacitors filters

Abstract

The performance of a shunt capacitor low-pass filter at high frequencies is limited by the mutual inductance appearing between the input loop, defined by the noise source and the capacitor, and the output loop created by the capacitor and the loading impedance. In this work we present a simple modification of the layout of the shunt capacitor filter that allows to decrease the impairing effect of this mutual inductance, thus increasing the attenuation provided by the shunt capacitor filter at high frequencies. We demonstrate that this attenuation can be increased up to approximately 10 dBs with proper filter design when comparing with a typical shunt-capacitor filter configuration. The interest of this technique resides in its simplicity, that allows to improve the filter performance without increasing the cost, the design time or the space required on the board by the filter. The proposed technique is based on forcing a mutual inductance coupling between currents flowing through the shunt capacitor and currents on the signal trace. In this work, we present a lumped-element equivalent circuit that explains the effect. Also, several filter test boards are designed, fabricated and measured, and key design parameters to maximize this technique's benefits are identified.