Resonator-type surface-acoustic-wave ladder filter with improved upper stopband attenuation and reduced insertion loss

A resonator-type surface-acoustic-wave filter having only one single series-arm surface-acoustic-wave resonator and at least one shunt-arm surface-acoustic-wave resonator coupled in a ladder circuit configuration, said series-arm surface-acoustic-wave resonator having an interdigital transducer and a pair of reflectors disposed on opposite sides of said interdigital transducer, wherein:

• said series-arm surface-acoustic-wave resonator satisfies the following condition A:
• (condition A) said reflectors are separated from said interdigital transducer by a distance t of at least (N/2+0.55).lambda..sub.1 but not more than (N/2+0.81).lambda..sub.1, where N is an arbitrary non-negative integer, and .lambda..sub.1 is a wavelength of a surface acoustic wave excited by said series-arm surface-acoustic-wave resonator;
• and
• every said shunt-arm surface-acoustic-wave resonator comprises an interdigital transducer with electrode fingers, and at least half of said electrode fingers are apodized.

A resonator-type surface-acoustic-wave filter having a plurality of series-arm surface-acoustic-wave resonators and at least one shunt-arm surface-acoustic-wave resonator coupled in a ladder circuit configuration, at least two of said series-arm surface-acoustic-wave resonators having respective interdigital transducers and respective pairs of reflectors disposed on opposite sides of the respective interdigital transducers, wherein:

• said at least two series-arm surface-acoustic-wave resonators satisfy the following condition A:
• (condition A) said reflectors are separated from said interdigital transducer by a distance t of at least (N/2+0.55).lambda..sub.1 but not more than (N/2+0.81).lambda..sub.1, where N is an arbitrary non-negative integer, and .lambda..sub.1 is a wavelength of a surface acoustic wave excited by said series-arm surface-acoustic-wave resonators;
• and
• said distance t is not identical in all series-arm surface-acoustic-wave resonators that satisfy said condition A.

A resonator-type surface-acoustic-wave filter having a plurality of series-arm surface-acoustic-wave resonators and at least one shunt-arm surface-acoustic-wave resonator coupled in a ladder circuit configuration, at least two of said series-arm surface-acoustic-wave resonators having respective interdigital transducers and respective pairs of reflectors disposed on opposite sides of the respective interdigital transducers, wherein:

• said at least two series-arm surface-acoustic-wave resonators satisfy the following condition A:
• (condition A) said reflectors are separated from said interdigital transducer by a distance t of at least (N/2+0.55).lambda..sub.1 but not more than (N/2+0.81).lambda..sub.1, where N is an arbitrary non-negative integer, and .lambda..sub.1 is a wavelength of a surface acoustic wave excited by said series-arm surface-acoustic-wave resonators;
• and
• said distance t differs in every series-arm surface-acoustic-wave resonator that satisfies said condition A.

A resonator-type surface-acoustic-wave filter having a plurality of series-arm surface-acoustic-wave resonators and at least one shunt-arm surface-acoustic-wave resonator coupled in a ladder circuit configuration, at least one of said series-arm surface-acoustic-wave resonators having an interdigital transducer and a pair of reflectors disposed on opposite sides of said interdigital transducer, wherein:

• said at least one of said series-arm surface-acoustic-wave resonators satisfies the following condition A:
• (condition A) said reflectors are separated from said interdigital transducers by a distance t of at least (N/2+0.55).lambda..sub.1 but not more than (N/2+0.81).lambda..sub.1, where N is an arbitrary non-negative integer and .lambda..sub.1 is a wavelength of a surface acoustic wave excited by said series-arm surface-acoustic-wave resonators;
• and
• every said shunt-arm surface-acoustic-wave resonator comprises an interdigital transducer with electrode fingers, and at least half of said electrode fingers are apodized.

A resonator-type surface-acoustic-wave filter having a plurality of series-arm surface-acoustic-wave resonators and at least one shunt-arm surface-acoustic-wave resonator coupled in a ladder circuit configuration, each of said series-arm surface-acoustic-wave resonators having a respective interdigital transducer, at least a first one of said series-arm surface-acoustic-wave resonators also having a pair of reflectors disposed on opposite sides of said interdigital transducer, wherein:

• said first one of said series-arm surface-acoustic-wave resonator satisfies the following condition A:
• (condition A) said reflectors are separated from said interdigital transducer by a distance t of at least (N/2+0.55).lambda..sub.1 but not more than (N/2+0.81).lambda..sub.1, where N is an arbitrary non-negative integer, and .lambda..sub.1 is a wavelength of a surface acoustic wave excited by said series-arm surface-acoustic-wave resonator;
• and
• at least a second one of said series-arm surface-acoustic-wave resonators does not satisfy said condition A but satisfies instead a condition B having three parts as follows:
• (first part of condition B) said interdigital transducers has electrode fingers, and at least thirty percent but not more than eighty percent of said electrode fingers are apodized, thereby creating within said interdigital transducers an excitation region for exciting surface acoustic waves and at least one dummy region for reflecting said surface acoustic waves;
• (second part of condition B) each of the electrode fingers that are apodized comprises a first part disposed in said excitation region and a second part disposed in said at least one dummy region;
• and
• (third part of condition B) said first part is offset from said second part by a distance t.sub.1 equal to at least 0.05 .lambda..sub.1 but not more than 0.31 .lambda..sub.1 in the direction of propagation of said surface acoustic waves.

A resonator-type surface-acoustic-wave filter having at least one series-arm surface-acoustic-wave resonator and at least one shunt-arm surface-acoustic-wave resonator coupled in a ladder circuit configuration, each said series-arm surface-acoustic-wave resonator having an interdigital transducer with electrode fingers, which excite surface acoustic waves with a wavelength .lambda..sub.1, wherein:

• at least one said series-arm surface-acoustic-wave resonator satisfies a condition B having three parts as follows:
• (first part of condition B) at least thirty percent but not more than eighty percent of the electrode fingers of said series-arm surface-acoustic-wave resonator are apodized, thereby creating within said interdigital transducer an excitation region and at least one dummy region;
• (second part of condition B) each of the electrode fingers that are apodized comprises a first part disposed in said excitation region and a second part disposed in said at least one dummy region;
• and
• (third part of condition B) said first part is offset from said second part by a distance t.sub.1 equal to at least 0.05.lambda..sub.1 but not more than 0.31.lambda..sub.1 in the direction of propagation of said acoustic waves.