Band reject filters

A radio frequency band reject filter comprising a plurality of shunt acoustic resonators each arranged to resonate generally at the reject frequency band and a plurality of series acoustic resonators each arranged to anti-resonate generally at the reject frequency band, the shunt and series acoustic resonators being arranged in a ladder configuration,

• wherein the plurality of series acoustic resonators includes a plurality of series connected SAW resonators formed on a common substrate, each SAW resonator sharing an input bus bar or output bus bar with an adjacent SAW resonator in the series.

A radio frequency band reject filter comprising a plurality of shunt acoustic resonators each arranged to resonate generally at the reject frequency band and a plurality of series acoustic resonators each arranged to anti-resonate generally at the reject frequency band, the shunt and series acoustic resonators being arranged in a ladder configuration,

• wherein the plurality of shunt acoustic resonators includes a plurality of SAW transducers formed on a common substrate, each SAW transducer being coupled in series with the fingers of one transducer feeding via a transition zone into the fingers of the next transducer in series, the transition zones each being formed as a plurality of nonparallel elongate conductors formed on the substrate and extending between the fingers of each successive transducer in the series, and the first and last transducer in series being coupled to an input and output bus bar respectively.

A base station power amplifier for a cellular radio network, the power amplifier including at least one inter-stage band reject filter comprising a shunt acoustic resonator and a series acoustic resonator, the shunt acoustic resonator being arranged to resonate generally at the reject frequency band and the series acoustic resonator being arranged to anti-resonate generally at the reject frequency band,

• wherein the series acoustic resonator includes a plurality of series connected SAW resonators formed on a common substrate, each SAW resonator sharing an input bus bar or output bus bar with an adjacent SAW resonator in the series.

A duplexer for a mobile telephone handset including a radio frequency band reject filter comprising a shunt acoustic resonator and a series acoustic resonator, the shunt acoustic resonator being arranged to resonate generally at the reject frequency band and the series acoustic resonator being arranged to anti-resonate generally at the reject frequency band,

• wherein the shunt acoustic resonator includes a plurality of SAW transducers formed on a common substrate, each SAW transducer being coupled in series with the fingers of one transducer feeding via a transition zone into the fingers of the next transducer in series, the transition zones each being formed as a plurality of nonparallel elongate conductors formed on the substrate and extending between the fingers of each successive transducer in the series, and the first and last transducer in series being coupled to an input and output bus bar respectively.

A low noise amplifier input stage including a band reject filter comprising a shunt acoustic resonator and a series acoustic resonator, the shunt acoustic resonator being arranged to resonate generally at the reject frequency band and the series acoustic resonator being arranged to anti-resonate generally at the reject frequency band, wherein the resonators are arranged such that there is substantially no acoustic resonance over a predetermined pass band, and

• wherein the shunt acoustic resonator includes a plurality of SAW transducers formed on a common substrate, each SAW transducer being coupled in series with the fingers of one transducer feeding via a transition zone into the fingers of the next transducer in series, the transition zones each being formed as a plurality of nonparallel elongate conductors formed on the substrate and extending between the fingers of each successive transducer in the series, the first and last transducer in series being coupled to an input and output bus bar respectively.