JPH10233646A - Saw filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the SAW filter that realizes an electric characteristic providing a small ripple within a pass band and a low insertion loss. SOLUTION: A distance between an IDT electrode 3b and a grating reflector 5b of a 2nd SAW filter is made different from a distance between other IDT electrode 2b and a grating reflector 4b by a 1/4 wavelength, a 1st output IDT electrode 3a and the 2nd output IDT electrode 3b are placed in opposite phase, and IDT electrodes 2a, 2b, 3a, 3b have a directivity toward grating reflectors 4a, 4b, 5a, 5b.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a SAW filter used for a mobile communication device such as a mobile phone and a pager.

[0002]

2. Description of the Related Art In recent years, it has been desired that a SAW filter has low insertion loss and small ripple in a pass band.

[0003] A conventional SAW filter will be described below. FIG. 2 shows an electrode configuration of a conventional SAW filter. In FIG. 2, 1 is a piezoelectric substrate, 2 is a bidirectional input IDT electrode, 3 is a bidirectional output IDT electrode, 4 is an input terminal, 5 is an output terminal, 6 and 7 are ground terminals, and 8 is an elastic surface. Waves. S configured as described above
The operation of the AW filter will be described below.

When an electric signal is input to the input terminal 4, a surface acoustic wave 8 is excited by the input IDT electrode 2.
The excited surface acoustic wave 8 is detected as an output signal by the output IDT electrode 3 and output from the output terminal 5.

[0005]

However, in the above-mentioned conventional configuration, the input IDT electrode 2 and the output IDT electrode 3 are bidirectional, so that the insertion loss is 6 dB, and the IDT electrode 2 is reflected due to the secondary effect and the like. If a low insertion loss is realized by increasing the logarithm, the length of the piezoelectric substrate 1 must be increased, and a large ripple occurs in the band.

An object of the present invention is to solve the above-mentioned conventional problems and to provide a SAW filter having low insertion loss and small ripple in a pass band.

[0007]

In order to achieve this object, a SAW filter according to the present invention comprises first and second SAW filters having grating reflectors formed on both sides of an input IDT electrode and an output IDT electrode. Are connected in parallel in two stages, the distance between one IDT electrode of the second SAW filter and the grating reflector is different from the distance between the other IDT electrode and the grating reflector by a quarter wavelength. And the output IDT electrode of the first SAW filter is connected to the output IDT electrode of the second SAW filter.
DT electrodes are arranged in opposite phase, and each IDT
The electrode is characterized in that it has directionality toward an adjacent grating reflector.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the structure of claim 1 of the present invention,
When an electric signal is input to the input terminal, surface acoustic waves are excited by the first and second input IDT electrodes. Since the first and second input IDT electrodes are unidirectional toward the adjacent grating reflector, they propagate only in the direction of the adjacent grating reflector and reflect in the opposite direction. Each of the reflected surface acoustic waves passes through the first and second output IDT electrodes, propagates to other grating reflectors outside thereof, is reflected in the opposite direction, and is output as an output signal via each output IDT electrode. It is detected from one output terminal. Here, by disposing the surface acoustic wave propagation path length of the first SAW filter by a quarter wavelength from the surface acoustic wave propagation path length of the second SAW filter, the propagation to the first output IDT electrode is performed. And the surface acoustic wave propagating to the second output IDT electrode have an opposite phase relationship. However, the first output ID
By making the relationship between the T electrode and the second output IDT electrode opposite phases, the output signals to be detected become in phase and reinforce each other, so that the insertion loss becomes low, and the S signal having bidirectionality is further obtained.
Compared with the AW filter, the propagation path length of the surface acoustic wave is secured with a small number of IDT logarithms, so that a frequency characteristic with a good ripple characteristic in a pass band can be obtained.

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, 1a is a piezoelectric substrate, 2a and 2b are input IDT electrodes, 3a and 3b are output IDT electrodes, 4a, 4b, 5a and 5b are grating reflectors, 6a is an input terminal, 7a is an output terminal, and 8a. Is a ground terminal. The distances between the input IDT electrodes 2a, 2b and the grating reflectors 4a, 4b are La and L, respectively.
b, when the distances between the output IDT electrodes 3a and 3b and the grating reflectors 5a and 5b are Lc and Ld, respectively,
The following relationship holds:

Ld = La ± λ / 4 (λ: wavelength of center frequency) = Lb ± λ / 4 = Lc ± λ / 4 The operation of the SAW filter configured as described above will be described below. When an electric signal is input to the input terminal 6a, the surface acoustic waves 9a and 9b are excited by the input IDT electrodes 2a and 2b, and the input IDT electrodes 2a and 2b are singly directed toward the grating reflectors 4a, 4b, 5a and 5b. Due to the directionality, the light propagates only in the direction of the grating reflectors 4a and 4b, and is reflected in the opposite direction. The reflected surface acoustic waves 9a and 9b pass through the output IDT electrodes 3a and 3b, propagate to the grating reflectors 5a and 5b, are reflected in the opposite direction, and are detected as output signals from the output IDT electrodes 3a and 3b from the output terminal 7a. Is done. Here, the propagation path lengths of the surface acoustic waves 9a and 9b are Le and L, respectively.
Assuming that f, Lf = Le ± λ / 4 holds. Therefore, the surface acoustic waves 9a and 9b propagating to the output IDT electrodes 3a and 3b have an opposite phase relationship. However, the output signals detected by the output IDT electrodes 3a and 3b have the same phase because of the opposite phase relations, and are strengthened.

Regarding the IDT logarithm, even if an insertion loss equivalent to that of a bidirectional SAW filter is realized, the propagation path length of the surface acoustic wave can be ensured with an IDT logarithm smaller than the bidirectionality.

From the above, it is possible to obtain a frequency characteristic with low insertion loss and good ripple characteristics in the pass band.

[0013]

As described above, the present invention provides an input / output IDT.
In a SAW filter in which two stages of SAW filters having grating reflectors formed on both sides of an electrode are connected in parallel,
The surface acoustic wave of the first SAW filter and the surface acoustic wave of the second SAW filter are in opposite phases, the output IDT electrodes are arranged in opposite phases, and all IDT electrodes are unidirectional toward the grating reflector. By doing so, a SAW filter with low insertion loss and small ripple in the pass band can be realized.

[Brief description of the drawings]

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

FIG. 2 is a plan view of a conventional SAW filter.

[Explanation of symbols]

 1a Piezoelectric substrate 2a, 2b Input IDT electrode 3a, 3b Output IDT electrode 4a, 4b, 5a, 5b Grating reflector 6a Input terminal 7a Output terminal 8a Grounding terminal

Claims (1)

[Claims] 1. A SAW filter in which first and second SAW filters each having a grating reflector formed on both sides of an input IDT electrode and an output IDT electrode are connected in parallel in two stages, one of the second SAW filters. The distance between the IDT electrode and the grating reflector is different from the distance between the other IDT electrode and the grating reflector by a quarter wavelength, and the output IDT electrode of the first SAW filter is connected to the second SAW filter. A SAW filter characterized in that the output IDT electrodes of the filter are arranged in phase opposition to each other, and each IDT electrode has directivity toward an adjacent grating reflector.