JPH09199987A - Surface acoustic wave multiple mode filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent deterioration in an out-band suppression due to the produced harmonic spurious mode by connecting surface acoustic wave multiple mode filters using lateral coupling effect in two-stage cascade. SOLUTION: The filter is constituted of plural interdigital electrodes 2a, 2b, 5a, 5b stimulating a surface acoustic wave and grating reflectors 3a, 3b, 6a, 6b to reflect the excited surface acoustic wave at both sides of the interdigital electrodes each on a piezoelectric substrate 1. Two surface acoustic wave resonators are connected in parallel. Then in the configuration of cascade connection of two stages of the surface acoustic wave multiple mode filters utilizing the lateral coupling effect, number of pairs of the 1st stage interdigital electrodes is selected different from number of pairs of the 2nd stage interdigital electrodes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a narrow band filter used for a communication device such as a UHF / VHF band radio and the like, and more particularly to improving the characteristics of a surface acoustic wave multimode filter.

[0002]

2. Description of the Related Art Regarding the surface acoustic wave multimode filter, for example, the 15th EM symposium (1986).
Entitled "Narrowband Dual-Mode SAW Filter" and discussed by Tanaka et al. FIG. 6 shows the basic configuration of a surface acoustic wave multimode filter utilizing lateral coupling. When two identical SAW resonators 30 and 31 are arranged close to each other in parallel,
When the distance G between the two resonators is sufficiently small, 1 shown by A, B, and C in the direction (transverse direction) perpendicular to the SAW propagation direction.
Resonant modes with second, third, and higher order displacement distributions are excited. Also, regarding the SAW propagation direction (vertical direction), the primary, secondary, and tertiary directions indicated by D, E, and F,
And, a resonance mode having a higher-order displacement distribution is excited. Generally, the mode A is called a symmetric mode and the mode B is called an antisymmetric mode.

A surface acoustic wave multimode filter utilizing lateral coupling utilizes symmetric and antisymmetric modes having different resonance frequencies near the center frequency, that is, in the pass band, and the frequency difference between these two modes is , Configuring SAW
Electrode finger cross width W of the resonator, two identical SAW resonators 3
It is determined by the interval G when 0 and 31 are closely arranged in parallel. In addition, the frequency characteristics of the surface acoustic wave multimode filter (fourth-order filter) having a configuration in which two stages of the surface acoustic wave multimode filter utilizing the lateral coupling effect are connected in cascade are shown together with the displacement distribution corresponding to the spurious response. It shows in FIG.

In addition, as a conventional technique of the surface acoustic wave multimode filter, there is JP-A-4-94207. This technique consists of two surface acoustic waves, which are composed of a plurality of interdigital electrodes that excite surface acoustic waves on a piezoelectric substrate and grating reflectors on both sides of the electrodes that reflect the excited surface acoustic waves. Between the first-stage resonator and the second-stage resonator in a configuration in which two-stage surface acoustic wave multimode filters utilizing the lateral coupling effect, in which the resonators are arranged in parallel adjacent to each other, are connected in series. By providing a slit in the, unnecessary spurious due to the bulk wave outside the band is suppressed.

[0005]

The prior art has the following problems in terms of out-of-band suppression. A conventional surface acoustic wave multi-mode filter has a structure in which two stages of surface acoustic wave multi-mode filters utilizing a lateral coupling effect are connected in cascade as a means for improving the out-of-band suppression degree. However, as can be seen from the displacement distribution diagram corresponding to the frequency characteristic and spurious response shown in FIG. 7, spurious noise occurs on both sides of the filter pass band.

The occurrence of this spurious is caused by the SA of the configuration of FIG.
Explaining in terms of the W resonator, even-order modes other than the second order in the direction (transverse direction) perpendicular to the SAW propagation direction do not appear as spurious responses due to the cancellation of charges in the interdigital transducer. In the higher-order modes, the resonance frequency increases as the order increases. Further, in the SAW propagation direction (longitudinal direction), the resonance frequency of the higher-order mode decreases as the order increases.

From the above, in the surface acoustic wave multimode filter, the direction (transverse direction) perpendicular to the SAW propagation direction,
Due to each higher-order spurious mode in the SAW propagation direction (longitudinal direction), unnecessary spurious is generated outside the band, and the out-of-band suppression degree is deteriorated.

Further, the surface acoustic wave multimode filter utilizing the lateral coupling effect is connected in two stages in cascade, and a slit is provided between the first-stage SAW resonator and the second-stage SAW resonator. Although it is possible to suppress this higher-order spurious mode, there is a problem that the manufacturing process becomes complicated because the slit is formed.

[0009]

In order to solve the above-mentioned conventional problems, a plurality of interdigital electrodes for exciting surface acoustic waves on a piezoelectric substrate, and for reflecting the excited surface acoustic waves on both sides thereof. , A surface acoustic wave multimode filter utilizing the lateral coupling effect, in which two surface acoustic wave resonators each composed of a grating reflector are connected in parallel adjacent to each other.
This is a filter configuration in which the number of pairs of interdigital electrodes in the first stage and the number of interdigital electrodes in the second stage are different from each other in a cascade connection configuration.

By adopting the above means, it is possible to improve the out-of-band suppression degree due to the generated higher-order spurious mode in a structure in which the surface acoustic wave multimode filters utilizing the lateral coupling effect are cascaded in two stages. That is, the number of pairs of the first interdigital transducers that form the surface acoustic wave multimode filter and 2
By changing the logarithm of the interdigital electrodes of the step, SAW
Of the high order spurious modes generated in the first and second stages are different from each other, and the spurious cancels or cancels the spurious.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The surface acoustic wave multimode filter of the present invention will be described below. FIG. 1 is a plan view of an electrode pattern of a surface acoustic wave multimode filter according to a first embodiment of the present invention.

As the piezoelectric substrate, a 38 ° ST-cut quartz substrate 1 is used, and an electrode film thickness of 0.3 μm is formed on this substrate.
The interdigital electrode 2a, the SAW resonator including the grating reflectors 3a on both sides thereof, and the SAW resonator including the interdigital electrode 2b and the grating reflectors 3b on both sides thereof are connected in parallel with each other at the common electrode 4 and The surface acoustic wave multimode filter is used as the stage. Further, an interdigital electrode 5a and an S including a grating reflector 6a on both sides of the interdigital electrode 5a.
The AW resonator and the SAW resonator composed of the interdigital transducer 5b and the grating reflectors 6b on both sides thereof are closely connected in parallel by the common electrode 7 to form a surface acoustic wave multimode filter of the second stage and a first stage of the surface acoustic wave. The surface acoustic wave multimode filter and the second-stage surface acoustic wave multimode filter are connected in cascade by the electrode pattern 8. Further, the bonding pad 9 is an input terminal of the package and the bonding pad 10 in order to convert the propagated SAW into an electric signal.
Are connected to the output terminals through aluminum wires each having a diameter of 25 μm. In addition, each grating reflector, from the bonding pad 11 to the ground terminal of the package,
Similarly, it is connected via an aluminum wire of 25 μmφ.
Further, the electrode pattern 8 is connected to the matching terminal of the package through an aluminum wire of 25 μmφ.
With this filter configuration, the number of interdigital electrodes 2a and 2b constituting the first-stage surface acoustic wave multimode filter is 30.
The number of pairs of the interdigital transducers 5a and 5b constituting the 0-th and second-stage surface acoustic wave multimode filters is different from 240 pairs, and the grating reflectors 3a, 3b, and 6a formed on both sides of each interdigital transducer. , 6b is set to 250. Further, the electrode width of the common electrodes 4 and 7 is 6 μm.

FIG. 2 shows the frequency characteristic of the surface acoustic wave multimode filter at this time. Compared to the conventional characteristic shown by the dotted line, the configuration of the present invention has a loss in the pass band of about 6 dB, and spurious due to a higher-order mode generated outside the band is
It can be seen that it is improved by about 8 dB.

3 and 4 show the logarithmic ratio (N1-N2) / N1 of the first and second interdigital transducers constituting the surface acoustic wave multimode filter and the bandwidth and out-of-band suppression degree ratio. The relationship is shown from the experimental results. Where N1 is 1
The second row, N2, is the logarithm of the second interdigital transducer. This logarithm is defined as (X-1) / 2 when the number of interdigital electrodes is an odd number and X / 2 when the number of interdigital electrodes is an even number when the number of interdigital electrodes is X. There is.

When the bandwidth ratio and the out-of-band suppression degree ratio in the conventional configuration are set to 1, the out-of-band suppression degree tends to be improved by increasing the logarithmic ratio. However, the bandwidth ratio tends to become narrower when the logarithmic ratio (N1-N2) / N1 is 0.2, that is, the loss in the pass band tends to deteriorate. Therefore, in order to make the pass bandwidth equal to the conventional level,
The logarithmic ratio needs to be (N1-N2) /N1≧0.2. In this case, the logarithmic ratio is (N1−N2) / N1 ≧
Although specified as 0.2, (N2-N1) /N2≧0.2
And the same result.

In the present invention, the interdigital electrode and the
Although Al-Cu was used as the electrode material of the grating reflector, Al and Al-based alloys such as Al-Ti may be used. Further, the piezoelectric substrate is not limited to the quartz substrate,
It may be a lithium tantalate single crystal substrate (LiTaO ₃ ) or a lithium niobate single crystal substrate (LiNbO ₃ ). Further, although the electrode film is formed by the EB vapor deposition method in the present invention, it may be formed by the sputtering method. Further, in a surface acoustic wave multi-mode filter that requires narrow band characteristics, the variation in center frequency poses a problem. Therefore, instead of wet chemical etching, high dimensional accuracy can be achieved by RIE (dry etching) for electrode formation. Technology may be used. Further, as the wire used for wire bonding, an Al wire of 25 μmφ was used.
And so on.

FIG. 5 shows a plan view of an electrode pattern of a surface acoustic wave multimode filter according to a second embodiment of the present invention. The interdigital electrode 12a, the SAW resonator including the grating reflectors 13a on both sides thereof, and the interdigital electrode 12
b, S consisting of grating reflectors 13b on both sides
A multi-mode filter in which an AW resonator and a common electrode 14 are closely connected in parallel, a SAW resonator including a comb-shaped electrode 15a and grating reflectors 16a on both sides thereof, a comb-shaped electrode 15b, and gratings on both sides thereof. The SAW resonator including the reflector 16b is connected to the common electrode 17
The surface acoustic wave multi-mode filter, which is closely connected in parallel with, is connected in parallel via the electrode patterns 18 and 19 to form the first-stage surface acoustic wave multi-mode filter. Furthermore, the interdigital electrode 20a and the grating reflectors 2 on both sides thereof are provided.
A surface acoustic wave multimode filter in which a SAW resonator composed of 1a, a comb-shaped electrode 20b, and a SAW resonator composed of a grating reflector 21b on both sides thereof are closely connected in parallel by a common electrode 22, and a comb-shaped electrode. 23a,
SAW consisting of grating reflectors 24a on both sides
A surface acoustic wave multimode filter, in which a resonator, a comb-shaped electrode 23b, and a SAW resonator including a grating reflector 24b on both sides thereof are closely connected in parallel by a common electrode 25, is arranged in parallel via electrode patterns 26 and 27. Connect, 2
Finally, the surface acoustic wave multimode filter of the stage is set to 1
The surface acoustic wave multi-mode filter of the second stage and the surface acoustic wave multi-mode filter of the second stage are connected in cascade by an electrode pattern 28. Further, in order to convert the propagated SAW into an electric signal, the electrode patterns 18 and 26 are connected to the input terminal or the output terminal of the package through a 25 μmφ aluminum wire. Further, each grating reflector is connected from the bonding pad 29 to the ground terminal of the package through the same aluminum wire of 25 μmφ. Further, the electrode pattern 28 is connected to the matching terminal of the package through an aluminum wire of 25 μmφ.

With this filter structure, the interdigital electrodes 12a, 1a constituting the first-stage surface acoustic wave multimode filter.
The number of logarithms 2b, 15a, and 15b is 200, and the interdigital transducer 20 that constitutes the second-stage surface acoustic wave multimode filter.
The number of pairs of a, 20b, 23a, 23b is different from 160 pairs, and the grating reflectors 13a, 13b, 16a, 16b, 21a, which are formed on both sides of each interdigital electrode,
The number of 21b, 24a, and 24b is 120. Further, the electrode width of the common electrodes 14, 17, 22, 25 is 6
μm. The frequency characteristics are the same as those of the first embodiment of the present invention, but by connecting the surface acoustic wave multimode filters of each stage in parallel, the number of pairs of interdigital electrodes and the number of grating reflectors constituting the filter. Can be significantly reduced and miniaturization is possible.

FIG. 8 shows a system diagram of a mobile radio device using the surface acoustic wave multimode filter of the present invention.

The signal input from the antenna 32 is received by the first stage receiving SAW filter 33, low noise amplifier 34,
The signals sent from the voltage controlled oscillator 36 and the local oscillation SAW filter 37 via the receiving SAW filter 35 of the second stage are mixed by the mixer 38, and the intermediate frequency using the surface acoustic wave multimode filter of the present invention is mixed. The signal is selected by the filter 39, and the demodulation circuit 40 and the logic circuit 41 are selected.
Is output to the audio output terminal 42 via. On the other hand, the reference signal for intermediate frequency conversion is sent from the logic circuit 41 to the control circuit 43 of the voltage controlled oscillator 36.

Similarly, the signal sent from the audio input terminal 44 is sent to the modulation circuit 45 via the logic circuit 41. The modulation circuit 45 modulates the signal by mixing the modulation signal from the voltage controlled oscillator 46. Furthermore, the modulated signal is transmitted to the second-stage transmission SAW filter 47, the high-power amplifier 48, and the first-stage transmission SAW filter 4.
It is output to the antenna 32 via 9. A control signal is also sent from the voltage controlled oscillator 46 to the reception control circuit 43 to synchronize with it.

By using the surface acoustic wave multimode filter of the present invention as the intermediate frequency filter 39, demodulation performance with good sensitivity can be obtained, and a high performance mobile radio system can be realized.

[0023]

According to the present invention, a plurality of interdigital electrodes for exciting surface acoustic waves and grating reflectors for reflecting the excited surface acoustic waves are provided on both sides thereof.
Two surface acoustic wave resonators, which are adjacent to each other in parallel and are connected in parallel, are composed of two stages of surface acoustic wave multimode filters that utilize the lateral coupling effect and are connected in cascade. And a filter configuration in which the logarithm of the second interdigital transducer is different, the loss of the pass band is not deteriorated, and the deterioration of the out-of-band suppression degree due to the higher-order spurious mode generated outside the band is improved. it can.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an embodiment of a first surface acoustic wave multimode filter of the present invention.

FIG. 2 is a frequency characteristic diagram of the first surface acoustic wave multimode filter of the present invention.

FIG. 3 is a characteristic diagram of a relationship between a logarithmic ratio and a bandwidth ratio of a surface acoustic wave multimode filter.

FIG. 4 is a characteristic diagram of the relationship between the logarithmic ratio and the out-of-band suppression ratio of the surface acoustic wave multimode filter.

FIG. 5 is an explanatory view of an embodiment of a second surface acoustic wave multimode filter of the present invention.

FIG. 6 is an operation explanatory diagram of a surface acoustic wave multimode filter.

FIG. 7 is a frequency distribution of a conventional surface acoustic wave multimode filter and a displacement distribution chart corresponding to spurious response.

FIG. 8 is a block diagram of a mobile radio system using the surface acoustic wave multimode filter of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Quartz substrate, 2a, 2b, 5a, 5b ... Interdigital electrode, 3a, 3b, 6a, 6b ... Grating reflector, 4, 7 ... Common electrode, 8 ... Electrode pattern, 9, 10, 11 ... Bonding pad.

Claims (3)

[Claims] 1. Elasticity comprising a plurality of interdigital electrodes for exciting surface acoustic waves on a piezoelectric substrate, and grating reflectors on both sides thereof for reflecting the excited surface acoustic waves. In a configuration in which surface acoustic wave multimode filters utilizing the lateral coupling effect, in which surface acoustic wave resonators are adjacently connected in parallel and adjacent to each other, are cascaded in two stages, the number of pairs of interdigital electrodes in the first stage and two stages A surface acoustic wave multimode filter, characterized in that the number of interdigital interdigital electrodes is different. 2. When the number of pairs of the first interdigital transducers is N1 and the number of the second interdigital transducers is N2, (N1−N2) /N1≧0.2. However,
N1> N2 or (N2-N1) /N2≧0.2, where N2> N1 is a surface acoustic wave multimode filter. 3. A mobile radio system using the surface acoustic wave multimode filter according to claim 1.