JP3160023B2 - Vertically coupled dual mode SAW filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a SAW filter,
The present invention relates to a longitudinally-coupled dual-mode SAW filter that realizes an extremely wide band and low loss performance in a high frequency region near GHz.

[0002]

2. Description of the Related Art A transversal SAW filter using a unidirectional IDT or a SAW filter using a multi-IDT has been widely used as a wide-band low-loss filter used in the high-frequency region of the VHF-UHF band. . The former has the advantage that the degree of freedom in designing the frequency characteristics of the filter is high, but requires the addition of a phase shifter to the filter element (when a group-type unidirectional IDT or a three-phase unidirectional IDT is used). In addition, the SAW, which has a defect of causing a problem in the production yield, has a small directional loss of the excited SAW, and is not satisfactory in terms of reducing the loss of the filter.

On the other hand, in the latter case, that is, in a SAW filter using a multi-IDT, if nine or more sets of IDTs are arranged, the directional loss becomes 1 dB or less, and a reduction in the filter loss can be realized. There is a defect that not only the size becomes large, but also a large number of spurious due to an increase in the reflection of the SAW inside the IDT appears, and the stop band attenuation of the filter becomes insufficient.

A resonator-type SAW called a multi-mode SAW filter has been conventionally used for the above-mentioned type of filter.
The filters are roughly classified into a laterally coupled dual mode SAW filter (see, for example, Japanese Patent Publication No. 2-16613) and a vertically coupled dual mode SAW filter (reference: Tanaka et al., No. 15).
Times EM symposium, pp5-10 (1986) 4.
In each case, a plurality of IDTs are arranged close to each other, and the resonance frequency difference between two waves having different modes (referred to as symmetric mode and anti-symmetric mode) generated when the SAWs excited by these are acoustically coupled to each other is filtered. It is superior to the above-mentioned type of filter in reducing the loss of the filter, but is applied to a wideband filter because the passband is limited by the capacitance ratio of the resonator. Had difficulties.

For example, the coupling coefficient becomes relatively large (5%).
Even if a longitudinally coupled dual mode SAW filter is configured using a 6 て Y cut-X propagation LiTaO ₃ substrate, it will only have a pass bandwidth of at most about 2% with respect to the center frequency, while Shimizu et al. The coupling coefficient found is extremely large (3
Called Love wave resonator (literature = IEICE techniques, US86-37 (1986) also 0%) becomes Y-cut -X-propagating Li / Nb / _{O 3} to excite Love waves at Au electrodes using a substrate by using the Although the capacitance ratio of the resonator was extremely small (approximately 3), it was thought that an extremely wide band low-loss filter could be realized. However, since the coupling coefficient was too large, the vibration energy was almost completely confined in each IDT, and mutual acoustic waves were generated. It was found that the coupling was not sufficiently generated, and the pass band of the filter was rather narrowed.

Although the inventor of the present application has not yet obtained a very wide pass band, Kanda has a possibility that the pass band width may be wider than that of a conventional longitudinally coupled double mode SAW filter using the vibration of the first and second order modes. longitudinally coupled double mode SAW filter (literature etc. to use the vibration of the X-cut Li Ta O ₃ primary and tertiary mode on the substrate during the study: 1988 Institute of Electronics, information and communication Engineers spring national Meeting Proceedings A-
238) Coupling coefficient 11% discovered by Shibayama et al.
About 64 ° Y cut-X direction propagating Li NbO ₃ substrate (Literature: J. Appl. Phys., Vol. 43, N.
o. 3, pp 856-862 (1972)) to construct a longitudinally coupled dual mode leaky SAW filter. However, SAW using this substrate
There have been few examples of prototypes of devices, and even no such devices have been applied to longitudinally coupled dual-mode SAW filters, and it was unknown at all what electrode configuration to use.

[0007]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned shortcomings of the generally known SAW filter at a glance and solves the problem at 1 GHz.
It is an object of the present invention to realize a longitudinally coupled double mode SAW filter having a fractional bandwidth of as much as 4% in a high frequency region close to the above and having low loss.

[0008]

SUMMARY OF THE INVENTION In order to achieve the above object, a longitudinally coupled dual mode SAW filter according to the present invention is disposed close to the SAW propagation direction for exciting three IDTs and has reflections on both sides of the IDT train. Place the container and the three IDs
In a dual-mode SAW filter utilizing first- and third-order vibration modes generated by acoustic coupling between T, (1) the intervals between each IDT are brought close to a certain range, and the IDTs facing each other The innermost electrode fingers are integrated, and (2) the above-mentioned 64 ° Y cut-X direction propagation L
i Nb O ₃ each IDT electrode finger pairs when using the substrate, it was to determine the optimal value of the cycle ratio and the electrode thickness of the IDT electrode fingers and the reflector grating is intended.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the embodiments shown in the drawings and experimental data. Prior to the description of the embodiments, a longitudinally coupled dual mode SAW
I will explain a little about the basic principle of the filter. FIG.
6A and 6B are diagrams respectively showing the configuration of the most basic longitudinally coupled dual mode SAW filter and the vibration energy distribution on the piezoelectric substrate, wherein two IDTs are provided on the surface of the piezoelectric substrate 1;
2 and 2 are arranged close to each other along the propagation direction 3 of the SAW excited by them, and reflectors 4 and 4 are provided on both sides of these IDTs 2 and 2, respectively, and vibrations confined in the IDTs 2 and 2 (not shown) ) Are acoustically coupled to each other to generate a pass-band filter using two vibrations of a first-order (symmetric) mode and a second-order (anti-symmetric) mode.

At this time, if the resonance frequency of the primary mode and the secondary mode vibrations is f ₁ and f ₂ , respectively, and the tight bandwidth of the filter is B, BＢ2 (f ₁ −f ₂ ) and the center frequency The number is f ₁
Is well known. However, the above f ₁ and f ₂
As described above, the difference is inversely proportional to the capacitance ratio of the resonator. Therefore, when a wider pass band is required in the resonator filter of the same type, the vibration of the first mode and the third or higher mode is used. Become. FIGS. 12A and 12B
3A and 3B are a basic electrode configuration diagram and a diagram showing a distribution of vibration energy of a longitudinally coupled dual mode SAW filter using first-order mode and third-order mode vibration, respectively. This type of S
In the AW filter, three IDTs are arranged on the piezoelectric substrate 1 along the propagation direction of the SAW excited by them, and at least 6, 6 provided on both sides of the central IDT 5 have the same number of electrode pairs and reflect outside these. The containers 4, 4 are arranged. As shown in FIG. 1B, the vibration modes generated as a result of the acoustic coupling between the IDTs 5, 6 and 6 are three modes of primary, secondary and tertiary. As is clear from the above, the vibration of the second mode is caused by the three IDs.
Filters having such an electrode configuration utilize the resonance frequencies f ₁ and f ₃ of the primary and tertiary modes of oscillation, since both are canceled on T 5 and 6, 6, and the center frequencies f ₁ ,
Pass bandwidth B may comprise a filter to be 1 to 2 times the difference between f ₁ and f _3.

The principle of the longitudinally-coupled dual-mode SAW filter according to the present invention is as described above. However, the inventor of the present application has specified this as a specific requirement, for example, a fractional band such as an RF filter of a 900 MHz band portable / car telephone. The following inferences were made as to what electrode configuration should be adopted for the requirement of an input / output impedance of 50Ω without tuning for a very wide band low loss filter with 3 to 4% and insertion loss of 2 to 35 dB. An experiment was performed.

First, as is well known, in a filter of the type utilizing acoustic coupling between IDTs, the interval between a plurality of IDTs determines the magnitude of acoustic coupling, which is substantially proportional to the pass band width of the filter. It has been considered that the periodic arrangement of the IDT electrode fingers is deviated and brought closer. Such ID
A change in the T interval generally causes an increase in the impedance of the resonator, but a 64 ° Y cut-X propagation LiNbO ₃
Since a resonator using a piezoelectric substrate having a large coupling coefficient as described above originally has low impedance, no serious problem should occur even if the IDT interval is appropriately changed.

Therefore, as shown in FIG.
16 pairs of central IDTs 5 are disposed on a propagation LiNbO ₃ substrate, and 10 pairs of IDTs 6 and 6 are disposed on both sides thereof, the IDT cross width is 40λ, the reflectors 4 are 250 right and left, and an Al electrode film is used.
An electrode having a thickness of 4% λ is formed, a filter element having a center frequency of 881.5 MHz is prototyped, and an experiment is conducted on how the pass band width B changes when only the innermost electrode finger center l of each IDT is changed. At the same time, theoretical calculations were also performed. As a result, as shown in FIG. 2, it can be seen that a peak of the pass bandwidth appears every time the value of 1 changes by λ / 2. Considering the range of 1 that satisfies the fractional bandwidth of 3 to 4% based on the above results, it is found that (n / 2−1 / 3) λ < l <(rn / 2−1 / 1 ).
5) As long as it is λ, the value of n takes into account that the peak of the pass bandwidth B appears at λ / 2 period as described above.
Referring to FIG. 3 showing the relationship between n and the pass bandwidth B when l ₋ = <n / 2−1 / 4) λ, it is sufficient if 2 ≦ n ≦ 5.

Considering FIG. 2 showing the relationship between the IDT interval 1 and the passband B, the maximum passband when the value of l is λ / 4, that is, when the innermost electrode finger of the IDT is in close contact. B is shown. Therefore, to study an IDT electrode structure that can realize l = λ / 4, for example, the structure shown in FIG. 4 may be used. That is, it was confirmed that when the bus bars 9 and 10, respectively, where the innermost electrode fingers 7 and 8 of the adjacent IDTs 5 and 6 facing each other were grounded, the pass bandwidth B almost corresponding to the theoretical value could be realized. . In addition, the above idea is, as shown in FIG. 5, one of the innermost electrode fingers facing the adjacent IDTs 5 and 6, for example, in this embodiment, the electrode finger of the IDT 6 that is separated from the bus bar 10 of the IDT 6. 7 may be combined. At this time, it is obvious that the hot terminal and the ground terminal can be exchanged with each other.

When the embodiment shown in FIG. 5 is applied to a dual mode SAW filter in which three IDTs according to the present invention are arranged in close proximity, as shown in FIGS. 6 (a) and 6 (b). The electrode finger 11 having a width of λ / 2 may be attached to both sides of the central IDT 5 or a central ID located between the both IDTs 6.
It may be attached to the surface adjacent to T5 so as to be symmetrical with respect to the center of the IDT array so as to match the phase of the reflected SAW. As described above, the electrode structure of the dual mode SAW filter using the vibration of the first and third order modes has been described with the embodiment using the 64 ° Y cut-X direction propagating LiNbO ₃ substrate. It goes without saying that the present invention is applicable not only to a specific piezoelectric substrate but also to a filter using another piezoelectric substrate.

Finally, a large coupling coefficient of 64 ゜ Y for use in a 900 MHz RF filter for portable / car phones.
Cut-X propagation LiNbO ₃ substrate using fractional band 3 to 4
% Of the dual-mode leaky SAW filter according to the present invention which realizes a low-bandwidth low-loss filter, which further satisfies the required conditions of an insertion loss of 2 to 3 dB and an input / output impedance of 50Ω without a tuning circuit. The following describes an experiment for optimizing various parameters on the electrode configuration, which was conducted. Among the added requirements described above, the factor that determines the input / output impedance is the center I
If DT5 is an input / output IDT, it is the number of electrode fingers. In order to sharpen the skirt characteristic of this filter and obtain a sufficient amount of attenuation, it is assumed that the above-described filter elements are connected in cascade in two stages. Then, whether or not a matching circuit (L or C) is required between the stages is determined by the central IDT.
5 shows the number of pairs of electrode fingers of the IDTs 6 and 6 arranged on both sides of FIG.
Therefore, when the combination of the number of electrode fingers of both IDTs was examined by experiment, the result as shown in FIG. 7 was obtained.

That is, if the number of electrode fingers of the central IDT 5 is 14 or less in the range of electrode finger crossing length (15λ to 50λ) which can be generally used, the input / output impedance becomes 5
It has been found that the number of electrode finger pairs of the central IDT 5 must be in the range of 15 to 20 after exceeding 0 Ω and 50 Ω or less for 21 pairs or less. On the other hand, both sides IDT6,
The pass band is not flat unless the number of pairs of electrode fingers is 6 or less, and if the number of pairs of electrode fingers is 7 or less, and if the number of pairs of electrodes is 13 or more, C is not inserted between the steps. It turned out that the number of finger pairs should be selected in the range of 8 to 12 respectively. However, among the combinations of these two, some may not be able to satisfy the above-mentioned requirements, but in general, it may be possible to insert a tuning circuit at the input / output end or between stages. The allowable range of the number of electrode finger pairs described above is sufficiently reasonable.

A parameter related to the magnitude of the insertion loss, which is another important requirement, is the thickness of the electrode ( Al).
FIG. 8 shows a result of examining how the insertion loss changes when the electrode film thickness is changed under the condition of the electrode finger cross width 40λ and the two-stage cascade connection . However, since it is well known that the electrode film thickness has the same effect on the pass band width B as the magnitude of the insertion loss, this was also measured. That is, as is apparent from FIG. 8, the electrode film thickness satisfying both the specific band of 3.5 to 4.5% and the insertion loss of 2 to 3 dB is 3 to 5% of the wavelength λ of the leaky SAW to be excited. . The ratio _{L T} when the electrode thickness is 3 to 5% lambda and IDT electrode fingers period _{L T} to the period _{L R} of the reflectors grating
/ _LR is theoretically required to be 0.990 to 0.975. By doing so, the resonance frequencies of the above-mentioned first and third-order mode vibrations can be located in the region where the reflectance with respect to the frequency of the reflector is maximum, and the maximum pass bandwidth B can be obtained.

As described above, a filter element in which various parameters are selected and the number of reflectors is 250 on the left and right is shown in FIG.
As shown in FIG. 10, a dual mode leaky SAW filter cascaded in two stages has a center frequency of 881.5 MHz as shown in FIG.
It is an extremely wideband and low-loss filter having a ratio band of 3.9% and an insertion loss of 2 dB, which satisfies the requirements for a 900 MHz band RF filter for portable / car telephones and the like, which requires a large number of channels. In addition, since the impedance can be adjusted to 50Ω without inserting a tuning circuit between the filter input / output terminal and the filter element stage, it is particularly suitable as a component of such a communication device that requires strict miniaturization.

[0020]

Since the filter according to the present invention is constructed as described above, it becomes particularly important in the future.
It has a remarkable effect in realizing low-loss and small-sized RF filters for public communication radio equipment, etc., which must have an extremely wide fractional band to allow a large number of channels in a high frequency band near z. I do.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a basic configuration of a longitudinally coupled dual mode SAW filter according to the present invention.

FIG. 2 is a diagram of theoretical and experimental results showing a relationship between an IDT interval 1 and a pass bandwidth B (fractional band) of a longitudinally coupled dual mode SAW filter.

FIG. 3 shows that an IDT interval 1 is {(n / 2) − (1/4)} λ
The figure which shows the relationship between the value of n and the pass band width B (fractional band) in the case where it is set to.

FIG. 4 shows an IDT used in the filter according to the present invention.
FIG. 3 is a configuration diagram showing one embodiment of an innermost electrode finger structure.

FIG. 5 shows an IDT used in the filter according to the present invention.
The block diagram which shows the other Example of the innermost electrode finger structure.

FIG. 6 (a) is a configuration diagram showing an embodiment of an IDT innermost electrode finger array used in the filter according to the present invention,
(B) is a block diagram showing another embodiment.

FIG. 7 is a diagram showing the results of an experiment performed to determine the optimal combination of the number of electrode fingers of the central IDT and both sides thereof and the IDT.

FIG. 8 is a diagram of an experimental result showing a relationship between a thickness of an IDT electrode finger electrode, an insertion loss, and a bandwidth ratio.

FIG. 9 is a configuration diagram of a two-stage cascaded dual-mode leaky SAW filter according to the present invention.

FIG. 10 is a characteristic diagram of the filter shown in FIG. 9;

11A is a configuration diagram of a longitudinally-coupled dual-mode SAW filter using first- and second-order mode vibrations, and FIG. 11B is a diagram illustrating an energy distribution of vibration in a mode to be used.

12A is a configuration diagram of a longitudinally-coupled dual-mode SAW filter using first- and third-order mode vibrations, and FIG. 12B is a diagram illustrating an energy distribution of vibration in a mode to be used.

[Description of Signs] 1 ... Piezoelectric substrate, 2, 5 and 6 ... IDT, 4 ...
・ Reflector, 7, 8 ・ ・ ・ Innermost electrode finger, l ・ ・ ・ Innermost electrode finger interval

 ──────────────────────────────────────────────────続 き Continuing on the front page Referee Mikio Kawana Judge Nobuaki Yoshimi Judge Masahiro Hashimoto (56) References JP-A-1-231417 (JP, A) JP-A 1-212015 (JP, A) JP Showa 61-192112 (JP, A) Actually open Showa 55-79124 (JP, U)

Claims (4)

(57) [Claims] 1. Three interdigital transducer (IDT) electrodes are arranged on a piezoelectric substrate along the propagation direction of surface acoustic waves (SAW) excited or received by the IDTs, and reflectors are provided on both sides thereof. A dual mode filter that substantially confine the excited SAW vibration energy within the three IDTs and utilizes two primary and tertiary vibration modes generated by acoustic coupling between the respective IDTs, wherein each IDT is (equal SAW of the wavelength λ substantially excited) facing the center-to-center spacing said l electrode finger period of the IDT _{L T} of the innermost electrode fingers to each other with respect to (n / 2-1 / 3) A longitudinally coupled dual mode SAW filter, wherein λ <l <(n / 2−1 / 5) λ (where n = 2 to 5). 2. The method according to claim 1 , wherein three interdigital transceivers are provided on a piezoelectric substrate.
The transducer (IDT) electrode is excited or excited by these IDTs.
Arranged along the propagation direction of the received surface acoustic wave (SAW)
Further, reflectors are provided on both sides of the SAW to excite the excited SAW.
When the kinetic energy is almost confined in the three IDTs,
Both are due to the acoustic coupling between the IDTs of these vibrations.
Using the first and third vibration modes generated by
In the dual mode filter, the IDTs are
The distance l between the centers of the innermost electrode fingers facing each other is
The electrode finger period L _T (substantially equal to the wavelength λ of the excited SAW)
Equal) to the 1 = lambda / 4 and close contact of the IDT innermost electrode finger each other and electrode fingers of width lambda / 2 from each other by
A longitudinally coupled dual mode SAW filter. 3. An IDT having an electrode finger having a width of λ / 2 is connected to one IDT.
The electrode is separated from the pass bar.
3. The longitudinally coupled dual mode SAW filter according to 2. 4. The method according to claim 1, wherein the piezoelectric substrate has a large coupling coefficient of 64 ° Y.
Cut-X direction propagation LiNbO ₃ substrate
The number of electrode pairs at the center of the IDT is 15 to 20 pairs.
8 to 12 pairs each on the side, the period of the IDT electrode finger
And L _T ratio _L T / _{L R} of the period _{L R} of the reflectors 0.990
To 0.975, and the excitation of the Al electrode thickness H
3-5% of the wavelength λ of the key SAW.
3. A longitudinally coupled dual mode SAW film according to claim 1 or 2.
Ta.