JPH1032462A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a two-stage cascade-connected dual mode surface acoustic wave filter for suppressing the spurious waves of a low frequency side attenuation band near a passing band. SOLUTION: For this filter, first and second dual mode surface acoustic wave filters 11a and 11b provided with plural surface acoustic wave converters and reflectors on the same surface acoustic wave propagation path of a piezoelectric substrate are cascade-connected. In this case, when the number of the reflection strips of the reflector 15a of the first dual mode surface acoustic wave filter 11a is defined as N1, a pitch is defined as P1, the number of the reflection strips of the reflector 15b of the second dual mode surface acoustic wave filter 11b is defined as N2 and the pitch is defined as P2, the values of the N1, P1, N2 and P2 are set so as to mutually offset the frequency responses of the reflectors 15a and 15b of the first and second dual mode surface acoustic wave filters 11a and 11b at the outside of the passing band.

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 two-stage cascade-connected double mode surface acoustic wave filter having excellent attenuation characteristics near a pass band.

[0002]

2. Description of the Related Art In recent years, mobile communication devices starting from automobile telephones have shifted to portable telephones and are rapidly spreading.
With the spread of portable telephones, demands for smaller, lighter, and lower loss are increasing, and individual components used therein are also required to be smaller, lighter, and have lower loss. A surface acoustic wave filter is being used as a high-frequency filter, which is a main component in a mobile communication device, to meet this demand. A surface acoustic wave filter is a filter using a surface acoustic wave propagating on a solid surface, and a number of configuration methods have been reported so far.

[0003] A double mode surface acoustic wave filter is one of the preferred configurations as a surface acoustic wave filter used for a high frequency filter for a cellular phone or a mobile communication terminal. A specific configuration of this type of filter is described in, for example, Japanese Patent Application Laid-Open No. 5-5587.
No. 1, Materials of the Institute of Electronics, Information and Communication Engineers, Ultrasonic Research Group US92-8 pp.7
-10 (1992).

FIGS. 8 and 10 schematically show a conventional two-stage cascaded double-mode surface acoustic wave filter. FIG. 8 shows three surface acoustic wave converters 12 and 13 on a piezoelectric substrate 10.
14 are arranged close to each other on the same propagation path, and reflectors 15 are provided on both outer sides thereof to constitute dual mode surface acoustic wave filters 11a and 11b. These two double mode surface acoustic wave filters 11a and 11b are cascaded. I have. FIG. 9 shows an outline of operation of a single dual-mode surface acoustic wave filter in this configuration. That is, a plurality of different modes of resonance occur between the reflectors provided on both outer sides, and in this case, the primary mode 31 and the tertiary mode 32 are particularly strongly excited. The dual mode surface acoustic wave filter realizes the filter characteristics using the two modes of the first and third order.

FIG. 10 shows another configuration example of a conventional two-stage cascade-connected double mode surface acoustic wave filter. Piezoelectric substrate 10
On the upper side, two surface acoustic wave converters 22 and 23 are arranged close to each other on the same propagation path, and dual mode surface acoustic wave filters 21a and 21b provided with reflectors 24 on both outer sides thereof are formed. Dual mode surface acoustic wave filters 21
a and 21b are connected in cascade. FIG. 11 shows an operation outline of a single dual-mode surface acoustic wave filter in this configuration. That is, a plurality of different modes of resonance occur between the reflectors disposed on both outer sides, and in this case, the primary mode 41 and the secondary mode 42 are particularly strongly excited. A dual mode surface acoustic wave filter as another conventional configuration example realizes the filter characteristics using the two modes of the primary and secondary modes.

Although the configuration of the prior art has been described above using a specific example, the main purpose of forming a two-stage configuration by cascading two dual-mode surface acoustic wave filters is to provide an out-of-band attenuation near the pass band. Is to improve significantly. When a two-stage cascade connection is used, the attenuation outside the band can be almost twice as high as that of a single stage in dB display. Obtaining a high attenuation outside the band by connecting a plurality of filters in cascade has been conventionally performed not only for surface acoustic wave filters but also for other filters.
Heretofore, the cascade-connected first double mode surface acoustic wave filter and the second double mode surface acoustic wave filter have been the same.

When a two-stage cascaded double-mode surface acoustic wave filter is used, for example, as an interstage filter of a mobile phone, the steepness near the pass band is particularly important. First, this will be briefly described. The frequency used for the mobile phone differs according to each country and each system. In the AMPS system used in the United States, the transmitting side frequency 824 to 849 is used.
MHz, the receiving side frequency is 869 to 894 MHz, and the transmission / reception interval is set as narrow as 869-849 = 20 MHz. In the ETACS system used in the UK, the transmitting side frequency is 872-905 MHz, the receiving side frequency is 917-950 MHz, and the transmission / reception interval is 917-9.
It is set as narrow as 05 = 12 MHz. Further, in the GSM system which is rapidly spreading in Europe in recent years, the transmitting side frequency is 890 to 915 MHz and the receiving side frequency is 935 to 960 MHz, and the transmitting and receiving interval is 935.
−915 = 20 MHz is set narrowly.

[0008] The surface acoustic wave filter is often used on the receiving side with a small power load in the high frequency section of these portable telephones. In this case, the receiving side frequency is used as a pass band and the transmitting side frequency is used as a stop band. It is said. Therefore, as described above, the transition width from the pass band to the stop band is set to be very narrow, and therefore, a sharp filter characteristic is required. In particular, the steepness on the low frequency side near the pass band, that is, on the transmission frequency side is important. However, on the low frequency side in the vicinity of the pass band, saw-tooth spurious repetitions of peaks and valleys appear, making it difficult to secure a sufficient amount of attenuation. The spurious component that degrades the attenuation near this passband is the reflection response of the reflectors located on both outer sides in the dual mode surface acoustic wave filter.
It is already pointed out in the above-mentioned Japanese Patent Application Laid-Open No. 5-55871 that the fact that the frequency does not become zero in this band is a factor. In JP-A-5-55871,
To suppress spurious near the passband and on the low frequency side,
It is disclosed that the number of reflector strips is set to a number larger than that at which Q is saturated, and it is shown that spurious is suppressed by about 7 dB by this means, and the attenuation is improved. However, what is inherently changed by this means is that the interval between the peaks and valleys is reduced, the spurs having a sawtooth shape are not removed, and the effect of the two-stage cascade configuration is sufficiently exhibited. Has been a challenge.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to eliminate the above-mentioned problems, and a two-stage cascaded double-mode surface acoustic wave filter which suppresses spurious components in a low-frequency attenuation band near a pass band. The purpose is to provide.

[0010]

This and other objects are attained by the present invention which is defined below as (structure 1) to (structure 3). (Configuration 1) A surface acoustic wave filter in which first and second double-mode surface acoustic wave filters provided with a plurality of surface acoustic wave converters and reflectors on the same surface acoustic wave propagation path of a piezoelectric substrate are cascaded. The number of reflection strips of the reflector of the first double mode surface acoustic wave filter is N1, the pitch is P1, and the number of reflection strips of the reflector of the second double mode surface acoustic wave filter is N2. When the pitch is P2, the values of N1, P1, N2 and P2 are the first
And the frequency response of the reflectors of the second dual mode surface acoustic wave filter is set so as to cancel each other out of the pass band.

(Structure 2) The surface acoustic wave filter according to Structure 1, wherein the piezoelectric substrate is a 64 ° rotated Y-cut lithium niobate, N1 = N2 = N, and N is 150.
At ~ 300, the ratio of P1 and P2 is

[0012]

(Equation 3)

It is characterized in that it is set so that

(Structure 3) The surface acoustic wave filter according to Structure 1, wherein the piezoelectric substrate is a 36 ° rotated Y-cut lithium tantalate, N1 = N2 = N, and N is 15
When 0 to 300, the ratio between P1 and P2 is

[0015]

(Equation 4)

It is characterized in that it is set so that

(Function) With the above configuration, the maximum and minimum positions of the frequency response outside the pass band of the first double mode surface acoustic wave filter reflector and the second double mode surface acoustic wave can be obtained. The maximum and minimum positions of the frequency response outside the pass band of the filter reflector can be different from each other,
As a result, the response outside the entire pass band including the first and second filters in the cascade connection is effectively brought close to zero, and spurious near the pass band is suppressed.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments. FIG. 1 is a diagram schematically illustrating a surface acoustic wave filter applied to the following embodiments. Two dual-mode surface acoustic wave filters 11a and 11b were formed on a 64 ° rotated Y-cut lithium niobate piezoelectric substrate 10 so that the propagation direction of the surface acoustic wave was in the crystal X-axis direction. further,
The pitch P1 of the reflection strip of the reflector of the first double mode surface acoustic wave filter 11a and the pitch P2 of the reflection strip of the second double mode surface acoustic wave filter 11b
The two dual-mode surface acoustic wave filters 11a and 11b are formed as follows so that the maximum and minimum positions outside the frequency response band of the reflector are offset from each other.

First double mode surface acoustic wave filter 11
In a, three surface acoustic wave converters 12a, 13a, and 14a are arranged close to each other on the same surface acoustic wave propagation path of a piezoelectric substrate, and the number of reflection strips N1 is 250 on both outer sides and the pitch P1 is 2.355 μm. Are provided respectively. Second double mode surface acoustic wave filter 1
1b, three surface acoustic wave converters 12b, 13b, and 14b are arranged close to each other on the same surface acoustic wave propagation path of a piezoelectric substrate, and the number of reflection strips N2 is 250 and the pitch P2 is 2.358 μm on both outer sides. And the reflectors 15b are provided. Here, the center frequency of the filter is set to 94
The electrode finger pitch of the surface acoustic wave converter (the surface acoustic wave wavelength at the center frequency) is set to 7.5 MHz (the center frequency of the receiving side in the GSM mobile phone) and the input / output impedance of the filter is adjusted to 50 ohms of the measuring system. λ) to λ
= 4.65 μm, surface acoustic wave converter 12a (12b),
The number of electrode finger pairs of 14a (14b) is 11, and 13a (13
The logarithm of b) was 17.5 pairs. The electrode finger intersection width is 5
6λ.

The first and second double mode surface acoustic wave filters 11a and 11b are connected in two stages by connecting the surface acoustic wave converters 12a and 12b and 14a and 14b by wiring patterns 16 and 17. The connection configuration was adopted. The surface acoustic wave converter 13a was used as an electric signal input terminal, and the surface acoustic wave converter 13b was used as an electric signal output terminal. Thus, in this case, the ratio of the pitches of the reflecting strips of the reflector, P1 / P2 = 0.
It can be seen that 9987 is set. Note that an Al-0.5 wt% Cu alloy was used as the electrode metal material. In this case, the film thickness was about 0.17 μm, which matched with 50 ohm of the measurement system. Further, for comparison with the present invention, P1 = P2 = 2.355 μm (P1 / P2 = 1) according to the prior art.
Other than that, a two-stage cascaded double-mode surface acoustic wave filter having the same configuration as that of the present embodiment was also prepared at the same time.

FIG. 2 shows the measurement result of the frequency characteristic of the above embodiment, and FIG. 3 shows the measurement result of the frequency characteristic by the conventional configuration. By changing the pitch P1 of the reflection strip of the reflector of the first double mode surface acoustic wave filter 11a and the pitch P2 of the reflection strip of the second double mode surface acoustic wave filter 11b, the reflection amount frequency of the reflector is changed. Since the maximum and minimum positions outside the response band are shifted from each other and cancel each other, the region A in FIG. 2 and the region B in FIG. According to the configuration of the present invention, it is understood that spurious components are removed.

It should be noted that N1 / N2, which is the structural factor of the reflector,
The same effect was obtained by changing both N1 / N2 and P1 / P2.

Next, it was experimentally examined in which range the ratio of P1 / P2 was set to obtain the effect of removing spurious. The results are shown in FIGS. FIG. 4 shows a case where a 64-degree rotated Y-cut lithium niobate is used as a piezoelectric substrate, and FIG. 5 shows a case where a 36-degree rotated Y-cut lithium tantalate is used. In each case, the surface acoustic wave propagation direction was the crystal X-axis direction.

The experimental results shown in FIG. 4 will be described.
The horizontal axis is the reflection strip pitch ratio P1 / P2 of the first and second reflectors, and the vertical axis is the ripple value R indicating the difference between the adjacent local maximum and minimum values of the passband low-frequency spurious.
p1. Assuming that the number of reflectors is N1 = N2 = N, N = 1
P1 / P2 for 4 types of 50, 200, 250, 300
The Rp1 value was measured by changing the ratio. In the figure, the cross (X) mark and the broken line indicate N = 150, and the triangle (△) mark and the two-dot chain line indicate N.
= 200, square (□) mark and dashed line indicate N = 250, diamond (菱) mark and solid line indicate N = 300, respectively. In the region where the improvement effect appears, the Rp1 value is equal to that of the prior art (P1 / P2 =
When an appropriate range of P1 / P2 is obtained as an area that is halved compared to 1.000), when N = 150, 0.99 is obtained.
When 72 ≦ (P1 / P2) ≦ 0.9990 and N = 200, 0.9979 ≦ (P1 / P2) ≦ 0.9992, N =
In the case of 250, 0.9985 ≦ (P1 / P2) ≦ 0.9
9998, N = 300, 0.9988 ≦ (P1 /
P2) ≦ 0.9994.

FIG. 6 shows the appropriate range of the P1 / P2 ratio obtained as a result of the above, with the horizontal axis representing the number N of reflectors. Based on these experimental data, when the lower and upper limits are approximated by a quadratic polynomial for N,

[0026]

(Equation 5)

Can be approximated by FIG. 6 also shows curves of these approximate expressions.

FIG. 5 shows a pitch ratio P1 / P when a 36-degree rotated Y-cut lithium tantalate is used as the piezoelectric substrate.
P2 and the ripple value R of the spurious near the pass band on the low frequency side
This is the result of experimentally examining the relationship with p1 as in the case of the above-described 64-degree rotated Y-cut lithium niobate substrate. FIG. 7 shows the range of the appropriate pitch ratio based on the result, with the number of reflectors N as the horizontal axis. As in the above-described case, the appropriate range is set so that the ripple value Rp1 is half or less of that of the related art. From this figure, as in the case described above, the pitch ratio P1
The lower and upper limits of the appropriate range of / P2 can be expressed as a second-order polynomial relating to the number N of reflectors as follows.

[0029]

(Equation 6)

As described above, the effects of the present invention have been described based on the embodiments with respect to a multi-mode filter in which three surface acoustic wave transducers are sandwiched by reflectors. However, the effects of the present invention are limited to this configuration. It is apparent that a similar effect can be obtained even in a type in which two surface acoustic wave converters are sandwiched between reflectors. In addition, although the embodiment in which the lithium substrate of 64 degrees rotation Y-cut lithium niobate and the lithium substrate of 36 degrees rotation Y-cut lithium tantalate are used as the piezoelectric substrate has been described, other piezoelectric substrates and cut angles may be used in the same manner. Obviously, a similar effect can be obtained. Further, in the present embodiment, the case where the number N1 of the reflectors of the first double mode surface acoustic wave filter is equal to the number N2 of the reflectors of the second double mode surface acoustic wave filter is described.
It will be apparent that a similar effect can be obtained if the values of N1 and N2 do not differ greatly from each other.

[0031]

As described above, in a double mode surface acoustic wave filter having a two-stage cascade configuration, spurious components appearing on the low frequency side in the vicinity of the pass band can be removed, whereby the out-of-band attenuation and steepness of the filter can be reduced. An improved surface acoustic wave filter can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a surface acoustic wave filter applied to an embodiment of the present invention.

FIG. 2 is a diagram showing frequency characteristics of a surface acoustic wave filter according to one embodiment of the present invention.

FIG. 3 is a diagram illustrating a frequency characteristic of a surface acoustic wave filter according to the related art.

FIG. 4 is a diagram showing a relationship between a reflector pitch ratio and a spurious suppression using the number of reflectors as a parameter when a 64-degree rotated Y-cut lithium niobate piezoelectric substrate is used.

FIG. 5 is a diagram showing a relationship between spurious suppression and a reflector pitch ratio using the number of reflectors as a parameter when a 36-degree rotated Y-cut lithium tantalate piezoelectric substrate is used.

FIG. 6 is a diagram showing an appropriate pitch ratio range when a 64-degree rotated Y-cut lithium niobate piezoelectric substrate is used.

FIG. 7 is a diagram showing an appropriate pitch ratio range when a 36-degree rotated Y-cut lithium tantalate piezoelectric substrate is used.

FIG. 8 is a diagram showing a cascade connection configuration of a dual mode surface acoustic wave filter using three general surface acoustic wave converters.

FIG. 9 is a diagram showing a distribution of two modes of a dual mode surface acoustic wave filter using three general surface acoustic wave converters.

FIG. 10 is a diagram showing a cascade connection configuration of a dual mode surface acoustic wave filter using two general surface acoustic wave converters.

FIG. 11 is a diagram showing distributions of two modes of a dual mode surface acoustic wave filter using two general surface acoustic wave converters.

[Explanation of symbols]

Reference Signs List 10 piezoelectric substrate 11a, 11b, 21a, 21b double mode surface acoustic wave filter 12, 12a, 12b, 13, 13a, 13b, 14,
14a, 14b, 22, 23 Surface acoustic wave converter 15, 15a, 15b, 24 Reflector 31, 32, 41, 42 Amplitude distribution

Claims (3)

[Claims] 1. A surface acoustic wave filter in which first and second double mode surface acoustic wave filters provided with a plurality of surface acoustic wave converters and reflectors on the same surface acoustic wave propagation path of a piezoelectric substrate are cascaded. The number of reflection strips of the reflector of the first double mode surface acoustic wave filter is N1, the pitch is P1, and the second
When the number of reflection strips of the reflector of the dual mode surface acoustic wave filter is N2 and the pitch is P2,
The values of 1, P1, N2 and P2 are set such that the frequency responses of the reflectors of the first and second dual-mode surface acoustic wave filters cancel each other out of the passband. Surface acoustic wave filter. 2. A surface acoustic wave filter according to claim 1, wherein said piezoelectric substrate is a 64 ° rotated Y-cut lithium niobate, and N1 = N2 = N. And the ratio of P2 is A surface acoustic wave filter characterized by being set so that: 3. The surface acoustic wave filter according to claim 1, wherein the piezoelectric substrate is a 36 ° rotated Y-cut lithium tantalate, and N1 = N2 = N. And the ratio of P2 is A surface acoustic wave filter characterized by being set so that: