Surface acoustic wave filter

Claim 2

The SAW filter as claimed in claim 1, characterized in that an aperture length (Ap) of the first SAW resonator (R1A) is larger than an aperture length (As) of the second SAW resonator.

Claim 3

The SAW filter as claimed in claim 1, characterized in that a number (Np) of electrode finger pairs of the first SAW resonator (R1B) is larger than a number (Ns) of electrode finger pairs of the second SAW resonator (R2).

Claim 4

The SAW filter as claimed in claim 1, characterized in that the first SAW resonator comprises an exciting interdigital electrode (131), and first and second reflectors (132, 133) respectively located on opposite sides of the exciting electrode so that β is equal to 0.4 in which said β is defined in the following equation: d = (n + β)·λ where d denotes a distance between the exciting electrode and each of the first and second reflectors, n is an integer, β is a real number equal to or less than 1, and λ denotes a period of the exciting interdigital electrode corresponding to the resonance frequency.

Claim 5

The SAW filter as claimed in claim 1, characterized in that: the exciting interdigital electrode and the first and second reflectors comprises either aluminum or an aluminum alloy containing a few percentage of metal other than aluminum by weight; and film thicknesses of the exciting interdigital electrode and the first and second reflectors are equal to 0.06 - 0.09 times the period of the exciting interdigital electrode.

Claim 6

The SAW filter as claimed in claim 1, characterized in that: the exciting interdigital electrode and the first and second reflectors comprise either gold or a gold alloy containing a few percentage of metal other than gold by weight; and film thicknesses of the exciting interdigital electrode and the first and second reflectors are equal to 0.0086 - 0.013 times the period of the exciting interdigital electrode.

Claim 7

The SAW filter as claimed in claim 1, characterized in that said inductance element comprises a bonding wire.

Claim 8

The SAW filter as claimed in claim 1, characterized in that said inductance element comprises: a ceramic package (81) accommodating a filter chip (82) on which the first and second SAW resonators are formed; and a microstrip line (220) which is formed on the ceramic package and extends from the first SAW resonator to a terminal (84₋₃).

Claim 9

The SAW filter as claimed in claim 1, characterized in that said inductance element comprises: a ceramic chip (82) on which the first and second SAW resonators are formed; and a microstrip line (230) which is formed on the ceramic chip and extends from the first SAW resonator.

Claim 10

A SAW filter characterized by comprising: a plurality of first SAW resonators (R1, R3, R5), each having a pair of terminals and a predetermined resonance frequency (f rp ), said first SAW resonators being respectively provided in parallel arms (62-64) of the SAW filter; a plurality of second SAW resonators (R2, R4), each having a pair of terminals and a predetermined resonance frequency (f rs ) approximately equal to a predetermined antiresonance frequency of the first SAW resonator (f ap ), said second SAW resonators being provided in a series arm (61) of the SAW filter; and inductance elements (Ls) respectively connected in series to the second SAW resonators.

Claim 11

The SAW filter as claimed in claim 10, characterized in that: each of the first SAW resonators comprises an exciting interdigital electrode (131), and first and second reflectors (132, 133) respectively located on opposite sides of the exciting electrode so that β is equal to 0.4 in which said β is defined in the following equation: d = (n + β)· λ where d denotes a distance between the exciting electrode and each of the first and second reflectors, n is an integer, β is a real number equal to or smaller than 1, and λ denotes a period of the exciting interdigital electrode corresponding to the resonance frequency.

Claim 12

The SAW filter as claimed in claim 11, characterized in that said inductance elements comprise: a ceramic package (81) accommodating a filter chip (82) on which the first and second SAW resonators are formed; and microstrip lines (220) which are formed on the ceramic package and extend from the first SAW resonators to terminals.

Claim 13

The SAW filter as claimed in claim 11, characterized in that said inductance elements comprise: a ceramic chip (82) on which the first and second SAW resonators are formed; and microstrip lines (230) which are formed on the ceramic chip and extend from the first SAW resonators.

Claim 14

A SAW filter characterized by comprising: a plurality of first SAW resonators (R1, R3, R5), each having a pair of terminals and a predetermined resonance frequency (f rp ), said first SAW resonators being respectively provided in parallel arms (62-64) of the SAW filter; a plurality of second SAW resonators (R2, R4), each having a pair of terminals and a predetermined resonance frequency (f rs ) approximately equal to the predetermined antiresonance frequency of the first SAW resoantor (f ap ), said second SAW resonators being provided in a series arm (61) of the SAW filter; and inductance elements (L1, L2, L3) respectively connected in series to the first SAW resonators in the parallel arms.

Claim 15

A SAW filter characterized by comprising: a first SAW resonator (R p ₁-R p ₃) having a pair of terminals, a first resonance frequency (f rp ), and a first antiresonance frequency (f ap ) based on the first resonance frequency and a first capacitance ratio (τ) and higher than the first resonance frequency, said first SAW resonator being provided in a parallel arm of the SAW filter; and a second SAW resonator (R s1 , R s ₂) having a pair of terminals, a second resonance frequency (f rs ), and a second antiresonance frequency (f as ) based on the second resonance frequency and a second capacitance ratio (τ) and higher than the second resonance frequency, said second SAW resonator being provided in a series arm of the SAW filter, wherein: the second resonance frequency is higher than the first antiresonance frequency; and a difference between the second resonance frequency and the first antiresonance frequency is equal to a value which provides an allowable ripple range and an allowable insertion loss.

Claim 16

The SAW filter as claimed in claim 15, characterized in that: the first and second SAW resonators are formed on a piezoelectric substrate; the second SAW comprises an interdigital electrode which is formed on the pizeoelectric substrate and which has a predetermined period; and a normalized value obtained by normalizing said difference by the second resonance frequency is larger than zero, and is smaller than a defined by the following equation: α = 1/ P(τ²+τ)/0.06 - τ) where P is a ratio of an electrostatic capacitance based on an aperture length and a number of finger pairs of the interdigital electrode of the second SAW resonator to an electrostatic capacitance based on an aperture length and a number of finger pairs of an interdigital electrode of the first SAW resonator.

Claim 17

The SAW filter as claimed in claim 15, characterized in that: the first and second SAW resonators are formed on a piezoelectric substrate; the second SAW comprises an interdigital electrode which is formed on the piezoelectric substrate and which has a predetermined period; and said piezoelectric substrate comprises 36° Y-cut X-propagation LiTaO₃ (241); and the predetermined period of the interdigital electrode of the second SAW resonator is selected so that said normalized ratio is larger than zero and is smaller than said a defined by the following equation: α = 6.67x10⁻²/(4.22 P -1) where P is a ratio of an electrostatic capacitance based on an aperture length and a number of finger pairs of the interdigital electrode of the second SAW resonator to an electrostatic capacitance based on an aperture length and a number of finger pairs of an interdigital electrode of the first SAW resonator.

Claim 18

The SAW filter as claimed in claim 15, characterized in that: the first and second SAW resonators are formed on a piezoelectric substrate; the second SAW comprises an interdigital electrode which is formed on the piezoelectric substrate and which has a predetermined period; and said piezoelectric substrate comprises 64° Y-cut X-preparation LiTaO₃ (241); and the predetermined period of the interdigital electrode of the second SAW resonator is selected so that said normalized ratio is larger than zero and is smaller than said α defined by the following equation: α = 1.47x10⁻²/(4.37 P -1) where P is a ratio of an electrostatic capacitance based on an aperture length and a number of finger pairs of the interdigital electrode of the second SAW resonator to an electrostatic capacitance based on an aperture length and a number of finger pairs of an interdigital electrode of the first SAW resonator.

Claim 19

The SAW filter as claimed in claim 15, characterized in that: the first and second SAW resonators are formed on a piezoelectric substrate; the second SAW comprises an interdigital electrode which is formed on the piezoelectric substrate and which has a predetermined period; and said piezoelectric substrate comprises 41° Y-cut X-propagation LiTaO₃ (241); and the predetermined period of the interdigital electrode of the second SAW resonator is selected so that said normalized ratio is larger than zero and is smaller than said a defined by the following equation: α = 2.273x10⁻¹/(4.52 P -1) where P is a ratio of an electrostatic capacitance based on an aperture length and a number of finger pairs of the interdigital electrode of the second SAW resonator to an electrostatic capacitance based on an aperture length and a number of finger pairs of an interdigital electrode of the first SAW resonator.

Claim 20

A SAW filter characterized by comprising: a plurality of first SAW resonators (R p ₁-R p3 ), each having a pair of terminals, a first resonance frequency (f rp ), and a first antiresonance frequency (f ap ) based on the first resonance frequency and a first capacitance ratio (τ) and higher than the first resonance frequency, said first SAW resonators being respectively provided in parallel arms of the SAW filter; and a plurality of second SAW resonators (R s1 , R s ₂), each having a pair of terminals, a second resonance frequency (f rs ), and a second antiresonance frequency (f as ) based on the second resonance frequency and a second capacitance ratio (τ) and higher than the second resonance frequency, said second SAW resonators being provided in a series arm of the SAW filter, wherein: the first and second SAW resonators are connected so that a ladder-type filter structure is formed; the second resonance frequency is higher than or equal to the first antiresonance frequency; a first outermost arm closest to either an input or an output of the SAW filter is said series arm, and a second outermost arm closest to a remaining one of the input and the output is one of the parallel arms; and one of the second SAW resonators provided in the first outermost arm has an impedance smaller than that of each of remaining second SAW resonators provided in the series arm and located inside said one of the second SAW resonators.

Claim 21

The SAW resonator as claimed in claim 20, characterized in that one of the first SAW resonators provided in the second outermost arm has an admittance smaller than that of each of remaining first SAW resonators provided in the parallel arms and located inside said one of the first SAW resonators.

Claim 22

The SAW resonator as claimed in claim 20, characterized in that the impedance of said one of the second SAW resonators is half that of each of the remaining second SAW resonators.

Claim 23

The SAW resonator as claimed in claim 21, characterized in that the impedance of said one of the second SAW resonators is half that of each of the remaining second SAW resonators.

Claim 24

The SAW resonator as claimed in claim 20, characterized in that: a first electrostatic capacitance based on a first product is larger than a second electrostatic capacitance based on a second product; the first product is a product of an aperture length of said one of the second SAW resonators in the first outermost arm, a number of finger pairs thereof, and a dielectric constant of a substrate of the first and second SAW resonators; and the second product is a product of an aperture length of each of said remaining second SAW resonators, a number of finger pairs thereof, and said dielectric constant.

Claim 25

The SAW resonator as claimed in claim 20, characterized in that each of said remaining second SAW resonators in the series arm comprises a plurality of SAW resonators connected in series, and has the same capacitance as said one of the second SAW resonators provided in the first outermost arm.

Claim 26

The SAW resonator as claimed in claim 21, characterized in that: a first electrostatic capacitance based on a first product is larger than a second electrostatic capacitance based on a second product; the first product is a product of an aperture length of said one of the second SAW resonators in the first outermost arm, a number of finger pairs thereof, and a dielectric constant of a substrate of the first and second SAW resonators; and the second product is a product of an aperture length of each of said remaining second SAW resonators, a number of finger pairs thereof, and said dielectric constant.

Claim 27

The SAW resonator as claimed in claim 21, characterized in that each of said remaining second SAW resonators in the series arm comprises a plurality of SAW resonators connected in series, and has the same impedance as said one of the second SAW resonators provided in the first outermost arm.

Claim 28

The SAW resonator as claimed in claim 21, characterized in that the admittance of said one of the first SAW resonators is half that of each of the remaining first SAW resonators.

Claim 29

The SAW resonator as claimed in claim 21, characterized in that: a first electrostatic capacitance based on a first product is larger than a second electrostatic capacitance based on a second product; the first product is a product of an aperture length of said one of the first SAW resonators in the second outermost arm, a number of finger pairs thereof, and a dielectric constant of a substrate of the first and second SAW resonators; and the second product is a product of an aperture length of each of said remaining first SAW resonators, a number of finger pairs thereof, and said dielectric constant.

Claim 30

The SAW resonator as claimed in claim 21, characterized in that each of said remaining first SAW resonators in the parallel arms comprises a plurality of SAW resonators connected in series, and has the same capacitance as said one of the first SAW resonators provided in the second outermost arm.

Claim 31

The SAW resonator as claimed in claim 23, characterized in that: a first electrostatic capacitance based on a first product is larger than a second electrostatic capacitance based on a second product; the first product is a product of an aperture length of said one of the second SAW resonators in the first outermost arm, a number of finger pairs thereof, and a dielectric constant of a substrate of the first and second SAW resonators; and the second product is a product of an aperture length of each of said remaining second SAW resonators, a number of finger pairs thereof, and said dielectric constant.

Claim 32

The SAW resonator as claimed in claim 23, characterized in that each of said remaining second SAW resonators in the series arm comprises a plurality of SAW resonators connected in series, and has the same capacitance as said one of the second SAW resonators provided in the first outermost arm.

Claim 33

The SAW resonator as claimed in claim 28, characterized in that: a first electrostatic capacitance based on a first product is larger than a second electrostatic capacitance based on a second product; the first product is a product of an aperture length of said one of the first SAW resonators in the second outermost arm, a number of finger pairs thereof, and a dielectric constant of a substrate of the first and second SAW resonators; and the second product is a product of an aperture length of each of said remaining first SAW resonators, a number of finger pairs thereof, and said dielectric constant.

Claim 34

The SAW resonator as claimed in claim 28, characterized in that each of said remaining first SAW resonators in the parallel arms comprises a plurality of SAW resonators connected in series, and has the same capacitance as said one of the first SAW resonators provided in the second outermost arm.

Claim 35

A SAW filter characterized by comprising: a plurality of first SAW resonators (R p1 -R p3 ), each having a pair of terminals, a first resonance frequency (f rp ), and a first antiresonance frequency (f ap ) based on the first resonance frequency and a first capacitance ratio (τ) and higher than the first resonance frequency, said first SAW resonators being respectively provided in parallel arms of the SAW filter; and a plurality of second SAW resonators (R s1 , R s2) , each having a pair of terminals, a second resonance frequency (f rs ), and a second antiresonance frequency (f as ) based on the second resonance frequency and a second capacitance ratio (τ) and higher than the second resonance frequency, said second SAW resonators being provided in a series arm of the SAW filter, wherein: the first and second SAW resonators are connected so that a ladder-type filter structure is formed; the second resonance frequency is higher than or equal to the first antiresonance frequency; a first outermost arm closest to either an input or an output of the SAW filter is said series arm, and a second outermost arm closest to a remaining one of the input and the output is one of the parallel arms; and one of the first SAW resonators provided in the second outermost arm has an admittance smaller than that of each of remaining first SAW resonators provided in the parallel arms and located inside said one of the first SAW resonators.

Claim 36

The SAW resonator as claimed in claim 35, characterized in that the admittance of said one of the first SAW resonators is half that of each of the remaining first SAW resonators.

Claim 37

The SAW resonator as claimed in claim 35, characterized in that: a first electrostatic capacitance based on a first product is larger than a second electrostatic capacitance based on a second product; the first product is a product of an aperture length of said one of the first SAW resonators in the second outermost arm, a number of finger pairs thereof, and a dielectric constant of a substrate of the first and second SAW resonators; and the second product is a product of an aperture length of each of said remaining first SAW resonators, a number of finger pairs thereof, and said dielectric constant.

Claim 38

The SAW resonator as claimed in claim 35, characterized in that each of said remaining first SAW resonators in the parallel arms comprises a plurality of SAW resonators connected in series, and has the same capacitance as said one of the first SAW resonators provided in the second outermost arm.

Claim 39

A SAW filter characterized by comprising: a plurality of first SAW resonators (R p1 -R p3 ), each having a pair of terminals, a first resonance frequency (f rp ), and a first antiresonance frequency (f ap ) based on the first resonance frequency and a first capacitance ratio (τ) and higher than the first resonance frequency, said first SAW resonators being respectively provided in parallel arms of the SAW filter; and a plurality of second SAW resonators (R s1 , R s ₂), each having a pair of terminals, a second resonance frequency (f rs ), and a second antiresonance frequency (f as ) based on the second resonance frequency and a second capacitance ratio (τ) and higher than the second resonance frequency, said second SAW resonators being provided in a series arm of the SAW filter, wherein: the first and second SAW resonators are connected so that a ladder-type filter structure is formed; the second resonance frequency is higher than or equal to the first antiresonance frequency; a first outermost arm closest to an input of the SAW filter is one of the parallel arms, and a second outermost arm closest to an output of the SAW filter is another one of the parallel arms; and one of the first SAW resonators provided in at least one of the first and second outermost arms has an admittance smaller than that of each of remaining first SAW resonators provided in the parallel arms and located inside said one of the first SAW resonators.

Claim 40

The SAW resonator as claimed in claim 39, characterized in that the admittance of said one of the first SAW resonators is half that of each of the remaining first SAW resonators.

Claim 41

The SAW resonator as claimed in claim 40, characterized in that: a first electrostatic capacitance based on a first product is larger than a second electrostatic capacitance based on a second product; the first product is a product of an aperture length of said one of the first SAW resonators in the second outermost arm, a number of finger pairs thereof, and a dielectric constant of a substrate of the first and second SAW resonators; and the second product is a product of an aperture length of each of said remaining first SAW resonators, a number of finger pairs thereof, and said dielectric constant.

Claim 42

The SAW resonator as claimed in claim 40, characterized in that each of said remaining first SAW resonators in the parallel arms comprises a plurality of SAW resonators connected in series, and has the same capacitance as said one of the first SAW resonators provided in one of the first and second outermost arms.

Claim 43

A SAW filter characterized by comprising: a plurality of first SAW resonators (R p1 -R p3 ), each having a pair of terminals, a first resonance frequency (f rp ), and a first antiresonance frequency (f ap ) based on the first resonance frequency and a first capacitance ratio (τ) and higher than the first resonance frequency, said first SAW resonators being respectively provided in parallel arms of the SAW filter; and a plurality of second SAW resonators (R s1 , R s ₂), each having a pair of terminals, a second resonance frequency (f rs ), and a second antiresonance frequency (f as ) based on the second resonance frequency and a second capacitance ratio (τ) and higher than the second resonance frequency, said second SAW resonators being provided in a series arm of the SAW filter, wherein: the first and second SAW resonators are connected so that a ladder-type filter structure is formed; the second resonance frequency is higher than or equal to the first antiresonance frequency; a first outermost arm closest to an input of the SAW filter is said series arm, and a second outermost arm closest to an output of the SAW filter is also said series arm; and one of the second SAW resonators provided in at least one of the first and second outermost arms has an impedance smaller than that of each of remaining second SAW resonators provided in the series arms and located inside said one of the second SAW resonators.

Claim 44

The SAW resonator as claimed in claim 43, characterized in that the impedance of said one of the second SAW resonators is half that of each of the remaining second SAW resonators.

Claim 45

The SAW resonator as claimed in claim 43, characterized in that each of said remaining first SAW resonators in the parallel arms comprises a plurality of SAW resonators connected in series, and has the same capacitance as said one of the first SAW resonators provided in the series arm.

Claim 46

A SAW filter characterized by comprising: a first SAW resonator (21, R1A, R1B) having a pair of terminals and a predetermined resonance frequency (f rp ), said first SAW resonator being provided in a parallel arm (24) of the SAW filter; and a second SAW resonator (23) having a pair of terminals and a predetermined resonance frequency (f rs ) approximately equal to or higher than a predetermined antiresonance frequency of the first SAW resonator (f ap ), said second SAW resonator being provided in a series arm (24) of the SAW filter, a first electric resistance (rs) of an interdigital electrode of said second SAW resonator is smaller than a second electric resistance (rp) of an interdigital electrode of said first SAW resonator.

Claim 47

The SAW resonator as claimed in claim 46, characterized in that: an aperture length (ls) of the interdigital electrode of said second SAW resonator is smaller than that (lp) of the interdigital electrode of said first SAW resonator; and a number (Ns) of finger pairs of the interdigital electrode of said second SAW resonator is larger than a number (Np) of finger pairs of the interdigital electrode of said first SAW resonator.

Claim 48

The SAW resonator as claimed in claim 46, characterized in that a film thickness of the interdigital electrode of said first SAW resonator is smaller than a film thickness of the interdigital electrode of said second SAW resonator.

Claim 49

A band-pass filter characterized by comprising: a plurality of SAW filters (F1, F2), the SAW filters having respective pass bands and SAW resonators; a pair of input terminals commonly connected to the plurality of SAW filters; and a plurality of pairs of output terminals respectively connected to the plurality of SAW filters.

Claim 50

The band-pass filter as claimed in claim 49, characterized in that each of the SAW filters comprises a series-arm resonator located at a first stage of each of the SAW filters.

Claim 51

The band-pass filter as claimed in claim 50, characterized by further comprising an inductance element (L) located between at least one of the SAW filters and the pair of input terminals, and connected in parallel to said at least one of the SAW filters.

Claim 52

The band-pass filter as claimed in claim 51, characterized by further comprising a capacitance element (C) connected in series between said inductance element and said series-arm resonator.

Claim 53

The band-pass filter as claimed in claim 49, characterized in that: a first one (F1) of the SAW filters comprises a series-arm SAW resonator located at a first stage of said first one of the SAW filters, and a parallel-arm SAW resonator connected to said series-arm SAW resonator; a second one (F2) of the SAW filter comprises a parallel-arm SAW resonator located at a first stage of said second one of the SAW filters, and a series-arm SAW filter connected to said parallel-arm SAW resonator located at the first stage of the second one of the SAW filters, and that the band-pass filter further comprises a line (S) used for phase rotation and connected in series between one of the pair of input terminals and the second one of the SAW filters.

Claim 54

The band-pass filter as claimed in claim 53, characterized by further comprising an inductance element located between the first one of the SAW filters and the pair of input terminals, and connected in parallel to the first one of the SAW filters.

Claim 55

The band-pass filter as claimed in claim 54, characterized by further comprising a capacitance element (C) connected in series between said inductance element and the series-arm resonator of said first one of the SAW filters.