Shigeru Tsuzuki

A design technique of balanced longitudinal coupled mode SAW filters for RF-stage with a desired matching impedance

A design technique for balanced longitudinal coupled mode SAW filters, in which the matching impedance can be easily controlled, has been developed. This is achieved by adoption of a divided IDT structure for the center IDT of the filter, which has three IDTs. The center IDT is connected to the balanced terminal. The IDTs on the both sides are connected to the unbalanced terminal. The center IDT is divided into three divided IDT portions, which are connected in series/parallel. It can make the impedance higher than that of the original filter. The finger number ratio for the divided IDTs can control the impedance. Applying this technique to a center IDT of 50ohm design, the matching impedance in the range approximately from 50ohm to 200ohm can be realized.

An analysis and improvement of balanced-type SAW filters

The degradation mechanism of balance performance for balanced-type SAW filters was analyzed. The authors have confirmed that the parasitic coupling between input and output IDTs makes the balance characteristics degrade. The measurement results show good agreement with simulations by assuming the parasitic capacitances. The balance characteristics can be improved by decreasing the parasitic capacitances. Further, the much better performance can be obtained by connecting inductors between input and output IDTs. Parallel resonance circuit with additional inductor and the parasitic capacitance can solve the problem successfully. The experimental balanced-type SAW filter shows an excellent performance. These values satisfy the system requirements.

A miniaturized SAW reflector filter for IF stage of CDMA cellular phone applications

A SAW reflector filter for IF stage of CDMA cellular phone applications has been developed. Because the CDMA systems use the spread spectrum technique, a bandwidth of the modulated signal becomes wide. It is about 1.25 MHz or wider. In addition, an excellent passband flatness and a very steep cutoff characteristic are required for the IF stage filter. The SAW reflector filter consists of two pairs of input IDTs, output IDTs and reflectors. This filter is miniaturized by folding the SAW propagation path using the reflectors. The reflectors are weighted for filter characteristics. Although the concept of this filter was already reported, we have studied improved design technique for reflector weighting and transducer structures. The EWC-SPUDT is introduced to the input IDTs for low insertion loss, and the double split finger structures are introduced to the output IDTs for suppressing acoustic reflection. The filter samples show excellent performances. The size of the filters are 13.5 mm ×6.5 mm ×1.8 mm, which is 30% smaller than a conventional transversal filter.

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 differential-mode 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.

P1K-2 Improvement of Balance Performance of SAW Filters by Common-Mode Signal Suppression

A new design concept for improvement of balance performance of SAW filters has been investigated. The balance performance is degraded by common-mode signals in the balance terminals. The concept employed is the suppression of the common-mode signals using a common-mode signal suppression circuit, connected to the balanced terminals. In the circuit, the impedance of balance terminals for the differential-mode signals becomes open to the ground level by the resonance circuit. And, the impedance of balance terminals for the common-mode signals becomes short to the ground level. And, the circuit has an impedance matching scheme. The improvement in balance performance was confirmed. This design concept could be applied to the circuit design for balanced-type SAW filter circuit designs