Analog Signal-Interference Narrow-Band Bandpass Filters With Hybrid Transmission-Line/SAW-Resonator Transversal Filtering Sections

A technique to design analog signal-interference narrow-band bandpass filters by merging transmission lines and surface-acoustic-wave (SAW) resonators is reported in this work. By inserting one-/two-port SAW resonators into the transversal filtering sections (TFSs) of the signal-interference filter, narrow-band filtering actions with remarkable electrical properties can be synthesized. The latter particularly refers to the untypicallyhigh effective unloaded quality factor (Q(u)) associated with the generated bandpass filtering response given its fully-planar realization. This leads to perceptible benefits in terms of in-band transmission loss and occupied physical area when compared to a classic pure signal-interference filter design. Moreover, for a given type of SAW resonator, adjustable bandwidth and transmission zeros (TZs) can be produced to attain a large variety of high-selectivity filtering functions. For practical validation, a three-stage microstrip narrow-band bandpass filter with measured S-parameters of a 433.9-MHz one-port SAW resonator is shown. The spurious-mode effect of the measured SAW resonator, as a limiting factor in the overall filter performance, is also discussed.