Osamu Ikata

Surface acoustic wave filter

A surface acoustic wave (SAW) filter includes a plurality of interdigital transducers located on a piezoelectric substrate along a surface wave propagation direction, at least a single one-port SAW resonator connected in series with an output side of the SAW filter, while the resonance frequency of the SAW resonator is set at a level which is higher than the passband of the SAW filter.

Development of low-loss band-pass filters using SAW resonators for portable telephones

A bandpass filter using surface acoustic wave (SAW) resonators in a ladder circuit structure for portable telephone systems is reported. For filter design, a simulation tool is used to consider the effects of electrodes (their apertures, the number of paired elements, thickness, and bulk wave radiation). Filter input and output impedance conditions are designed by the resonator capacitance to match the line impedance. The insertion loss and stopband rejection values are traded off against each other and are controlled by the ratio of capacitances between the parallel arm and series arm resonators, 800-MHz and 1.9-GHz band filters mounted in 3.8 mm*3.8-mm*1.5-mm ceramic packages were fabricated. They are suitable for surface mount technology (SMT). Typical frequency response characteristics are 2.5-dB maximum insertion loss in the 25-MHz passband and a stopband rejection of 25 dB.<<ETX>>

Optimal cut for leaky SAW on LiTaO/sub 3/ for high performance resonators and filters

The paper describes how the characteristics of leaky surface acoustic wave (LSAW) propagation depend on the thickness of Al grating electrodes on rotated Y-X LiTaO/sub 3/. It is shown that the propagation loss arising from leaky nature changes parabolically with both the grating electrode thickness and rotation angle and becomes zero when electrode thickness and rotation angle are properly determined. This means that even when thick grating electrodes are needed in device design, zero propagation loss is always realized by properly determining the rotation angle. When the grating electrode thickness is 0.07 to 0.1 in wavelength for example, LSAWs on 40-42/spl deg/Y-X LiTaO/sub 3/ give zero propagation loss without deteriorating other characteristics. Ladder-type filters for the 800-MHz range were fabricated, which essentially need thick Al grating electrodes of about 0.1 wavelength thickness. As predicted by theoretical calculation, experimental results showed that if the rotation angle is increased to circa 42/spl deg/ from a conventional value of 38/spl deg/, the insertion loss and shape factor are markedly improved compared with devices based on 36/spl deg/Y-X LiTaO/sub 3/. This is essentially a result of the minimized propagation loss.

Temperature compensated LiTaO/sub 3//sapphire bonded SAW substrate with low loss and high coupling factor suitable for US-PCS application

A novel temperature-compensated SAW substrate was developed based on bonded LiTaO/sub 3//sapphire. Because of its small TEC (thermal expansion coefficient) and large Youngs modulus, sapphire is an ideal material for a support substrate. The frequency and temperature characteristics of the bonded LiTaO/sub 3//sapphire SAW substrate and its application to a US-PCS duplexer are described.

SAW duplexer metallizations for high power durability

The improvement on power durability of SAW filters in order to realize a SAW duplexer is described in this study on IDT metallization. Compared with the conventional single layer metallizations such as Al-Cu, the laminated films are found to show the higher power durability. Many kinds of laminated film were tested changing the material of the middle layer for three layered films of which the top and bottom layers were made of Al-Cu or pure Al. Among them, two types of films such as Al-Cu/Cu/Al-Cu and Al/Mg/Al showed the best power durability. Al-Cu/Cu/Al-Cu had a lifetime 250 times longer than that of current Al-Cu single film. Al/Mg/Al further had a longer lifetime than Al-Cu/Cu/Al-Cu. These three layered films are being successfully applied to SAW antenna duplexers of cellular phones, indicating sufficient power durability for practical use.

Optimum cut of LiTaO/sub 3/ for high performance leaky surface acoustic wave filters

The paper describes how leaky surface acoustic wave (LSAW) propagation characteristics on LiTaO/sub 3/ depend upon its rotation angle when the Al electrode thickness is increased. It is shown that the propagation loss arising from the leaky nature changes parabolically with both the film thickness and rotation angle, and becomes zero when they are properly chosen. This means that even when relatively thick electrodes are needed, zero propagation loss is always achieved by properly determining the rotation angle for LiTO/sub 3/. When the electrode thickness is 0.07/spl sim/0.1 wavelengths, for example, LSAWs on the 40/spl sim/42/spl deg/ Y-X LiTaO/sub 3/ offer zero propagation loss without deteriorating other characteristics. Ladder filters for 800 MHz range were fabricated, which essentially need thick Al electrodes. As is predicted by theoretical calculation, experimental results showed that if the rotation angle a is increased to circa 42/spl deg/ from the conventional value of 36/spl deg/, the insertion loss and shape factor are improved by 0.7 dB and 0.26, respectively. The result gives a valuable guideline for determining the optimal rotation angle of LiTaO/sub 3/ to be used for high performance LSAW filters, which are one of the key components in RF sections of mobile communication systems.

A design of antenna duplexer using ladder type SAW filters

This paper describes a design technology of a small antenna duplexer, especially package conditions, by using ladder type SAW filters. This requires not only optimizing the filter design but also the electrode materials and the package design which has matching circuitry for the duplexer configuration. The matching circuitry was incorporated into a microstrip structural pattern which is designed onto the surface of a multilayer ceramic package. To avoid the deterioration of in-band VSWR and reflection coefficient of the stop-band after rotating phase caused by the matching pattern, the characteristic impedance of the matching pattern was designed to be 1.06 times Zo. To obtain the high stop-band attenuation and Tx-Rx isolation characteristics, the package parasitic inductance was used by the multilayer structure design of the package.

High-frequency SAW duplexer with low-loss and steep cut-off characteristics

In CDMA (code division multiple access), analog, and partly TDMA (time division multiple access) cellular phone systems, transmitting and receiving are carried out simultaneously. Thus, an antenna duplexer is needed. An antenna duplexer using ladder-type SAW filters was first applied in an AMPS (Advanced Mobile Phone System)-CDMA system in 1998. The package size was 9.5 /spl times/ 7.5 /spl times/ 2 mm. The development of SAW antenna duplexers will require SAW filter designs that enable lower loss and a sharper cut-off, electrode materials that provide higher power durability, and package designs that enable lower loss and a smaller size. The miniaturization of this duplexer has been achieved by improving the filter chip and reducing the package size to 5 mm square. However, SAW duplexer with low-loss and steep cut-off characteristics is needed for high frequency applications. Through improved SAW filter characteristics, a duplexer for a 1.9 GHz PCS (personal communication services)-CDMA system has been realized. This paper describes these advanced technologies.

L and S band low-loss filters using SAW resonators

This paper describes L and S band low-loss filters using one-port SAW resonators in a ladder circuit structure. Four kinds of filters were developed. Three types have a wide frequency band, with fractional bandwidth of about 4%. A 36° y-x LiTaO3 substrate was used for these. The last type of filter has a narrow frequency band, with 0.03% of fractional bandwidth, and used an ST-cut quartz substrate. To design these ladder type SAW filters, several important factors were considered, the static capacitance of IDT, the period of IDT and film thickness. For fabrication, i-line stepper and reactive ion etching was used to delineate fine IDT patterns with line and gap width of 0.4 to 0.7 μm. Minimum insertion losses of 2 dB were obtained for the three wide band filters. The insertion loss of the narrow band filter was 6 dB. The input and output impedance of these filters were 50 Ω. The filters were mounted in 3 mm×3 mm×1 mm or 3.8 mm×3.8 mm×1.5 mm SMT packages

Improved Power Durability of Surface Acoustic Wave Filters for an Antenna Duplexers

For portable telephone applications, we improved the power durability of ladder-type filters composed of one-port surface acoustic wave (SAW) resonators. In these ladder-type filters, the chip temperature was raised and power durability was weak around both cut-off frequencies in transmission characteristics. Power durability tests of a duplexer composed of ladder-type filters showed that the receiver filter (Rx) in which the power was leaked from the transmitter one (Tx) was weaker than the Tx. We therefore studied Rx in this work. In filter design, reducing the resonator resistance to suppress the rise in chip temperature improved power durability. The lifetime of electrode material was improved to up to 10,000 times longer than that of an Al-Cu single-layer film by use of a 3-layered film of Al-Cu/Cu/Al-Cu.