JP2003087096A - Ladder-type surface accoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ladder-type SAW filter, which has superior filter characteristic which oppresses a ripple within a passing band and is small in an insertion loss and an in-band deviation, in the ladder-type SAW filter. SOLUTION: A ladder-type surface accoustic wave filter employs a surface accoustic wave resonator. In the surface accoustic wave resonator, an interdigital transducer of an electrode cycle λand grating reflectors on both sides of the interdigital transducer are disposed along the propagation direction of a surface accoustic wave on a main surface of a piezoelectric substrate. With the arrangement of the ladder-type surface accoustic wave filter, ratio Lt/Lr of a pitch Lt of the interdigital transducer to a pitch Lr of the grating reflector is set to be smaller than 1, additionally a center-to-center distance Ltr between neighboring electrode fingers of the interdigital transducer and the grating reflector is set to be smaller than 0.5 λ, and further, Lts of the surface accoustic wave resonator of at least two or more from among a plurality of surface accoustic wave resonators which are disposed in a serial arm are made to differ from each other.

Description

Description: BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a ladder type surface acoustic wave filter, and more particularly, to suppressing a spurious appearing near a resonance frequency of a surface acoustic wave resonator used in a series arm. The present invention relates to a ladder type surface acoustic wave filter in which the in-band deviation is reduced by doing so. [0002] In recent years, Surface Acoustic W
Ave (hereinafter, SAW) devices are often used in, for example, mobile phones because of their excellent features such as high performance, small size, and mass productivity. FIG. 5 is a view showing an example of a SAW resonator, in which interdigital transducers (Interdigital Tras) are formed on the main surface of the piezoelectric substrate 1 along the SAW propagation direction.
nsducer: Hereinafter, IDT) 2 is arranged, and SAW
The ID to confine energy on the IDT 2
A grating reflector (hereinafter referred to as “reflector”) 3 is arranged at a position on both sides of T2 in the SAW propagation direction. The IDT electrode 2 is formed of a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween, and a pair of input lead terminals (IN) and output lead terminals (OUT) extending from each of the comb-shaped electrodes. Thus, a one-port SAW resonator is provided. [0003] Recently, SAW devices have been replaced by GPS (Global P
Ositioning System) Receivers are increasingly used. This is because the SAW device has advantages in addition to the above features, such as being suitable for high frequency operation and low insertion loss. In the GPS system, a terrestrial GPS receiver receives a radio wave transmitted from a GPS satellite, synchronizes a signal subjected to spread spectrum modulation (SS signal), extracts time data therefrom, and compares it with a reference time. This system also receives radio waves from a plurality of GPS satellites, derives relative time differences, converts these time differences into distances, and calculates the position of the receiver with high accuracy. A carrier transmitted from a GPS satellite has 157 carriers.
Since the spread spectrum modulation method is used for modulating the signal at 5.42 MHz, the pass bandwidth is ± 1.0.
23 MHz. However, the pass band width ± 1.023
To guarantee MHz, change in bandwidth due to temperature change,
Taking into account the variation in the resonance frequency of each SAW resonator at the time of manufacture, a pass band width of about ± 10 MHz is required, and the maximum insertion loss in the band is 1.5 dB.
Is required. Therefore, the applicant of the present application has designed the RF stage filter of the GPS receiver using a ladder type SAW filter which is suitable for relatively wide band. FIG. 6 is an electrical equivalent circuit diagram of a ladder-type SAW filter designed for use in the RF stage of a GPS receiver, and shows a piezoelectric resonator (the SAW resonator is one of the piezoelectric resonators).
Are represented by the symbols Xs, Xp
Xs is a SAW resonator having a serial arm, and Xp is a SAW resonator having a parallel arm. FIG. 7 shows an electrode pattern actually formed on the piezoelectric substrate 1 based on the equivalent circuit diagram shown in FIG. 6, in which the SAW resonators 4, 5, 6, and 7 are the series arm resonators Xs,
The SAW resonators 8 and 9 are the parallel arm resonators Xp. Here, as shown in FIG. 8, the electrode period of the IDT electrode 2 of the SAW resonator is λ, the electrode pitch is Lt (Lt = λ / 2), the cross width is W, the electrode pitch of the reflector 3 is Lr, and the IDT electrode is 2
The distance between the centers of electrode fingers adjacent to each other and the reflector 3 is L
Let it be tr. FIG. 9 shows a ladder type SA shown in FIGS. 6 and 7 using 42 ° Y-cut X-propagation LiTaO ₃ for the piezoelectric substrate.
IDT of SAW resonators Xs and Xp constituting W filter
The electrode film thickness is 2300 °, various constants (number of IDT electrode pairs: N, number of reflectors: M, IDT electrode period: λ, cross width: W, Lt /
Lr, Ltr) are shown in Table 1 below. As shown in FIG. 9, in the pass band characteristics of a ladder type SAW filter, a ripple R1 near the center frequency is obtained.
And the insertion loss and the in-band deviation are degraded. In order to analyze and investigate the cause, a SAW resonator was prototyped and return loss characteristics and impedance characteristics were measured. As shown in FIG. 10, in the return loss characteristic and the impedance characteristic, it was found that spurious appeared at Sp1 immediately near the low frequency side of the resonance frequency. That is, the serial arm SAW
In a ladder-type SAW filter that forms a pass band near the resonance frequency of a resonator, the above-described spurious component causes a ripple R1 in the pass band, which causes insertion loss and in-band deviation to deteriorate. The results of the trial production, experiment and analysis have been revealed. Therefore, an attempt was made to suppress the ripple R1 generated in the pass band of the ladder type SAW filter by suppressing the spurious signal Sp1. In order to find a solution for suppressing the spurious Sp1,
From the filter characteristics obtained by a simulation based on the filter theory by setting various constants of the SAW resonator and the filter characteristics of the actually manufactured SAW resonator,
The behavior of spurious components appearing on the low frequency side near the resonance frequency of the resonator was analyzed. If Lt / Lr is set to be smaller than 1, the frequency ft of the maximum radiation conductance exhibited by the IDT electrode is lower than the center frequency fr of the stop band presented by the reflector, so that the pitch Lr of the reflector is made larger than Lt. As a result, the center frequency fr of the stop band can be approximated to ft, and SA
It is well known that the Q value of a W device can be improved. As a result of various studies, as described in Japanese Patent Application No. 2001-217616 filed by the same applicant, SAW
When Lt / Lr of the resonator is set to 0.98 and Ltr is set to 0.45λ, spurious near the low frequency side of the resonance frequency is greatly improved in the return loss characteristic and the impedance characteristic as shown in FIG. Was. still,
In the curve of FIG. 11, the solid line represents the SAW resonator after the improvement,
The broken line is the characteristic data of the conventional SAW resonator. Based on the experimental results, a ladder type SAW
The IDT electrode film thickness of the SAW resonators Xs and Xp constituting the filter is 2300 °, and various constants (number of IDT electrode pairs: N, number of reflectors: M, IDT electrode period: λ, cross width: W, Lt / L)
(r, Ltr) was set as shown in Table 2 below, and a trial production was performed. As shown in FIG. 12, a pass band characteristic in which the ripple was reduced and the insertion loss and the in-band deviation were improved was obtained. In the curve of FIG. 12, the solid line shows the pass band characteristics of the improved ladder type SAW filter, and the broken line shows the pass band characteristics of the conventional ladder type SAW filter. [Table 2] [0011] However, FIG.
In the pass band characteristics of the above, a slight ripple R2 is still observed in the vicinity of the center frequency, and there has been a problem that a market requirement specification, for example, a specification of ripple: 0.2 dB or less cannot be satisfied. The reason that this ripple appears is that the SAW resonator constituting the ladder type SAW filter,
In particular, as described above, the spurs are generated near the lower frequency side of the resonance frequency of the SAW resonator arranged in the series arm. As can be seen from the SAW resonator return loss characteristics after the improvement shown in FIG. 11, the spurious Sp2 is still slightly observed near the low frequency side of the resonance frequency. Ladder type SAW
Lt / Lr of SAW resonator of series arm constituting filter
Is 0.98 (less than 1) and Ltr is 0.45λ.
(Smaller than 0.50λ), the ripple generated in the pass band is reduced. However, in order to further improve the ripple, optimization of the above-described parameter 2 condition or the parameter 2 condition In addition to this, there was a need to find further solutions. In addition, considering the mass production of the ladder type SAW filter, there has been a problem that the size of the ripple may vary depending on the production lot. SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to limit the reduction of ripples in the pass band of a ladder type SAW filter and to provide a good filter characteristic with small insertion loss and in-band deviation. Ladder type SAW
The purpose is to provide a filter. According to a first aspect of the present invention, there is provided a ladder-type SAW filter according to the present invention. The ladder-type SAW filter is provided on a main surface of a piezoelectric substrate along a SAW propagation direction. In a ladder type SAW filter using an IDT having an electrode period λ and a SAW resonator having reflectors arranged on both sides of the IDT, the ratio Lt / Lr of the pitch Lt of the IDT to the pitch Lr of the reflector is made smaller than 1. Along with
The distance Ltr between the centers of adjacent electrode fingers of the IDT and the reflector is made smaller than 0.5λ, and at least two or more SAW resonators among a plurality of SAW resonators arranged in a series arm are used.
A ladder-type SAW filter characterized in that the resonators have different Lt values. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings. FIG. 1 is data showing pass band characteristics of a ladder-type SAW filter according to the present invention. As shown in FIG. 2, the series-arm SAW resonators Xs1 to Xs4 and the parallel-arm SAW resonator constituting the ladder-type SAW filter are shown in FIG. The XT IDT electrode film thickness is set to 2300 °, and various constants (number of IDT electrode pairs: N, number of reflectors: M, IDT electrode period: λ, cross width: W, Lt / Lr, Ltr) are set as shown in Table 3 below. 7 shows data obtained by measuring the pass band characteristics of a prototype ladder-type SAW filter. [Table 3] As shown in FIG. 3A, the basic section of the ladder type resonator filter is composed of a parallel arm resonator Xp and a series arm resonator Xs, and the reactance curve of each arm is shown in FIG. It is set as in b). That is, by setting the anti-resonance frequency of the parallel arm resonator Xp and the resonance frequency of the series arm resonator Xs to be substantially the same, the band-pass filter as shown in FIG. Is well known. That is, the serial arm SA which is a problem discussed in the present invention.
If spurious components exist near the low frequency side of the resonance frequency of the W resonator, ripples appear near the center frequency of the filter. Heretofore, it has been common practice among those skilled in the art that a plurality of SAW resonators used in a series arm are connected to the same SAW resonator to form a ladder-type SAW filter.
However, when designing a filter for the RF stage of a GPS receiver, to satisfy the required specifications, realization with a conventional ladder-type SAW filter is equally difficult, and it is necessary to suppress ripples that have not been recognized as a problem. Is extremely severe, and the conventional design concept has failed to satisfy the required specifications. The applicant of the present invention has made various trial productions, experiments, and studies, and as a result, has found that the spurious near the low frequency side of the resonance frequency of the series arm resonator, which causes ripple in the pass band, can be further reduced. Determined that it was severe, and made it possible to suppress the ripple based on a completely new idea.
In other words, a plurality of SAW resonators having the same common sense have been arranged in series arms until now, but the resonance characteristics are almost the same.
If the frequency position of the spurious appearing on the low frequency side near the resonance frequency is connected in multiple stages to different series SAW resonators, the spurs of a plurality of SAW resonators arranged in the series arm will cancel each other out. They have found that ripples in the pass band of a ladder-type SAW filter can be suppressed. To explain in more detail, assuming that the width of the ripple generated in the pass band shown in FIG. 4 is x MHz, the ripple appears immediately below the low frequency side of the resonance frequency of the plurality of SAW resonators arranged in the series arm. According to the analysis of the present invention, it is possible to correct the IDT electrode period λ of the SAW resonator having the center frequency fo and to derive the corrected IDT electrode period λ ′ from the following equation in order to make the frequency position of the spurious different. It was confirmed from the results. λ ′ = λ × (fo + kX) / fo (1) X ≧ x (2) In the four series arm SAW resonators constituting the ladder type SAW filter shown in FIG. '1 (Xs1) to λ'4 (Xs4) are respectively k at 1.0 intervals, k1 = −1.5, k2 = −0.5, k3 =
By setting +0.5, k4 = + 1.5, the IDT electrode cycle of each SAW resonator is calculated. Center frequency fo = 157
The electrode periods λ′1 to λ′4 of each SAW resonator were derived as follows from 5.42 MHz, X = 1.6 MHz, and the reference (conventional) electrode period λ = 2.46360 μm. λ′1 = λ × (fo−1.5X) /fo=2.4598
4 μm λ′2 = λ × (fo−0.5X) /fo=2.4623
6 μm λ′3 = λ × (fo + 0.5X) /fo=2.4648
4 μm λ′4 = λ × (fo + 1.5X) /fo=2.4673
A ladder-type SAW using four serial arm SAW resonators Xs1 to Xs4 in which the IDT electrode period is set as described above and a parallel arm SAW resonator Xp of a conventional design.
When the filter was configured as shown in FIG. 2, the pass band characteristics shown in FIG. 1 were obtained. Ripple in the passband is suppressed to the extent that it can hardly be confirmed, good filter characteristics with small insertion loss and in-band deviation are obtained, and the required specification ripple: 0.2 dB or less is also fully satisfied. Was confirmed. As described above, the feature of the present invention is that, among the SAW resonators forming the ladder type SAW filter, at least the SAW resonators of the serial arm have a pitch L of the reflector.
r, the ratio Lt / Lr of the pitch Lt of the IDT electrode to Lt is smaller than 1, and the distance Ltr between centers of adjacent electrode fingers of the IDT electrode and the reflector is smaller than 0.5λ. At least two placed in series arm of
That is, a ladder-type SAW filter is configured by changing the pitch Lt of the IDTs of the SAW resonators or more. Although the ladder-type SAW filter according to the present invention has been described using a 42 ° Y-cut X-propagation LiTaO ₃ for the piezoelectric substrate, it is not necessary to limit the cutting angle to this.
Other cutting angles may be used. LiNb is used as a piezoelectric substrate.
It goes without saying that O ₃ , langasite (La ₃ Ga ₅ SiO ₁₄ ), lithium tetraborate (Li ₂ B ₄ O ₇ ) and the like can be applied. Furthermore, the ladder-type SAW filter according to the present invention has been described by taking the RF stage of a GPS receiver as an example, but it is not limited to this, and it goes without saying that the present invention can be applied to an RF filter of a cellular type cellular phone or the like. No. The ladder type SAW filter according to the present invention has the following excellent effects because it is configured as described above. According to a first aspect of the present invention, there is provided a ladder-type SAW filter in which a plurality of SAW resonators having different spurious frequencies appearing in the vicinity of the low frequency side of the resonance frequency and having substantially the same resonance characteristics are connected in multiple stages to a series arm. Thus, an excellent effect is obtained that a good filter characteristic with a small insertion loss and a small in-band deviation that suppresses the ripple in the pass band can be obtained. Furthermore, since the frequency position of the spurious component appearing near the lower frequency side of the resonance frequency of the SAW resonator is designed to be different, the filter characteristics of the ladder type SAW filter depending on the production lot in mass production, especially, the ripple characteristics are reduced. Since there is almost no variation, there is an excellent effect that it is suitable for mass production.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing pass band characteristics of a ladder type SAW filter according to the present invention. FIG. 2 is an equivalent circuit configuration of a ladder-type SAW filter according to the present invention. FIG. 3A is a diagram illustrating a circuit configuration of a ladder-type basic section, and FIG. 3B is a diagram illustrating filter characteristics of filters configured with respective reactance curves. FIG. 4 is a diagram illustrating pass band characteristics of a filter. FIG. 5 is a plan view showing a configuration of a SAW resonator. FIG. 6 is a diagram illustrating an equivalent circuit configuration of a ladder-type SAW filter. FIG. 7 is a plan view showing a ladder-type SAW filter formed on a piezoelectric substrate. FIG. 8 is a plan view for explaining various constants of the SAW resonator. FIG. 9 shows pass band characteristics of a ladder type SAW filter. FIG. 10 shows impedance characteristics and return loss characteristics of a SAW resonator. FIG. 11 shows impedance characteristics and return loss characteristics of a SAW resonator. FIG. 12 shows a pass band characteristic of a ladder-type SAW filter. [Description of Signs] 1 Piezoelectric substrate 2 IDT 3 Reflector 4, 5, 6, 7, 8, 9 SAW resonator 10 Filter characteristics

Claims (1)

Claims: 1. An elastic surface having an interdigital transducer having an electrode period λ on a main surface of a piezoelectric substrate along a propagation direction of a surface acoustic wave and grating reflectors on both sides of the interdigital transducer. In a ladder type surface acoustic wave filter using a wave resonator, the ratio Lt / Lr of the pitch Lt of the interdigital transducer to the pitch Lr of the grating reflector is set to 1
In addition to making the distance smaller, the center-to-center distance Ltr between adjacent electrode fingers of the interdigital transducer and the grating reflector is made smaller than 0.5λ, and at least two of the plurality of surface acoustic wave resonators arranged in the series arm are arranged. A ladder type surface acoustic wave filter, wherein Lt of two or more surface acoustic wave resonators are different from each other.