JPH04249906A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To obtain a surface acoustic wave filter provided with steep selectivity without increasing insertion loss and narrowing a passing band width. 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, respectively. 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.

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. 5 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. 5 are shown in FIGS. 6 and 7. In the equivalent circuits shown in FIGS. 6 and 7, 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 surface acoustic wave filters are used, particularly in the mobile communication equipment market, even steeper selectivity, that is, a better shape factor is often required. In such cases, a method commonly used is to connect a plurality of surface acoustic wave filters in cascade to steepen the selectivity.

However, when a plurality of surface acoustic wave filters are connected in series, the selectivity becomes steep, the insertion loss becomes large, and the passband width becomes narrower than necessary.

Therefore, the main object of the present invention is to provide a surface acoustic wave filter that can steeply selectivity without increasing insertion loss or narrowing the passband width. .

[0007]

[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. A surface acoustic wave filter with a capacitance bridged between its input and output.
It is a surface acoustic wave filter.

[0008]

[Operation] By connecting capacitance between input and output, the surface acoustic wave filter has a steep frequency characteristic.

[0009]

According to the present invention, a surface acoustic wave filter having steep selectivity can be obtained without increasing the insertion loss or narrowing the passband width.

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.

[0011]

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. The logarithm of these comb-shaped electrodes 18a, 18b is
In this embodiment, 130 pairs are formed. Then, the wavelength of the surface acoustic wave excited by this surface acoustic wave filter 10 is λ
Then, the intersection width of the interdigital transducer 18 is set to 9λ. One comb-shaped electrode 18a of the interdigital transducer 18 is connected to the input terminal 24, and the other comb-shaped electrode 18b is connected to the ground terminal 26.

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. In this embodiment, the number of electrodes of each of the reflectors 20 and 22 is 135. Further, the aperture length of the reflectors 20 and 22 is set to 9λ, where λ is the wavelength of the surface acoustic wave. The interdigital transducer 18 and reflectors 20, 22 are made of, for example, aluminum.

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,
The reflectors 30 and 32 are formed of, for example, 135 electrodes and are connected to the ground terminal 26. This surface acoustic wave resonator 16 is formed to face another surface acoustic wave resonator 14 in line symmetry. The gap width between the two surface acoustic wave resonators 14 and 16 is set to 0.9λ, where λ is the wavelength of the surface acoustic wave.

Between the input terminal 24 and the output terminal 34,
A capacitance 36 is bridged. The capacitance 36 is formed on the piezoelectric substrate 12, for example. This capacitance 36
Accordingly, the waveform of the frequency characteristic of the surface acoustic wave filter 10 can be made steep.

Equivalent circuits of this surface acoustic wave filter 10 are shown in FIGS. 2 and 3. In FIGS. 2 and 3, L1 = Ls = La, R1 = Rs = Ra,
1/Cm = 1/Ca -1/Cs. Also, C
B indicates the capacitance 36 between the input terminal 24 and the output terminal 34.

FIG. 4 shows the frequency characteristics of the surface acoustic wave filter 10 of the present invention and a conventional surface acoustic wave filter. This frequency characteristic is a characteristic when surface acoustic wave filters are connected in cascade in two stages. In FIG. 4, A shows the characteristics using a capacitance 36 of 0.035 pF, B shows the characteristics using a capacitance 36 of 0.01 pF, and C shows the characteristics using a conventional surface acoustic wave filter. shows the characteristics of

As can be seen from FIG. 4, in the surface acoustic wave filter 10 of the present invention, the slope of the rising portion in the passband is larger than that in the conventional filter in which no capacitance is formed between the input and output terminals. ing. In other words, it can be seen that the surface acoustic wave filter 10 of the present invention has improved selectivity compared to the conventional filter. Therefore, the selectivity of the surface acoustic wave filter can be improved with a small number of multi-stage connections, and when the selectivity is the same, the insertion loss can be reduced.

[0019] Also, the surface acoustic wave filter 10 of the present invention
In this case, the shape factor is improved compared to the conventional one, and the passband width is not made narrower than necessary when the selectivity is the same.

Note that the frequency characteristics in FIG. 4 show the characteristics when two stages of surface acoustic wave filters are connected in cascade, but when there is only one stage of surface acoustic wave filters or when three or more stages are cascaded, A similar effect can also be obtained.

Furthermore, the logarithm of the interdigital transducers 18, 28 and the reflectors 20, 22, 30
, 32 can be arbitrarily set as required.

[Brief explanation of the drawing]

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

FIG. 2 is an equivalent circuit diagram of the surface acoustic wave filter shown in FIG. 1.

FIG. 3 is an equivalent circuit diagram equivalent to the equivalent circuit shown in FIG. 2;

FIG. 4 is a graph showing frequency characteristics of a surface acoustic wave filter of the present invention and a conventional surface acoustic wave filter.

FIG. 5 is an illustrative diagram showing an example of a conventional surface acoustic wave filter that is the background of the present invention.

6 is an equivalent circuit diagram of the conventional surface acoustic wave filter shown in FIG. 5. FIG.

FIG. 7 is an equivalent circuit diagram equivalent to the equivalent circuit shown in FIG. 6;

[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. A surface acoustic wave filter with capacitance bridged between its input and output.