JP3191473B2 - Surface acoustic wave filter - Google Patents

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 for a high frequency circuit of a mobile communication device.

[0002]

2. Description of the Related Art FIG. 18 is, for example, a document “Technical Research Report of the Institute of Electronics, Information and Communication Engineers (Ultrasonics)”, US92-52, pp.
FIG. 9-16 shows a configuration of a conventional surface acoustic wave filter of this type shown in FIG. In FIG. 18, 4a,
4b is a one-port surface acoustic wave resonator, 6 is an input terminal, and 7 is an output terminal. In the figure, a one-port pair surface acoustic wave resonator 4a inserted in a series arm and a one-port pair surface acoustic wave resonator 4b inserted in a parallel arm are provided in a ladder form between an input terminal 6 and an output terminal 7. Connected. FIG. 1 shows a general configuration of one-port surface acoustic wave resonators 4a and 4b in FIG.
It is shown in FIG. In FIG. 19, 1 is a piezoelectric substrate, 2 is an interdigital electrode, and 3 is a reflector. An IDT 2 and two reflectors 3 are arranged on a piezoelectric substrate 1 to form a one-terminal surface acoustic wave resonator 4.

Next, the operation will be described. When an electric signal is applied between the terminals in FIG. 19, a surface acoustic wave is excited from the interdigital transducer 2. The reflectors 3 provided on both sides of the interdigital transducer 2 reflect surface acoustic waves. Therefore, the excited surface acoustic waves cause multiple reflections between the reflectors 3 on both sides, and resonance occurs.

FIG. 20 shows the impedance characteristics of the one-port surface acoustic wave resonator 4 shown in FIG. In the figure, the vertical axis indicates the imaginary part of the impedance. The impedance becomes zero at the resonance frequency fr and becomes infinite at the anti-resonance frequency fa. Further, the impedance becomes inductive between the resonance frequency fr and the anti-resonance frequency fa, and the impedance becomes capacitive at other frequencies.

In FIG. 18, the resonance frequency fr of the one-port surface acoustic wave resonator 4a of the serial arm is matched with the anti-resonance frequency fa of the one-port surface acoustic wave resonator 4b of the parallel arm. Assuming that this frequency is f0, near the frequency f0,
Since the impedance of the one arm of the serial arm to the surface acoustic wave resonator 4a is small and the impedance of the one arm of the parallel arm to the surface acoustic wave resonator 4b is large, the electric signal input to the input terminal 6 is hardly attenuated. Output from output terminal 7,
The passing power increases. On the other hand, at a frequency apart from f0, the impedance of the one terminal / surface acoustic wave resonator 4a of the serial arm is large, and the one terminal / surface acoustic wave resonator 4a of the parallel arm is large.
Since the impedance of b decreases, the electric signal input to the input terminal 6 is hardly output from the output terminal 7 and the passing power decreases. Therefore, it operates as a bandpass filter having a pass band near f0 and an attenuation band at other frequencies.

FIG. 21 shows the pass characteristics of the surface acoustic wave filter shown in FIG. Corresponding to the anti-resonance frequency of the one-port pair surface acoustic wave resonator 4a of the serial arm and the resonance frequency of the one-port pair surface acoustic wave resonator 4b of the parallel arm, attenuation occurs on the higher and lower sides of the pass band, respectively. A pole occurs. However, at frequencies away from the attenuation pole, the passing power increases again. This is because the one-port surface acoustic wave resonator 4a of the serial arm and the one-port surface acoustic wave resonator 4b of the parallel arm both have capacitive impedance, and a part of the electric signal input to the input terminal 6 is output. This is because the signal is output to the terminal 7.
For this reason, there is a disadvantage that it is difficult to obtain a sufficient amount of out-of-band attenuation at a frequency distant from the pass band.

In practice, the reflector 3 does not reflect surface acoustic waves of any frequency, and the frequency band in which reflection occurs is limited. FIG. 22 shows the frequency characteristics of the reflection efficiency of the reflector 3. In a band called a stop band, the surface acoustic wave incident on the reflector 3 is almost completely reflected, but in other bands, the reflection efficiency is significantly reduced. The width of the stop band of the reflector 3 is
The thickness can be changed depending on the thickness of the metal film forming the reflector 3 and the like. In general, the thickness can be increased as the thickness of the metal film increases. However, the thicker the metal film, the greater the bulk conversion loss and the like, and the lower the reflection efficiency as a whole. For this reason,
The width of the stop band of the reflector 3 is limited.

If the reflection efficiency of the reflector 3 is low, the surface acoustic wave is not reflected, and a loss occurs in the power input to the one-port surface acoustic wave resonator 4. Therefore, when a surface acoustic wave filter is configured, the loss in the pass band increases.
In addition, the stop band of the reflector 3 of the one-terminal pair surface acoustic wave resonator 4a of the series arm and the stop band of the reflector 3 of the one terminal pair surface acoustic wave resonator 4b of the parallel arm generally deviate in band. Therefore, power loss occurs outside the frequency range where the two overlap, and the filter loss increases. For this reason, the pass band width of the surface acoustic wave filter is limited by the width of the overlap between the stop bands of the reflectors 3 of the serial arm and the parallel arm.

Further, in FIG. 18, the connection between the one terminal and the surface acoustic wave resonators 4a and 4b is actually connected by a metal wire or a line made of a metal film on the piezoelectric substrate 1. It is formed and connected. For this reason, when the length of the wire or the line between the input terminal 6 and the output terminal 7 is increased, these resistance components are increased, and the loss of the surface acoustic wave filter is increased as a whole.

Next, another configuration of the conventional surface acoustic wave filter will be described. FIG. 23 shows another configuration of a conventional surface acoustic wave filter of this type shown in, for example, a document "1990 IEICE Autumn National Convention", SA-10-3. In FIG. 23, 1 is a piezoelectric substrate, 2 is an interdigital electrode, 3 is a reflector, 5 is a two-terminal surface acoustic wave resonator, 6 is an input terminal, and 7 is an output terminal. In FIG. 23, two interdigital electrodes 2 are provided on a piezoelectric substrate 1.
And the reflectors 3 arranged on both sides thereof, thereby forming a two-terminal pair surface acoustic wave resonator 5. One of the two interdigital transducers 2 of the two-terminal surface acoustic wave resonator 5 is connected to the input terminal 6 and the other is connected to the output terminal 7.

Next, the operation will be described. Input terminal 6
When an electric signal is input to the electrode, a surface acoustic wave is excited from one of the interdigital transducers 2. Since the surface acoustic wave is reflected by the reflector 3, it is multiple-reflected between the two reflectors 3 and resonates at a specific frequency. A part of the resonated surface acoustic wave is converted again into an electric signal by the other IDT 2 and output from the output terminal 7. FIG. 24 shows an amplitude distribution at a resonance frequency of the two-terminal surface acoustic wave resonator 5 shown in FIG. In the figure, a symmetric mode indicated by a solid line and an anti-symmetric mode indicated by a broken line occur, and the resonance frequencies of these two modes are slightly different. By setting the difference between the resonance frequencies to a required value, a dual mode filter having bandpass characteristics can be obtained.

FIG. 25 shows the pass characteristics of the surface acoustic wave filter shown in FIG. At a frequency away from the pass band, the surface acoustic wave is not much excited from the interdigital transducer 2, so that a larger attenuation is obtained as compared with FIG. However, in the vicinity of the pass band, there is a portion where the amount of attenuation is slightly higher than the pass band. This is because, even at a frequency slightly higher than the pass band, multiple reflection of the surface acoustic wave occurs in the interdigital electrode 2, and this resonance becomes spurious. In order to reduce the level of spurious, it is conceivable to increase the number of stages by cascading a large number of two-port surface acoustic wave resonators 5, but at the same time, the insertion loss in the pass band increases.

[0013]

As described above, in the conventional surface acoustic wave filter, the one-port surface acoustic wave resonator 4
Or the configuration using only the two-port surface acoustic wave resonator 5, there is a problem that the out-of-band attenuation becomes small at a frequency far from the pass band or near the pass band. is there. Further, when the number of stages is increased in order to increase the out-of-band attenuation, a drawback such as an increase in insertion loss occurs. Further, in the case where the one-port surface acoustic wave resonator 4 is connected in a ladder shape, disadvantages such as an increase in insertion loss and a decrease in the pass band due to the limitation of the stop band of the reflector 3 are encountered. Occurs. The present invention
An object of the present invention is to solve the above-mentioned problem, and an object of the present invention is to provide a surface acoustic wave filter having a small loss and a large out-of-band attenuation. It is another object of the present invention to obtain a surface acoustic wave filter having a small loss and a wide pass band.

[0014]

According to a first aspect of the present invention, there is provided a surface acoustic wave filter in which a surface acoustic wave resonator is electrically connected to a one-port surface acoustic wave resonator and a two-terminal surface acoustic wave resonator. It is characterized by being connected.

According to a second aspect of the present invention, in the surface acoustic wave filter, the one-port surface acoustic wave resonator and the two-terminal surface acoustic wave resonator are connected to an input terminal and an output terminal of the surface acoustic wave filter. It is characterized in that it is connected symmetrically.

According to a third aspect of the present invention, there is provided a surface acoustic wave filter comprising a two-terminal surface acoustic wave resonator having three or more interdigital electrodes.

A surface acoustic wave filter according to a fourth aspect of the present invention comprises a series arm and a parallel arm.
A one-port pair surface acoustic wave resonator having interdigital electrodes and a reflector is used, and the one-port pair surface acoustic wave resonator of the series arm and the one-port pair surface acoustic wave resonator of the parallel arm are formed in a ladder shape. The one terminal of the parallel arm is connected to one terminal of the parallel arm, and the electrode finger arrangement period of the interdigital electrode of the serial arm and the lattice arrangement period of the reflector of the surface arm of the surface arm are Lis and Lrs, respectively. When the electrode arrangement period of the interdigital transducer and the lattice arrangement period of the reflector of the surface acoustic wave resonator are Lip and Lrp, respectively, Lis / Lrs
<Lip / Lrp.

A surface acoustic wave filter according to a fifth aspect of the present invention is a surface acoustic wave filter in which a plurality of surface acoustic wave resonators are electrically connected to each other. One terminal pair of the plurality of series arms and one terminal pair of the one or more parallel arms are provided between the input terminal and the output terminal of the surface acoustic wave filter using a terminal pair surface acoustic wave resonator. A surface acoustic wave resonator is connected in a ladder form, and an inductor is connected in parallel across two or more of the one terminal of the plurality of series arms and the surface acoustic wave resonator. .

[0019]

According to the first aspect of the present invention, a one-port surface acoustic wave resonator and a two-terminal surface acoustic wave resonator are electrically connected in cascade to form a surface acoustic wave filter. The spurious of the surface acoustic wave resonator can be canceled by the attenuation pole of the one-port pair surface acoustic wave resonator, and a surface acoustic wave filter with low loss and large out-of-band attenuation can be obtained.

According to the second aspect of the present invention, the one-port surface acoustic wave resonator and the two-port surface acoustic wave resonator are symmetrically connected to the input terminal and the output terminal of the surface acoustic wave filter. The impedance of the input terminal and the output terminal can be equalized, matching with an external circuit can be easily achieved, and a low-loss surface acoustic wave filter can be obtained.

According to the third aspect of the present invention, since a two-terminal surface acoustic wave resonator having three or more interdigital electrodes is provided, a surface acoustic wave filter having a wide pass band can be obtained.

According to the fourth aspect of the present invention, the one-terminal pair surface acoustic wave resonator of the serial arm and the parallel arm has a predetermined relationship between the electrode finger arrangement period of the interdigital transducer and the lattice arrangement period of the reflector. Since the condition is satisfied, the stop band of the reflector can be more effectively used, and a surface acoustic wave filter having a wider pass band can be obtained.

According to the fifth aspect of the present invention, in a surface acoustic wave filter in which a plurality of surface acoustic wave resonators are electrically connected, one terminal pair surface acoustic surface is used as a component of the series arm and a component of the parallel arm. A wave resonator is used, and one terminal of a plurality of series arms and one surface of one or more parallel arms are arranged between an input terminal and an output terminal of the surface acoustic wave filter.
Since the terminal and the surface acoustic wave resonator are connected in a ladder form, and the inductor is connected in parallel across two or more of the one terminal and the surface acoustic wave resonator of the plurality of series arms, the out-of-band The passing signals can cancel each other, and an attenuation pole can be created in the passing characteristics. Accordingly, a surface acoustic wave filter having a large out-of-band attenuation can be obtained.

[0024]

Embodiment 1 An embodiment of the present invention will be described with reference to FIG. FIG. 1 is a configuration diagram showing a first embodiment of the present invention.
In FIG. 1, 1 is a piezoelectric substrate, 2 is an interdigital electrode, 3
Is a reflector, 4 is a one-port surface acoustic wave resonator, 5 is a two-terminal surface acoustic wave resonator, 6 is an input terminal, and 7 is an output terminal. In the figure, a one-terminal pair surface acoustic resonator 4 composed of one interdigital electrode 2 and a two-terminal pair surface acoustic wave resonator 5 composed of two interdigital electrodes 2 are arranged on a piezoelectric substrate 1. The one-port surface acoustic wave resonator 4 and the two-port surface acoustic wave resonator 5 are electrically connected.

Next, the operation will be described. The one-port surface acoustic wave resonator 4 in FIG. 1 has the impedance characteristic as shown in FIG. 20, similarly to the one used in FIG. That is, the impedance becomes zero at the resonance frequency fr, and becomes infinite at the anti-resonance frequency fa. Accordingly, if the one-port surface acoustic wave resonator 4 is connected in series as shown in FIG. 2 to form a two-port circuit, the electric signal input to the input terminal 6 has a resonance frequency fr.
All pass through the output terminal 7 and do not pass at all at the anti-resonance frequency fa, resulting in an attenuation pole. Therefore, FIG.
It shows the pass characteristics as follows.

On the other hand, the two-terminal pair surface acoustic wave resonator 5 in FIG. 1 has a pass characteristic as shown in FIG. 3B similarly to FIG. 23, and a spurious is generated in the vicinity fs near the high frequency side of the pass band. .

In FIG. 1, however, the one-port pair surface acoustic wave resonator 4 and the two-port pair surface acoustic wave resonator 5
Are connected in cascade. Further, in FIG. 1, the one-port surface acoustic wave resonator 4 and the two-port surface acoustic wave resonator 2 are arranged such that the attenuation pole fa in FIG. 3A is substantially equal to the frequency fs at which the spurious in FIG. The terminal pair surface acoustic wave resonator 5 is formed. Therefore, the entire transmission characteristic is as shown in FIG. 3C, and the spurious of the two-port surface acoustic wave resonator 5 is canceled by the attenuation pole of the one-port surface acoustic wave resonator 4. Out-of-band attenuation can be increased.

Further, at this time, the pass band of the two-port surface acoustic wave resonator 5 and the resonance frequency fr of the one-port surface acoustic wave resonator 4 can be substantially equalized. The increase in the insertion loss due to the connection of the one-terminal surface acoustic wave resonator 4 is small with respect to the insertion loss of a single unit. Therefore, the insertion loss can be reduced as compared with a case where the two-terminal surface acoustic wave resonator 5 is connected in multiple stages. As described above, according to the first embodiment of the present invention, a surface acoustic wave filter having a low loss and a large amount of out-of-band attenuation can be obtained.

Second Embodiment FIG. 4 is a configuration diagram showing a second embodiment of the present invention. FIG.
, 1 to 7 are the same as those in FIG. In the figure, a two-terminal pair surface acoustic wave resonator 5 is provided on a piezoelectric substrate 1.
And one terminal-pair surface acoustic wave resonator 4 is provided on both sides thereof.
Two one-terminal to surface acoustic wave resonators 4
And the two-terminal surface acoustic wave resonator 5 are electrically connected to the input terminal 6 and the output terminal 7 so as to be symmetrical.

Next, the operation will be described. The operations of the one-port SAW resonator 4 and the two-port SAW resonator 5 in FIG. 4 are the same as those in the first embodiment. However, unlike FIG. 1, FIG. 4 uses two one-port surface acoustic wave resonators 4, so that the effect of the attenuation pole in FIG. 3A is increased, and the out-of-band attenuation can be further increased. .

Further, in FIG. 4, the configuration of the surface acoustic wave filter is symmetrical with respect to the input terminal 6 and the output terminal 7. Therefore, the input impedance as viewed from the input terminal 6 and the input impedance as viewed from the output terminal 7 are equal to each other. Since the impedance of the external circuit connected to each of the input terminal 6 and the output terminal 7 of the surface acoustic wave filter is usually the same, the configuration of FIG. 4 facilitates impedance matching with the external circuit. Therefore, the mismatch loss of the filter and the loss due to the external matching circuit are reduced, and a low-loss surface acoustic wave filter is obtained.

Third Embodiment FIG. 5 is a configuration diagram showing a third embodiment of the present invention. FIG.
, 1 to 7 are the same as those in FIG. In the figure, a two-terminal pair surface acoustic wave resonator 5 is provided on a piezoelectric substrate 1.
And one terminal pair surface acoustic wave resonator 4 is provided on both sides thereof.
And the two one-port surface acoustic wave resonators 4 and the two-terminal surface acoustic wave resonators 5 are electrically connected. Further, as the two-terminal surface acoustic wave resonator 5, a so-called three-electrode type using three interdigital electrodes 2 is used.

Next, the operation will be described. The operations of the one-port SAW resonator 4 and the two-terminal SAW resonator 5 in FIG. 5 are the same as those in the first and second embodiments, respectively. However, in FIG. 5, there are three IDTs 2 of the two-terminal SAW resonator 5, the IDT 2 at the center is the input side, and the IDTs 2 at both ends are connected to the output side. . FIG. 6 shows the amplitude distribution at the resonance frequency of the two-terminal surface acoustic wave resonator 5 in FIG. In the figure, a zero-order symmetric mode shown by a solid line and a second-order symmetric mode shown by a broken line occur, and the anti-symmetric mode shown in FIG. 24 is not excited. At this time, as in the case of FIG. 23, if the difference between the resonance frequencies of the zero-order symmetric mode and the second-order symmetric mode is set to a required value, a dual mode filter having bandpass characteristics can be obtained.

In addition, the difference between the resonance frequencies of the 0-order symmetric mode and the second-order symmetric mode in FIG. 6 can be made larger than the difference between the resonance frequencies of the symmetric mode and the anti-symmetric mode in FIG. 23 can have a wider pass band than the two-electrode surface acoustic wave resonator 5 of the two-electrode type shown in FIG. Therefore, the third embodiment has an effect of obtaining a surface acoustic wave filter having a wider pass band than the first embodiment.

In the first to third embodiments, the case where the one-port surface acoustic wave resonator 4 and the two-port surface acoustic wave resonator 5 are arranged on the same piezoelectric substrate 1 has been described. The present invention is not limited to this, and the one-port surface acoustic wave resonators 4 and 2
The terminal pair surface acoustic wave resonator 5 may be disposed on a separate piezoelectric substrate 1, and at this time, different types of piezoelectric substrates 1 may be used. Further, the number of one-port surface acoustic wave resonators 4 and the number of two-port surface acoustic wave resonators 5 are not limited to those described in the above embodiments. In general, if a large number of one-port surface acoustic wave resonators 4 and two-port surface acoustic wave resonators 5 are cascaded, the out-of-band attenuation increases and the insertion loss increases as the number of connected devices increases. Therefore, the number of connections may be arbitrarily selected according to the required values of the out-of-band attenuation and the insertion loss.

Further, in the above-described first to third embodiments, the one-port-pair surface acoustic wave resonator 4 and the two-port-pair surface acoustic wave resonator 5 having the reflector 3 are used.
However, the present invention is not limited to this, and a surface acoustic wave resonator using only the resonance of the interdigital transducer 2 itself without the reflector 3 may be used.

Fourth Embodiment FIG. 7 is a configuration diagram showing a fourth embodiment of the present invention. FIG.
, 1 is a piezoelectric substrate, 2 is an interdigital electrode, 3 is a reflector, 4 is a one-terminal surface acoustic wave resonator, 6 is an input terminal,
7 is an output terminal. In the figure, on a piezoelectric substrate 1,
A one-terminal surface acoustic wave resonator 4 and an interdigital electrode 2 are arranged, and these are electrically connected to each other to form an inverted L-shaped circuit.

Next, the operation will be described. FIG. 8 is a diagram for explaining the operation of the fourth embodiment of the present invention.
Is a capacitor. 8, a one-terminal surface acoustic wave resonator 4 is inserted in series between an input terminal 6 and an output terminal 7, and a capacitor 8 is inserted in parallel. 1
As described above, the terminal-pair surface acoustic wave resonator 4 is
, The impedance becomes inductive between the resonance frequency fr and the anti-resonance frequency fa. At this time, since the capacitor 8 has a capacitive impedance, the circuit of FIG. 8 can provide a low-loss bandpass filter similar to a constant K filter well known in transmission circuit theory. However, since an attenuation pole is generated at the anti-resonance frequency fa of the one-port SAW resonator 4, the pass characteristic has an attenuation pole on the higher side of the pass band as shown by the solid line in FIG.

FIG. 7 shows a configuration in which the capacitor 8 of FIG. The impedance of the interdigital transducer 2 exhibits the same capacitive characteristics as the capacitor 8 at frequencies other than the frequency at which the surface acoustic wave is excited. Therefore, the surface acoustic wave filter shown in FIG.
At a frequency other than the frequency at which the interdigital transducer 2 excites the surface acoustic wave, the filter exhibits characteristics similar to those of the filter shown in FIG.
Therefore, the pass band characteristic can be reduced as in FIG. However, at a frequency at which the interdigital transducer 2 excites a surface acoustic wave, an electric signal input to the interdigital transducer 2 is converted into a surface acoustic wave, and a power loss occurs. Therefore, as shown by the broken line in FIG. 9, the power of the input signal suffers a loss at the frequency fi at which the interdigital transducer 2 excites the surface acoustic wave,
The power of the output signal decreases. Therefore, the frequency fi
Thus, the attenuation of the passage characteristic can be increased. At this time, the frequency fi at which the surface acoustic wave is excited can be freely changed by changing the electrode finger arrangement cycle of the interdigital transducer 2, so that the attenuation can be increased at an arbitrary frequency outside the band. .

Fifth Embodiment FIG. 10 is a block diagram showing a fifth embodiment of the present invention. In FIG. 10, 1 to 7 are the same as those in FIG. In the figure, two one-terminal surface acoustic wave resonators 4 and interdigital electrodes 2 are arranged on a piezoelectric substrate 1 and electrically connected to each other to form a T-shaped circuit.

Next, the operation will be described. In the configuration of FIG. 10, two surface acoustic wave filters shown in FIG. 7 are connected to each other by the output terminals 7, and two parallel IDTs 2 formed at the center are arranged in parallel. Therefore, it operates as a bandpass filter similar to that of FIG.
In addition, since the number of filter stages is increased as compared with FIG. 7, the amount of attenuation outside the band can be increased, and steep filter characteristics can be obtained.
Also in this case, the attenuation can be further increased at an arbitrary frequency outside the band by changing the frequency at which the SAW is excited from the IDT 2.

Sixth Embodiment FIG. 11 is a configuration diagram showing a sixth embodiment of the present invention. In FIG. 11, reference numerals 1 to 7 are the same as those in FIG. In the figure, a one-terminal surface acoustic wave resonator 4 and an interdigital electrode 2 are arranged on a piezoelectric substrate 1 and electrically connected to each other to form an inverted L-shaped circuit. However, in FIG. 11, unlike FIG. 7, between the input terminal 6 and the output terminal 7,
The IDTs 2 are connected in series, and the one-terminal surface acoustic wave resonator 4
Are inserted in parallel.

FIG. 11 also operates as a bandpass filter as in FIG. In this case, since an attenuation pole is generated at the resonance frequency fr of the one-port surface acoustic wave resonator 4, the transmission characteristic has an attenuation pole on the lower side of the pass band. However, in FIG. 11 as well as in FIG. 7, it goes without saying that the attenuation can be further increased at an arbitrary frequency outside the band by changing the frequency at which the surface acoustic wave is excited from the interdigital electrode 2.

Seventh Embodiment FIG. 12 is a configuration diagram showing a seventh embodiment of the present invention. 12, reference numerals 1 to 7 are the same as those in FIG. In the figure, two interdigital electrodes 2 are provided on a piezoelectric substrate 1.
And a one-terminal surface acoustic wave resonator 4 are arranged and electrically connected to each other to form a T-shaped circuit.

The configuration shown in FIG. 12 is such that two surface acoustic wave filters shown in FIG. 11 are connected to each other at output terminals, and two parallel one-port surface acoustic wave resonators formed at the center are combined into one. It is a summary. Therefore, it operates as a band-pass filter similar to that of FIG. 11, and a filter characteristic steeper than that of FIG. 11 is obtained. Also in this case, the attenuation can be further increased at an arbitrary frequency outside the band by changing the frequency at which the SAW is excited from the IDT 2. Further, for the upper and lower two interdigital electrodes 2,
By making the frequencies at which the surface acoustic waves are excited different from each other, there is an effect that the attenuation can be increased in two different bands.

In the above embodiments 4 to 7, the circuit configuration of the surface acoustic wave filter is shown as an inverted L type or a T type. However, the present invention is not limited to this. And the number of stages may be increased. further,
A part of the capacitor 8 may be left instead of the IDT 2. When there are a large number of interdigital electrodes 2, the frequencies at which the surface acoustic waves are excited may be changed individually or may be the same.

Further, in the above Examples 4 to 7,
Although the case where the one-terminal pair surface acoustic wave resonator 4 and the interdigital electrode 2 are arranged on the same piezoelectric substrate 1 has been described, the present invention is not limited to this, and the one-terminal pair surface acoustic wave resonator 4 and The interdigital transducers 2 may be arranged on separate piezoelectric substrates 1, and at this time, different types of piezoelectric substrates 1 may be used.

Eighth Embodiment FIG. 13 is a block diagram showing an eighth embodiment of the present invention. In FIG. 13, 1 is a piezoelectric substrate, 2 is an interdigital electrode, 3
Is a reflector, 4a and 4b are one-port surface acoustic wave resonators, 6
Is an input terminal and 7 is an output terminal. In the figure, a plurality of one-terminal surface acoustic wave resonators 4 are arranged on a piezoelectric substrate 1 and are electrically connected to form a ladder circuit. Further, in the one-terminal pair surface acoustic wave resonator 4a of the serial arm, the electrode finger arrangement period Lis of the interdigital electrodes 2 (hereinafter, referred to as the pitch of the interdigital electrodes 2) and the lattice arrangement period of the reflector 3 (hereinafter, reflection). Lrs) (Litch <Lrs).

Next, the operation will be described. In FIG. 13, the resonance frequency fr of the one-terminal pair surface acoustic wave resonator 4a of the series arm and the anti-resonance frequency fa of the one-terminal pair surface acoustic wave resonator 4b of the parallel arm are made substantially coincident to operate as a bandpass filter. ing. This is the same as the conventional surface acoustic wave filter shown in FIG. Therefore, the resonance frequency fr of the one arm pair surface acoustic wave resonator 4a of the serial arm is
This is the center frequency of the filter.

FIG. 14 is a circuit diagram similar to the one shown in FIG.
In the terminal-to-surface acoustic wave resonator 4, a change in the resonance frequency fr and the anti-resonance frequency fa when only the pitch Li of the interdigital transducer 2 is changed while keeping the pitch Lr of the reflector 3 constant is an equivalent circuit. It is calculated using a model. Since the horizontal axis is Li / Lr, Li / Lr is 1
In this case, the pitch between the interdigital transducer 2 and the reflector 3 becomes equal. Even if the pitch Li of the interdigital transducer 2 is changed, the distance between the interdigital transducer 2 and the reflector 3 is kept constant. From the figure, it was found that when the pitch Li of the interdigital transducer 2 was changed, both the resonance frequency fr and the anti-resonance frequency fa changed, and the frequency difference between the resonance frequency fr and the anti-resonance frequency fa did not change much.

The dotted line in FIG.
Are the lower limit frequency and upper limit frequency of the stop band. When the pitch between the interdigital transducer 2 and the reflector 3 is equal,
The resonance frequency fr is substantially equal to the lower limit frequency of the stop band of the reflector 3. Therefore, if the frequency is slightly lower than the resonance frequency fr, the frequency deviates from the stop band of the reflector 3, and the reflection efficiency of the reflector 3 decreases.

In FIG. 13, the pitch Lis of the interdigital electrodes 2 in the one-terminal surface acoustic wave resonator 4a of the serial arm is shown.
And the pitch Lrs of the reflector 3 are different, and Lis <L
rs. Therefore, as can be seen from FIG.
The resonance frequency fr becomes higher than the lower limit frequency of the stop band of the reflector 3. At this time, since a frequency slightly lower than the resonance frequency fr is included in the stop band, a large reflection efficiency can be obtained.

As described above, the resonance frequency fr of the one-terminal surface acoustic wave resonator 4a of the serial arm is the center frequency of the filter. Therefore, by changing the pitch between the interdigital transducer 2 and the reflector 3 as shown in FIG. 13, it is possible to reduce the loss due to the decrease in the reflection efficiency of the reflector 3 at a frequency lower than the center frequency of the filter, and to reduce the loss of the filter in the pass band. Insertion loss can be reduced. Further, since the frequency width at which the stop bands of the reflectors 3 of the series arm and the parallel arm overlap can be widened, the pass band of the filter can be made wider.

Ninth Embodiment FIG. 15 is a block diagram showing a ninth embodiment of the present invention. In FIG. 15, reference numerals 1 to 4 and 6, 7 are the same as those in FIG. In the figure, a plurality of one-terminal surface acoustic wave resonators 4 are arranged on a piezoelectric substrate 1 and are electrically connected to form a ladder circuit. Further, as shown in the figure, the pitch between the IDT 2 and the reflector 3 of the one-terminal-pair surface acoustic wave resonator 4a of the serial arm is set to Li, respectively.
s, Lrs, one terminal of a parallel arm vs. a surface acoustic wave resonator 4
When the pitch of the interdigital transducer 2 and the reflector 3 of b is Lip and Lrp, respectively, Lis / Lrs <1 and Lip / Lrp> 1.
At this time, Lis / Lrs <Lip / Lrp holds.

The operation of the ninth embodiment is the same as that of the eighth embodiment. However, in the ninth embodiment, the pitch Li of the interdigital transducer 2 is also set in the one-terminal surface acoustic wave resonator 4b of the parallel arm.
p and the pitch Lrp of the reflector 3 are different, and Lip / L
rp> 1. At this time, as can be seen from FIG. 14, the anti-resonance frequency fa can be made closer to the center frequency of the stop band of the reflector 3 as compared with the case where the pitch is equal. Therefore, at a frequency higher than the anti-resonance frequency fa, the frequency band included in the stop band of the reflector 3 becomes wider.

Since the anti-resonance frequency fa of the one-port surface acoustic wave resonator 4b of the parallel arm is also the center frequency of the filter, the reflection efficiency of the reflector 3 at a frequency higher than the center frequency of the filter is shown in FIG. Can be increased, and the frequency width at which the stop bands of the reflectors 3 of the series arm and the parallel arm overlap can be made wider, so that the pass band of the filter can be made wider than that of FIG.

In FIG. 15, there are two one-terminal surface acoustic wave resonators 4a of the serial arm, and their cross widths are different from each other. As described above, the crossing width of the one-terminal surface acoustic wave resonator 4 may vary. Similarly, when there are three or more one-terminal surface acoustic wave resonators 4a in the series arm, the respective intersection widths may be changed, and there are various ways of change. This is the same for the one-port surface acoustic wave resonator 4b of the parallel arm.

At this time, by changing the crossing width of the one-port surface acoustic wave resonator 4a of the series arm and the one-port surface acoustic wave resonator 4b of the parallel arm, the crossing width is set to a required value.
The band pass characteristics of the filter can be changed to the amplitude flat characteristics, amplitude wavy characteristics (Chebyshev characteristics),
It can approximate various characteristics such as delay flat characteristics,
Any property can be obtained.

Further, although not explicitly shown in FIG. 15, the resonance frequencies fr of the one-terminal surface acoustic wave resonators 4a of the plurality of serial arms need not be completely equal, and may be changed. This is because, as described above, the pitch Li of the interdigital transducer 2, the pitch Lr of the reflector 3, and the ratio L
It can be easily realized by changing i / Lr and the like.
The anti-resonance frequency fa of the one-port surface acoustic wave resonator 4b of the parallel arm may be changed. Also in this case, the resonance frequency fr and the anti-resonance frequency fa
Is set to a required value, it is possible to approximate the bandpass characteristic of the filter to various characteristics.

Embodiment 10 FIG. 16 is a block diagram showing a tenth embodiment of the present invention.
In FIG. 16, 1 to 4 and 6, 7 are the same as those in FIG. As in FIG. 13, on the piezoelectric substrate 1,
A plurality of one-port surface acoustic wave resonators 4 are arranged and electrically connected to form a ladder circuit. Further, in FIG. 13, among the five one-port surface acoustic wave resonators 4,
There are three series arm one-port surface acoustic wave resonators 4a, and three series arm one-terminal surface acoustic wave resonators 4a are arranged adjacent to each other.

The operation of the tenth embodiment is the same as that of the eighth and ninth embodiments. However, in the tenth embodiment, by arranging three one terminals of the serial arm to the surface acoustic wave resonator 4a adjacent to each other, the same number of one terminal of the serial arm to the surface acoustic wave resonator 4a and one terminal of the parallel arm are arranged. As compared with the case where the surface acoustic wave resonators 4b are staggered, the input terminal 6 and the output terminal 7
Can be shortened. Therefore, the length of the wire or line connecting the one-terminal surface acoustic wave resonators 4a of the serial arm to each other can be reduced, and the effect of these resistance components can be reduced, so that a surface acoustic wave filter with a small insertion loss can be obtained. .

At this time, on the contrary, the length of the wire or line connecting the one terminal of the parallel arm and the surface acoustic wave resonator 4b increases, and the resistance component increases. However, since the impedance of the parallel arm is originally substantially infinite in the pass band of the filter, even if the resistance component increases, the insertion loss of the filter hardly changes and a low-loss characteristic can be obtained.

In the above-described embodiments 8 to 10, the case where the one-port surface acoustic wave resonator 4 is arranged on the same piezoelectric substrate 1 has been described. The surface acoustic wave resonator 4 and the interdigital transducer 2 may be arranged on separate piezoelectric substrates 1, and different types of piezoelectric substrates 1 may be used at this time. Further, the number of one-port surface acoustic wave resonators 4a and 4b is not limited to the number shown in the above embodiment, and may be arbitrarily selected according to required values of the out-of-band attenuation and the insertion loss.

Embodiment 11 FIG. 17 is a block diagram showing Embodiment 11 of the present invention.
In FIG. 17, 4a, 4b, 6, and 7 correspond to FIGS.
Similar to 3 and the like, 9 is an inductor. Similarly to FIG. 13, one terminal of a serial arm vs. a surface acoustic wave resonator 4a.
And one terminal-pair surface acoustic wave resonator 4b of the parallel arm is connected between the input terminal 6 and the output terminal 7 in the form of a ladder.
However, in FIG. 17, unlike FIG.
The terminal is connected in series or parallel to the surface acoustic wave resonator 4. Further, the inductor 9 is connected across the plurality of one-terminal surface acoustic wave resonators 4.

The operation of the eleventh embodiment is similar to that of the eighth to tenth embodiments, and operates as a low-loss bandpass filter. However, in the eleventh embodiment, the inductor 9 is set to 1
For example, the terminal-to-surface acoustic wave resonator 4 is connected in series. At this time, considering the combined impedance characteristics of the one-port surface acoustic wave resonator 4 and the inductor 9, the inductor 9 is connected to the impedance of the single-port surface acoustic wave resonator 4 shown in FIG. As a result, the imaginary part of the impedance increases as a whole. Therefore, the anti-resonance frequency fa does not change but the resonance frequency fr decreases, and the frequency difference between fr and fa increases. When the inductors 9 are connected in parallel, the imaginary part of the reciprocal of the impedance becomes smaller as a whole by the connection of the inductors 9. At this time, the resonance frequency fr does not change, but the anti-resonance frequency fa increases, and the frequency difference between fr and fa also increases.

As described above, by connecting the inductor 9, the resonance frequency fr of the one-port surface acoustic wave resonator 4 is obtained.
And the anti-resonance frequency fa can be apparently increased. Therefore, in the pass characteristic of the filter shown in FIG. 21, the frequency interval between the two attenuation poles can be widened, and a surface acoustic wave filter having a wide pass bandwidth can be obtained.

Further, in the eleventh embodiment, the inductor 9 is connected across a plurality of one-terminal surface acoustic wave resonators 4. As described above, at a frequency apart from the pass band of the filter, the one-port surface acoustic wave resonator 4 exhibits a capacitive impedance characteristic. Therefore, by connecting the inductor 9 having an inductive impedance, signals passing outside the band can be canceled each other,
An attenuation pole can be created in the transmission characteristic. Accordingly, a surface acoustic wave filter having a large out-of-band attenuation can be obtained.

In the eleventh embodiment, the inductor 9 is
It is not necessary to use all of the components shown in FIG. 7 and the effect of the present invention can be obtained by connecting at least one inductor 9. The inductor 9 may have any structure. For example, the inductor 9 may be formed on the same piezoelectric substrate 1 as the one-terminal surface acoustic wave resonator 4, or may be formed of a metal wire or the like. When the one-terminal surface acoustic wave resonator 4 is sealed in a package, the inductor 9 may be housed in the same package, or the inductor 9 may be provided outside and connected. Further, the connection method of the inductor 9 is shown in FIG.
7, various methods can be used.

In the first to eleventh embodiments, the material of the piezoelectric substrate 1 may be a single crystal or a substrate in which a piezoelectric thin film is formed on another substrate, and any material that excites a surface acoustic wave may be used. I do not care. Further, the surface acoustic wave is not limited to a Rayleigh wave, and a surface wave such as a so-called pseudo-surface acoustic wave may be used. Further, in the above embodiment,
One terminal pair surface acoustic wave resonator 4 used as a circuit element
Instead of the interdigital transducer 2, two or more identical or different one-terminal-pair surface acoustic wave resonators 4 or interdigital transducers 2 connected in series or in parallel may be used. The effect of is obtained.

[0070]

According to the first aspect of the present invention, the one-terminal surface acoustic wave resonator and the two-terminal surface acoustic wave resonator are electrically connected in cascade to form a surface acoustic wave filter. The spurious of the surface acoustic wave resonator can be canceled by the attenuation pole of the one-port surface acoustic wave resonator, and a surface acoustic wave filter having a low loss and a large out-of-band attenuation can be obtained.

According to the second aspect of the present invention, the one-port surface acoustic wave resonator and the two-port surface acoustic wave resonator are symmetrically connected to the input terminal and the output terminal of the surface acoustic wave filter. The impedance of the input terminal and the output terminal can be equalized, matching with an external circuit can be easily performed, and a low-loss surface acoustic wave filter can be obtained.

According to the third aspect of the present invention, since a two-terminal pair surface acoustic wave resonator having three or more interdigital electrodes is provided, a surface acoustic wave filter having a wide pass band can be obtained.

According to the fourth aspect of the present invention, the one-terminal pair surface acoustic wave resonator of the serial arm and the parallel arm has a predetermined relationship between the electrode finger arrangement period of the interdigital transducer and the lattice arrangement period of the reflector. Since the condition is satisfied, the stop band of the reflector can be more effectively used, and the surface acoustic wave filter having a wider pass band can be obtained.

According to the fifth aspect of the present invention, in a surface acoustic wave filter in which a plurality of surface acoustic wave resonators are electrically connected, one terminal pair surface acoustic surface is used as a component of the series arm and a component of the parallel arm. A wave resonator is used, and one terminal of a plurality of series arms and one surface of one or more parallel arms are arranged between an input terminal and an output terminal of the surface acoustic wave filter.
The terminal-to-surface acoustic wave resonator is connected in a ladder form, and the inductor is connected in parallel across two or more of the one-terminal to surface-acoustic wave resonators of the plurality of series arms, so that the attenuation is caused to pass characteristics. The poles can be formed, and a surface acoustic wave filter having a large out-of-band attenuation can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram for explaining the operation of the first embodiment of the present invention.

FIG. 3 is a diagram for explaining the operation of the first embodiment of the present invention.

FIG. 4 is a configuration diagram showing a second embodiment of the present invention.

FIG. 5 is a configuration diagram showing a third embodiment of the present invention.

FIG. 6 is a diagram for explaining the operation of the third embodiment of the present invention.

FIG. 7 is a configuration diagram showing a fourth embodiment of the present invention.

FIG. 8 is a diagram for explaining the operation of the fourth embodiment of the present invention.

FIG. 9 is a diagram for explaining the operation of the fourth embodiment of the present invention.

FIG. 10 is a configuration diagram showing a fifth embodiment of the present invention.

FIG. 11 is a configuration diagram showing a sixth embodiment of the present invention.

FIG. 12 is a configuration diagram showing a seventh embodiment of the present invention.

FIG. 13 is a configuration diagram showing Embodiment 8 of the present invention.

FIG. 14 is a diagram for explaining the operation of the eighth embodiment of the present invention.

FIG. 15 is a configuration diagram showing a ninth embodiment of the present invention.

FIG. 16 is a configuration diagram showing a tenth embodiment of the present invention.

FIG. 17 is a configuration diagram showing Embodiment 11 of the present invention.

FIG. 18 is a configuration diagram showing a conventional surface acoustic wave filter.

FIG. 19 is a view for explaining the operation of a conventional surface acoustic wave filter.

FIG. 20 is a diagram for explaining the operation of a conventional surface acoustic wave filter.

FIG. 21 is a view for explaining the operation of a conventional surface acoustic wave filter.

FIG. 22 is a view for explaining the operation of a conventional surface acoustic wave filter.

FIG. 23 is a configuration diagram showing a conventional surface acoustic wave filter.

FIG. 24 is a diagram for explaining the operation of a conventional surface acoustic wave filter.

FIG. 25 is a diagram for explaining the operation of a conventional surface acoustic wave filter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Piezoelectric substrate 2 IDT 3 Reflector 4 1 terminal vs. surface acoustic wave resonator 5 2 terminal vs. surface acoustic wave resonator 6 input terminal 7 output terminal 8 capacitor 9 inductor

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Tomonori Kimura 5-1-1, Ofuna, Kamakura City Mitsubishi Electric Corporation Electronic Systems Laboratory (72) Inventor Koji Murai 8-1-1, Tsukaguchi Honcho, Amagasaki City Mitsubishi (56) References JP-A-5-83084 (JP, A) JP-A-58-131810 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) ) H03H 9/145 H03H 9/64

Claims (5)

(57) [Claims] 1. A surface acoustic wave filter in which a plurality of surface acoustic wave resonators are electrically connected, wherein the surface acoustic wave resonator is a one-port surface acoustic wave resonator, and the one-port surface acoustic wave resonator. A surface acoustic wave filter comprising: a two-terminal pair surface acoustic wave resonator cascaded to a wave resonator. 2. The one-port pair surface acoustic wave resonator and the two-port surface acoustic wave resonator.
2. The surface acoustic wave filter according to claim 1, wherein a terminal-to-surface acoustic wave resonator is symmetrically connected to an input terminal and an output terminal of the surface acoustic wave filter. 3. The surface acoustic wave resonator according to claim 2, wherein the two-terminal surface acoustic wave resonator has three or more interdigital electrodes.
The surface acoustic wave filter according to any one of the preceding claims. 4. A one-port pair surface acoustic wave resonator having an interdigital transducer and a reflector as a component of the series arm and a component of the parallel arm, wherein the one-terminal pair surface acoustic wave resonator of the series arm is used. And a one-terminal pair surface acoustic wave resonator of the parallel arm connected in a ladder form, wherein the electrode fingers of the interdigital transducers of the one-terminal pair surface acoustic wave resonator of the serial arm are connected. The period and the lattice arrangement period of the reflector are Lis and Lrs, respectively, and the electrode finger arrangement period of the interdigital transducer and the lattice arrangement period of the reflector of the one-port surface acoustic wave resonator of the parallel arm are Lip, respectively. ,
When Lrp is set, Lis / Lrs <Lip / Lr
A surface acoustic wave filter characterized by p. 5. A surface acoustic wave filter in which a plurality of surface acoustic wave resonators are electrically connected, wherein a one-port pair surface acoustic wave resonator is used as a component of a series arm and a component of a parallel arm. A plurality of one-arm surface acoustic wave resonators of the series arm and one or more one-arm surface acoustic wave resonators of one or more parallel arms are connected in a ladder shape between an input terminal and an output terminal of the surface acoustic wave filter. A surface acoustic wave filter in which an inductor is connected in parallel across two or more of the one-terminal pair surface acoustic wave resonators of the plurality of serial arms.