A saw filter includes a first saw resonator having a pair of terminals and a predetermined resonance frequency (frp), the first saw resonator being provided in a parallel arm of the saw filter. A second saw resonator has a pair of terminals and a predetermined resonance frequency (frs) approximately equal to a predetermined antiresonance frequency of the first saw resonator (fap). The second saw resonator is provided in a series arm of the saw filter. An inductance element is connected in series to the first saw resonator.
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0. 6. A band-pass filter having a pair of band-pass filter input terminals and plural pairs of band-pass filter output terminals, comprising:
a pair of saw filters having respective, different pass bands and each saw filter having a pair of saw filter input terminals and a pair of saw filter output terminals and comprising a plurality of one-port saw resonators connected in a latter structure between the input and output terminals and including at least a first stage having a series-arm saw resonator connected to one of the pair of input terminals; a pair of saw filters having respective pass bands and comprising a plurality of one-port saw resonators connected in a ladder structure, each having at least a first stage located at a side of the pair of band-pass filter input terminals and a series-arm resonator located at the first stage, a pair of input terminals and a pair of output terminals; the pair of band-pass filter input terminals being commonly connected to the respective pairs of input terminals of the pair of saw filters; the plurality of pairs of band-pass filter output terminals being connected to the respective pairs of output terminals of the pair of saw filters.
0. 1. A band-pass filter having a pair of band-pass filter input terminals and plural pairs of band-pass filter output terminals, comprising:
a pair of saw filters having respective pass bands and comprising a plurality of one-port saw resonators connected in a ladder structure, each having at least a first stage located at a side of the pair of band-pass filter input terminals and a series-arm resonator located at the first stage, a pair of input terminals and a pair of output terminals; the pair of band-pass filter input terminals being commonly connected to the respective pairs of input terminals of the pair of saw filters; the plurality of pairs of band-pass filter output terminals being connected to the respective pairs of output terminals of the pair of saw filters; and an inductance element located between at least one of the saw filters located at the first stage and the pair of band-pass filter input terminals and directly connected between the respective pair of input terminals of the at least one of the saw filters and thereby in parallel to said at least one of the saw filters.
4. A band-pass filter having a predetermined pass-band characteristic and comprising:
a plurality of saw resonators connected in a ladder formation, said plurality of resonators being connected in respective said serial arms and parallel arms; and bonding inductance elements, said parallel arms of said ladder formation being connected to ground via respective said bonding inductance elements.
7. A band-pass filter having a predetermined pass-band characteristic and comprising:
a plurality of saw resonators connected in a ladder configuration of respective serial arms and parallel arms, said plurality of saw resonators being connected in respective said serial arms and parallel arms; and bonding inductance elements respectively connecting said parallel arms of said ladder configuration to ground.
0. 16. A saw filter comprising:
a first saw resonator in a parallel arm of said saw filter and having a predetermined resonance frequency; a second saw resonator in a series arm of said saw filter and having a predetermined resonance frequency approximately equal to an antiresonance frequency of said first saw resonator; and an inductance element connected in series with said first saw resonator, said inductance element functioning to increase the admittance of the parallel arm and decrease the resonance frequency.
2. A saw filter comprising:
a plurality of first saw resonators, each having a pair of terminals and a predetermined resonance frequency (frp), said first saw resonators being connected in respective, parallel arms of the saw filter; a plurality of second saw resonators, each having a pair of terminals and a predetermined resonance frequency (frs) approximately equal to an antiresonance frequency (fap) of each of the first saw resonators, said second saw resonators being provided in series arms of the saw filter; and inductance elements respectively connected in series with the first saw resonators in the parallel arms and formed of wires.
0. 9. An rf saw filter having a relative bandwidth equal to or greater than 2%, a suppression factor equal to or larger than 20 db, and an insertion loss less than or equal to 5 db comprising:
a plurality of first saw resonators, each having a pair of terminals and a predetermined resonance frequency, said first saw resonators being connected in respective, parallel arms of said saw filter; a plurality of second saw resonators, each having a pair of terminals and a predetermined resonance frequency approximately equal to an antiresonance frequency of each of said first saw resonators, said second saw resonators being provided in series arms of said saw filter; and #20# inductance elements respectively connected in series with said first saw resonators in the parallel arms.
0. 8. A saw filter comprising:
a plurality of first saw resonators, each having a pair of terminals and a predetermined resonance frequency, said first saw resonators being connected in respective, parallel arms of said saw filter; inductance elements connected in series to respective ones of said first saw resonators in the parallel arms of said saw filter; a plurality of second saw resonators, each having a pair of terminals and a predetermined resonance frequency approximately equal to an antiresonance frequency of each of said first saw resonators, said second saw resonators being provided in series arms of said saw filter; a first product of an aperture length and a number of electrode finger pairs of each of said first saw resonators being larger than a second product of an aperture length and a number of electrode finger pairs of each of said second saw resonators. #20#
3. The saw filter as claimed in
a package accommodating the first and second resonators and the inductance elements; and lead terminals extending from interiorly of the package to exteriorly thereof, said wires of the inductance elements being bonded to said lead terminals.
5. The band-pass filter as claimed in
0. 10. A saw filter as claimed in
0. 11. A saw filter as claimed in
0. 12. A saw filter as claimed in
0. 13. A saw filter as claimed in
0. 14. A saw filter as claimed in
0. 15. A saw filter as claimed in
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Application Ser. No. 09/314,943, filed May 4, 1999 and copending application Ser. No. 09/925,942, filed Aug. 10, 2001, are each reissues of U.S. Pat. No. 5,631,612 (application Ser. No. 08/369,492, filed Jan. 6, 1995).
This application is a continuation of application No. 07/965,774, filed Oct. 23, 1992, now U.S. Pat. No. 5,559,481, patented Sep. 24, 1996.
1. Field of the Invention
The present invention generally relates to surface acoustic wave (SAW) filters, and more particularly to a ladder-type SAW filter suitable for an RF (Radio Frequency) filter provided in pocket and mobile telephones, such as automobile phone sets and portable phones.
2. Description of the Prior Art
In Japan, an automobile phone or portable phone system has a specification in which a transmission frequency band is ±8.5 MHz about a center frequency of 933.5 MHz. The ratio of the above transmission band to the center frequency is approximately 2%.
Recently, SAW filters have been employed in automobile phone or portable phone systems. It is required that the SAW filters have characteristics which satisfy the above specification. More specifically, it is required that the pass band width is so broad that 1) the ratio of the pass band to the center frequency is equal to or greater than 2%, 2) the insertion loss is small and equal to 5 dB-2 dB, and 3) the suppression factor is high and equal to 20 db-30 dB.
In order to satisfy the above requirements, SAW filters are substituted for conventional transversal filters. Generally, SAW elements are so connected that a ladder-type filter serving as a resonator is formed.
The SAW filter 1 shown in
It is a general object of the present invention to provide a SAW filter in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a SAW filter having a large band width, a large suppression factor, and a small insertion loss.
The above objects of the present invention are achieved by a SAW filter comprising: a first SAW resonator (21, R1A, R1B) having a pair of terminals and a predetermined resonance frequency (frp), the first SAW resonator being provided in a parallel arm (24) of the SAW filter; a second SAW resonator (23) having a pair of terminals and a predetermined resonance frequency (frs) approximately equal to the predetermined antiresonance frequency of the first SAW resonator (fap), the second SAW resonator being provided in a series arm (24) of the SAW filter; and an inductance element (25, L1) connected in series to the first SAW resonator.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which:
The principle of the SAW filter 20 will now be described. Use of image parameters is convenient to verify whether or not a resonance circuit has a filter characteristic. The details of image parameters are described in the following document: Yanagisawa et al., "Theory and Design of Filters", Sanpo Shuppan, Electronics Sensho, pp.192-pp.203, 1974.
First of all, a basic ladder-type circuit having a filter characteristic will be described with reference to FIG. 4. Two black boxes 30 and 31 shown in
According to the image parameter method, an image transfer quantity γ (a complex number) defined in the following equation has the important meaning:
where V1 and I1 denote an input voltage and an input current, respectively, and V2 and I2 denote an output voltage and an output current, respectively. The equation (1) can be rewritten as follows:
where A, B, C and D denote parameters of an F matrix showing the whole circuit shown in FIG. 4. When the value expressed by the equation (2) is an imaginary number, the two-terminal-pair circuit shown in
Hence, the following equation (4) can be obtained from the equation (2) using the above ABCD parameters:
When 0<bx<1, that is, when b and x have the same sign and are small values, the entire circuit shown in
In order to qualitatively understand the frequency characteristics of b and x, the impedance and admittance of the SAW resonators will not be considered.
As shown in
A description will now be given of the factors that determine the band width in the resonator-type SAW filters. As is seen from
where τ denotes the capacitance ratio. The ratio of the pass band to the center frequency (Δf/fo) is mainly dependent on the difference between fr and fa, and is therefore expressed in the following expression, using the equations (6) and (7):
It can be seen from the equation (8) that the capacitance ratio τ is the main factor which determines the ratio of the pass band to the center frequency. However, as set forth in Japanese Laid-Open Patent Publication No. 52-19044, the capacitance ratio is much dependent on the type of substrate material used for the interdigital electrode. For example, an ST-cut crystal having a small electromechanical coupling coefficient has a capacitance ratio τ equal to or greater than 1300, while a 36°C Y-cut X-propagation LiTaO3 substrate having a large electromechanical coupling coefficient has a capacitance ratio τ of approximately 15. The ratio of the pass band to the center frequency is 0.04% for ST-cut crystal, and 3.3% for the 36°C Y-cut X-propagation LiTaO3 substrate. Hence, the band width is much dependent on the substrate material.
The band width decreases as the equivalent parallel capacitance COB increases in order to improve the side lobe suppression factor according to Japanese Laid-Open Patent Publication No. 52-19044.
The above phenomenon will now be described with reference to
The following two conditions must be satisfied in order to eliminate the above disadvantages. The first condition is to increase the difference between the resonance frequency fr and the antiresonance frequency fa in at least one of the resonators provided in the series and parallel arms (see FIG. 8C). The second condition is to increase either the impedance or admittance of the above-mentioned one of the resonators. As the impedance or admittance increases, the side lobe attenuation quantity increases. When the above two conditions awe satisfied, the side lobe attenuation quantity can be improved while the pass band is improved or prevented from being narrowed.
Regarding the first condition, it is effective to provide an inductor L connected in series to a SAW resonator having a pair of terminals in order to increase the difference between fr and fa.
It can be seen from
Regarding the aforementioned second condition, the admittance value increases due to the inductance L, as shown in FIG. 9B. However, as shown in
In order to increase the band width, it is also possible to select the antiresonance frequency fap of the parallel arm resonator and the resonance frequency frs of the series arm resonator so that frs>fap. In this case, the condition bx<0 occurs around the center frequency, and hence the aforementioned pass band condition is not satisfied. Hence, there is a possibility that an insertion loss and a ripple may increase. However, by controlling Δf=frs-fap, it is possible to substantially suppress the increase in the insertion loss and the ripple and to expand the increase in the pass band.
A description will now be given of embodiments of the present invention. The embodiments which will be described are based on a simulation. Hence, this simulation will be described first, as well as the results of comparisons between the experimental results and the simulation in order to show the validity of the simulation.
The equivalent circuit shown in
Hence, it will be apparent from the above that the results of a simulation of the filter with the combination of the resonators disposed in the parallel and series arms match the results of the experiment. The embodiments described below are based on the result of simulations.
As shown in
A variation of the configuration shown in
The band center frequencies f1 and f2 of the sixteenth through nineteenth embodiments of the present invention are not limited to 887 MHz and 932 MHz.
The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.
Matsuda, Takashi, Satoh, Yoshio, Ikata, Osamu, Miyashita, Tsutomu, Takamatsu, Mitsuo
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