JPH1197966A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide balanced type output terminals, to increase the impedance, and to prevent a filter from being largely scaled due to this by dividing a central surface acoustic wave transformer (IDT) into two part, and constituting it so that acoustic cascade connection and electric serial connection can be obtained. SOLUTION: A central IDT 11 is arranged on a piezoelectric substrate, and outside IDT 12 and 13 are arranged at the both outside parts, and reflectors 14 and 14 are arranged at the further both outside parts. The polarities of the right and left acoustic ports of the central IDT 11 are made opposite so that the outside IDT 12 and 13 can be formed so as to be vertically inverted. Output terminals 15 and 16 connected with the central IDT 11 are commonly formed as ungrounded balanced output type output terminals. The central IDT 11 is divided into two parts, and serially connected so that output impedance can be turned into 200 Ω. An input terminal 17 connected with the outside IDT 12 and 13 is obtained as an unbalanced type input terminal.

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 cellular phone or the like, and more particularly, to three surface acoustic wave converters (Interdigital Trasfer).
nsducer (hereinafter abbreviated as IDT), using 0th order and 2nd order.
A longitudinally-coupled double-mode surface acoustic wave filter that utilizes the coupling of the following two symmetric modes, enables balanced output of electric signals, and enables direct signal input to a semiconductor active device.
And two IDTs, utilizing the coupling between the 0th-order symmetric mode and the 1st-order antisymmetric mode, enabling balanced output of electric signals, and enabling direct signal input to a semiconductor active device. The present invention relates to a dual mode surface acoustic wave filter.

[0002]

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

The frequencies used for mobile communication, especially for mobile phones, are 800 MHz to 1 GHz, around 1.5 GHz,
8 to 2 GHz, and 2 GHz with increasing demand
It is expected that frequency bands exceeding the above will also be used. Filters used in these frequency bands are required to have low loss and a wide passband, and it is already known that there is a longitudinally-coupled double-mode filter using surface acoustic waves as a suitable configuration method to achieve this requirement. It is known.

[0004] As one example of the configuration of a longitudinally coupled double mode filter, one disclosed in Japanese Patent Publication No. Hei 7-1859 is cited. The publication discloses a piezoelectric substrate (in this case,
On the X-cut 112 ° rotation Y-propagation lithium tantalate is used), three IDTs are arranged along the surface acoustic wave propagation direction, and grid-like reflectors are arranged on both outer sides thereof. It is shown that a so-called even-order mode, which is symmetric with respect to the center, is excited in the 0th and 2nd-order modes, and a combination of these excites a double-mode filter. Here, the term "longitudinal coupling" means that these two modes are excited in the same direction as the surface acoustic wave propagation direction. Such a dual mode filter has been known for a long time as a monolithic filter using a quartz bulk wave, and it is known that the frequency arrangement of two modes is important in designing the filter. In this publication as well, this frequency arrangement may be such that the resonance frequency of the zero-order mode and the anti-resonance frequency of the second-order mode are substantially matched, that is, the normalized frequency difference between the two frequencies is smaller than 0.0005. It is shown. And with this configuration,
Fractional bandwidth (value obtained by dividing pass bandwidth by center frequency) 0.4
It is shown that 0% of the filter was obtained.

On the other hand, as an example of the configuration of a vertically coupled double mode filter in which two IDTs are arranged close to each other in the surface acoustic wave propagation direction and reflectors are arranged on both outer sides thereof, for example,
One described in Japanese Patent Publication No. 3-51330 is exemplified. In this case, a zero-order mode symmetric with respect to the center and an anti-symmetric first-order mode are excited, and a double mode filter is formed by coupling these. In the conventional example cited here, lithium tantalate as the piezoelectric substrate,
STD crystal is used, IDT formed on the substrate
It has been shown that by controlling the total logarithm and the width of the intersection, the pass bandwidth of the configured filter can be controlled. In this publication, although there is no clear description about the cut angle and the propagation direction of the surface acoustic wave of lithium tantalate, it is expected from the description of the temperature characteristics that X-cut 112 ° rotation Y propagation.

[0006] Mobile communication currently in practical use at home and abroad, that is, mobile phones and cordless telephone systems include the AMPS system in the United States, the GSM, EGSM and CT-2 systems in Europe, and the PHS and PDC systems in Japan. And NT
There are various types such as ACS system, and furthermore, CDM
The A method and the W-CDMA (Wideband CDMA) method are also being put to practical use. Filters used in the high-frequency circuit sections of these systems are required to have various specifications having a pass band width ranging from several megahertz to several tens of megahertz. It is said to be able to meet various specification requirements. Specifically, when the piezoelectric substrate is made of a material having a large electromechanical coupling coefficient, the pass band width can be widened, and two IDTs are provided so that the 0th-order symmetric mode and the 1st-order antisymmetric mode can be used. Rather than using three IDTs, the use of the zero-order symmetric mode and the second-order symmetric mode enables a wider band. A structure in which a 64 ° -rotation Y-cut lithium niobate having a large electromechanical coupling coefficient is used as a piezoelectric substrate and two IDTs are provided on this substrate is disclosed in, for example, Japanese Patent Application Laid-Open No. Hei 4-207615. A structure in which three IDTs are provided on lithium cut niobate is disclosed in, for example, Japanese Patent Application Laid-Open No. 5-267990. As described above, the piezoelectric substrate used in the longitudinally-coupled double mode surface acoustic wave filter and the number of IDTs to be installed are appropriately combined according to required specifications.

As described above, a surface acoustic wave filter is frequently used in a high frequency circuit section of a mobile communication terminal due to its small size and light weight. In order to meet various required specifications, a vertically coupled double mode filter having a large degree of design freedom is appropriate.

By the way, considering the structure of a mobile communication terminal, specifically, a cellular phone, the surface acoustic wave filter is
Usually, it is arranged between the low-noise amplifier and the mixer in the receiving high-frequency circuit section or in front of the low-noise amplifier. Active elements such as mixers and amplifiers usually have an input / output impedance of 200Ω. In order to widen the dynamic range at low voltage and obtain high gain,
Input and output are becoming balanced. However, in the conventional surface acoustic wave filter described above, the input / output impedance is 50Ω, and one terminal of each input / output terminal pair is of an unbalanced type. Cannot be directly connected, and the effect of noise cannot be removed due to grounding conditions.

[0009] Regarding the problem of the balance between the input and output of the active element and the balance of the input and output of the surface acoustic wave filter used therein, see, for example, “Transactions of the IEICE General Conference (Basic / Boundary)”. No. A-11-17, p29
2, 1997 ". In this report, four 100Ω SAW resonator filters were used as RF SAW filters capable of coping with unbalanced 50Ω matching on the antenna side and balanced 200Ω matching on the amplifier side.
By connecting the input side in parallel and the output side in series, 50Ω
A -200Ω impedance is realized. Then, on the balanced output side, the input / output phase of one SAW filter is inverted by reversing the direction of the IDT, and a balanced signal is output. However, in this configuration, there is a problem that the element size becomes large, and as a result, the number of pieces to be taken per wafer decreases and the cost increases.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a vertically coupled dual mode filter having both input and output signals.
Another object of the present invention is to provide a balanced type input or output terminal, raise the impedance from 50 Ω of the conventional configuration to about 200 Ω, which is four times as large as that of the conventional configuration, and avoid an increase in the size of the filter.

[0011]

The above object is achieved by the following (1).
And (2) are achieved. (1) Along the surface of the piezoelectric substrate,
IDTs are disposed close to each other, reflectors are disposed on both outer sides thereof, and a longitudinally-coupled double-mode surface acoustic wave filter using the coupling of a 0th-order symmetric mode and a 2nd-order symmetric mode. Is divided into two parts, and the surface acoustic wave filter is configured to be cascade-connected acoustically and series-connected electrically. (2) Along the surface of the piezoelectric substrate,
IDTs are arranged close to each other, reflectors are arranged on both outer sides of the IDTs, and a longitudinally-coupled double-mode surface acoustic wave filter using coupling of a 0th-order symmetric mode and a 1st-order antisymmetric mode,
A surface acoustic wave filter configured to divide one of the two IDTs into two and connect them acoustically in cascade and electrically in series.

[0012]

According to the present invention, one IDT conventionally set to 50Ω is divided into two parts, and these are arranged so as to be cascaded acoustically and electrically connected in series. The impedance can be increased to 200Ω which is four times the conventional value without increasing the element size as compared with the conventional longitudinally-coupled double mode filter, and can be matched with the input impedance of the peripheral active element. Further, since the bus bar for connection to the external terminal, which has been opposed to the conventional IDT electrode finger, can be formed on the same edge, both terminals of the balanced input / output terminal pair have the same electrical length when viewed from the outside. Which is preferable in terms of characteristics.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on embodiments.

FIG. 1 is a schematic diagram of an electrode structure showing an embodiment of a surface acoustic wave filter according to the present invention. In this surface acoustic wave filter, a central IDT 1 is provided on a piezoelectric substrate (not shown).
1 are arranged, outer IDTs 12 and 13 are arranged on both outer sides, and reflectors 14 and 14 are arranged on both outer sides.

In this embodiment, a 64 ° rotation Y-cut X-propagating lithium niobate is used for the piezoelectric substrate, and an Al-0.5 wt% Cu alloy formed by sputtering is used for the electrode. And the electrode material may be pure Al or other A
It may be an alloy. In this embodiment, the IDT
The period λ is set to 4.648 μm, and the electrode thickness normalized by this λ is set to 3.55%. These may be set as appropriate according to the required specifications.

Next, when describing the specifications of the IDT and the reflector, the effective logarithm of the central IDT 11 is 17 pairs,
The effective logarithm of each of the DTs 12 and 13 is 11 pairs, and the electrode finger intersection width is 56λ. In this configuration, the central IDT
Since the left and right acoustic ports of 11 have opposite polarities,
The outer IDTs 12 and 13 on both outer sides are formed to be upside down. However, by shifting the distance from the central IDT by a half wavelength, the outer IDTs 12, 1
3 may have the same structure without having an inverted relationship. The reflector 14 had 250 reflective stripes, and its pitch was set slightly wide so that the maximum position of the IDT conductance was within the reflector stop band.

The configuration shown in FIG. 1 is of a balanced output type in which both output terminals 15 and 16 connected to the central IDT 11 are not grounded, and the output terminals 15 and 16 are provided with substantially the same electrical length from the surface acoustic wave propagation path. It is characteristically preferable because it can be taken out. Since the central IDT 11 is divided into two and connected in series, the output impedance becomes 200Ω. In this configuration, it was separately confirmed that the input / output impedance was 50Ω when the central IDT 11 had a conventional structure without being divided and connected in series. In this configuration, since the input terminals 17 connected to the outer IDTs 12 and 13 are of an unbalanced type, the configuration is such that 50Ω unbalanced input-200Ω balanced output. However, if the output terminals 15 and 16 are used as input terminals and the input terminal 17 is used as an output terminal, a configuration of 200Ω balanced input and 50Ω unbalanced output can be obtained. Which configuration to use may be appropriately selected according to the application location of the surface acoustic wave filter. Incidentally, the fact that the output terminal and the input terminal may be reversed is the same in other embodiments described below.

It should be noted that the center ID of the 50Ω system which is the source of the division
When T has an even number of electrode fingers, it may be divided into two so that the electrode indices are equal as shown in FIG. 1. If so, the two IDTs in a split series configuration are:
The electrode indices differ by one.

FIG. 2 shows another embodiment of the present invention. Conventionally, it has been known that a vertically coupled double mode filter has a two-stage cascade connection configuration in order to secure out-of-band attenuation of the filter. However, an example in which the present invention is applied to this two-stage cascade connection configuration Is the embodiment shown in FIG. In this configuration, the central IDT 11a is divided and connected in series in the same manner as the central IDT 11 shown in FIG. 1 in the first-stage vertically coupled double mode filter 1a having three IDTs sandwiched between the reflectors 14a on both sides. In this connection configuration, the output terminals 15a and 16a are of a balanced type. The second-stage longitudinally-coupled double-mode filter 1b has three sides sandwiched between reflectors 14b and 14b.
Central IDT to which input terminal 17 is connected among IDTs
11b is a conventional configuration. And both outer IDTs, that is, outer IDT 12a and outer IDT 1
By connecting the outer IDT 13a and the outer IDT 13b and the outer IDT 13b to form a cascade connection, a filter having a 50Ω unbalanced input-200Ω balanced output and a large out-of-band attenuation is obtained.

FIG. 3 shows another embodiment of the present invention. In this configuration, the first-stage vertically coupled double mode filter 1a and the second-stage vertically coupled double mode filter 1b
Each central IDT 11a and central IDT 11b
Are divided and connected in series in the same manner as the central IDT 11 shown in FIG. 1, and the output terminals 15a and 16a and the input terminal 15
b and 16b are of a balanced type. And both outer IDTs, that is, outer IDT 12a and outer IDT 1
2b, and the outer IDT 13a and the outer IDT 13b are connected to form a cascade connection configuration. That is, a two-stage cascade connection configuration in which the vertically coupled double mode filter shown in FIG. 1 is arranged so that the unbalanced input side is an interstage connection part. With this configuration, a balanced-balanced input / output relationship can be obtained, and the input / output impedance can be set to 200Ω. Therefore, the configuration is suitable for application to a circuit section having active elements before and after the filter. In this configuration, both input and output terminal pairs can have the same electrical length.

The configuration in which the present invention is applied to a longitudinally coupled double mode filter using three IDTs has been described above.
The present invention can also realize a balanced input / output configuration in a longitudinally-coupled double-mode filter that utilizes coupling with a second-order antisymmetric mode.

FIGS. 4, 5 and 6 show examples in which the present invention is applied to a vertically coupled double mode filter provided with two IDTs.

FIG. 4 shows that two IDTs 21 and 22 are arranged close to each other along a surface acoustic wave propagation direction on a piezoelectric substrate (not shown), and reflectors 14 and 14 are arranged on both outer sides thereof. This is a longitudinally-coupled double-mode surface acoustic wave filter that utilizes the coupling between a first-order symmetric mode and a first-order antisymmetric mode.
T21 is divided into two parts, and is configured to have a cascade connection acoustically and a series connection electrically, similarly to the central IDT11 in FIG. Also in this configuration, similarly to the configuration of FIG. 1, the output terminals 15 and 16 are of a balanced type, the input terminal 17 is of an unbalanced type, and a configuration of 50Ω unbalanced input and 200Ω balanced output is provided.

Although the material of the piezoelectric substrate in this configuration is not particularly limited, the configuration in which two IDTs are provided is a case where relatively narrow band characteristics are required. 36 ° rotation Y-cut X-propagating lithium tantalate having a small coupling coefficient, X-cut 11
2 ° rotation Y-propagating lithium tantalate is frequently used.

In the case of the configuration in which two IDTs are provided, as in the case of the configuration in which the three IDTs are provided, two-stage cascade connection is usually used to improve the out-of-band attenuation.

FIG. 5 shows an embodiment in which two stages are connected in cascade.
The relationship between FIG. 4 and FIG. 5 is the same as the relationship between FIG. 1 and FIG. That is, in this configuration, one IDT 21a is connected to one IDT 21a in the first-stage vertically coupled double mode filter 1a in FIG.
In this case, the output terminals 15a and 16a are of a balanced type as in the case of the IDT 21. The second-stage vertical-coupling double-mode filter 1b has one IDT 22b to which the input terminal 17 is connected, which has a conventional configuration. And the other IDTs, that is, IDT 22a
And the IDT 21b are connected in a cascade configuration to provide a 50Ω unbalanced input-200Ω balanced output configuration.
In addition, a filter having a large out-of-band attenuation is obtained.

FIG. 6 shows another embodiment in which two stages are cascaded. The relationship between FIG. 4 and FIG. 6 is the same as the relationship between FIG. 1 and FIG. That is, in this configuration, one IDT 21a is provided in each of the first-stage vertically coupled double-mode filter 1a and the second-stage vertically coupled double-mode filter 1b.
Further, one IDT 22b has a split series connection configuration like the IDT 21 of FIG. 4, and the output terminals 15a and 16a and the input terminals 15b and 16b are of a balanced type.
The other IDTs, ie, IDTs 22a and I
The DT21b is connected to form a cascade connection configuration. That is, a two-stage cascade connection configuration in which the vertically coupled double mode filter shown in FIG. 4 is arranged so that the unbalanced input side is an interstage connection part. With this configuration, 200Ω
Since an input / output relationship of balanced -200Ω balance is obtained, it is suitable for application to a circuit section having active elements before and after a filter.

[Brief description of the drawings]

FIG. 1 is a plan view showing a configuration of an IDT in a longitudinally coupled double mode surface acoustic wave filter according to the present invention.

FIG. 2 is a plan view showing a configuration of an IDT in a case where the longitudinally-coupled double-mode surface acoustic wave filter of the present invention has a balanced-unbalanced configuration by two-stage cascade connection.

FIG. 3 is a plan view showing the configuration of an IDT when the longitudinally-coupled double-mode surface acoustic wave filter of the present invention has a balanced-balanced configuration by two-stage cascade connection.

FIG. 4 is a plan view showing a configuration of an IDT in a longitudinally coupled double mode surface acoustic wave filter according to the present invention.

FIG. 5 is a plan view showing a configuration of an IDT when the longitudinally-coupled double-mode surface acoustic wave filter of the present invention has a balanced-unbalanced configuration by two-stage cascade connection.

FIG. 6 is a plan view showing the configuration of an IDT when the longitudinally coupled double mode surface acoustic wave filter of the present invention has a balanced-balanced configuration by two-stage cascade connection.

[Explanation of symbols]

1a, 1b Vertically-coupled double-mode filter having one-stage configuration 11, 11a, 11b Central IDT 12, 12, 12b, 13, 13a, 13b Outside I
DT 14, 14a, 14b Reflector 15, 15a, 16, 16a Output terminal 15b, 16b, 17 Input terminal 21, 21a, 21b, 22, 22a, 22b IDT

Claims (2)

[Claims] 1. Three IDTs are arranged close to each other along a surface acoustic wave propagation direction on a piezoelectric substrate, and reflectors are arranged on both outer sides of the three IDTs to couple a 0-order symmetric mode and a second-order symmetric mode. A longitudinally-coupled double-mode surface acoustic wave filter, wherein a central IDT is divided into two, and acoustically cascade-connected and electrically series-connected. 2. Two IDTs are arranged close to each other along the direction of surface acoustic wave propagation on a piezoelectric substrate, and reflectors are arranged on both outer sides thereof to couple a zero-order symmetric mode and a first-order antisymmetric mode. A longitudinally coupled double-mode surface acoustic wave filter using a surface acoustic wave filter, wherein one of the two IDTs is divided into two, and is acoustically cascaded and electrically connected in series. Wave filter.