JPH04249907A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To obtain a surface acoustic wave filter without generating an unrequired mode sprious, not being affected by inductance due to wiring, and provided with steep selectivity. CONSTITUTION:Two surface acoustic wave resonators 14, 16 are formed on a piezoelectric substrate 12. The two surface acoustic wave resonators 14, 16 are formed by confronting with each other so as to form axial symmetry. The surface acoustic wave resonators 14, 16 include interdigital transducers 18, 28 and reflectors 20, 22 and 30, 32 formed at both sides of the transducers. An input terminal 24 and an output terminal 34 are drawn out from the interdigital transducers 18, 28. An electrostatic capacitor 36 is bridge-connected between the input terminal 24 and the output terminal 34 by utilizing part of the reflectors 22, 32.

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 filter, and more particularly to a surface acoustic wave filter in which two surface acoustic wave resonators each formed of an interdigital transducer and a reflector are formed on a piezoelectric substrate.

0002

2. Description of the Related Art FIG. 3 is an illustrative diagram showing an example of a conventional surface acoustic wave filter, which is the background of the present invention. Surface acoustic wave filter 1 includes piezoelectric substrate 2 . Two surface acoustic wave resonators 3 are formed on the piezoelectric substrate 2 . The surface acoustic wave resonator 3 includes an interdigital transducer 4,
and two reflectors 5 formed on both sides of the interdigital transducer 4. These two surface acoustic wave resonators 3 are formed to face each other so as to be line symmetrical. Then, the input terminal 6 and the output terminal 7 are drawn out from the two interdigital transducers 4, and the interdigital transducer 4 and the reflector 5 are connected to each other.
The ground terminal 8 is pulled out so as to be connected to the ground terminal 8.

Equivalent circuits of the surface acoustic wave filter 1 shown in FIG. 3 are shown in FIGS. 4 and 5. In the equivalent circuits shown in FIGS. 4 and 5, L1 = Ls = La, R1
=Rs =Ra, 1/Cm =1/Ca -1/C
It is s. This surface acoustic wave filter 1 is formed, for example, as a bandpass filter.

[0004] When such a surface acoustic wave filter 1 is used, especially in the mobile communication equipment market, a steeper selectivity, that is, a better shape factor is often required. In such cases, a method is generally used in which a plurality of surface acoustic wave filters 1 are connected in cascade to steepen the selectivity. However, if multiple surface acoustic wave filters 1
When multiple are connected in cascade, the selectivity becomes steeper, and at the same time,
The passband width also becomes narrower than necessary.

[0005] Therefore, as shown in FIG. 6, it is conceivable to bridge-connect a capacitance 9 between the input terminal 6 and the output terminal 7. An equivalent circuit of such a surface acoustic wave filter 1 is a circuit in which a capacitance CB is connected between input and output, as shown in FIGS. 7 and 8. In this surface acoustic wave filter 1, the selectivity can be made steeper without narrowing the pass band width compared to a filter in which the capacitance 9 is not formed. This capacitance 9 is formed on the piezoelectric substrate 2, for example.

[0006]

[Problems to be Solved by the Invention] Considering the case where two surface acoustic wave filters 1 are connected in cascade, as shown in FIG.
A method is adopted in which wire bonding is performed over the interdigital transducer 4. In such a case, the surface acoustic waves excited by the surface acoustic wave filter 1 may affect the other surface acoustic wave filter 1. To prevent this, a damping agent is usually applied between the two surface acoustic wave filters 1.

However, in order to perform wire bonding while avoiding the area where the damping agent is applied, the area of the piezoelectric substrate 2 must be increased, resulting in an increase in the size of the surface acoustic wave filter. Furthermore, wire bonding beyond the top of the interdigital transducer 4 is undesirable in terms of manufacturing and electrical characteristics. Therefore, it is conceivable to connect the capacitance 9 by routing electrodes on the piezoelectric substrate 2, but in this case as well, the area of the piezoelectric substrate 2 increases, and there is an effect of inductance generated by the routing electrodes.

Therefore, as shown in FIG. 10, it is conceivable to form a capacitance 9 by partially crossing two interdigital transducers 4. however,
In such a surface acoustic wave filter, since a part of the interdigital transducer 4 is sacrificed, there is a problem in that unnecessary mode spurious is generated.

[0009] Therefore, the main object of the present invention is to be able to easily manufacture without increasing the size, without generating unnecessary mode spurious, and to obtain steep selectivity without narrowing the passband width. , to provide a surface acoustic wave filter.

0010

[Means for Solving the Problems] The present invention includes a piezoelectric substrate, and two surface acoustic wave resonators each consisting of an interdigital transducer and a reflector are formed on the piezoelectric substrate so as to be line-symmetrically opposed to each other. This is a surface acoustic wave filter in which capacitance is bridge-connected between input and output by crossing parts of the reflectors of two surface acoustic wave resonators.

[0011]

[Operation] By crossing parts of the reflectors of two surface acoustic wave resonators, a capacitance is formed. This capacitance is bridged between the input and output.

0012

According to the present invention, since the capacitance is bridge-connected between the input and output of the surface acoustic wave filter, the selectivity can be made steeper without narrowing the passband width. Furthermore, since this capacitance is formed by crossing a portion of the reflector, the capacitance can be formed without wire bonding, routing electrodes, or the like. Therefore, there is no need to increase the area of the piezoelectric substrate, and the surface acoustic wave filter does not become large. Furthermore, the influence of inductance caused by wire bonding and routing electrodes can be avoided. Furthermore, this surface acoustic wave filter does not generate unnecessary mode spurious noise, unlike when a capacitance is formed using a part of an interdigital transducer. Moreover, this surface acoustic wave filter can be manufactured by the same method as a conventional surface acoustic wave filter that does not form capacitance.

The above objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments with reference to the drawings.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an illustrative diagram showing an embodiment of the present invention. Surface acoustic wave filter 10 includes a piezoelectric substrate 12 . As the material of the piezoelectric substrate 12, for example, ST cut crystal is used. Two surface acoustic wave resonators 14 and 16 are formed on the piezoelectric substrate 12. One surface acoustic wave resonator 14 includes an interdigital transducer 18, and this interdigital transducer 1
Two reflectors 20, 22 are formed on both sides of the 8.

The interdigital transducer 18 is formed at the center of the piezoelectric substrate 12 in the longitudinal direction. The interdigital transducer 18 is formed such that two comb-shaped electrodes 18a and 18b alternately intersect. One of the comb-shaped electrodes 18a is connected to the input terminal 2.
4, and the other comb-shaped electrode 18b is connected to the ground terminal 2.
Connected to 6.

The reflectors 20 and 22 are arranged around the interdigital transducer 18 and the piezoelectric substrate 12.
It is formed on both sides of the longitudinal direction. Reflectors 20, 22
has a plurality of electrodes extending in the width direction of the piezoelectric substrate 12, and these electrodes are connected to the ground terminal 26. Interdigital transducer 18 and reflector 20
, 22 are made of aluminum, for example.

Similar to the surface acoustic wave resonator 14, the other surface acoustic wave resonator 16 includes an interdigital transducer 28, and reflectors 30 and 32 are formed on both sides thereof. The interdigital transducer 28 is 2
It includes two comb-shaped electrodes 28a and 28b. One comb-shaped electrode 28a is connected to the output terminal 34, and the other comb-shaped electrode 28b is connected to the ground terminal 26. Also,
Reflectors 30 and 32 are connected to ground terminal 26. This surface acoustic wave resonator 16 is formed to face another surface acoustic wave resonator 14 in line symmetry.

The reflector 22 of one surface acoustic wave resonator 14 and the reflector 32 of the other surface acoustic wave resonator 16 are formed so that their electrodes intersect with each other near their centers. Thereby, a capacitance 36 is formed between the reflectors 22 and 32. This capacitance 36 is bridge-connected between the input terminal 24 and the output terminal 34. Note that if the pitch of the electrodes in the capacitance 36 portion is set to be the same as that in other reflector portions, it functions as a capacitance and also functions as a reflector.

FIG. 2 shows the surface acoustic wave filter 10 shown in FIG.
FIG. 2 is a plan view showing a state in which two of As shown in FIG. 2, even when two surface acoustic wave filters 10 are connected, they can be easily connected using short wiring. The surface acoustic wave filter 10 of the present invention has steeper selectivity than conventional filters. Moreover, its passband width is almost the same as that of conventional surface acoustic wave filters.

In this surface acoustic wave filter 10, the reflectors 22 and 32 are used to form the capacitance 36, so there is no need to form wire bonding or routing electrodes. Therefore, there is no need to increase the size of the substrate 12, and it can be made the same size as a conventional surface acoustic wave filter, and the surface acoustic wave filter 10 does not need to be increased in size. Furthermore, this surface acoustic wave filter 10 is not affected by inductance due to wire bonding or routing electrodes.

[0021] Furthermore, in this surface acoustic wave filter 10,
Since the capacitance 36 is formed using the electrodes of the reflectors 22 and 32, it can be manufactured in the same process as a conventional surface acoustic wave filter in which no capacitance is formed. Furthermore, this surface acoustic wave filter 10 does not generate unnecessary mode spurious noise unlike conventional filters in which capacitance is formed in the interdigital transducer portion.

[0022] If this surface acoustic wave filter 10 is used, frequency characteristics with steep selectivity can be obtained whether the surface acoustic wave filter is used in one stage or in two or more stages connected in cascade. Obtainable. Furthermore, in FIGS. 1 and 2, the capacitance 36 is formed approximately at the center of the reflectors 22, 32, but the capacitance 36 may be formed at the ends of the reflectors 22, 32. . Furthermore, the capacitance 36 may be formed in separate portions of the reflectors 20, 30.

[Brief explanation of the drawing]

FIG. 1 is an illustrative diagram showing an embodiment of the present invention.

FIG. 2 is an illustrative view showing a state in which two stages of surface acoustic wave filters shown in FIG. 1 are connected in cascade.

FIG. 3 is an illustrative diagram showing an example of a conventional surface acoustic wave filter.

FIG. 4 is an equivalent circuit diagram of the conventional surface acoustic wave filter shown in FIG. 3;

FIG. 5 is an equivalent circuit diagram equivalent to the equivalent circuit shown in FIG. 4;

FIG. 6 is an illustrative diagram showing another example of a conventional surface acoustic wave filter.

7 is an equivalent circuit diagram of the surface acoustic wave filter shown in FIG. 6. FIG.

8 is an equivalent circuit diagram equivalent to the equivalent circuit shown in FIG. 7. FIG.

FIG. 9 is an illustrative view showing a state in which two stages of the conventional surface acoustic wave filters shown in FIG. 6 are connected in cascade.

FIG. 10 is an illustrative view showing still another example of a conventional surface acoustic wave filter.

[Explanation of symbols]

10 Surface acoustic wave filter 12 Piezoelectric substrate 14 Surface acoustic wave resonator 16 Surface acoustic wave resonator 18 Interdigital transducer 20 Reflector 22 Reflector 24 Input terminal 28 Interdigital transducer 30 Reflector 32 Reflector 34 Output terminal 36 Capacitance

Claims (1)

[Claims] 1. A surface acoustic wave filter including a piezoelectric substrate, on which two surface acoustic wave resonators each consisting of an interdigital transducer and a reflector are formed facing each other in a line-symmetrical manner. and a surface acoustic wave filter in which capacitance is bridge-connected between input and output by crossing parts of the reflectors of the two surface acoustic wave resonators.