S-band Low-loss Wide-bandwidth Microwave Surface Acoustic Wave Filter

A novel S-band low-loss wide-bandwidth microwave microstrip, surface acoustic wave (SAW) filter used in spread spectrum communication systems was designed and implemented. The 16 micron input/output interdigital transducer (IDT) on the 64°-rotated Y-cut LiNbO₃ 69.0 MHz SAW filter, with microwave square open-loop resonators, has an insertion loss of 3.987 dB. This device is also a 2.5 GHz microwave microstrip SAW filter, with four cross-coupled microstrip square open-loop resonators and two planar interdigital capacitors, with an insertion loss as low as 2.41 dB and a 3 dB bandwidth as wide as 450 MHz (18 percent). The characteristics of these low-loss wide-bandwidth microwave microstrip SAW filters are influenced by the interaction between electromagnetic waves and the piezoelectric SAW.

From: Vol. 52 | No. 9 | September 2009 | Page 144

by Na-Fu Wang and Yu-Zen Tsai, Cheng Shiu University, Taiwan; Tseng Tang, National University of Tainan; Mau-Phon Houng, Institute of Microelectronics, National Cheng Kung University, Taiwan

Recently, many types of wireless communication systems have rapidly developed and the demand for bandwidth has increased. The spread spectrum communication systems, such as the code division multiple access (CDMA) systems have attracted increasing attention because of their unique features that make it possible to transmit signals through noisy communication channels with a high degree of security. Therefore, high-performance microwave devices having low insertion loss and wide bandwidth are required.

The filter is an important component for radio frequency integrated circuits (RFIC) in wireless communication systems and used in both receivers and transmitters. The reduction of filter size has been the focus of several studies. The problems caused by the larger, heavier and discrete conventional filters can be solved by using a planar filter. Many forms of the planar filter exist; therefore, surface acoustic wave (SAW) filters1,2 and microwave microstrip filters^3-12 are small, light and simply fabricated. No convenient and low cost resonators for high-frequency applications are better than planar resonators such as microstrip resonators. The microstrip version is the most popular structure of a planar transmission line. Moreover, cross-coupled microstrip filters have attracted much attention in recent years due to their quasi-elliptical function or flat group delay responses. This type of microstrip resonator with appropriately designed circuits, effectively reduces the volume of circuits by using ULSI techniques.

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