JPH11312951A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave filter which adjusts a pass-band width into a decreasing direction and has excellent shield characteristics near the pass band. SOLUTION: This device is a surface acoustic wave filter F which is composed by connecting a surface acoustic wave resonator that has comb-type electrodes to a ladder type circuit, and it is constituted so that plural surface acoustic wave resonators 1a and 1b including resonators at least one of which is different in a resonance frequency are serially connected to a serial arm of the ladder type circuit and plural surface acoustic wave resonators 2a and 2b including resonators at least one of which is different in the resonance frequency are connected in parallel to a parallel arm of the ladder type circuit.

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 used in a wireless communication circuit of a mobile communication device or the like, and more particularly to a surface acoustic wave filter in which a surface acoustic wave resonator is connected to a ladder type circuit. It concerns bandwidth control.

[0002]

2. Description of the Related Art In recent years, many surface acoustic wave elements have been used as elements such as filters, delay lines, and transmitters of electronic equipment utilizing radio waves. In particular, in the field of mobile communication, a surface acoustic wave filter which is small and lightweight and has high sharp cutoff performance as a filter has been widely used as an RF stage and an IF stage filter of a portable terminal device. There is a need for a surface acoustic wave filter having various fractional bandwidths having excellent loss and out-of-band cutoff characteristics.

Until now, various types of surface acoustic wave filters, such as a ladder type, a transversal type, and a longitudinal mode coupling resonator type, have been put into practical use from the viewpoint of electrode configuration.
Among them, the ladder type surface acoustic wave filter has low loss and good cutoff characteristics near the pass band, and has high reliability in terms of power durability due to electrode miniaturization due to high frequency, and is very promising. This is a surface acoustic wave filter.

[0004] In the case of such a ladder-type filter, the fractional bandwidth (pass bandwidth normalized by the center frequency) is:
It is almost determined by standardizing Δf, which is the difference between the resonance frequency and the antiresonance frequency of the surface acoustic wave resonator constituting the filter, with the resonance frequency, and this is one of the material constants of the piezoelectric substrate, the electromechanical coupling coefficient. Therefore, the filter is manufactured by selecting a piezoelectric substrate having an appropriate electromechanical coupling coefficient in order to obtain a desired specific bandwidth.

[0005]

However, the electromechanical coupling coefficient of a piezoelectric substrate typified by lithium niobate and lithium tantalate currently in practical use is determined by the plane orientation of each crystal and the propagation orientation of a surface acoustic wave. Even if it includes the propagation modes of surface acoustic waves such as Rayleigh waves and leaky waves, since there are only discrete and finite types, it has not been possible to easily realize a variety of required fractional bandwidths.

It is known that in the ladder type surface acoustic wave filter, the technique of adjusting the relative bandwidth in the direction of increasing the bandwidth can be realized by providing an inductance component between the parallel arm resonator and GND (for example, However, since the relative bandwidth cannot be adjusted in the decreasing direction, if the pass band width of the filter becomes too wide, the attenuation near the pass band can be sufficiently secured. However, there was a problem that the cutoff characteristics deteriorated.

Accordingly, the present invention has been made to address such a problem, and provides a surface acoustic wave filter having a good cut-off characteristic in the vicinity of the pass band by adjusting the pass band width in a decreasing direction. The purpose is to do.

[0008]

In order to solve the above problems, a surface acoustic wave filter according to the present invention comprises a surface acoustic wave filter having a surface acoustic wave resonator having comb-shaped electrodes connected to a ladder type circuit. Wherein a plurality of surface acoustic wave resonators including at least one resonator having a different resonance frequency are connected in series to the series arm of the ladder-type circuit, and at least one of the resonance arms having different resonance frequencies is connected to the parallel arm of the ladder-type circuit. Characterized in that a plurality of surface acoustic wave resonators including resonators are connected in parallel.

[0009]

For example, when two surface acoustic wave resonators having slightly different resonance frequencies are connected in series, and this is equivalently regarded as one resonator, one resonance frequency is generated, and one resonance frequency is generated. Two resonance frequencies occur. Here, a combination having a small difference between the resonance frequency and the anti-resonance frequency corresponds to the resonator frequency and the anti-resonance frequency of the conventional surface acoustic wave resonator provided only in the series arm and the parallel arm. The difference between the frequency and the anti-resonance frequency can be made smaller.

When three or more surface acoustic wave resonators whose resonance frequencies are slightly different from each other are connected in series, the same as the above-described two series connection except that the anti-resonance frequencies are generated by the number of connections. In this case as well, the difference between the resonance frequency and the anti-resonance frequency can be further reduced.

On the other hand, when a plurality of surface acoustic wave resonators having the same resonance frequency are connected in series, the impedance becomes twice or more as a single equivalent resonator. There is no change in the anti-resonance frequency. Therefore, the difference between the resonance frequency and the anti-resonance frequency does not change. In some cases, two or more capacitors may be intentionally connected to adjust impedance or adjust a voltage applied to one resonator.

Further, when two surface acoustic wave resonators having slightly different resonance frequencies are connected in parallel to each other and equivalently viewed as one resonator, two resonance frequencies are generated, and One resonance frequency occurs. Here, a combination having a small difference between the resonance frequency and the anti-resonance frequency corresponds to the resonator frequency and the anti-resonance frequency of the conventional surface acoustic wave resonator provided only in the series arm and the parallel arm. The difference between the frequency and the anti-resonance frequency can be made smaller.

When three or more surface acoustic wave resonators having slightly different resonance frequencies are connected in parallel, the same as the above-described two parallel connection except that the resonance frequency is generated by the number of connections. In this case, similarly, the difference between the resonance frequency and the anti-resonance frequency can be further reduced.

On the other hand, when a plurality of surface acoustic wave resonators having the same resonance frequency are connected in parallel, the impedance becomes 1 / number of connection times when one equivalent resonator is considered. There is no change in frequency and anti-resonance frequency. Therefore, the difference between the resonance frequency and the anti-resonance frequency does not change. In some cases, two or more capacitors may be intentionally connected to adjust impedance or adjust a current applied to one resonator.

As described above, by connecting a plurality of surface acoustic wave resonators of the present invention in series, it is possible to control in a direction in which the difference between the resonance frequency and the anti-resonance frequency is reduced while the impedance characteristics near the resonance frequency are good.

Further, by connecting a plurality of surface acoustic wave resonators according to the present invention in parallel, it is possible to control the difference between the resonance frequency and the anti-resonance frequency in a direction in which the impedance characteristic near the anti-resonance frequency is good.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a surface acoustic wave filter according to the present invention will be described below with reference to the drawings. A simplified basic ladder type surface acoustic wave filter J is composed of a surface acoustic wave resonator 1a arranged in a series arm and a surface acoustic wave resonator 2a arranged in a parallel arm as shown in FIG. . In addition, each of these surface acoustic wave resonators is configured by disposing a reflector composed of a ladder-like electrode at both ends of a pair of comb-like electrodes, for example. 3a and 3b are input terminals, and 4a and 4b are output terminals.

FIG. 3 shows an example of the impedance characteristics of such a surface acoustic wave resonator. In the impedance characteristics of FIG. 3, the frequency at which the impedance has a minimum value is the resonance frequency fr, and the frequency at which the impedance has a maximum value is the anti-resonance frequency fa.
The difference between the anti-resonance frequency fa and the resonance frequency fr is referred to as Δf of the resonator.

Normally, a good bandpass filter can be obtained by making the resonance frequency of the surface acoustic wave resonator arranged in the series arm substantially equal to the resonance frequency of the surface acoustic wave resonator arranged in the parallel arm. I have.

Further, the resonance frequency and the antiresonance frequency of the surface acoustic wave resonator can be easily controlled by changing the period of a comb-shaped electrode formed on the piezoelectric substrate.
At this time, the pass band width of the band-pass filter is substantially equal to Δf of the surface acoustic wave resonators arranged in the series arm and the parallel arm. Therefore, in order to control the passband, the surface acoustic wave resonators arranged in the
It is necessary to control f.

The present invention provides a means for controlling in a direction in which the pass band width is reduced by controlling in a direction in which Δf of the surface acoustic wave resonator is reduced. It is intended to prevent the attenuation near the pass band from deteriorating and to improve the attenuation characteristics near the pass band.

A method for controlling Δf of the surface acoustic wave resonator will be described below. FIG. 1 shows a surface acoustic wave filter F comprising a basic circuit of the present invention. As shown in FIG.
For example, by changing the electrode line width or the space between the electrode lines, the impedance characteristic of the series resonator group 1 in which two first series resonators 1a and second series resonators 1b having different frequencies by δf are connected in series. Has only one resonance frequency at which the impedance has a minimum value, but generates two anti-resonance frequencies at which the impedance has a maximum value.

FIG. 4 shows an example of the impedance characteristic at this time. As can be seen from FIG. 4, when this resonator group is arranged in a series arm, the difference between the anti-resonance frequency and the resonance frequency closer to the resonance frequency can be treated as Δf of the surface acoustic wave resonator. It is obvious from the principle of the filter.

Here, the first and second series resonators 1a, 1
The difference between the anti-resonance frequency and the resonance frequency of b is denoted as Δfo,
Assuming that the resonance frequency difference between the first series resonator 1a and the second series resonator 1b is δf, and the difference between the equivalent anti-resonance frequency and resonance frequency of the series resonator group 1 is Δf, δ normalized by Δfo.
The relationship between f and Δf is as shown in FIG. As is clear from FIG. 5, as δf is increased from 0, Δf becomes Δ
fo is reduced by 20% or more.

As shown in FIG. 1, the resonance frequency is δf
The impedance characteristic of the parallel resonator group 2 in which the two first parallel resonators 2a and the second parallel resonator 2b which are different from each other are connected in parallel is as follows. There are two resonance frequencies at which is a minimum value.

FIG. 6 shows an example of the impedance characteristic at this time. As can be seen from FIG. 6, when this resonator group is arranged on the parallel arm, the difference between the anti-resonance frequency and the resonance frequency closer to the anti-resonance frequency can be treated as Δf of the surface acoustic wave resonator. This is obvious from the principle of the ladder filter.

Here, the first and second parallel resonators 2a, 2a
The difference between the anti-resonance frequency and the resonance frequency of b is denoted as Δfo,
If the frequency difference between 2a and 2b is δf, and the difference between the equivalent anti-resonance frequency and resonance frequency of the parallel resonator group 2 is Δf, then Δf
The relationship between δf and Δf normalized by fo is as shown in FIG. As is clear from FIG. 7, as δf increases from 0, Δf decreases by 20% or more from Δfo.

As δf / △ fo becomes larger than 0, the magnitude of the ripple appearing on the low frequency side and the high frequency side of the pass band increases. In addition, since the insertion loss of the pass band (the sign of the attenuation of the pass band is inverted) increases,
0 <δf / △ fo <1. In particular, when δf / △ fo is about 0.6, the attenuation level of the ripple and the attenuation level of the pass band become substantially equal, so that a favorable range of δf / △ fo is 0 <δf / △ fo <0.6, and Preferably, the level difference between the pass band and the ripple needs to be about 5 dB, so that 0 <δf / △ fo <0.3.

As described above, in the ladder-type surface acoustic wave filter, a series resonator group in which two surface acoustic wave resonators having different frequencies are connected in series is connected to a series arm, and a parallel resonator group connected in parallel is connected to a parallel arm. By arranging them, a surface acoustic wave filter controlled in a direction to reduce the pass band width can be obtained. At this time, the magnitude of the pass band width can be controlled by the frequency difference δf between the two surface acoustic wave resonators.

[0030]

Embodiments of the present invention will be described below. A surface acoustic wave resonator having a comb-shaped electrode made of aluminum or an aluminum alloy and a ladder-like reflector provided on a 36 ° to 42 ° Y-cut lithium tantalate substrate so that the surface acoustic wave propagates in the X direction. Was formed in the most basic connection arrangement as shown in FIG.

Here, the electrode and the reflector have a thickness of 4000Å.
The cross width of the first series resonator provided in the series arm is 20λ (where λ is the wavelength of the surface acoustic wave), and the logarithm is 5
0, the number of reflectors is 20, the electrode line width (or the space between electrode lines) is 1.000, the cross width of the second series resonator is 20λ, the logarithm is 50, the number of reflectors is 20, and the electrode lines are The width (or the space between the electrode wires) is 1.014, the cross width of the first parallel resonator provided in the parallel arm is 20λ (where λ is the wavelength of the surface acoustic wave), the logarithm is 150, and the number of reflectors is 20, the electrode line width (or the space between the electrode lines) is 1.050, the cross width of the second parallel resonator is 20λ,
The logarithm was 150, the number of reflectors was 20, and the electrode line width (or the space between the electrode lines) was 1.036.

The serially connected 2 provided on the serial arm
The normalized frequency difference between the two surface acoustic wave resonators and the two surface acoustic wave resonators connected in parallel provided on the parallel arm was δf / Δfo = 0.15. Ten or more surface acoustic wave filters having such a configuration were manufactured.

These many average filter characteristics are shown in FIG.
Shown in For comparison, the conventional characteristics (the resonators connected to the series arm and the parallel arm, respectively) are also shown in FIG. FIG.
As is clear from the above, when the required pass band width is 16 MHz, the filter of the present invention has better attenuation characteristics near the pass band than the conventional example.

In the embodiment, the resonator group connected in series and the resonator group connected in parallel have been described using two surface acoustic wave resonators. However, the same applies to the connection of three or more resonators. It is possible to apply.

Further, the present invention can be applied to a surface acoustic wave filter in which the embodiment shown in FIG. 1 is connected in multiple stages. As the piezoelectric substrate, a single crystal substrate of lithium niobate, lithium tetraborate, quartz, langasite or the like can be used other than the lithium tantalate single crystal.

[0036]

According to the surface acoustic wave filter of the present invention,
By connecting a plurality of surface acoustic wave resonators according to the present invention in series, it is possible to control the difference between the resonance frequency and the anti-resonance frequency in a direction in which the impedance characteristic near the resonance frequency is good.

Further, by connecting a plurality of surface acoustic wave resonators of the present invention in parallel, it is possible to control in a direction in which the difference between the resonance frequency and the anti-resonance frequency is reduced while the impedance characteristic near the anti-resonance frequency is good.

Further, a plurality of surface acoustic wave resonators each including at least one resonator having a different resonance frequency are connected in series to the series arm of the ladder-type circuit, and at least one surface acoustic wave resonator including a resonator having a different resonance frequency is connected to the parallel arm. By connecting a plurality of surface acoustic wave resonators in parallel, control in a direction to reduce the pass bandwidth of the ladder type surface acoustic wave filter having a pass bandwidth that is excessively larger than the desired pass bandwidth,
It is possible to provide a very excellent surface acoustic wave filter which has good cut-off characteristics in which a sufficient amount of attenuation near the outside of the pass band is ensured and has no ripple in the pass band.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating a basic circuit of a surface acoustic wave filter according to the present invention.

FIG. 2 is a circuit diagram illustrating a basic circuit of a conventional general ladder type surface acoustic wave filter.

FIG. 3 is a characteristic diagram showing impedance characteristics of a conventional surface acoustic wave resonator.

FIG. 4 is a characteristic diagram showing impedance characteristics when the surface acoustic wave resonator according to the present invention is connected in series.

FIG. 5 is a graph showing the relationship between δf / △ fo and Δf / △ fo when the surface acoustic wave resonator according to the present invention is connected in series.

FIG. 6 is a characteristic diagram showing impedance characteristics when the surface acoustic wave resonator according to the present invention is connected in parallel.

FIG. 7 is a graph showing the relationship between δf / △ fo and Δf / △ fo when the surface acoustic wave resonator according to the present invention is connected in parallel.

FIG. 8 is a characteristic diagram showing filter characteristics of the embodiment of the present invention and a conventional example.

[Description of Signs] 1: Series resonator group 1a: First series resonator 1b: Second series resonator 2: Parallel resonator group 2a: First parallel resonator 2b: Second parallel resonator 3a: Input terminal 3b : Input terminal 4a: output terminal 4b: output terminal F: surface acoustic wave filter

Claims (1)

[Claims] 1. A surface acoustic wave filter comprising a surface acoustic wave resonator having comb-shaped electrodes connected to a ladder type circuit, wherein at least one of the series arms of the ladder type circuit has a different resonance frequency. And a plurality of surface acoustic wave resonators including resonators having different resonance frequencies are connected in parallel to a parallel arm of the ladder-type circuit. Characteristic surface acoustic wave filter.