A New Design Concept for Balanced-Type SAW Filters Using a Common-Mode Signal Suppression Circuit

A new design concept for a common-mode signal suppression circuit for a balanced-type filter has been investigated. The degradation mechanism of the balance characteristics was studied. The degradation is caused by the common-mode signals combined with the differentialmode signals in the balanced terminals. The concept employed is the reduction of the common-mode signal using a common-mode signal suppression circuit, connected to the balanced terminals. A serial resonance circuit is formed, in which the common-mode signals are shorted to ground. The circuit was applied to the balanced-type Surface Acoustic Wave (SAW) filter. The improvement in balance characteristics, without increasing in the insertion loss, was confirmed by experiments for Global System Mobile (GSM) applications. key words: SAW filter, balanced-type, common-mode, differential-mode, balance characteristics