SURFACE WAVE DEVICE WITH CAPACITANCE

摘要

A surface wave device filter comprises two 2-pole waveguide coupled double mode resonators (WCRs) electrically connected together in cascade. To permit narrowing of the filter passband for improved out-of-band rejection, e.g. for use of t he filter as an intermediate frequency channel filter in a wireless communications system, w hilemaintaining good matching for low ripple in the pass band, a shunt capacitance isconnected in parallel with the static capacitanc es of the two WCRs. The shunt capacitance is constituted by interdigital fingers provided on the same piezoelectric su bstrate as one or both of the WCRs, perpendicularly to the WCR fingers to avoid surface wave interference, conveniently in space otherwise used by rails of the WCRs, opt ionally with oppositely phased fingers to cancel surface waves generated by these fingers . A capacitance can be similarly provided on the same substrate as any other typ e of surface wave device.