A low‐loss bandpass filter using SAW resonators

A filter structure having a ladder network of one‐port surface acoustic waves (SAW) resonators was studied, as a development of low‐loss, high‐frequency bandpass filters. A simulation tool was developed first and used for the design of the filters. It was found that the loss and the stopband attenuation can be controlled by the ratio of the capacitance for the parallel arm resonator C op and that for the serial arm resonator C os , i.e., C op C os . It was revealed also that the input and output impedances can be matched to 50 Ω by maintaining a certain relationship between C op and C os . Based on these findings, 900—MHz and 1.5—GHz RF filters were designed and prototyped for portable telephone applications. The 900—MHz filter has the insertion loss of 1.5 to 3 dB, the stopband attenuation of 20 to 30 dB, and the input output VSWR of less than 2, and requires no impedance matching circuit. Similar characteristics were achieved by the 1.5—GHz filter. The insertion loss turned out to be approximately half of what was achieved by the conventional transversal filters.