JP3089611B2 - Surface acoustic wave multimode 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 surface acoustic wave multi-mode filter used in a high-frequency circuit of a radio.

[0002]

2. Description of the Related Art In recent years, surface acoustic wave filters have been widely used as RF and IF stage filters in transmission / reception circuits of communication equipment. The conventional S will be described below with reference to the drawings.
An example of the AW multiplex mode filter will be described.

FIG. 4 shows an electrode pattern configuration of a conventional SAW multimode filter. In FIG. 4, 1
Reference numeral 1 denotes a single-crystal piezoelectric substrate, on which an electrode pattern is formed to excite a surface acoustic wave. 12a is the ID
An energy trap type SAW resonator is formed by arranging reflectors 14a and 15a on both sides of a T electrode. The IDT electrode 12b and the reflectors 14b, 15
A similar SAW resonator is formed by b, and the two resonators are arranged close to each other and acoustic coupling occurs therebetween, thereby forming a first-stage SAW multimode filter. Further, as described above, the second-stage SAW multimode filter
DT electrodes 13a, 13b and reflectors 16a, 17a, 1
6b and 17b, and the two-stage SAW multi-mode filter is electrically connected in cascade by the electrode pattern 18 to form a multi-stage connected SAW multi-mode filter.

The surface acoustic wave multi-mode filter configured as described above determines the mode frequency of the SAW to be excited on the piezoelectric substrate according to the cross width between the IDT electrodes 12 and 13, and determines the pass band width thereof. However, the characteristic obtained is a very narrow band, and the specific bandwidth of the filter realized by the configuration of FIG. 4 is at most about 0.1%. Has inserted an extension coil between the stages of the multimode filter and the ground to deal with it.

[0005]

However, the above configuration has a problem that the out-of-band attenuation characteristic of the filter is significantly deteriorated due to the effects of self-resonance and unnecessary radiation of the interstage extending coil. Was.

The present invention has been made in view of the above-mentioned problems of the conventional surface acoustic wave multimode filter, and has as its object to provide a surface acoustic wave multimode filter which realizes a wide band and sufficiently compensates for the out-of-band attenuation. I do.

[0007]

The surface acoustic wave multi-mode filter of the present invention has a structure in which electrodes between the stages of a plurality of SAW multi-mode filters are grounded via an LC parallel resonance circuit. .

[0008]

According to the present invention, the capacitor in the LC parallel resonance circuit functions as a bypass circuit to the ground in the out-of-band frequency band, and the out-of-band attenuation characteristic is improved.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A surface acoustic wave multimode filter according to an embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a configuration of a surface acoustic wave multi-mode filter according to an embodiment of the present invention. In FIG. 1, a first-stage SAW multimode filter composed of IDT electrodes 12a and 12b and reflectors 14a, 15a, 14b and 15b on a piezoelectric substrate 11, another IDT electrodes 13a and 13b and a reflector 16a, 16b, 17a, 1
The second-stage SAW multimode filter constituted by 7b is electrically connected by an inter-stage electrode pattern 18, and the electrode pattern 18 is grounded via an LC parallel resonance circuit 19.

The operation of the surface acoustic wave multimode filter configured as described above will be described below.

FIG. 2 shows a specific bandwidth of 0.1 in this embodiment.
It is a graph of the out-of-band characteristic obtained experimentally of a SAW multimode filter (ST cut quartz substrate) designed at 25%. In the SAW multimode filter shown in FIG. 1 of the present embodiment, the resonance frequency of the LC parallel resonance circuit 19 including the crossing capacitance of the IDT electrodes 12b and 13a and the stray capacitance of the line becomes near the pass band of the SAW multimode filter.
Further, the out-of-band characteristic when the admittance slope parameter of the resonance circuit is determined to be 0.015 is shown by a solid line, and an extension coil is connected between the stages of the conventional SAW multimode filter of FIG. The out-of-band characteristics when a similar fractional bandwidth is secured are indicated by broken lines.

As a result, in the conventional SAW multimode filter, an unnecessary peak appears near the pass band due to the resonance between the interstage extending coil and the stray capacitance of the line, so that the out-of-band characteristics deteriorate as a whole. doing. The position of this peak slightly varies depending on the value of the extension coil, but the out-of-band attenuation characteristic does not change so much. On the other hand, in the case of the SAW multimode filter of the present embodiment, since the capacitor in the LC parallel resonance circuit functions as a bypass circuit to the ground outside the pass band, this peak is suppressed.
It can be seen that the out-of-band attenuation is significantly improved as compared with the conventional example.

The above-mentioned LC parallel resonance circuit includes the effect of the stray capacitance of the substrate, so that the maximum value of the inductor is limited.
The value of the admittance slope parameter cannot be less than 0.01. Conversely, when this value increases, the ratio of the capacitor in the resonance circuit increases, and the signal in the pass band leaks to the ground side, so that the in-band loss increases. If the value exceeds 0.025, the in-band loss is considerably deteriorated, and sufficient filter characteristics cannot be obtained. After all, when the value of the admittance slope parameter is 0.01 to 0.025, good filter characteristics are obtained. Incidentally, when the admittance slope parameter is 0.025, the inductance value is about 25 nH in the 200 MHz band.

As described above, according to the present embodiment, a sufficient out-of-band attenuation can be obtained by grounding the stages of the multi-stage connected SAW multi-mode filter via the LC parallel resonance circuit having a predetermined element value. Wide band pass characteristics can be realized while securing.

Hereinafter, a second embodiment of the present invention will be described with reference to the drawings.

FIG. 3 is a configuration diagram of a surface acoustic wave multi-mode filter according to a second embodiment of the present invention. In FIG. 3, a first-stage SAW multimode filter composed of IDT electrodes 12a and 12b and reflectors 14a, 15a, 14b and 15b on a piezoelectric substrate 11 and IDT electrodes 13a and
A second-stage SAW multiplex mode filter constituted by 13b and reflectors 16a, 16b, 17a, 17b is electrically connected by an inter-stage electrode pattern 18, and the above is the same as the configuration in FIG. is there.

1 is different from FIG. 1 in that a T-branch is provided in the electrode pattern 18 to form an electrode pattern 20 which extends to near the end of the piezoelectric substrate 11, and is grounded via an LC parallel resonance circuit 19 therefrom. Is a point.

The surface acoustic wave multi-mode filter configured as described above can make the bonding wire for leading from the inter-stage electrode portion 18 of the multi-stage SAW multi-mode filter to the LC parallel resonance circuit 19 very short. For,
Unwanted radiation and stray capacitance to the SAW electrode pattern due to the influence of the bonding wire can be minimized, and the out-of-band characteristics of the SAW multimode filter are further improved.

As described above, by forming the inter-stage electrode pattern of the multi-stage connected SAW multi-mode filter so as to extend to the vicinity of the end of the piezoelectric substrate, the same effect as that of the first embodiment can be obtained and its characteristics can be improved. Can be further improved.

In the above embodiment, the LC parallel resonance circuit 19 is configured as an external circuit. However, all or a part of the elements in this circuit may be configured on the piezoelectric substrate 11.

In this embodiment, the multi-stage connection SAW
Although the description has been made on the assumption that the number of stages of the multi-mode filter is two, it is needless to say that the present invention is applicable to a multi-stage SAW multi-mode filter having three or more stages.

The interstage connection according to the present invention is not limited to the electrode as in the above embodiment, but may be an electrode pattern of another shape.

[0024]

As is apparent from the above description,
According to the present invention, by passing ground between the stages of a multi-stage connected SAW multi-mode filter via an LC parallel resonance circuit having a predetermined element constant, the SAW multi-mode filter can pass without deteriorating the out-of-band attenuation characteristics. Broadening of the band characteristics can be realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a surface acoustic wave multi-mode filter according to a first embodiment of the present invention.

FIG. 2 is an out-of-band characteristic diagram of a surface acoustic wave multimode filter comparing the first embodiment of the present invention with a conventional example.

FIG. 3 is a configuration diagram of a surface acoustic wave multi-mode filter according to a second embodiment of the present invention.

FIG. 4 is a configuration diagram of a conventional surface acoustic wave multimode filter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Piezoelectric substrate 12, 13 IDT electrode 18 Electrode pattern for connection between stages 19 LC parallel resonance circuit

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-6-244676 (JP, A) JP-A-5-129884 (JP, A) JP-A-6-53772 (JP, A) 38130 (JP, U) Hira 4-38128 (JP, U) (58) Fields surveyed (Int. Cl. ⁷ , DB name) H03H 9/145 H03H 9/64

Claims (2)

(57) [Claims] 1. An interdigital transducer (ID)
T) Surface acoustic wave (SA) with reflectors on both sides of the electrode
W) A plurality of SAW multi-mode filters in which resonators are closely arranged and acoustically coupled are formed on the same piezoelectric substrate, and the SAW multi-mode filters are connected in cascade and have a resonance frequency including a stray capacitance. A surface acoustic wave multi-mode filter, characterized in that the connection point is grounded by an LC parallel resonance circuit substantially matching a pass band of the SAW multi-mode filter. 2. The connection between a plurality of SAW multi-mode filters is performed using an electrode pattern formed on the piezoelectric substrate and having a T-branch extending to near the end of the piezoelectric substrate. The surface acoustic wave multi-mode filter according to claim 1, wherein: