JPH0555855A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To easily select an appropriate value for coupling capacity to be impressed between SAW resonator filters in an SAW filter obtained by forming plural cascade-connected SAW resonator filters on a piezo-electric base. CONSTITUTION:Plural capacitors 6, 7 are formed between a node of two SAW resonator filters 2, 3 and ground. These capacitors 6, 7 are arranged between the two filters 2, 3. The coupling capacity value can be changed by cutting out at least one of the connection parts 8, 9 between the capacitors 6, 7 and the ground.

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 including a plurality of surface acoustic wave resonator filters connected in cascade.

[0002]

2. Description of the Related Art Generally, a surface acoustic wave (SAW) resonator filter is often used by connecting a plurality of these in cascade connection. In addition, a plurality of these SAWs connected in cascade
The resonator filter can be formed on a single piezoelectric substrate by forming a capacitor (coupling capacitor) at the connection point.

[0003]

In such a SAW filter, in order to obtain a high selectivity characteristic, it is necessary to appropriately select the coupling capacitance value given by the above-mentioned capacitor.

The optimum coupling capacitance value can be obtained to some extent by simulation, but in reality, it is not so easy to find the optimum value due to the influence of the stray capacitance and the like. Further, since the pattern formed in the SAW device is fine, once the pattern is formed, it is impossible to set the optimum capacitance value by cut and try. Therefore, in many cases, the number of trial productions is actually increased in order to find the optimum capacitance value.

Therefore, an object of the present invention is to provide a SAW filter in which an appropriate coupling capacitance value can be easily selected.

[0006]

The present invention is directed to a SAW filter in which a plurality of cascade-connected SAW resonator filters are formed on a single piezoelectric substrate, and is directed to the above-mentioned technique. In order to solve the above-mentioned problem, a plurality of capacitors are formed between the connection point of the plurality of SAW resonator filters and the ground, and the formation positions of the capacitors are between two SAW resonator filters adjacent to each other. It is characterized by being selected in.

[0007]

In the present invention, the connection between the capacitor and the ground is cut so that at least one of the plurality of capacitors does not function, so that the coupling capacitance having an appropriate value can be obtained.

[0008]

Therefore, according to the present invention, it becomes easy to find an appropriate coupling capacitance value in one pattern formation. That is, the selectivity characteristic of the filter can be changed particularly in the high frequency side by changing the number of capacitors that function.

Further, according to the present invention, since the capacitor is located between the adjacent SAW resonator filters, the distance between these filters becomes long and the acoustic coupling can be reduced. Therefore, spurious characteristics can be improved. If at least a part of such a capacitor is covered with a sound absorbing material, the above becomes more effective.

[0010]

1 is a plan view showing a SAW filter 1 according to an embodiment of the present invention. FIG. 1 clarifies the electrical connection of the elements constituting the SAW filter 1, and also shows the mechanical arrangement relationship. These elements are formed on a single piezoelectric substrate. In addition, FIG.
FIG. 2 is an equivalent circuit diagram of the SAW filter 1 shown in FIG.

The SAW filter 1 comprises a SAW resonator filter 2 on the input side and a SAW resonator filter 3 on the output side. The resonator filters 2 and 3 are connected in cascade. Each of the resonator filters 2 and 3 has two interdigital transducers (IDTs).
4 and reflectors 5 positioned so as to sandwich them, respectively.

Two capacitors 6 and 7, for example, are formed between the connection point of these resonator filters 2 and 3 and the ground. These capacitors 6 and 7 are located between two adjacent resonator filters 2 and 3. These capacitors 6 and 7 are, in this embodiment,
Each is composed of a comb-shaped electrode.

In the SAW filter 1 described above, both the connection portions 8 and 9 are connected, the connection portions 8 or 9 are cut, or both the connection portions 8 and 9 are cut. , The coupling capacitance value can be changed.

The SAW filter 1 described above was actually manufactured in the following manner. Piezoelectric substrate: ST cut quartz Electrode material: Aluminum (SAW resonator filters 2 and 3) Center frequency: about 130 MHz IDT logarithm: 130 vs. IDT cross width: 11λ Number of reflectors: 135 Reflector aperture length: 11λ Gap width between resonators 1λ (λ is the wavelength) (Capacitors 6 and 7) Capacitance value: about 1.1 pF The frequency characteristics of the SAW filter 1 obtained by the above procedure are shown in FIG. In FIG. 3, in the state where both the connecting portions 8 and 9 are still connected,
The state in which one of the connecting portions 8 is cut represents the characteristics when both of the connecting portions 8 and 9 are in the cut state. As can be seen from FIG. 3, by changing the number of functioning capacitors, especially on the high frequency side,
The selectivity characteristic of the filter can be changed.

In FIG. 1, if some or all of the capacitors 6 and 7 are coated with a sound absorbing material such as silicone rubber, acoustic coupling between the two resonator filters 2 and 3 can be effectively prevented. it can.

It is desirable that the pitch of the comb teeth of the capacitors 6 and 7 formed of the comb-shaped electrodes be greatly different from the pitch of the IDT 4 of the filters 2 and 3, but the capacitance value to be obtained. Alternatively, when the pitch is close to the IDT 4 pitch due to dimensional reasons, the sound absorbing material may be used to damp the vibration.

[Brief description of drawings]

FIG. 1 is a plan view showing a circuit configuration of a SAW filter 1 according to an embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram of the SAW filter 1 shown in FIG.

3 is a diagram showing an example of frequency characteristics obtained by the SAW filter 1 shown in FIG.

[Explanation of symbols]

 1 SAW filter 2,3 SAW resonator filter 6,7 Capacitor

Claims (2)

[Claims] 1. A surface acoustic wave filter having a plurality of cascaded surface acoustic wave resonator filters formed on a single piezoelectric substrate, wherein a connection point of the plurality of surface acoustic wave resonator filters. A plurality of capacitors are formed between the surface acoustic wave filter and a ground, and a position where the capacitors are formed is selected between two adjacent surface acoustic wave resonator filters. 2. The surface acoustic wave filter according to claim 1, wherein at least a part of the capacitor is covered with a sound absorbing material.