JP3478280B2 - Surface acoustic wave filter, communication device - 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, and more particularly to a surface acoustic wave filter having a balance-unbalance conversion function and a communication device having the same.

[0002]

2. Description of the Related Art Heretofore, there have been remarkable technological advances in recent years in reducing the size and weight of communication devices such as mobile phones. As a means for achieving this, not only the reduction and miniaturization of each component but also the development of a component having a plurality of functions has been advanced. Against this background, a surface acoustic wave filter used in the RF stage of a mobile phone having a balanced-to-unbalanced conversion function, that is, a so-called balun function has been actively researched in recent years, and the GSM method (Global System). fo
r Mobile communications) etc. have come to be used mainly.

When a balanced line such as a parallel two-wire feeder and an unbalanced line such as a coaxial cable are directly connected to each other, an unbalanced current flows and the feeder line (feeder) itself operates as an antenna. Since it is not desirable, it refers to a circuit that blocks unbalanced currents and matches balanced and unbalanced lines.

Several patents have been filed concerning a surface acoustic wave filter having such a balanced-unbalanced conversion function. As a surface acoustic wave filter having a balanced-unbalanced conversion function in which the input impedance and the output impedance are substantially equal, as shown in FIG.
The configuration described in Japanese Patent No. 781 is widely known.

In the surface acoustic wave filter shown in FIG. 11, a comb-shaped electrode portion (also called a comb-shaped electrode portion, which is called a comb-shaped electrode portion, is formed on a piezoelectric substrate.
Inter-Digital Transducer (hereinafter referred to as IDT) 51
Is provided, and the IDTs 52 and 53 are arranged on the left and right sides of the IDT 51 (positions along the propagation direction of the surface acoustic wave).

Further, in the surface acoustic wave filter, the reflectors 54 and 55 for reflecting the surface acoustic waves and improving the conversion efficiency are arranged so as to sandwich the respective IDTs 52, 51 and 53 from the left and right. Is
Further, there are provided balanced signal terminals 58 and 59 respectively connected to the IDT 51 and unbalanced signal terminals 60 connected to the IDTs 52 and 53.

Such a surface acoustic wave filter has a 3ID
It is called a T type longitudinally coupled resonator type surface acoustic wave filter, and has a balanced-unbalanced conversion function by conversion using surface acoustic waves between the IDTs 52, 51, 53.

In the above surface acoustic wave filter, the ID
An electrode finger (a portion of the electrode finger 56 in FIG. 11) adjacent to each other in T51 and IDT52, and IDT5
1 and the IDT 53 are adjacent to the electrode fingers (see FIG. 1).
1, the pitch of several electrode fingers (the portion of the electrode finger 57) is set narrower than the pitch of the other electrode fingers, and further, the IDT-IDT interval on one side, for example, the electrode finger 57 side, is set around it. By making it about 0.5 times the wavelength of IDT,
Loss due to components emitted as bulk waves in surface acoustic waves is reduced.

By the way, in FIG. 11, the number of electrode fingers is reduced to simplify the illustration. Also,
In the surface acoustic wave filter, the ID is adjusted in order to adjust the impedance in the pass band and to secure the attenuation outside the pass band.
Two surface acoustic wave resonators 61 and 62 are connected in series between the T52 and IDT 53 and the unbalanced signal terminal 60.

[0010]

However, in the above-mentioned conventional surface acoustic wave filter, the balanced signal terminals 58 and 59 are provided.
The problem is that the equilibrium between them deteriorates. That is, in the surface acoustic wave filter having the balanced-unbalanced conversion function, in the transmission characteristics within the pass band between the unbalanced signal terminal and the balanced signal terminals, the amplitude characteristics are equal and the phase is 180 degrees. Are required to be inverted, which are called amplitude balance degree and phase balance degree between the balanced signal terminals, respectively.

The degree of amplitude balance and the degree of phase balance are the balance-
Considering a surface acoustic wave filter having an unbalance conversion function as a 3-port device, for example, assuming that the unbalanced input terminal is the first port and the balanced output terminals are the second port and the third port, respectively, the degree of amplitude balance = | A |, A = | 20log
(S21) | − | 20log (S31) |, phase balance = | B−180 |, B = | ∠S21−∠S31 |, respectively. Note that S21 is a transfer coefficient from the first port to the second port, and S31 is a transfer coefficient from the first port to the third port. Regarding the balance between the balanced signal terminals, ideally, the amplitude balance is 0 dB and the phase balance is 0 within the pass band of the surface acoustic wave filter.
It is said to be degree.

However, the conventional configuration of FIG. 11 has a problem that the degree of balance between the balanced signal terminals 58 and 59 is deteriorated.

That is, in the above configuration, each balanced signal terminal 5
There is no neutral point between 8 and 59, and output signals are output from both poles of the same IDT 51. Therefore, the polar relationship between the output IDT 51 and the adjacent IDTs 52 and 53 affects the degree of balance.

From this, in the configuration shown in FIG. 11, in order to exert the balance-unbalance conversion function, the IDT5
Since the total electrode index of 1 is an odd number, the electrode fingers of both ends (both ends along the propagation direction of the surface acoustic wave) on the side connected to the balanced signal terminal 58 in the IDT 51 are respectively connected to the ground. While the electrode fingers are adjacent, the IDT
Electrode fingers connected to the signal line (balanced signal terminal 58) are adjacent to the electrode fingers on both ends of 51 on the side connected to the balanced signal terminal 59.

Therefore, in the above conventional configuration, the IDT 5
Since the polarities of the electrode fingers adjacent to the electrode fingers facing each other in 1 are different,
This causes a problem that the degree of balance between 8 and 59 deteriorates.

The influence of this deterioration becomes greater as the center frequency of the surface acoustic wave filter becomes higher (larger).
Therefore, a surface acoustic wave filter having a center frequency near 1.9 GHz for DCS and PCS has a problem that a sufficient degree of balance cannot be obtained.

The object of the present invention is to correct the above-mentioned difference between the balanced signal terminals, which is one of the causes of the deterioration of the balanced degree between the balanced signal terminals, so that the balanced-unbalance between the balanced signal terminals is good. An object is to provide a surface acoustic wave filter having a balance conversion function.

[0018]

[Means for Solving the Problems]

The SAW filter of the present invention, in order to solve the above problems, on a piezoelectric substrate, an IDT having two comb-shaped electrodes having a respective adjacent electrode fingers, the propagation direction of the surface acoustic wave Along a plurality of surface acoustic wave filters having adjacent balanced signal terminals for input and output, which are adjacent to each other and are connected to the IDT, one of the balanced signal terminals is connected, Electrodes adjacent to each other of the outermost electrode fingers of the comb-shaped electrode are connected to a signal terminal together, and the other of the balanced signal terminals is connected to the electrodes of the outermost electrode fingers of the comb-shaped electrode adjacent to each other. Of the fingers, one is connected to the signal side, the other is connected to the ground side, and the other of the balanced signal terminals is connected, the outermost electrode finger of the comb-shaped electrode is on the propagation path of the surface acoustic wave. by being separated One and the other of the balanced signal terminals
The outermost electrode fingers of the connected comb-shaped electrodes are both on the signal side.
The electrode finger connected to the
It is characterized by being adjacent to the extreme finger .

According to the above structure, a plurality of IDTs having electrode fingers are provided on the piezoelectric substrate along the propagation direction of the surface acoustic wave, and at least one of the input and output balances connected to the IDTs is balanced. By having the signal terminal, the conversion function between the balanced signal and the unbalanced signal can be exhibited together with the filter function.

By the way, conventionally, one of the electrode fingers of each adjacent IDT, which faces the electrode fingers at both ends of the IDT connected to the balanced signal terminal, is connected to the signal line and the other is connected to the ground line. . By this,
The electrode fingers in the IDT connected to the balanced signal terminals have different ambient environments such that one faces the signal line and the other faces the ground line, and the degree of balance between the balanced signal terminals deteriorates. Was there.

However, in the above structure, at least one electrode finger of at least one IDT connected to the balanced signal terminal is separated on the propagation path of the surface acoustic wave. One can be connected to the ground line and the other, not separated, can be connected to the signal line.

As a result, in the above configuration, the difference in the surrounding environment of each electrode finger in the IDT connected to the balanced signal terminal can be reduced more than before, and the balanced signal terminals for at least one of the input and output can be balanced. You can improve the degree.

In the surface acoustic wave filter, the separated electrode fingers may be connected to the ground. Also,
In the above surface acoustic wave filter, the separated electrode fingers are
Connected to a matching IDT or grounded reflector
It is grounded and the separated electrode fingers
The electrode fingers that are adjacent to each other are I to which the separated electrode fingers are connected.
It is preferably connected to a DT or reflector . According to the above configuration, the effect of adjusting the degree of balance between the balanced signal terminals connected to the signal line can be increased.

In the surface acoustic wave filter, the balanced signal terminals may be connected to at least one IDT. In the surface acoustic wave filter, three I
It may have DT. In the SAW filter, when it has three IDT, the medium is balanced signal terminals
It may be connected to the central IDT. According to the above configuration, the effect of adjusting the balance can be increased.

In the above surface acoustic wave filter, it is preferable that the separated electrode fingers are separated at substantially the center in the cross width direction. According to the above configuration, the adjustment of the degree of balance can be made more reliable.

In the surface acoustic wave filter, at least one surface acoustic wave resonator may be connected to the IDT in at least one of series and parallel. The surface acoustic wave filter may have a balanced-unbalanced conversion function.

According to the above configuration, the attenuation amount outside the connection pass band can be increased by connecting at least one surface acoustic wave resonator in at least one of series and parallel.

In the above-mentioned surface acoustic wave filter, a ground line may be inserted between adjacent signal lines connected to the IDT. According to the above configuration, the balance can be further improved by inserting the ground line.

A communication device of the present invention is characterized by having any one of the surface acoustic wave filters described above.
According to the above configuration, the filter function and the excellent balanced signal-
By having a surface acoustic wave filter having a conversion function between unbalanced signals, the functions can be combined to reduce the size, and the excellent conversion function can improve the transmission characteristics.

[0031]

BEST MODE FOR CARRYING OUT THE INVENTION The following will describe each embodiment of the present invention in reference to FIGS. 1 to 10. FIG. 1 shows the configuration of the first embodiment of the present invention. In each of the following embodiments, a DCS reception filter will be described as an example.

In the first embodiment, as shown in FIG.
A longitudinally coupled resonator type surface acoustic wave filter 1 is provided on a piezoelectric substrate 20 made of 40 ± 5 ° YcutX propagation LiTaO _3.
And aluminum (Al) in which the surface acoustic wave resonator 2 and the surface acoustic wave resonator 3 connected in series to the surface acoustic wave filter 1 are formed by a photolithography method or the like.
It is formed by electrodes. The surface acoustic wave filter 1, the surface acoustic wave resonator 2 and the surface acoustic wave resonator 3 are each composed of an IDT (comb-shaped electrode portion) and a reflector.

The IDT is a strip-shaped base end portion (bus bar portion).
And two comb-teeth-shaped electrodes, each of which has a plurality of parallel electrode fingers extending in a direction orthogonal to the base end from one side of the base end. The above-mentioned comb-teeth-shaped electrodes are provided in a state in which the side portions of the electrode fingers of each comb-teeth-shaped electrode are interdigitated between the electrode fingers so as to face each other substantially in parallel.

In such an IDT, by setting the length and width of each electrode finger, the distance between adjacent electrode fingers, and the cross width indicating the face-to-face length in a complicated state between the electrode fingers, respectively. The signal conversion characteristics and pass band can be set.

The reflector has a function of reflecting the surface acoustic wave propagating to the reflector and returning it to the propagating direction. That is, the reflector extends in a direction orthogonal to the longitudinal direction of the base end portion from a pair of strip-shaped base end portions (bus bar portions) and one side portion of the base end portions, and each of the base end portions. A plurality of parallel and parallel electrode fingers connecting the parts are provided on the piezoelectric substrate.

As a result, the reflector is excited by the propagating surface acoustic wave, the surface acoustic wave generated by the exciting electric signal cancels the advancing surface acoustic wave, and the surface acoustic wave propagates. It is set to generate a new surface acoustic wave in the opposite direction. Therefore, the reflector quasi-reflects the propagating surface acoustic wave.

The structure of the surface acoustic wave filter 1 is the IDT 4
The IDTs 5 and 6 are formed so as to sandwich them from the left and right (along the propagation direction of the surface acoustic wave), and the reflectors 7 and 8 are formed on both sides thereof. Therefore, each IDT4,
The longitudinal directions of the electrode fingers of 5, 6 and the respective reflectors 7, 8 are set to be substantially orthogonal to the propagation direction of the surface acoustic wave.

Further, in the surface acoustic wave filter 1, as is apparent from FIG. 1, there are several parts between the IDT 4 and the IDT 5 (adjacent parts) and between the IDT 4 and the IDT 6 (adjacent parts). Electrode fingers (narrow pitch electrode fingers)
Is set to be smaller than other portions of the IDT (locations 9 and 10 in FIG. 1, wavelength λI ₂ ).

The balanced signal terminals 11 and 12 are connected to both poles (one electrode finger and the other electrode finger) of the IDT 4, respectively. The unbalanced signal terminal 13 is the IDT 4
And the IDT 6 are provided. In both poles of such IDT4, the phases are opposite to each other (that is, 180
Therefore, in each of the embodiments, a balanced-unbalanced conversion function can be exerted between the balanced signal terminals 11 and 12 and the unbalanced signal terminal 13.

Surface acoustic wave resonator 2 and surface acoustic wave resonator 3
Are connected in series via the signal line 18 between the unbalanced signal terminal 13 and the IDTs 5 and 6. Therefore, the balanced signal terminal 11 is set so as to be adjacent to and surrounded by the signal line 18. By the way, in FIG. 1, the number of electrode fingers is reduced to simplify the drawing.

The surface acoustic wave resonator 2 is composed of the IDT 2a.
And reflectors 2b and 2c arranged so as to sandwich it from both sides (along the propagation direction of the surface acoustic wave). The surface acoustic wave resonator 3 includes an IDT 3a,
Each reflector 3 arranged so as to sandwich it from both sides
b and 3c.

In the first embodiment, the ID of FIG.
As shown in FIG. 2, which is an enlarged view of a main part related to T4 and IDT5, the outermost electrode fingers 15 of the IDT4 connected to the balanced signal terminal 11 are adjacent to the IDTs 5 and 6, respectively. The electrode fingers 15a and 15b are separated on the propagation path of the surface acoustic wave. The separated electrode fingers 15b are connected to the ground by being respectively connected to the bus bar portions 5a and 6a connected to the ground side of the IDTs 5 and 6, respectively.

A detailed design example of the longitudinally coupled resonator type surface acoustic wave filter 1 is shown below. Crossing width W: A portion where electrode fingers are not separated (5b in FIG. 2): 71.2λI
Part where _one electrode finger is separated (4a in FIG. 2): 35.6λI ₁ Number of IDTs (in the order of 5, 4, 6): 21 (4) / (4) 3
5 (4) / (4) 21 (number of electrode fingers with a small pitch in parentheses) IDT wavelength λI ₁ : 2.18 μm, IDT λI ₂ : 1.
96 μm Reflector wavelength λR: 2.18 μm Number of reflectors: 150 IDT-IDT Interval: 0.50 λI ₂ Distance between wavelength λI ₁ and wavelength λI ₂ between electrode fingers (14 in FIG. 1): 0.25λI ₁ +0.25 λI ₂ Wavelength λI ₂ The gap between electrode fingers (2 in Fig. 1)
5): 0.50 λI ₂ IDT-reflector interval: 0.460 λR IDT duty: non-narrowed portion: 0.63 narrowed portion: 0.60 reflector duty: 0.57 electrode film thickness: 0.09 λR The above-mentioned "spacing" means that the two electrode fingers adjacent to each other are
It is the distance between the centers in the width direction.

The surface acoustic wave filter according to the first embodiment is characterized in that the electrode fingers adjacent to the electrode finger connected to the ground and connected to the balanced signal terminal 11 are located at both ends of the central IDT 4, respectively. 15 is separated and the electrode finger 15a
And the electrode finger 15b is formed, and the electrode finger 15a is connected to the balanced signal terminal 11, while the electrode finger 15b is connected to the ground.

The operation and effect of such a surface acoustic wave filter will be described below. First, FIG. 3 shows a graph of amplitude balance between balanced signal terminals with respect to frequency in the configuration of the first embodiment. 4 shows a graph of the separation ratio in the cross width direction and the phase balance degree. As a comparison,
Also shown in FIG. 3 is the amplitude balance degree in the configuration of the conventional example in which the electrode fingers at both ends adjacent to the IDTs 52 and 53 in the IDT 51 shown in FIG. 1 are not separated.
The frequency range of the pass band in the DCS-Rx (for reception) filter is 1805 MHz to 1880 MHz.

The maximum deviation in the amplitude balance degree in this pass band range is 3.2 dB in the conventional example, whereas
In the first embodiment, it is 2.0 dB. Therefore, in the first embodiment, the amplitude balance degree is improved by 1.2 dB as compared with the conventional example.

In the first embodiment, this is the IDT4
This is because the difference in the surrounding environment of the electrode fingers 15 respectively located at both ends of is reduced compared with the conventional example. That is, each ID
The electrode fingers 15 at both ends of the IDT 4, which are adjacent to the electrode fingers connected to the ground of T5 and T6, are separated into two along the longitudinal direction of the electrode finger 15 on the surface acoustic wave propagation path and separated. Each electrode finger 15b that has been applied to each IDT 5 on both sides,
It is connected to the ground via 6. As a result, the electrode fingers 1 at both ends of the IDT 4 connected to the balanced signal terminal 11 are connected.
5a will face the electrode fingers 5c and 6b of the IDTs 5 and 6, respectively, while the electrode fingers 4b at both ends of the IDT 4 connected to the balanced signal terminal 12 will be the electrode fingers 15b connected to the IDTs 5 and 6, respectively. Will face each other.
Therefore, since the electrode fingers 15a and the electrode fingers 4b can have the same polarity relationship (ambient environment) with the electrode fingers 15a and the electrode fingers adjacent to the electrode fingers 4b, the respective balanced signal terminals 11 , 12 will be improved.

Next, the result of investigation of the optimum value in the cross width direction at the separation positions of the electrode fingers 15 at both ends will be described. The method of investigation is as shown in FIGS. 1 and 2, in which the crossing width 4b of the part to be separated is 4
The ratio of "a" (hereinafter, the separation ratio in the cross width direction) was gradually changed, and the change in the amplitude balance degree in the pass band was examined.

The ratio of separation in the cross width direction is not separated.
The case of 1 was set to 1, the case of FIG. 1 was set to 1/2, and the amplitude balance degree was investigated for the cases where the separation ratios in the cross width direction were 1/4, 1/2, and 3/4. FIG. 4 shows the value of the amplitude balance degree when the separation ratio in the cross width direction is changed. From FIG. 4, when the separation ratio in the cross width direction is set to about 0.5 (1/2), that is, at the position substantially in the center in the cross width direction, the electrode finger 1 is
When 5 was separated, the degree of amplitude balance in the pass band became the minimum.

This is because the balance between the balanced signal terminals 11 and 12 is improved. That is, the electrode fingers 15 at both ends of the central IDT 4 are separated at a substantially central position in the cross width direction, and the separated electrode fingers 15b are connected to the ground, so that the balanced signal terminal 11 is connected.
Electrode fingers 5a and 6b, which are connected to the ground, to the electrode fingers 15a on both ends of the IDT 4 and the electrode fingers 4b on both ends of the IDT 4, which are connected to the balanced signal terminal 12.
b are adjacent to each other at about half of the entire cross width. For this reason, the polarities of the adjacent electrode fingers are the same, including the weight of the crossing, so that the degree of balance between the balanced signal terminals 11 and 12 is improved.

As described above, in the first embodiment, in the 3IDT type longitudinally coupled resonator type surface acoustic wave filter for extracting a balanced signal from both poles of the central IDT 4 having an odd total electrode index, the central electrode The electrode fingers 15 at both ends of the IDT 4 are separated at a substantially central position in the cross width direction of the propagation path of the surface acoustic wave, and the separated electrode fingers 15b are connected to the ground, so that the amplitude balance degree is higher than that of the conventional example. Improved, and
A 3IDT type longitudinally coupled resonator type surface acoustic wave filter having a balanced-unbalanced conversion function can be obtained.

In the first embodiment, two SAW resonators 2 and 3 are connected in series to obtain a balanced signal from the IDT 4 at the center of a 3IDT longitudinally coupled resonator type SAW filter. However, the present invention is not limited to the above-described configuration, and similar effects can be obtained in a surface acoustic wave filter having any configuration having balanced signal terminals.

For example, as shown in FIG. 5, two IDTs 2
In the case of a longitudinally coupled resonator type surface acoustic wave filter having 4, 25, the IDT 24 connected to the balanced signal terminal 11.
Since the electrode fingers 24a on both ends of the IDT 25 are adjacent to the electrode finger 25a connected to the outermost ground of the IDT 25 and the reflector 7a connected to the ground, the electrode fingers on both ends of the IDT 24 connected to the balanced signal terminal 11 are connected. 24a is separated on the surface acoustic wave propagation path, and the separated electrode finger 24b is ID
By connecting to the ground via T25 and the reflector 7a, the amplitude balance can be improved more than before.

In the first embodiment, the balanced signal terminal 11
The electrode fingers 15 on both ends of the IDT 4 connected to the above are described as being adjacent to the electrode fingers 5c and the like connected to the ground on both sides, but, for example, as shown in FIG. 6, the electrode fingers 34a and 34b on both ends of the IDT 34 are connected. However, the electrode finger 35a connected to the ground on one side and the signal line 1 on the other side.
Also in the surface acoustic wave filter configured to be adjacent to the electrode finger 36a connected to the electrode 8, the electrode finger 34a on the side adjacent to the electrode finger 35a connected to the ground.
By separating only the electrode on the propagation path of the surface acoustic wave and connecting the separated electrode finger 34c to the ground via the IDT 35, the difference in the surrounding environment can be reduced and the amplitude balance can be improved as compared with the conventional example.

In the first embodiment, one ID
Although the present invention has been described with the configuration in which the balanced signal is taken out from both poles of T4, for example, as shown in FIG.
Also in the configuration in which the balanced signal is taken out from both electrodes of T37 and T38, the outermost electrode fingers 37a and 38a of the two IDTs 37 and 38 are separated on the propagation path of the surface acoustic wave, and the separated electrode fingers 37b and 38b are separated. By connecting to the ground via the IDT 39 and the reflectors 7a and 8a, it is possible to reduce the difference in the surrounding environment and improve the amplitude balance degree as compared with the conventional example.

Furthermore, in the first embodiment of the present invention, the present invention has been described with the surface acoustic wave filter having a 3 IDT longitudinally coupled resonator type structure. For example, as shown in FIG. Also in the case of a longitudinally coupled resonator type surface acoustic wave filter having 45, the IDT 4 for extracting a balanced signal
By separating the electrode fingers 42a, 44a on both ends of 2, 44 on the surface acoustic wave propagation path, and connecting the separated electrode fingers 42b, 44b to the ground via the ground side bus bar portion of the IDTs 41, 43, 45. , The amplitude balance can be improved as compared with the conventional one.

As described above, according to the present invention, in a longitudinally coupled resonator type surface acoustic wave filter having a balanced signal terminal, at least one outermost electrode finger of at least one IDT from which a balanced signal is taken out is elastic surface. By separating on the wave propagation path and connecting the separated electrode fingers to the ground, a surface acoustic wave filter with improved amplitude balance between the balanced signal terminals can be realized.

Next, a second embodiment according to the present invention will be described with reference to FIG. In the second embodiment of the present invention,
In the surface acoustic wave filter according to the first embodiment, the signal line 28 connected to the balanced signal terminal 11 and the signal line 18 connected to the unbalanced signal terminal 13 are adjacent to each other while grounding. The line 22 is inserted on the piezoelectric substrate (not shown). In the second embodiment of the present invention, the bridging capacitance generated between the signal lines 18 and 28 can be reduced by the ground line 22.

Next, the operation and effect of the second embodiment will be described. First, each signal line 18, 2
When the ground line 22 is inserted between the eight, the bridging capacitance between the signal lines 18 and 28 is
It is reduced by 2. If a bridging capacitance having a certain phase angle exists between adjacent signal lines, the influence of the bridging capacitance is completely opposite in the transmission characteristics from the balanced signal terminal 11 and the balanced signal terminal 12. become. As a result, the degree of balance between the balanced signal terminals 11 and 12 deteriorates according to the magnitude of the bridging capacitance.

Therefore, in the present invention, in addition to separating the electrode fingers 15 at both ends of the IDT 4 connected to the balanced signal terminal 11 on the surface acoustic wave propagation path and connecting the separated electrode fingers 15b to the ground. And each signal line 1
By inserting the ground line 22 between the adjacent 8 and 28, the degree of balance between the balanced signal terminals 11 and 12 can be further improved.

Next, the communication device according to the present invention will be described with reference to FIG. As shown in FIG. 10, the communication device 10
0 is an antenna 101, an antenna common part / RFTop filter 10 as a receiver side (Rx side) for receiving.
2, amplifier 103, Rx interstage filter 104, mixer 1
05, 1st IF filter 106, mixer 107, 2n
dIF filter 108, 1st + 2nd local synthesizer 111, TCXO (temperature compensated cr
ystal oscillator (temperature-compensated crystal oscillator) 112,
It is configured to include a divider 113 and a local filter 114.

Rx interstage filter 104 to mixer 105
It is preferable to transmit each balanced signal in order to secure balance, as shown by the double line in FIG.

Further, the communication device 100 uses the antenna 101 as the transceiver side (Tx side) for transmission.
And the antenna common part / RFTop filter 102
And TxIF filter 121, mixer 122, Tx interstage filter 123, amplifier 124, coupler 125, isolator 126, APC (automatic power).
er control (automatic output control) 127.

Then, the above Rx interstage filter 104,
1stIF filter 106, TxIF filter 121,
For the Tx interstage filter 123, the surface acoustic wave filter described in the first and second embodiments of the present embodiment described above can be preferably used.

The surface acoustic wave filter according to the present invention has not only a filter function but also an unbalanced-balanced conversion function.
It has excellent characteristics that the amplitude characteristics and phase characteristics between each balanced signal are closer to the ideal.

Therefore, the communication device of the present invention having the surface acoustic wave filter can be downsized as the frequency band used increases by using the surface acoustic wave filter, and the communication signal between the balanced signal and the unbalanced signal can be reduced. With the excellent conversion function of, the transmission characteristics can be improved.

[0067]

As described above, the surface acoustic wave filter of the present invention is provided with the adjacent electrode fingers on the piezoelectric substrate.
A comb-shaped electrode section with two comb-shaped electrodes
Along the propagation direction, having a plurality, adjacent to each other, the above
A small number of input and output connected to the comb electrode
Surface acoustic wave filter with at least one balanced signal terminal
At one of the balanced signal terminals connected to
The outermost electrode fingers of the tooth-shaped electrode are adjacent to each other.
Connected to the other terminal of the balanced signal terminals.
The outermost electrode fingers of the comb-shaped electrodes that are connected next to each other
One of the electrode fingers is connected to the signal side and the other is connected to the ground side
And the other of the balanced signal terminals is connected.
The outermost electrode finger of the toothed electrode is on the surface acoustic wave propagation path.
By being separated, one and the other of the balanced signal terminals
The outermost electrode finger of the comb-shaped electrode connected to the
No. connected to the electrode side and the ground side.
The adjacent electrode fingers are adjacent to each other .

Therefore, in the above configuration, since at least one electrode finger of at least one IDT connected to the balanced signal terminal is separated on the propagation path of the surface acoustic wave, the separated electrode fingers are separated. One can be connected to the ground line and the other to the signal line.

[0069] Thus, in the above arrangement, as in the prior art, one is the signal line of the electrode fingers at both ends of the IDT connected to the balanced signal terminal and the other facing pair to the earth line, between the balanced signal terminals It is possible to avoid the deterioration of the equilibrium degree by the separation, and it is possible to improve the equilibrium degree.

As described above, the communication device of the present invention has the above-mentioned surface acoustic wave filter.

Therefore, the above configuration has a filter function and
Since the surface acoustic wave filter provided with the excellent conversion function between the balanced signal and the unbalanced signal is provided, it is possible to improve the transmission characteristics (communication characteristics) by the excellent conversion function.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a surface acoustic wave filter according to a first embodiment of the present invention.

FIG. 2 is an enlarged view of a main part of the surface acoustic wave filter.

FIG. 3 is a graph showing a difference in amplitude balance between the first embodiment and the conventional example.

FIG. 4 is a graph showing a change in the degree of amplitude balance with respect to the separation ratio in the cross width direction in the first embodiment.

FIG. 5 is a schematic configuration diagram showing a modification of the first embodiment.

FIG. 6 is a schematic configuration diagram showing another modification of the first embodiment.

FIG. 7 is a schematic configuration diagram showing still another modified example of the first embodiment.

FIG. 8 is a schematic configuration diagram showing still another modified example of the first embodiment.

FIG. 9 is a schematic configuration diagram of a surface acoustic wave filter according to a second embodiment of the present invention.

FIG. 10 is a block diagram of a communication device according to the present invention.

FIG. 11 is a schematic configuration diagram of a conventional surface acoustic wave filter.

[Explanation of symbols]

4 IDT (comb type electrode part) 5 IDT (comb type electrode part) 6 IDT (comb type electrode part) 11 balanced signal terminals 12 balanced signal terminals 15 electrode fingers 15a electrode finger 15b electrode finger 20 Piezoelectric body

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-55-5573 (JP, A) JP 2002-528987 (JP, A) US Patent 5835990 (US, A) International Publication 00/25423 (WO, A1) ) P. J. Edmonson, CK Campbell, Effect of Strray Coupling on the Balance of a Differential LSAW Front-End Resonator UR-ESR E, E-Eur, EF, 1999-1999. 1, p. 361-364 M.I. Koshino, K .; Kanas aki, N.M. AKAHORI, M .; Kawase, R .; Chujo and Y. Ebata, A Wide-band Balanced SAW Filter with longitudinal multi-mode Resonator, Proceedings of 2000 IEEE ULTRAS ONICS SYMPOSIUM, 2000. 1, p. 387-390 (58) Fields investigated (Int.Cl. ⁷ , DB name) H03H 9/145 H03H 9/64

Claims (11)

(57) [Claims] 1. A plurality of comb-shaped electrode portions having two comb-teeth-shaped electrodes each having adjacent electrode fingers are arranged adjacent to each other along a propagation direction of a surface acoustic wave on a piezoelectric substrate. In a surface acoustic wave filter having at least one balanced signal terminal for input and output, which is connected to the comb-shaped electrode portion, an outermost electrode finger of a comb-shaped electrode to which one of the balanced signal terminals is connected. The electrode fingers adjacent to each other are both connected to a signal terminal, and the other one of the balanced signal terminals is connected, among the electrode fingers adjacent to each other of the outermost electrode fingers of the comb-shaped electrode, one is on the signal side. and the other is connected to the ground side, the other of the balanced signal terminal is connected, the outermost electrode fingers of the interdigital electrodes, separated by the propagation path of the surface acoustic wave Tei
By doing so, one and the other of the balanced signal terminals are connected.
Also, the outermost electrode fingers of the comb-shaped electrode are both connected to the signal side.
That is connected to the grounded electrode finger.
A surface acoustic wave filter characterized by being adjacent to each other . 2. The surface acoustic wave filter according to claim 1, wherein the separated electrode fingers are connected to the ground. 3. The surface acoustic wave filter according to claim 2, wherein the separated electrode fingers are grounded by being connected to adjacent comb-shaped electrode portions or grounded reflectors, and are also separated from each other. The surface acoustic wave filter, wherein electrode fingers adjacent to each other are connected to a comb-shaped electrode portion or a reflector to which the separated electrode fingers are connected. 4. The surface acoustic wave filter according to any one of claims 1 to 3, the surface acoustic wave filter, wherein the balanced signal terminals are connected to the poles of at least one interdigital transducer . 5. A surface acoustic wave filter according to any one of claims 1 to 4, the surface acoustic wave filter characterized by having a 3 interdigital electrode portion. 6. The surface acoustic wave filter according to claim 5 , wherein the balanced signal terminal is connected to the central comb-shaped electrode portion. 7. The surface acoustic wave filter according to any one of claims 1 to 6, the electrode fingers are separated, the surface acoustic wave filter characterized in that it is separated at substantially the center of the cross width direction . 8. A surface acoustic wave filter according to any one of claims 1 to 7, in series, and in parallel at least one surface acoustic wave resonator is connected at least one, the interdigital transducer A surface acoustic wave filter characterized in that 9. The surface acoustic wave filter according to any one of claims 1 to 8, the balanced - SAW filter, characterized in that it has unbalanced conversion function. 10. A surface acoustic wave filter according to any one of claims 1 to 9, characterized respectively connected to the interdigital electrode portion, between the signal lines adjacent to each other, that the earth line is inserted And a surface acoustic wave filter. 11. A communication apparatus characterized by having a surface acoustic wave filter according to any one of claims 1 to 10.