6B-1 Complex Coefficient SAW Filters for Double Quadrature Down-Converter

This paper presents a newly developed complex coefficient surface acoustic wave (SAW) filter for a double quadrature down-converter, to achieve a high image rejection ratio in a low-IF topology. We realize the function of the complex coefficient filter, by using SAW transversal filter techniques. The complex coefficient SAW filter has stable characteristics for the amplitude and phase errors, due to its high accuracy of the pattern. Moreover, the SAW filter has band-pass characteristics so that the unwanted signals out of the system band can be suppressed. We demonstrate 900 MHz and 2.4 GHz complex coefficient SAW filters on 128degY-X LiNbO₃, to show the basic performance. The fabricated complex coefficient SAW filter consists of two tracks corresponding to the I- and Q-channels, with slanted transducers. They had excellent characteristics such as an amplitude error below plusmn0.3 dB and a phase error below plusmn2deg in the pass-band. They correspond to an excellent image rejection ratio of more than 35 dB