JPH0555871A - Vertically connected multiple mode saw filter - Google Patents

Abstract

PURPOSE:To obtain a vertically connected multiple mode SAW filter capable of suppressing spuriousness generated in a low band side attenuation band near a pass band by sharply increasing the number of reflectors as compared with the number of reflector to be required for the practical arrival of a Q value at a saturated value. CONSTITUTION:An input inter-digital transducer(IDT) electrode 2 and an output IDT electrode 3 are adjacently arranged on a piezo-electric base 1 along the propagating direction of a surface acoustic wave(SAW) excited from the IDT 2. Reflectors 4, 4' are formed on the face sides of these IDTs 2, 3 to confine the excited SAW in the IDTs 2, 3. In an SAW filter utilizing primary and secondary mode surge generated due to acoustic coupling between the IDTs 2, 3, the number of reflectors 4, 4' is sharply increased as compared with the number of reflectors to be required for the practical arrival of the Q value at the saturation value. Namely the Q value arrives at the saturation value by 80 to 100 reflectors, and when the number of reflectors is increased up to 250 reflectors, filter characteristics are improved so that relative attenuation value is about 32.5dB and spuriousness is suppressed to 4.5dB. Thereby the filter having a sufficient attenuation value can be obtained.

Description

Detailed Description of the Invention

This invention relates to a plurality of I's on a piezoelectric substrate.
The present invention relates to a longitudinally coupled multimode SAW filter that utilizes the waves of different modes generated by a DT and a reflector.

2. Description of the Related Art Conventionally, a resonator type SAW filter called a multimode SAW filter has existed and is put to practical use, especially in the high frequency region of the VHF to UHF band. As one type of SAW filter of this type, an input IDT and an output IDT are vertically arranged close to each other along the traveling direction of the SAW excited by the input IDT, and reflectors are provided on both sides of the input IDT and the output IDT for excitation. There is a longitudinally coupled multimode SAW filter that confines the SAW within the IDT and utilizes the waves of different modes generated by the acoustic coupling between the IDTs. In the resonator type SAW filter as described above, it is known that the Q value of the resonator changes with the IDT logarithm and the like as a parameter, and the number of reflectors is also one of the parameters for changing the Q. As shown in FIG. 2, the larger the number of reflectors, the larger the Q becomes. However, in the vicinity of n in the figure, Q almost reaches the saturation value, and even if the number is further increased, Q hardly changes. Therefore, even if the number of reflectors is increased beyond the saturation point of Q, the element is simply lengthened. Therefore, conventionally, the number of reflectors is generally set to about n in the figure where Q reaches a saturation value. Met. However, the characteristic of the SAW filter configured in this way is that a relatively large spurious is generated in the low-side attenuation band near the pass band.
If it is not applied to the RF filter of a band mobile phone, the transmission / reception frequency interval is set to a value very close to 45 MHz. Therefore, the low frequency side spurious of the filter with the higher center frequency has the lower center frequency. There was a defect that it almost coincided with the pass band of and affected it.

An object of the present invention is to eliminate the defects of the conventional longitudinally coupled multimode SAW filter as described above, and to suppress spurious generated in the low side attenuation band near the pass band. It is an object to provide a multimode SAW filter.

SUMMARY OF THE INVENTION To achieve the above object, in the present invention, input / output IDTs are arranged on the surface of a piezoelectric substrate along the traveling direction of a wave excited by an interdigital transducer (IDT) electrode. In a SAW filter having reflectors on both sides, the number of the reflectors is significantly increased as compared with the number of the reflectors whose Q value substantially reaches the saturation value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the drawings showing the embodiments. First, in order to facilitate understanding of the present invention, the basic configuration of the longitudinally coupled dual-mode SAW filter will be described in a little detail together with its principle. FIG. 1A is a conceptual diagram showing a basic configuration of a longitudinally coupled dual mode SAW filter, in which an input IDT 2 and an output IDT 3 are provided on a piezoelectric substrate 1.
Are arranged close to each other along the propagation direction of the SAW excited by the input IDT2, and reflectors 4 and 4'are provided on both sides of the two to confine the excited SAW in the IDTs 2 and 3 and acoustic coupling between the IDTs 2 and 3. Thus, the fact that the waves of the primary and secondary modes exist as standing waves along the SAW propagation direction as shown in FIG. In the longitudinally coupled dual mode SAW filter as described above, Q varies depending on the number of reflectors, and for example, substrate material 64 ° Y cut-X propagation LiNbO ₃ , IDT pairs of 12 pairs each, and the same crossover length. A center frequency of 836.5 M formed under the conditions of 75λ (λ is the wavelength of the excited SAW), the same pitch 5.18 μm, the electrode material Al, and the same film thickness 0.2 μm.
In the case of a filter having a Hz pass band width of 35 MHz, the relationship shown by the solid line in FIG. 2 is shown. As is clear from the figure, Q increases as the number of reflectors increases. However, if the number of reflectors increases unnecessarily, the size of the parts will increase, so that Q will reach a saturation value. It was generally about this. Vertically coupled dual mode S constructed in this way
The two-stage cascade connection of the AW filter elements exhibits the filter characteristic as shown in FIG. 3, and a relatively large spurious is generated in the low frequency side attenuation band about 30 MHz away from the center frequency in the vicinity of the pass band. As described above, was not obtained. Therefore, the inventors of the present application consider that the spurious on the low frequency side is caused by the side lobes in the frequency characteristics of the reflector, and paying attention to the fact that the side lobe spacing becomes finer as the number of reflectors increases, An experiment was conducted using this as a parameter. FIG. 4 shows the filter characteristics when the number of reflectors is set to 250, which is about three times that of the conventional one. Spurious that occurs in the low-side attenuation band near the pass band is suppressed by about 7 dB, and the relative attenuation amount is reduced. Shows a good attenuation characteristic of 29 dB. In this embodiment, the relationship between the number of reflectors and the relative attenuation amount in the low-side attenuation band near the pass band is about 3 as shown in FIG.
Although it changes almost linearly up to around 00, it is predicted from FIGS. 3 and 4 that the change becomes small as shown by the dotted line in the figure, and it is predicted from FIGS. 3 and 4 that the number of reflectors is 150 to 300. Choose a book. As described above, a large increase in the number of reflectors leads to lengthening of the components. As described above, in the high frequency region of this embodiment, the pitch of the reflectors is narrow, and the number of reflectors increases. The overall dimensions are not significantly affected. Further, for example, as shown in FIG. 6A, the input IDT 5 and the output IDTs 6 and 7 on both sides of the input IDT 5 are arranged close to each other along the propagation direction of the SAW excited by the input IDT 5, and the reflectors 4 are provided on both sides thereof. 4'is provided and the excited SAW is converted to the IDT.
A longitudinally coupled dual mode which is confined in 5, 6, and 7 and which utilizes standing waves of first and third modes generated between the reflectors 4 and 4'by acoustic coupling between the IDTs as shown in FIG. In the SAW filter, the input IDT5
Of the output IDTs 6 and 7 is 9 pairs,
Even when the IDT crossing length is 40λ and the other conditions are the same as those in the above-described embodiment, Q and the number of reflectors have the relationship as shown by the dotted line in FIG. At Q reaches the saturation value. At this time, the characteristics of the filter in which the above-mentioned filter elements are cascade-connected in two stages have a relative attenuation amount of 28 dB in the low-side attenuation band as shown in FIG.
On the other hand, when the number of reflectors is increased to 250, the filter characteristic shows that the relative attenuation is about 3 as shown in FIG.
2.5 dB and spurious are suppressed by 4.5 dB. Although the present invention has been described above with reference to two embodiments and the experimental results thereof, the present invention is not limited to these, and experimental values and the like are omitted to avoid complexity of the specification, but other piezoelectric substrates are used. It has been confirmed that a similar result can be obtained even with a longitudinally coupled multimode SAW filter using other modes. That is, in this type of longitudinally coupled dual-mode SAW filter, if the number of reflectors is increased by about 50% or more relative to that at which Q saturates, the relative attenuation amount in the vicinity of the low band side of the pass band is greatly increased. Can be improved.

Since the present invention is configured as described above, it is particularly useful for realizing a filter having a sufficient amount of attenuation by suppressing spurious in the low-side attenuation band near the pass band. It exerts a remarkable effect in the area.

[Brief description of drawings]

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

FIG. 2 is a diagram of experimental data showing the relationship between the number of reflectors and Q of a longitudinally coupled dual mode SAW filter.

FIG. 3 is a diagram of experimental data showing characteristics of a conventional longitudinally coupled dual mode SAW filter.

FIG. 4 is a diagram of experimental data showing characteristics of an example of a longitudinally coupled dual mode SAW filter according to the present invention.

5 is a diagram of experimental data showing the relationship between the number of reflectors and the relative attenuation amount in the embodiment of FIG.

FIG. 6 is a conceptual diagram showing the configuration and principle of another embodiment of the longitudinally coupled dual mode SAW filter according to the present invention.

FIG. 7 is a diagram of experimental data showing characteristics of a conventional longitudinally coupled dual mode SAW filter.

8 is a diagram of experimental data showing the characteristics of the example of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Piezoelectric substrate 2 and 5 ... Input IDT 3, 6 and 7 ... Output IDT 4 and 4 '... Reflector

Front page continuation (72) Inventor Tatsuro Kaneko 2-1-1 Kotani, Samukawa-cho, Takaza-gun, Kanagawa Toyo Communication Equipment Co., Ltd.

Claims (1)

[Claims] 1. An interdigital transducer (ID
T) A plurality of IDTs are closely arranged on the piezoelectric substrate along the propagation direction of the surface acoustic wave (SAW) excited or received by the electrode, and reflectors are provided on both sides of these IDTs to excite the excited SAW vibrational energy in the IDT. In a longitudinally coupled multi-mode SAW filter that utilizes the waves of a plurality of modes generated by acoustic coupling between the IDTs of these vibrations, the number N of the reflectors is set to a substantially saturated value. A longitudinally coupled multimode SAW filter characterized in that the number n of reflectors reached is significantly increased.