JP3403866B2 - Composite 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, and more particularly to a filter having a low loss and a steep attenuation gradient in a medium band required for mobile communication and the like.

[0002]

2. Description of the Related Art Conventionally, 70 MHz used for mobile communication
A transversely coupled double mode SAW filter using a quartz substrate is generally used as an intermediate frequency (IF) filter in a high frequency region of up to 250 MHz. This filter uses two 1-port SAW resonators arranged close to each other in parallel on a quartz substrate and utilizes the waves of the primary and secondary vibration modes generated by the acoustic coupling. 0.1%
Suitable for narrow band filters.

However, with the digitization of mobile communication such as mobile phones and cordless phones, the specific bandwidth is 0.1 to 0.1%.
A 0.3% mid-band filter has become necessary.

As one method for satisfying this demand, there is a method of narrowing the crossing length and the coupling gap of a laterally coupled double mode SAW filter using a quartz substrate to strengthen acoustic coupling and widen the bandwidth. However, widening the bandwidth of the laterally coupled double-mode SAW filter has a drawback in that the impedance becomes extremely high and it cannot be used practically.

As another method, there is a vertically coupled double mode SAW filter using a quartz substrate. This utilizes two vibration modes generated by acoustic coupling of two or more IDTs arranged close to each other in the SAW propagation direction. Since the coupling is stronger than the lateral coupling, a wide bandwidth can be obtained. However, in the case of the longitudinally coupled dual mode SAW filter, it is necessary to secure the amount of attenuation in two sections because of spurious due to the characteristics of the IDT appearing on the high frequency side.

The pass band width of the double mode SAW filter is limited by the capacitance ratio of the SAW resonators constituting the double mode SAW filter, and the capacitance ratio of the resonator is large and the wide band width is wide in a substrate having a small electromechanical coupling coefficient such as crystal. An extension coil is required when trying to obtain.

Therefore, in the two-section structure, it is necessary to insert an extension coil between the stages, and since the impedance is high, the sensitivity due to the inductance of the coil is extremely high and the filter is very difficult to use.

On the other hand, LiTa having a large electromechanical coupling coefficient
There is also a method of using a substrate such as O ₃ or LiNbO ₃ to construct a longitudinally coupled double-mode SAW filter with a SAW resonator with a small capacitance ratio, but these substrates have poor frequency-temperature characteristics, so the amount of frequency change due to temperature changes. Is large and the amount of attenuation in the vicinity cannot be sufficiently obtained.

On the other hand, recently, Yamamoto et al. Invented a composite longitudinal mode SAW filter, and as shown in FIG.
As shown in FIG. 7A, two longitudinally coupled dual-mode SAW filters arranged such that the phase relationship of T is different by 180 ° have the same phase relationship between one primary mode and the other secondary mode. Shift the center frequency and connect them in parallel.
A wide bandwidth was achieved by constructing a 3-pole filter with a section configuration (reference: 1993 Spring Conference of the Institute of Electronics, Information and Communication Engineers, A-339). In this configuration, the bandwidth of one longitudinally coupled double-mode SAW filter is approximately half the desired bandwidth, which is advantageous because the impedance is low.

By applying this technique, Yamada et al., In a laterally coupled double mode SAW filter, by changing the arrangement of IDTs between two laterally coupled double mode SAW filters as shown in FIG. As shown in FIG. 7 (b), two filters having different phase relationships of IDTs are connected in parallel by shifting the center frequencies so that the phase relationship between one secondary mode and the other primary mode becomes equal, Similarly, a 1-section 3-pole filter was used to increase the bandwidth (Reference: 1994 Autumn Meeting of the Institute of Electronics, Information and Communication Engineers, SA).
-11-3).

However, even if these methods are used, the impedance of the filter is still high, the pass band width is insufficient depending on the demand, and a filter having a higher attenuation gradient is desired, and there is a drawback that the required characteristics are not satisfied. Was.

[0012]

SUMMARY OF THE INVENTION The present invention is a composite dual mode S as described above.
This is done to eliminate the drawbacks of the AW filter, and the impedance of the medium band filter with a specific bandwidth of 0.1 to 0.3% in the high frequency region is made low, and the composite double mode with a high attenuation gradient is provided. An object is to provide a SAW filter.

[0013]

SUMMARY OF THE INVENTION In order to achieve the above object, a composite double mode SAW filter according to the present invention uses a laterally coupled double mode SAW filter or a longitudinally coupled double mode SAW filter to obtain two vibration mode frequencies. It is characterized in that two identical filters having center frequencies separated by about twice the difference are connected in parallel to form a filter.

[0014]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in detail below with reference to the drawings showing the embodiments.

FIG. 1 is an electrode pattern diagram of a composite double mode SAW filter showing an embodiment of the present invention. The IDTs 2 and 2'are arranged close to each other on the surface of the piezoelectric substrate 1 and the reflectors 3 are placed on both sides of the IDTs 2 and 2 ', and the laterally coupled double mode SAW filter 4 is provided.
Make up. Furthermore, the same laterally coupled double mode SAW filter 5 is arranged in parallel with the filter 4 in the same propagation direction on the same substrate, and these two filters are connected in parallel. At that time, the cycle of the IDT is set so that the center frequencies of the two filters are separated by about twice the frequency difference between the primary mode and the secondary mode.

FIG. 2 is a diagram showing the frequency arrangement of the filter of the present invention in comparison with the conventional method. FIG. 2 (a) is shown in FIG. 7 (b).
7 is a frequency arrangement of a composite double-mode SAW filter using the laterally coupled double-mode SAW filter of the conventional method shown in FIG. As described above, the phase relationship between the two filters is reversed to
By arranging them apart from each other by the frequency difference between the two vibration modes, and by matching the frequency and phase of the second-order mode f _{2 of} one filter and the first-order mode f ₁ 'of the other filter,
After the parallel connection, the pass band is from f ₁ to f ₂ ′.

On the other hand, in the present invention, as shown in FIG. 2B, the two filters connected in parallel have the same phase relation, and the frequency difference between the two vibration modes (f ₂ −f ₁ = f ₂ '-F
By separating the center frequency by twice as much as ₁ '), f ₁ ~
The frequency intervals of f _{2 to} f ₁ ′ to f ₂ ′ are almost the same, and the phase relationship is, for example, 0 ° to 180 ° to 0 ° to 180 °, so that the filter can be configured.

In FIG. 3, for example, the substrate is a 38 ° rotated Y-cut, X-propagation crystal in FIG. 1, the electrode is aluminum, the film thickness is 0.15 μm, the IDTs 2 and 2'are 300 pairs, and the cross length is 10λ (λ is SAW wavelength), IDT 2, 2'spacing (coupling gap) is 1λ, 300 reflectors each, I
It is a figure which shows the experimental result of this invention which comprised the filter 4 as 11.18 micrometers and the filter 5 as 11.165 micrometers as DT period. The frequency transmission characteristics of each of the two laterally coupled dual-mode SAW filters are shown. The solid line shows the characteristics of the filter 4, and the broken line shows the characteristics of the filter 5. As shown in the figure, the filters 4 and 5 show the same frequency transmission characteristics only with different center frequencies. In addition, the phase relationship is 0 in the primary mode.
The second mode is 180 ° at 180 °, which is the same.

In the figure, the frequency arrangement of each vibration mode of the two filters 4 and 5 is 0 ° to 180 ° as described above.
It is 0 ° to 180 °, and the frequency intervals of these are almost the same. From this, as apparent from the filter theory, if two filters are connected in parallel and combined, a bandpass filter can be constructed under an appropriate termination impedance.

FIG. 4 shows the frequency transmission characteristics of the composite dual-mode SAW filter when the two filters shown in FIG. 3 are connected in parallel and terminated with a terminating impedance of 400Ω / -1pF. As is clear from the figure, a filter having a flat pass band, a specific bandwidth of about 0.2%, a low loss, and a high steepness is obtained.

This filter is equivalent to a 4-pole filter, has a steeper attenuation gradient than the conventional 3-pole filter, and has a low impedance despite its wide bandwidth. .

Although the present invention has been described by taking the composite dual mode SAW filter using the laterally coupled double mode SAW filter as shown in FIG. 1 as an example, the present invention is described as a composite dual mode SAW filter. It goes without saying that it can also be applied to a dual mode SAW filter.

Further, the substrate is not necessarily a quartz substrate and may be applied to a composite double mode SAW filter using other surface waves such as a LiTaO ₃ substrate, a LiNbO ₃ substrate and a Li ₂ B ₄ O ₇ substrate.

[0024]

Since the present invention is constructed as described above, it exhibits a remarkable effect in realizing a filter having a medium frequency band, a low loss and a steep attenuation gradient in a high frequency region.

[Brief description of drawings]

FIG. 1 is a diagram showing an electrode pattern of a composite double mode SAW filter according to the present invention.

2A and 2B are diagrams showing the frequency arrangement of a composite double-mode SAW filter, FIG. 2A is a diagram showing the frequency arrangement of a conventional composite double-mode SAW filter, and FIG. The figure which shows the frequency arrangement | positioning of the compound double mode SAW filter of invention.

FIG. 3 is a diagram showing frequency transmission characteristics of two laterally coupled double mode SAW filters that form the composite double mode SAW filter of the present invention.

FIG. 4 is a diagram showing an example of frequency transmission characteristics of the composite double-mode SAW filter of the present invention.

FIG. 5 is an electrode pattern diagram of a composite double mode SAW filter using a conventional longitudinally coupled double mode SAW filter.

FIG. 6 is an electrode pattern diagram of a composite double mode SAW filter using a conventional laterally coupled double mode SAW filter.

7A and 7B are conventional composite dual mode SAs.
In the figure showing the frequency arrangement of the W filter, (a) shows the vertical coupling 2
The figure which shows the frequency arrangement | positioning of a double mode SAW filter, (b)
Is a diagram showing a frequency arrangement of a laterally coupled dual mode SAW filter.

[Explanation of symbols]

1 ... Piezoelectric substrate 2, 2 '... IDT 3 ... Reflector 4, 5 ... Lateral coupling dual mode SAW filter

Claims (2)

(57) [Claims] 1. A piezoelectric substrate having at least one interdigital transducer (IDT) electrode for exciting and receiving surface acoustic wave (SAW) and reflectors on both sides of the electrode for reflecting SAW excited by the IDT. The vibration energy is confined inside the IDT to form a resonator, and two resonators are arranged in parallel and close to each other to utilize the waves of the primary and secondary vibration modes generated by the acoustic coupling of these vibrations. A composite dual-mode SAW filter, characterized in that a coupled dual-mode SAW filter is formed by connecting two of the filters having a center frequency separated by about twice the frequency difference between these two vibration modes in parallel. filter. 2. The two dual-mode SAW filters,
Two or more IDTs that excite and receive SAW on a piezoelectric substrate
Electrodes are arranged close to each other in the propagation direction of the SAW, and reflectors are provided on both sides of the electrodes so that the vibration energy of the excited SAW is
The composite double mode SAW filter according to claim 1, wherein the composite double mode SAW filter is a longitudinally coupled double mode SAW filter that is confined in a DT and utilizes the waves of two vibration modes generated by acoustic coupling between the IDTs.