JPH08125489A - Multiple mode surface acoustic wave filter - Google Patents

Abstract

PURPOSE: To provide the multiple mode surface acoustic wave(SAW) filter in which ripple is not almost included in a filter characteristic and suppression degree of a signal is high at out-band by specifying a distance between an IDT(interdigital transducer) of a resonator and a reflector. CONSTITUTION: A distance D between IDT electrodes 1, 1' being components of a SAW resonator of the SAW filter and reflectors 2, 2' is selected to be a multiple of 0.45 to 0.65 of a pitch (p) of IDT electrode digits 11, 11'. The distance D in a conventional filter has been decided to be 1/4 wavelength with respect to a center frequency so that SAWs are continuously matched and the distance D corresponds to a multiple of 0.25 of the pitch (p). However, in this case, a reflected wave includes a waveform component with a deviated phase when the wave returns to the IDT electrodes 1, 1' and ripple appears in the filter characteristic through the interference of the waves. On the other hand, the filter adopting the D in a range of 0.45 to 0.65 of the pitch (p) has no ripple and the attenuation is sharp an then a desired characteristic is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved multimode surface acoustic wave (SAW) filter.

[0002]

2. Description of the Related Art Conventionally, a multi-mode SAW filter (electrode finger reverse phase arrangement type) of the type shown in FIG. 1 is known (Toyo Communication Equipment Technical Report 1984 No. 35, page 13). This is an interdigital transducer electrode (IDT electrode) (1)
And a pair of grading reflectors (GR) (2) arranged on both sides of the SAW in the propagation direction, the SAW resonator.
(3) is formed, and this is coupled to another SAW resonator (3) 'having the same structure on the surface of the piezoelectric substrate (5) such as crystal through the earth electrode (4).

When an input signal is applied to the terminal (7) of the lead (6) which is in conduction with one of the IDT electrodes (1), the IDT electrode (1)
SAW is generated between (1) and the electrode fingers (11) and (41) of the ground electrode (4). SAW propagates to both sides of the IDT electrode (1), repeats reflection and / or deceleration propagation in the grid (21) of the reflectors (2) and (2), and the symmetrical mode (a) shown on the right side of FIG. And the antisymmetric mode (b) displacement distribution mode. The ground electrode (4) and the bus bars (42) (22) of the reflectors (2) (2) 'are grounded and have the same potential, and the IDT electrode (1)' of the other SAW resonator (3) 'resonates. Then, the output can be obtained from the terminal (9) of the lead (8).

The pitch p of the electrode fingers (11) of the IDT electrode (1)
Are formed at intervals that coincide with the wavelength λ of the center frequency of the input signal, and the pitch of the grids (21) of the reflectors (2) (2) ′ is at intervals of half the wavelength. It is formed. The electrode fingers (41) of the ground electrode (4) are also arranged at an interval corresponding to the wavelength λ of the center frequency and alternately with the electrode fingers (11) of the IDT electrode (1). The frequency difference between the symmetrical resonance mode (FIG. 1a) and the antisymmetric resonance mode (FIG. 1b) is mainly due to the ID.
Cross width W of the electrode fingers (11) and (41) of the T electrode (1) and the ground electrode (4)
And electrode fingers (11) (11) ′ of the two SAW resonators (3) (3) ′
Is determined by the distance between the tips of the, or bond gap G. This frequency difference also determines the bandwidth Q of the filter.

A desirable characteristic of the SAW filter is that an input signal within the band Q is allowed to pass through with no attenuation, but an input signal outside the band is sharply attenuated. In the band Q, the characteristics are flat and there is no fluctuation in the attenuation (ripple A).

[0006]

[Problems to be solved] In the conventional multimode SAW filter, the arrangement of the IDT electrode (1) and the reflector (2) is S
The separation distance D is set to 1/4 of the wavelength λ of the center frequency so that the AWs are continuously matched. As is clear from FIG. 2, the pitch p of the electrode fingers (11) is equal to the wavelength λ,
The width is λ / 4, and the duty ratio of the electrode finger (41) of the earth electrode (4) is usually 0.5, so the width of the earth electrode finger (41) is λ / 4, and the waveform is continuous. The distance D is λ / 4
Because it was supposed to be. This separation distance D
Corresponds to 0.25 times the pitch p of the electrode fingers (11) of the IDT electrode (1).

However, when a signal is input to such a SAW filter, it is inevitable that ripples appear in the filter characteristics. The reason is that even if the separation distance D between the IDT electrode (1) and the reflector (2) is set to λ / 4, the SAW passes through the electrode finger cross width W, the gap G, and the bus bar. (12) There are some which pass through the cross width W while being reflected at the same time while being reflected by (42), and SAW has a slow propagation speed when passing through the positions of the electrode fingers (11) (41), and the substrate between the electrode fingers.
Since the speed increases when passing through the point (5), it is considered that when the reflected waveform returns to the IDT electrode (1), the reflected wave components include phase-shifted waveform components, which interfere with each other.
The present invention clarifies a SAW filter that includes almost no ripple A in the filter characteristic and realizes a filter characteristic in which the degree of signal suppression is high outside the band.

[0008]

[Structure] In the present invention, the separation distance D between the IDT electrode (1) and the reflector (2) forming the SAW resonator of the SAW filter
Was set to 0.45 to 0.65 times the pitch p of the IDT electrode fingers (11).

[0009]

The multimode SAW filter according to the present invention has almost no ripple A in the characteristic curve,
The out-of-band suppression was large and the required filter characteristics were realized.

[0010]

EXAMPLE In the multimode SAW filter of the type shown in FIG. 1, the distance D between the IDT electrodes (1) (1) 'and the reflectors (2) (2)' is 0.44 times the electrode finger pitch p. , 0.45 times,
The filter characteristics were tested for 0.65 times and 0.66 times. The characteristic diagrams are shown in FIGS. 3 to 6.
However, the IDT electrode (1) has 700 electrode fingers, the reflector (2) has 150 grids, the electrode finger pitch is p = 10 μm, the center frequency is 250 MHz, and the electrode finger cross length W = 6λ = 60 μ.
m and the gap G = 1λ = 10 μm.

FIGS. 3 (a), 3 (b) and 6 (c) show the characteristics of the same filter with different magnifications. Figure (a) shows a signal with a center frequency of 250MHz.
The attenuation of the signal when the frequency changes in the range of ± 50 MHz is shown. The degree of attenuation means the degree of suppression. The degree of attenuation is 0 with respect to the center frequency, and the input signal is passed through, but it is desirable that the degree of attenuation be sharply increased outside the band. FIG. 6 (b) is an enlargement of the curve of FIG. 3 (a) in the horizontal axis direction for a signal at the center frequency within a range of ± 2 MHz. The vertical scale is unchanged. (c) Figure is (a)
The curve of the figure is ± 500KHz about the signal of the center frequency.
The range is enlarged in the abscissa direction and is enlarged 10 times in the ordinate direction.

FIG. 3 shows the case of D = 0.44p. Ripple A occurs in the band. 4 shows the case of D = 0.45p, and FIG. 5 shows the case of D = 0.65p. In-band ripple disappears and out-of-band attenuation is steep. Figure 6
Is the case of D = 0.66p. Ripple A in the band
Is appearing. Also, since the attenuation outside the band is gentle, the filter characteristics are inferior. From the results of the above filter characteristic test, D = 0.45 shown in FIG. 4 and FIG.
In the range of 0.65p, there is no ripple A and the attenuation is sharp, which is a desirable characteristic. Further, it has been found that when the value is out of the above range, ripples occur and the out-of-band attenuation degree becomes loose, resulting in poor filter characteristics.

The present invention can be implemented with other types of multi-mode SAW filters as shown in FIG. This is 2
Two SAW resonators (3) (3) 'are arranged side by side in the direction orthogonal to the SAW propagation direction without being grounded, and bus bars (12) (12)' of IDT electrodes are connected by conductors (13) (Japanese Patent Laid-Open No. 2-11012) [H03H 9/64]. Even in this type of SAW filter, the IDT electrode (1) and the reflector
The separation distance D of (2) is 0.45 to 0.6 of the electrode finger pitch p.
By setting it to 5 times, excellent filter characteristics can be obtained.

The above description of the embodiments is for explaining the present invention, and should not be construed as limiting the invention described in the claims or reducing the scope. The configuration of each part of the present invention is not limited to the above-mentioned embodiment, and it goes without saying that various modifications can be made within the technical scope described in the claims.

[Brief description of drawings]

FIG. 1 is a plan view of a multimode SAW filter, in which standing waves are shown on the right side as (a) and (b).

FIG. 2 is an enlarged sectional view taken along line XX in FIG.

FIG. 3 is a characteristic diagram of a multimode filter, (a)
The figure shows the attenuation in the range of the input signal frequency of 50 MHz. Figure (b) shows the attenuation in the 2MHz range. (c) 50 figures
The attenuation in the range of 0 KHz is shown enlarged.

FIG. 4 is a characteristic diagram of a multimode filter, (a)
The figure shows the attenuation in the range of the input signal frequency of 50 MHz. Figure (b) shows the attenuation in the 2MHz range. (c) 50 figures
The attenuation in the range of 0 KHz is shown enlarged.

FIG. 5 is a characteristic diagram of a multimode filter, (a)
The figure shows the attenuation in the range of the input signal frequency of 50 MHz. Figure (b) shows the attenuation in the 2MHz range. (c) 50 figures
The attenuation in the range of 0 KHz is shown enlarged.

FIG. 6 is a characteristic diagram of a multimode filter, (a)
The figure shows the attenuation in the range of the input signal frequency of 50 MHz. Figure (b) shows the attenuation in the 2MHz range. (c) 50 figures
The attenuation in the range of 0 KHz is shown enlarged.

FIG. 7 is another type of multi-mode SAW filter.

[Explanation of symbols]

 (1) IDT electrode (11) Electrode finger (2) Reflector (3) SAW resonator

Claims (1)

[Claims] 1. A resonator comprising an IDT electrode and a pair of reflectors arranged on both sides of the IDT electrode in the surface acoustic wave propagating direction is provided in a direction orthogonal to the surface acoustic wave propagating direction. In two surface acoustic wave filters arranged side by side, the separation distance D between the IDT electrode of the resonator and the reflector is
A multi-mode surface acoustic wave filter characterized in that it is 0.45 to 0.65 times the electrode finger pitch p of the T electrode.