Masakazu Mimura

Surface acoustic wave duplexer for US personal communication services with good temperature characteristics

The transmission (Tx) and receiving (Rx) passbands of the Personal Communication Services (PCS) mobile phone system in the US are 1850–1910 MHz and 1930–1990 MHz, respectively. The transition bandwidth between Tx and Rx is very narrow, 20 MHz. A duplexer for the US-PCS employing surface acoustic waves (SAWs) requires a substrate which has good temperature stability, an optimum electromechanical coupling factor, a large reflection coefficient, and a good resonant mechanical Q value. Previously, there did not exist any substrate suitable for the US-PCS SAW duplexer. In this paper, we describe a new substrate that is suitable for the US-PCS SAW duplexer and a US-PCS SAW duplexer constructed with this new substrate.

SAW substrate, with coupling factor and excellent temperature stability suitable for duplexer of PCS in US

The pass-bands of a transmission (Tx) and a receiving (Rx) of the Personal Communication Services (PCS) Handy-phone in US are 1850-1910 MHz and 1930-1990 MHz, respectively. The transition bandwidth between the Tx and the Rx is very narrow as 20 MHz compared with other systems. A duplexer for the PCS using surface acoustic wave (SAW) requires a SAW substrate, which has a the good temperature stability, an optimum electromechanical coupling factor, and a large reflection coefficient. Some Rayleigh waves and leaky SAW (LSAW) on various substrate or structures have a good temperature characteristic, but almost all of them have not an optimum coupling factor and a large reflection coefficient for the US-PCS duplexer. In 2003, the authors reported a US-PCS SAW duplexer having good temperature stability, a steep frequency characteristic in transition band, a low loss, and a large out-of-band suppression. This paper describes the detail of the new substrate having the good temperature stability, the optimum electromechanical coupling factor, and sufficient reflection coefficient.

SAW substrate for Duplexer with Excellent Temperature Characteristics and Large Reflection Coefficient realized by using Flattened SiO2 Film and Thick Heavy Metal Film

Authors previously proposed a SAW PCS-duplexer with an excellent temperature coefficient of frequency (TCF) and a good frequency characteristic composed of a thick-SiO2/thin-Au-electrodes/LiTaO3 structure. However, a sheet resistance of thin Au electrode is large, compared with thick Al-electrode, so thin Au-electrode is not suitable for filters requiring low loss. Authors tried to use thick Cu electrodes with small resistance. By flattening large convex portions on the SiO2 surface, a SAW substrate suitable for US-PCS duplexer with low insertion loss and an excellent TCF was realized

Improvement of Temperature Characteristics of Boundary Acoustic Wave Resonators Using Multilayered Electrodes

In this paper, we describe a new boundary acoustic wave structure employing multilayered metal electrodes with a high-density metal and a low-density metal. By using this structure, such as Pt/Al/Pt, the electromechanical coupling coefficient (k2) and temperature coefficient of frequency (TCF) of the boundary acoustic wave can be changed. We theoretically studied the dependence of the energy distribution of the boundary wave on the position of the total electrode gravity center when the electrode layer structure is changed. It was experimentally confirmed that k2 and TCF can be changed simultaneously. By using this structure, we developed a novel filter with good electrical characteristics, and a very small variation of the filter characteristic with temperature (almost zero TCF) was successfully realized.

A suppression method of second order mode spurious responses for boundary acoustic wave filters

In this paper, we describe a suppression method of spurious responses for boundary acoustic wave filters, which employ a shear-horizontal type boundary acoustic wave in SiN/SiO2/electrodes/LiNbO3 structure. In this structure, the second order mode of boundary acoustic wave is excited strongly and causes a spurious response which deteriorates out-of-band attenuation. We studied the excitation mechanism of the second order mode and found that the intensity of the second order mode depends highly on the relation between the acoustic velocity of the second order mode and the fast shear bulk wave velocity of LiNbO3. Moreover, we found that the change of the propagation direction from the X-axis on rotated V-cut LiNbO3 makes the fast shear bulk wave velocity lower. As a combination of these results, a new suppression method of the spurious responses was proposed. Using this method, a boundary acoustic wave filter without spurious responses was successfully realized.

Small sized band 20 SAW duplexer using low acoustic velocity Rayleigh SAW on LiNbO 3 substrate

Small sized surface acoustic wave (SAW) devices with high performances have been strongly demanded. In order to fulfill these requirements, we have developed a small sized temperature compensated SAW (TC-SAW) duplexer. The die size of current SAW devices highly depends on the size of interdigital transducers (IDTs) which is proportional to the acoustic velocity of the SAW. Therefore, we tried to lower the acoustic velocity of the Rayleigh SAW on LiNbO3 (LN) substrates, which is widely used for TC-SAW devices. In order to lower the acoustic velocity, we adopted multi layered IDT electrodes of Pt and Al, and increased the thickness of the Pt layer. From both several calculations and experimental results of one port resonators, the thickness of the Pt layer where the target acoustic velocity can be derived was determined, and the cut angle of LN substrates was optimized so as to minimize a spurious response. Moreover, good characteristics of high quality factor (Q-factor) and small temperature coefficient of frequency (TCF) were experimentally confirmed. Finally, by using this low acoustic velocity Rayleigh SAW, we successfully realized a small sized SAW duplexer for Band 20, which is 19 % smaller than the conventional one, maintaining high performances.