JP2002223147A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave filter the filter characteristic of which can be enhanced while suppressing the filter scale from being increased. SOLUTION: The ladder type surface acoustic wave filter having a 1st surface acoustic wave resonator with a prescribed resonance frequency placed in a parallel arm and having a 2nd surface acoustic wave resonator with an anti- resonance frequency corresponding to the resonance frequency placed in a serial arm, is provided with a 1st impedance means that is placed in the serial arm and connected in series with the 2nd surface acoustic wave resonator and with a 2nd impedance means placed in the serial arm and connected in parallel with the 2nd surface acoustic wave resonator.

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, which is suitable for being mounted on a small mobile communication terminal such as a portable telephone.

[0002]

2. Description of the Related Art In recent years, small and lightweight mobile communication device terminals such as portable telephones have been rapidly developed. Accordingly, there is a demand for miniaturization and high performance of components used, and RF (high frequency) components based on surface acoustic wave (SAW) elements have been developed and used.

The following document 1 describes a surface acoustic wave filter that can be used for this type of RF component.
There are ~ 4.

Reference 1: Japanese Patent Application Laid-Open No. 9-167937 Document 2: Japanese Patent Application Laid-Open No. 2000-13184 Document 3: Japanese Patent Application Laid-Open No. 10-303682 Document 4: Low-loss bandpass filter using a SAW resonator: Sato, Italy Kata, Miyashita, Matsuda, Nishihara: IEICE Transactions A, Vol. J76-A, No. 2, pp. 245-2
52, February, 1993 Among these Documents 1 to 4, the ladder-type SAW filter described in Document 4 is practically used and widely used because it is a device that greatly contributes to downsizing of the RF unit. However, the level of performance required for the SAW filter has been improved, and there is a strong demand for a high-performance ladder-type SAW filter having a lower insertion loss in the pass band and a higher attenuation in the attenuation band.

[0005] R using a ladder-type SAW filter of Document 4
As the F device, for example, a SAW duplexer (SAW duplexer) and the like have been developed, and some of them have been put to practical use.

FIG. 2 shows the configuration of a basic section (unit section) 100 of a ladder-type SAW filter described in Document 4.

Basic section 1 of the ladder type SAW filter of FIG.
The characteristic of 00 is that the cross length of the serial arm resonator is 100 μm, the logarithm is 100, the cross length of the parallel arm resonator is 70 μm,
If the logarithm is 70, the result is as shown in FIG. FIG. 4 shows impedance characteristics of the series arm resonator and the parallel arm resonator in this case.

As is apparent from a comparison between FIG. 3 and FIG.
The attenuation pole in the high-pass attenuation band of the passband shown in FIG. 3 appears at the frequency of the infinite point of the series arm resonator, and the attenuation pole in the low-pass attenuation band of the passband has the frequency where the parallel arm resonator is zero. Appears in.

That is, in FIG. 3, one attenuation pole is generated in each of the lower attenuation region and the higher attenuation region of the pass band.

FIG. 4 also shows the real part of the circuit of each SAW resonator when Q = 500.

[0011]

By the way, with the rapid increase in the demand for the mobile communication device terminal described above, 800 (MH)
Many types of mobile communication using the z) band and the 2 (GHz) frequency band have already been put into practical use and are currently being put into practical use. Tend to require increasingly higher performance.

For example, PCS (Pers) in the 2 (GHz) band
ona1 Communication Service
e) In the case of the method, the transmission band is 1850-1910 (MH
z), since the reception band is set to 1930-1990 (MHz), the interval between the transmission band and the reception band is 20 (M
Hz), the interval between the transmission band and the reception band of the conventional 800 (MHz) band mobile phone is also 20 (MHz), and the case of the 800 (MHz) band CDMA (code division multiple access) system is also 20. (MHz), the used frequency band (transmission band or reception band) is more than doubled (the frequency is increased from about 800 MHz to 190 MHz).
Not only has it more than doubled to about 0 MHz,
Although the passband has a bandwidth of 60 MHz, which is wider than in the past, the interval (20 MHz) has not changed, so that a steeper filter characteristic is required.

Therefore, as a filter used in the PCS-type duplexer, a dielectric filter which easily realizes a steep filter characteristic is often used.
There are few, but not few, practical applications of AW filters.

On the other hand, from the viewpoint of miniaturization, it is apparent that a SAW filter using a SAW (surface acoustic wave) having a slow propagation speed is more advantageous in principle than a dielectric filter using an electromagnetic wave.

However, the ladder-type SAW filter has not yet been able to obtain a steep and good filter characteristic equivalent to that of a dielectric filter. Therefore, the performance of a duplexer (SAW-DUP) using a ladder-type SAW filter is also improved.
P), it was lower.

Several studies have been made to improve the performance of such a SAW-DUP to the same level as that of a dielectric-DUP, and as a result, a ladder type that can be expressed by an equivalent circuit as shown in FIG. A SAW filter 900 has been devised.

The circuit configuration of FIG. 9 is a SAW filter circuit corresponding to the basic section 100 of Document 4 shown in FIG. 2 (or a two-stage π shown in the circuit diagram of FIG. 5 described in Document 2).
In this case, three inductances L (941, 942, 980) are connected to the ladder-type SAW filter itself.

However, the polarized (ie, having an attenuation pole on the filter characteristic curve) SAW filter 900 shown in FIG.
According to the method described above, it is possible to improve the filter characteristics as compared with the related art, but nonetheless, it has not been possible to obtain a filter characteristic as good and steep as that of a dielectric filter.

Further, it is considered effective to increase the number of attenuation poles in order to further improve the filter characteristics. However, a ladder type SAW constructed by connecting the basic sections 100 shown in FIG. In the filter, the attenuation pole is
Since only one band can be generated in each of the low-band attenuation region and the high-band attenuation region of the passband, there is a limit in improving the filter characteristics.

[0020]

According to the present invention, there is provided a first surface acoustic wave resonator having a predetermined resonance frequency arranged in a parallel arm, and a first surface acoustic wave resonator arranged in a series arm. A ladder-type surface acoustic wave filter comprising: a second surface acoustic wave resonator having an anti-resonance frequency corresponding to a resonance frequency; A first impedance unit connected in series; and a second impedance unit disposed on the series arm and connected in parallel to the second surface acoustic wave resonator. .

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (A) Embodiment Hereinafter, an embodiment will be described by taking as an example a case where a surface acoustic wave filter according to the present invention is used in a small mobile communication device such as a mobile phone.

Generally, SAW (Surface Acoustic Wav)
e) If a large number of SAW resonators are connected in multiple stages, the filter has a sufficiently large amount of attenuation in both the low-side attenuation band and the high-side attenuation band of the pass band, and is steep and ideal. Although it is possible to obtain filter characteristics, it is desirable to obtain good filter characteristics close to such ideal filter characteristics by using a SAW filter as small as possible using as few SAW resonators as possible. is important.

The first to third embodiments relate to ladder-type band-pass filters using SAW resonators, and to polarized SAW filters having an attenuation pole in an attenuation band.

(A-1) Configuration of First Embodiment FIG. 1 shows a ladder-type SAW filter 500 having a two-stage π-type configuration according to this embodiment. The circuit shown in FIG. 8 is an equivalent circuit of FIG.

In FIG. 1, the SAW filter 500
Are input terminals IN and E1, output terminals OUT and E2,
It includes three SAW resonators 510, 520, 521, one inductor 530, and six connection points B1 to B6.

The SAW resonator 510 is disposed in a series arm between the input terminal IN and the output terminal OUT. In addition to the SAW resonator 510, an additional inductor 570 is connected to the series arm in series.
1 and the additional capacitor 562 are connected in parallel via connection points B2 and B3.

A SAW resonator 520 is disposed on the parallel arm connected to the series arm via the connection point B1, and the parallel arm connected to the series arm via the connection point B4. , A SAW resonator 521 is arranged.

The parallel arm resonators 520, 521
Is connected to one end of an inductor 530 via a connection point B5, and the other end of the inductor 530 is connected to the input terminal E1 and the output terminal E2 via a connection point B6.

In order to clarify the function of the SAW filter 500 according to the present embodiment, the ladder-type SAW filter 100 shown in FIG. Try.

When the real part of the impedance of the series arm resonator 110 and the parallel arm resonator 120 shown in FIG. 2 having different resonance frequencies is set to zero (Q = ∞), that is, in the SAW filter 100 of FIG. When the impedance characteristic of the series arm resonator is jx and the admittance characteristic of the parallel arm resonator is jb, the image transmission amount γ is given by the following equation (1).
The attenuation characteristic of the ladder-type SAW filter 100 of FIG. 2 is evaluated.

Tanh (γ) = (bx / (bx−1)) ^1/2 (1) Therefore, the characteristic of the ladder-type SAW filter is expressed by the following equation (1).
, A pure imaginary number is a pass band, and a real number is an attenuation band. That is, the passband is 0 ≦ bx ≦ 1, and bx ≧ 1
Or, it becomes a pass band when bx ≦ 0. In the case of the equation (1), the loss in the pass band is zero, and the phase angle in the attenuation band is n times (π / 2).

As described above, the cross length of the serial arm resonator 110 is 100 μm, the logarithm is 100, and the parallel arm resonator 1
FIG. 3 shows an example of specific characteristics of the ladder-type SAW filter 100 shown in FIG. 2 when the intersection length of 20 is 70 μ and the logarithm is 70 lines.

In FIG. 3, the attenuation pole frequency is
The impedance of the series arm resonator 110 is at the frequency of the infinity point in the higher attenuation band of the pass band, and the impedance of the parallel arm resonator 120 is zero in the lower attenuation band of the pass band.

The impedance characteristics of the series arm resonator 110 and the parallel arm resonator 120 shown in FIG. 4 (that is, the imaginary part characteristic of the series arm resonator is jx, the imaginary part characteristic of the parallel arm resonator is jb, The real part characteristic of the arm resonator is rs,
The real part where the imaginary part characteristic of the parallel arm resonator is rp) is caused by the loss of each resonator.

This real part characteristic can be evaluated by the following equations (2), (3) and (4). Now, the Q of the resonator
Is finite, and assuming that Q0, a series arm resonator 110 including Q0
And the admittance Y of the parallel arm resonator 120 are given by equation (2).

Z = 1 / Y = Rd + jZO = 1 / (Gd + jYO) (2) Gd = ((ωC0 + ωC1 (1 + ω ＾ 2 * L1 * C1)) / ((1−ω ＾ 2 * L1 * C1) ＾ 2 ) / Q0 (3) Y0 = ω (C0 + C1 + ω ＾ 2 * L1 * C1 * CO) / (1 + ω ＾ 2 * L1 * C1) (4) where “＾ 2” represents the immediately preceding character or numerical value. "*" Is a symbol indicating multiplication.

When Q of each of the resonators 110 and 120 is ∞, the impedance of the series arm resonator 110 is jx,
The admittance of the parallel arm resonator 120 is jb, and the attenuation characteristic can be evaluated by the equation (1) as described above.
Since the Q of each resonator is finite, the real part and the imaginary part of each resonator are obtained from the equations (2), (3) and (4) instead of the equation (1). By applying the following equation (5) based on the imaginary part, the actual attenuation amount α is obtained.

In this case, unlike the equation (1), the loss (dB) in the pass band is not zero and becomes as shown in the equation (5).

[0039] α = 10 * LOG ((1 + bx / 2) 2 ten ^{((x + b) / 2} ) 2) ... (5) In addition, borrowing impedance Zin of the SAW filter
Is given by the following equation (6).

Zin = (1−bx + jx) / (1 + jb) (6) As is apparent from comparison between FIG. 3 and FIG. 4, the pass band (from about 863 MHz to about 911 MHz) shown in FIG. The attenuation pole in the high-frequency side attenuation region is such that the series arm resonator has a frequency at an infinity point (ie, about 42 dB) (ie, 91 dB).
At around 9 MHz), and the attenuation pole in the lower attenuation band of the passband is the frequency at which the parallel arm resonator is zero (that is, 855).
MHz).

Since the attenuation pole in the lower attenuation band of the pass band occurs at a frequency at which the impedance of the parallel arm resonator 120 is zero, the entire circuit of the SAW filter 100 shown in FIG. 2 is formed on one chip. If it is assumed that the chip is present, the attenuation pole frequency can be changed even after the chip is formed, for example, by using a bonding wire used for connecting the chip and the package by wire-bonding as an inductor.

That is, for example, the SAW filter 1 shown in FIG.
14 (A) and 14 (B) showing only the parallel arm portion of 00 (here, FIG. 14 (B) is an equivalent circuit of FIG. 14 (A)) and an inductor L _WB ( 144)
Is a bonding wire or the like, and its L value is changed, so that the attenuation pole frequency of the lower attenuation band can be changed.

The L value (inductance value) of the bonding wire can be changed by selecting the material of the bonding wire, or by changing the length or the three-dimensional shape of the bonding wire.

On the other hand, since the attenuation pole in the higher attenuation band of the pass band occurs at the frequency where the impedance of the series arm resonator 110 is infinite, the series arm resonator 110 itself must be changed to change the attenuation pole frequency. Configuration (IDT (interdigital transducer) electrode finger spacing, etc.)
Need to be changed, and the series arm resonator 1
At the stage after chip formation in which the configuration of No. 10 has been determined, it was impossible to change the attenuation pole frequency.

On the other hand, the present embodiment provides a means for changing the attenuation pole frequency in the higher attenuation band of the passband even after the chip is formed.

For this purpose, the SAW filter 5 of the present embodiment
00 is located inside the input terminal IN as shown in FIG. 1 or FIGS. 13A and 13B (here, FIG. 13B is an equivalent circuit of FIG. 13A). Between the connection point B1 and the connection point B2, a series additional inductor 570 is arranged in the series arm, and between the connection points B2 and B3, the parallel addition inductor 561 and the additional capacitor 562 are arranged in the series arm.

The SAW filter 5 of the present embodiment as described above
00 is the conventional SA shown in FIG. 2 in that the filter characteristic is determined by the impedance relationship between the series arm and the parallel arm.
Similar to W filter 100, but it is also possible to change the attenuation pole frequency of the higher attenuation band in the passband by adjusting the impedance of the series arm, so that the filter characteristics are controlled with a higher degree of freedom than before. be able to.

In the SAW filter 500 as well, the method of changing the attenuation pole frequency in the lower attenuation band of the pass band is as shown in FIG.
Like the SAW filter 100, the inductor 530 is formed of a bonding wire or the like and its L value is changed. However, to change the attenuation pole frequency in the high-frequency attenuation band, an additional inductor 570 is added. Inductor 56
1 and an additional capacitor 562 are utilized.

That is, when an additional capacitor 562 is added in parallel to the series arm resonator 510, its C value (capacitance value)
As the value is larger, the pole frequency in the higher-frequency attenuation band changes to the lower frequency side, and the attenuation slope becomes steeper.

When an additional inductor 561 is added in parallel to the series arm resonator 510, the larger the L value is,
The pole frequency in the high-frequency attenuation band changes to the high frequency side. In this case, the attenuation slope does not become steep, but the amount of attenuation increases.

Further, when an additional inductor 570 is added in series to the series arm resonator 510, the impedance characteristic of the filter is mainly improved. In this case, as the L value of the additional inductor 570 increases, the SAW filter 5
The input / output impedance of 00 becomes large.

The input terminal IN and the output terminal O
When a chip is connected to a package (lead frame) in a part of a UT or the like, it is considered that a bonding wire is usually used for the connection. However, a bonding wire of an input terminal IN and / or an output terminal OUT is used. In addition, by using a bonding wire as the additional inductor 570, there is an advantage that the adjustment range of the impedance can be expanded.

Although the L value of the bonding wire can be changed by the above-described method, the change range has an upper limit and a lower limit, and the additional inductor 570 of this embodiment is effective in increasing the upper limit. It is. Therefore, the additional inductor 570 is extremely useful in practical use.

It is preferable to use the bonding wire when a larger L value is required, but it is effective to use a microstrip when a smaller L value is required. In the equivalent circuit of FIG. 8, a bonding wire can be used for the inductors 841 and 842, and a microstrip can be used for the inductor 880).

In the SAW filter 500 of the present embodiment,
As an example, the configuration of the series arm resonator 510 has a crossing length of 90 (μm) and a logarithm of 60 (pairs), and the parallel arm resonator 52
Both the configurations of 0 and 521 have an intersection length of 86 (μm) and a logarithm of 60 (pair).

In this case, for example, the inductor 530 is 0.5 (nH) and the additional capacitor 562 is
Is 0.5 (pF) and the additional inductor 561 is 0.1
(NH), the additional inductor 570 is 0.1 (nH).
It may be.

The operation of the present embodiment having the above configuration will be described below with reference to FIGS. FIG. 10 is a simulation result showing the filter characteristics of the SAW filter 500 corresponding to each of the constant values listed here, and FIG. 15 is a summary of the points related to the simulation result.

That is, the case No. 1 in FIG.
0 corresponds to the curve CA11 indicated by the one-dot chain line.
o2 corresponds to the curve CA12 shown by the solid line in FIG.
Case No. 3 corresponds to a curve CA13 indicated by a dotted line in FIG.

(A-2) Operation of the First Embodiment Here, the case No. 1 in FIG. 15 relates to the SAW filter having the circuit configuration of FIG. 9 described above, and has the polarized L, that is, the inductor 980 ( The L value is L =
0.5 nH), but there is no series L, ie, the additional inductor 570, and there is no parallel L, ie, the additional inductor 5.
61, and there is no parallel C, that is, no additional capacitor.

Next, the case No. 2 in FIG.
A SAW filter having a configuration in which the series L, that is, the additional inductor 570 is removed from the AW filter 500 (this SAW filter is also an example of the present embodiment).
In addition to having the polarized L, that is, the inductor 530 (the L value is L = 0.5 nH), the parallel L, that is, the additional inductor 561 (the L value is L = 0.1 nH)
H) and parallel C, that is, the additional capacitor 562 (its C value is C = 0.5 pF).

Finally, the case No. 3 in FIG. 15 relates to the SAW filter 500 having all the components shown in FIG.
The value is L = 1.0 nH), the polarity L, that is, the inductor 530 (the L value is L = 0.5 nH), and the parallel L
That is, the additional inductor 561 (the L value of which is L =
0.1 nH) and the parallel C, that is, the additional capacitor 5
62 (its C value is C = 0.5 pF).

As compared to case No. 1, case Nos.
In No. 3, the attenuation pole frequency in the higher attenuation band is reduced by 10 MHz to 934 MHz.

From this, it can be seen that the parallel L and / or the parallel C can control the attenuation pole frequency in the high-frequency attenuation band.

Although the attenuation pole frequency in the lower attenuation band is hardly changed, the attenuation is 3 dB to 20 dB.
Indicates the frequency width required to change to (3-20)
Looking at the dB frequency width, the case N
For o2 and No3, it can be seen that the steepness of the filter characteristic (attenuation slope) is enhanced on both the low frequency side and the high frequency side.

That is, in case No. 1, the low slope indicating the steepness on the low frequency side is 9.4 MHz, whereas in case No. 2, it becomes 8.8 MHz, and in case No. 3, it becomes 7.6 MHz, and the steepness increases. Similarly, in the case of case No. 1, the high slope indicating the steepness on the high frequency side is also 14.
The frequency of 9 MHz is increased to 9.5 MHz in cases No. 2 and No. 3 and the steepness is increased.

Note that the 3 dB bandwidth indicating the passband is 40.7 MHz in case No. 2 compared to 41 MHz in case No. 1.
Although the MHz and 40.5 MHz of Case No. 3 are slightly narrower, a difference of this degree can be considered practically the same.

Further, in the conventional SAW filter 100 shown in FIG. 2, as shown in FIG. 3, only one attenuation pole exists on each of the high frequency side and the low frequency side. In cases No. 2 and No. 3, two appear on the low frequency side.

In the example described above, the attenuation pole frequency in the high-frequency attenuation band is reduced.
And the value of the parallel L, it is also possible to increase the attenuation pole frequency in the higher attenuation band.

(A-3) Effects of the First Embodiment According to the present embodiment, a SAW filter close to the dielectric filter and having good and steep filter characteristics can be configured with a small circuit scale. It is possible.

Further, according to the present embodiment, even in a stage after chip formation in which the configuration of the series arm resonator (510) has already been determined, the attenuation of the lower attenuation band in the passband can be achieved. It is possible to change not only the pole frequency but also the attenuation pole frequency in the high-frequency side attenuation band, so that there is little restriction on the point of change in the performance (specification) of the SAW filter, and the degree of freedom is improved.

Further, in the present embodiment, the series L (570)
Can be used to adjust the impedance of the series arm, so that the impedance can be changed in a wider range than before, and it is easy to obtain high-performance filter characteristics.

(B) Second Embodiment In the following, only the points of this embodiment different from the first embodiment will be described.

(B-1) Configuration and Operation of Second Embodiment FIG. 6 shows a SAW filter 600 of this embodiment.

In FIG. 6, the functions of the components denoted by the same reference numerals as those in FIG. 1 are the same as those in FIG.

Therefore, the difference between the present embodiment and the first embodiment is limited to the portion related to the additional inductor 660 connected in parallel with the series arm resonator 510 on the series arm.

In the first embodiment, what is connected in parallel to the series arm resonator 510 on the series arm is the inductor 5
In the present embodiment, there is only one element of the inductor 660, although the two elements are 61 and the capacitor 562.

Here, the inductor 660 in FIG.
All the other components are basically the same as the first embodiment (case 3), including their constants (intersection length of each resonator 510, 520, 521, logarithm, and L value of the inductor 530). is there. However, the L value of the inductor 570 (that is, the series L) is 0.5 nH.

The L value of the inductor 660 (ie, the parallel L) is 100 nH or 200 nH.

When having such constants, the operation of the SAW filter 600 according to the present embodiment will be described with reference to FIGS.
It becomes as shown in.

FIG. 11 shows a filter characteristic (simulation result) corresponding to FIG. 10, and FIG.
5, which summarizes the main points related to the simulation results.

That is, the case No. 1 in FIG.
1 corresponds to the curve CA21 indicated by the dashed line,
o2 corresponds to the curve CA22 shown by the solid line in FIG.
Case No. 3 corresponds to a curve CA23 indicated by a dotted line in FIG.

Also in the present embodiment, it can be seen that various characteristics of the filter are improved in case No. 2 and case No. 3 where these are present, as compared with case No. 1 where there is no series L and no parallel L.

In the first embodiment, the provision of the parallel L and the parallel C lowers the attenuation pole frequency in the higher attenuation band of the pass band.
And the parallel L (the parallel C is not provided), the attenuation pole frequency in the higher attenuation band increases.

That is, in the present embodiment, case No. 1
In this case, the attenuation pole frequency of the lower attenuation band was 944 MHz, the value of series L was 0.5 nH, and the value of parallel L was 200 n.
In case H2 of H, the frequency becomes 948 MHz,
Is 954 MHz in case No. 3 in which the value of the parallel L is 100 nH and the value of the parallel L is 100 nH.

It should be noted that the L value of the parallel L is 100 nH
And 200 nH are values that are too large to be realized in a normal package. However, since these values are examples, it is not always necessary to use such large values when mounting them on an actual mobile phone or the like. There is no.

By changing the selection of other constants, these L values can be further reduced.

(B-2) Effects of the Second Embodiment According to the present embodiment, even at a stage after chip formation where the configuration of the series arm resonator (510) has already been determined, It is possible to change not only the attenuation pole frequency in the lower attenuation band of the passband but also the attenuation pole frequency in the higher attenuation band, and there are few restrictions on the change time point of the performance (specification) of the SAW filter, and the degree of freedom is small. improves.

Further, in the present embodiment, contrary to the first embodiment, the attenuation pole frequency of the higher attenuation band of the passband is
Can be enhanced.

(C) Third Embodiment Hereinafter, only the points of this embodiment different from the first embodiment will be described.

The present embodiment is different from the first embodiment in that the SAW filter having a two-stage π-type configuration is replaced with a four-stage π-type configuration.

(C-1) Configuration and Operation of Third Embodiment A SAW filter 700 of this embodiment is shown in FIG.

In FIG. 7, the functions of the components denoted by the same reference numerals as those in FIG. 1 are the same as those in FIG.

Therefore, the difference between this embodiment and the first embodiment is that the components 760, 761, 511, 522, 5
71, and only the parts related to B7 to B9.

However, the connection point B4 of this embodiment has the first
An additional inductor 571 is connected instead of the output terminal OUT of the embodiment, and a parallel arm resonator 520 is connected to a connection point B5.
521 and a parallel arm resonator 522 are connected. For this reason, in the present embodiment, two serial arms are used in the first embodiment, and three parallel arms are used in the first embodiment.

In the present embodiment, the configuration of the two series arm resonators 510 and 511 is the same for the cross length 90 (μ),
Logarithm 80 (pair), three parallel arm resonators 520, 52
The configurations of 1,522 both have an intersection length of 86 (μ) and logarithm of
In the (pair), the L value of the polarized inductor 530 is 0.075.
nH.

Further, in the first embodiment, the two elements connected in parallel to the series arm resonator 510 on the series arm are the inductor 561 and the capacitor 562.
In this embodiment, only one element of the additional capacitor 760 (that is, the parallel C1) is connected in parallel with the series arm resonator 510 on the series arm.

Similarly, in this embodiment, only one element of the additional capacitor 761 (that is, the parallel C2) is connected in parallel to the other series arm resonator 511.

The C values of the parallel C1 and the parallel C2 are variously changed in FIGS. 12 and 17 showing the operation of the SAW filter 700 of the present embodiment.

FIG. 12 shows the filter characteristics corresponding to FIG. 10, and FIG. 17 corresponds to FIG. 15, and summarizes the points concerning the filter characteristics (simulation results).

That is, the case No. shown in FIG.
1 corresponds to a curve CA31 indicated by a dashed line in FIG. 12, and case No. 2 corresponds to a curve CA3 indicated by a solid line in FIG.
12, case No. 3 corresponds to the curve CA33 indicated by the dotted line in FIG. 12, and case No. 4 corresponds to the curve C33 in FIG.
This corresponds to CA34 indicated by a dotted line with a shorter pitch than A33.

Note that case No. 1 in the present embodiment
Is different from case No. 1 of the first and second embodiments in that the SAW filter 700 of the present embodiment has a four-stage π-type configuration.
5 shows a simulation result corresponding to a SAW filter (not shown) having a configuration in which the additional inductors 570 and 571 and the additional capacitors 760 and 761 are removed.

Since the SAW filter 700 of this embodiment has a four-stage π-type configuration, two attenuation poles appear in the high-frequency side attenuation band where there was only one attenuation pole in the first embodiment. I have.

Furthermore, the attenuation pole frequency decreases in each case where the values of the parallel C1 and the parallel C2 are larger than 0.0 pF, as compared with the case No. 1 in which the values of the parallel C1 and the parallel C2 are both 0.0 pF. ing.

From this, it can be seen that by appropriately selecting the values of the parallel C1 and the parallel C2, the decrease width of the attenuation pole frequency in the high-frequency attenuation band can be controlled.

As is clear from the (3-20) dB frequency width shown in FIG. 17, the steepness of the filter characteristic on the high frequency side is improved in cases Nos. 2 and 3 as compared with case No. 1. .

Further, comparing FIG. 17 with FIG. 15 regarding the (3-20) dB frequency width, the steepness of the filter characteristic of the present embodiment on the high frequency side is larger than that of the first embodiment. You can also see that it has improved.

The 3 dB bandwidth corresponding to the width of the pass band is wider in the present embodiment than in the first embodiment, and good filter characteristics are obtained.

(C) Effect of Third Embodiment According to this embodiment, although the circuit scale is slightly larger than that of the first embodiment, it is possible to obtain better filter characteristics than the first embodiment. It is possible.

In particular, the steepness of the filter characteristic on the high frequency side is
It is significantly higher than in the first embodiment.

(D) Other Embodiments In the above-described first to third embodiments, for the sake of simplicity, a number of specific numerical values have been shown. The scope of the invention is not limited by these numerical values.

In the first to third embodiments, the circuit configuration is changed by connecting an inductor or a capacitor in series or in parallel to the series arm. Fig. 18 is a generalization of the step π configuration.
The SAW filter 400 having the circuit configuration shown in FIG.

In FIG. 18, the SAW filter 40
0 indicates input terminals IN and E1 and output terminals OUT and E2.
, Three SAW resonators 510, 520, 521, and an impedance unit 470 connected in series to the series arm
, An impedance section 460 connected in parallel to the series arm, an inductor 530, and six connection points B1.
To B6.

The function of each component given the same reference numeral as in FIG. 1 may be the same as in FIG.

Either an inductor is arranged in the impedance section 470, or only a simple wiring pattern is present and no effective element is arranged.

Further, capacitors and / or inductors are arranged in impedance section 460.

On the other hand, in the first to third embodiments, the filter characteristics are variously changed by connecting an inductor or a capacitor in series or in parallel to the series arm to change the circuit configuration. To summarize the relationship between such a circuit configuration and filter characteristics, the following (1) to (1)
(3) can be said.

(1) When a capacitor is added in parallel to the series arm resonator, one (or more) attenuation pole frequency in the higher attenuation band changes to the lower frequency side. In this case, the attenuation slope becomes steep.

The magnitude of the change in the filter characteristics tends to increase as the C value of the capacitor increases.

(2) When an inductor is added in parallel to the series arm resonator, the attenuation pole frequency of one (or a plurality of) attenuation poles in the higher attenuation band changes to the higher frequency side. In this case, the attenuation slope does not become steep, but the amount of attenuation increases.

The magnitude of such a change in filter characteristics tends to increase as the L value of the inductor increases.

(3) When an inductor is added in series to the series arm resonator, the impedance characteristic of the filter is mainly improved.

The scope of the present invention is not limited to a two-stage π-type SAW filter such as the SAW filter 400 described above. An example includes a four-stage π-type configuration as in the third embodiment.

[0123]

As described above, according to the present invention, a surface acoustic wave filter having good and steep filter characteristics can be configured with a small circuit scale.

Further, according to the present invention, even if the configuration of the second surface acoustic wave resonator has already been determined, the attenuation pole frequency of the lower attenuation band in the pass band of the surface acoustic wave filter is determined. In addition, it is possible to change not only the attenuation pole frequency of the high-frequency side attenuation band, but also the restriction on the point of change in the performance of the surface acoustic wave filter, and the degree of freedom is improved.

Further, according to the present invention, the impedance of the series arm can be adjusted by using the first impedance means, so that the impedance can be changed in a wider range than before, and it is easy to obtain high-performance filter characteristics. become.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a ladder-type SAW filter according to a first embodiment.

FIG. 2 is a circuit diagram of a conventional ladder type SAW filter.

FIG. 3 is a diagram illustrating the operation of a SAW filter.

FIG. 4 is a diagram illustrating the operation of a SAW filter.

FIG. 5 is a circuit diagram of a conventional ladder-type SAW filter.

FIG. 6 is a circuit diagram of a ladder-type SAW filter according to a second embodiment.

FIG. 7 is a circuit diagram of a ladder-type SAW filter according to a third embodiment.

FIG. 8 is an equivalent circuit diagram of the ladder-type SAW filter according to the first embodiment.

FIG. 9 is a circuit diagram illustrating a configuration example of a ladder-type SAW filter for describing a problem to be solved by the invention.

FIG. 10 is an operation explanatory diagram of the first embodiment.

FIG. 11 is an operation explanatory diagram of the second embodiment.

FIG. 12 is an operation explanatory diagram of the third embodiment.

FIG. 13 is an operation explanatory diagram of the first embodiment.

FIG. 14 is an operation explanatory diagram of the first embodiment.

FIG. 15 is an operation explanatory diagram of the first embodiment.

FIG. 16 is an operation explanatory diagram of the second embodiment.

FIG. 17 is an operation explanatory diagram of the third embodiment.

FIG. 18 is an equivalent circuit diagram of the ladder-type SAW filter according to the first embodiment.

[Explanation of symbols]

100, 400, 500, 600, 700, 900 ... ladder type SAW filters, 510, 511, 520, 52
1,522: SAW resonator, 460, 470: additional impedance section, 530: polarized inductor, 561, 5
70, 660... Additional inductor, 562, 760, 76
1. Additional capacitor.

Claims (5)

[Claims] A first surface acoustic wave resonator having a predetermined resonance frequency disposed on a parallel arm; a first surface acoustic wave resonator disposed on a series arm;
A ladder-type surface acoustic wave filter comprising: a second surface acoustic wave resonator having an anti-resonance frequency corresponding to the resonance frequency; And first impedance means connected in series with the first arm, and second impedance means disposed on the series arm and connected in parallel to the second surface acoustic wave resonator. Surface acoustic wave filter. 2. The surface acoustic wave filter according to claim 1, wherein an inductor is used as said first impedance means, and a capacitor is used as said second impedance means. 3. The surface acoustic wave filter according to claim 1, wherein an inductor is used as said first impedance means, and an inductor and a capacitor are used as said second impedance means. 4. The surface acoustic wave filter according to claim 1, wherein an inductor is used as said first impedance means, and an inductor is used as said second impedance means. 5. A surface acoustic wave filter characterized by being a ladder type filter formed by cascading a plurality of surface acoustic wave filters according to claim 1.