JPH06276048A - Surface acoustic wave filter device - Google Patents

Abstract

PURPOSE:To provide the surface acoustic wave filter which is superior in both out-band attenuation characteristics and insertion loss. CONSTITUTION:Plural filter stages are formed on a piezoelectric substrate 1 and reflectors 6, 7, 8, and 9 are arranged adjacently to at least one transducer side between transmission-side transducers 2 and 4 and reception-side transducers 3 and 5 of the respective filter stages. Those reflectors 6-9 reflect surface acoustic waves, propagated in the opposite directions from the reception-side transducers 3 and 5 among surface acoustic waves excited by the transmission- side transducers 2 and 4, to the transmission-side transducers 2 and 4. Further, the surface acoustic waves passed through the reception-side transducers 3 and 5 are reflected toward the reception-side transducers 3 and 5.

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 device, and more particularly to a surface acoustic wave filter device with improved out-of-band attenuation characteristics and insertion loss.

[0002]

2. Description of the Related Art A surface acoustic wave filter device for extracting a signal in a specific frequency band by forming an interdigital transmitter and receiver on a piezoelectric substrate has been put into practical use. In this surface acoustic wave filter device, unidirectional transducers are used as transmitter and receiver converters to minimize insertion loss. A surface acoustic wave filter device using a unidirectional transducer has a relatively small insertion loss and is highly useful because the phase characteristic and the frequency characteristic can be controlled appropriately.
However, the side rope characteristic is insufficient, and it is strongly demanded to further improve the out-of-band attenuation characteristic.

As a method of improving the attenuation characteristic outside the band,
A method is conceivable in which several filter stages are formed on a piezoelectric substrate and these filter stages are connected in series. For example, 2
If a single surface acoustic wave filter device is configured by connecting two filter stages in series, the obtained attenuation characteristic is a characteristic obtained by multiplying the attenuation characteristics of the two filter stages, so that the out-of-band attenuation characteristic is significantly increased. Can be improved.

[0004]

When a plurality of filter stages are connected in series, the insertion loss becomes a characteristic obtained by multiplying the characteristics of the two filter stages, and the insertion loss is larger than that of the filter device using one filter stage. This causes a problem that the loss is slightly increased. That is, the insertion loss of the first filter stage is 4
When the insertion loss of the second filter stage is 4 dB in dB, the insertion loss of 8 dB occurs in series connection. Therefore, if a means for improving the insertion loss is coupled to the surface acoustic wave filter device in which a plurality of filter stages are connected in series,
It is possible to realize a surface acoustic wave filter device having an improved out-of-band attenuation characteristic and a good insertion loss. Therefore, it is an object of the present invention to provide a surface acoustic wave filter device having an improved out-of-band attenuation characteristic and a good loss characteristic.

[0005]

A surface acoustic wave filter device according to the present invention comprises a piezoelectric substrate and N (N is an integer of 2 or more) filters formed on the piezoelectric substrate and connected in series. Each filter stage having a transmitter-side converter, a receiver-side converter, and a reflector arranged adjacent to at least one of the transmitter-side converter and the receiver-side converter. The transmitter-side converter and the receiver-side converter are arranged between the interdigital positive electrode and the negative electrode and between the positive electrode and the negative electrode, and each electrode finger is an electrode finger of the adjacent positive electrode. A floating electrode arranged at a position deviated from an intermediate position between the negative electrode and the electrode finger,
A surface acoustic wave excited from the transmitter-side converter of each filter stage in a direction opposite to the corresponding receiver-side converter is reflected by the reflector toward the transmitter-side converter, and is excited by the transmitter-side converter. The surface acoustic wave that has passed through the corresponding receiving side transducer is reflected by the reflector toward the receiving side transducer.

[0006]

When a unidirectional transducer is used as the transmitter and receiver transducers, most of the surface acoustic wave excited by the transmitter propagates toward the receiver transducer.
Some surface acoustic waves propagate in the direction opposite to the receiving transducer. Also, most of the surface acoustic waves that have been incident on the receiving side transducer excited by the transmitting side transducer are converted to electrical signals by the receiving side transducer, but some surface acoustic waves are converted by the receiving side transducer. It passes without being passed. Therefore, if these surface acoustic waves can be effectively utilized, the conversion efficiency of the transmitter and receiver converters can be further improved and the insertion loss can be reduced. Based on such recognition, in the present invention,
A reflector is arranged adjacent to at least one of the transmitter-side converter and the receiver-side converter of each filter stage, and these surface acoustic waves are incident on the transmitter-side converter and the receiver-side converter, respectively. To reflect.

By using these reflectors, the insertion loss of each filter stage is further reduced, so that the insertion loss of the filter device as a whole is further reduced. As a result, it is possible to realize a surface acoustic wave filter device having further improved out-of-band attenuation characteristics and excellent loss characteristics. In particular,
When a quartz substrate is used as the piezoelectric substrate, the quartz substrate has good temperature characteristics (characteristics in which the band variation with respect to temperature change is small), but has a drawback that the electromechanical coupling coefficient is small. Therefore, if a quartz substrate is used as the piezoelectric substrate,
A surface acoustic wave filter device having excellent temperature characteristics and further improved out-of-band attenuation characteristics and loss characteristics can be realized.

[0008]

FIG. 1 is a schematic plan view showing the overall structure of a surface acoustic wave filter device according to the present invention, and FIG. 2 is a schematic plan view showing the detailed structures of a transmitter-side converter and a reflector. Yes,
FIG. 3 is a schematic plan view showing a detailed configuration of the receiving side converter and the reflector. In this example, the quartz substrate 1 is used as the piezoelectric substrate.
To use. The ST-cut crystal has a temperature coefficient (TCF) of 1.6pp with respect to frequency in the temperature range of -20 ℃ to 80 ℃.
It is extremely small at m / ° C. By the way, since the TCF of 128 ° rotation Y-cut × direction propagation LiNbO ₃ is -74ppm / ° C, the quartz substrate has extremely good temperature characteristics, and the fluctuation of the pass frequency band due to temperature change is maintained within an extremely minute range. can do. A first transmitter-side converter 2, a first receiver-side converter 3, a second transmitter-side converter 4, and a second receiver-side converter 5 are formed on a crystal substrate 1. The first transmitter-side converter 2 and the first receiver-side converter 3 constitute a first filter stage, and the second transmitter-side converter 4 and the second receiver-side converter 5 form a second filter. Make up the steps. First and second reflectors 6 and 7 are respectively arranged adjacent to the first transmitter-side converter 2 and the first receiver-side converter 3, and the second transmitter-side converter 4 and the second receiver-side converter 3 are arranged. Adjacent to the side converter 5 are formed third and fourth reflectors 8 and 9, respectively. The first and third reflectors 6 and 8 are the first and second transmitter side converters 2
, And 4 and the surface acoustic waves that are propagated in the opposite direction to the corresponding receiving side transducers 3 and 5 are reflected toward the corresponding transmitting side transducers 2 and 4, respectively. Most of the surface acoustic waves excited by the first and second transmitter-side converters 2 and 4 propagate toward the transmitter-side converter, respectively.
Part of the excited surface acoustic wave propagates in the direction opposite to the receiving side transducer. By reflecting the surface acoustic waves propagating in the opposite directions toward the transmitting side transducer by the reflectors 6 and 8, the excited surface acoustic waves can be effectively used to improve the insertion loss. Further, most of the surface acoustic waves incident on the first and second receiving side converters 3 and 5 are converted into electric signals by these converters,
Some surface acoustic waves pass through these transducers. Therefore, the second and fourth converters 7 and 9 are connected to the first and second converters.
Are arranged so as to be adjacent to the transmitting side transducers 3 and 5, respectively, and the surface acoustic waves that have not been converted are reflected toward the receiving side transducers 3 and 5, respectively. In this example, the reflectors are provided in both the transmitter-side converter and the receiver-side converter of the first and second filter stages. However, even if a reflector is provided adjacent to at least one converter, the insertion loss is reduced. It can be further improved.

FIG. 2 is a plan view showing a detailed structure of the first transmitting side converter 2 and the first reflector 6 adjacent thereto. The first transmitting-side converter 2 has an interdigital positive electrode 10 and a negative electrode 11, and short-circuit type floating electrodes 12 and 13 formed between the positive electrode and the negative electrode. still,
In order to make the drawing clear, the number of pairs of each electrode is 2 in FIG.
Although shown as a pair, various logarithms can be set according to the pass band width, and for example, in the case of a narrow band filter for digital communication, it can be set to 200 to 400 pairs. The pitch between the electrode fingers 10a and 10b of the positive electrode 10 and the pitch between the electrode fingers 11a and 11b of the negative electrode 11 are both set to be equal to the wavelength λ of the fundamental surface acoustic wave. The wavelength lambda of the fundamental surface acoustic wave, v is the average propagation velocity of the surface acoustic wave in the quartz substrate, when the center frequency f _O, is set to be λ = v / f _O. Further, the center-to-center distance between the electrode finger of the positive electrode 10 and the electrode finger of the negative electrode 11 is set to λ / 2. The floating electrodes 12 and 13 have paired electrode fingers 12a, 12b and 13a, 13b, respectively, and the pitch between these electrode fingers is set to λ / 2. The width in the propagation direction of the surface acoustic wave of each electrode finger of the positive electrode, the negative electrode and the floating electrode is set to λ / 12. Floating electrode 12
One electrode finger 12a is the electrode finger of the positive electrode 10 and 10a and λ /
6 and the electrode finger 11a of the negative electrode 11 are adjacent to each other with a center distance of λ / 3, and the other electrode finger 12b is adjacent to the electrode finger 11a of the negative electrode with a center distance of λ / 6. In addition, the positive electrode is adjacent to the electrode finger 10b with a center distance of λ / 3. In addition, the electrode finger 13a of the floating electrode 13
Similarly, the electrode fingers of the floating electrode 12 are also adjacent to the electrode fingers of the positive electrode and the negative electrode with a center distance of λ / 6 and λ / 3. Therefore, the electrode finger of the positive electrode, the electrode finger of the floating electrode, and the electrode finger of the negative electrode are λ along the propagation direction of the surface acoustic wave.
They are sequentially formed with the center distances of / 6 and λ / 3. With this configuration, each electrode finger 12 of the floating electrode
a, 12b, 13a, 13b are the distance of λ / 12 from the midpoint between the positive electrode electrode and the negative electrode electrode finger where these electrode fingers are adjacent to each other, in the direction opposite to the surface acoustic wave propagation direction. As a result, the mechanical reflection characteristics of the floating electrode due to the asymmetric structure can be more effectively utilized, and most of the excited surface acoustic wave propagates to the right side of FIG. 1, that is, the receiving side transducer. Can be made. As a result, the unidirectionality of the transducer is further enhanced and the insertion loss can be reduced. In the quartz substrate, the amount of deviation from the intermediate point between the positive electrode electrode finger and the negative electrode electrode finger of the floating electrode is extremely important for enhancing the unidirectionality. As a result of various studies conducted by the present inventor on this deviation amount, it is too small if the electrode fingers of the floating electrode are located at the midpoint between the positive electrode and the negative electrode. It was found that the loss characteristics could not be obtained. Therefore, the displacement amount of the floating electrode must be set so that the edge of the floating electrode on the surface acoustic wave propagation direction side is located closer to the front side than the intermediate position in the surface acoustic wave propagation direction. . Furthermore, from various examination results,
When the center position in the propagation direction of the surface acoustic wave of the electrode finger of the floating electrode is separated from the intermediate position between the positive electrode and the negative electrode by about the width of the electrode finger, the reflected wave by the floating electrode and the positive electrode and the negative electrode The phases of the surface acoustic wave excited by and the surface acoustic waves were matched with each other, and the optimum loss characteristics and phase characteristics were obtained.

The first reflector 6 is arranged so as to be separated from the first transmitter-side converter 2 by a distance of approximately an integral multiple of λ / 2. The first reflector 6 has electrode fingers 6a, 6b, 6c,
It has 6d, 6e and 6f. In this example, these electrode fingers 6a
The width of the surface acoustic wave of ˜6f in the propagation direction is set to λ / 4. The electrode fingers 6a to 6f have an arrangement pitch equal to the arrangement pitch of the electrode fingers of the positive electrode and the negative electrode of the first transmitter transducer 2, that is, λ / 2 along the surface acoustic wave propagation direction.
Sequentially formed with the distance between the centers of. Since the reflector 6 has a high reflection characteristic with respect to the surface acoustic wave excited by the first transmitting-side converter 2 and propagating in the direction of arrow b opposite to the propagation direction of the surface acoustic wave, the incident elastic force Most of the surface waves can be reflected toward the first transmitter side converter 2.

FIG. 3 is a schematic plan view showing a detailed structure of the first receiving side converter and the second reflector. The receiving side converter 3 includes an interdigital positive electrode 20 and a negative electrode 21.
And short-circuited floating electrodes 22 and 23 formed between the positive electrode and the negative electrode. Similar to the transmitter transducer, the pitch between the electrode fingers 20a and 20b of the positive electrode 20 and the pitch between the electrode fingers 21a and 21b of the negative electrode 21 are both equal to the wavelength λ of the surface acoustic wave. Set to. Further, the center-to-center distance between the electrode finger of the positive electrode 20 and the electrode finger of the negative electrode 21 is set to λ / 2. The floating electrodes 22 and 23 each have a pair of electrode fingers 22a, 22b and 23a, 23b,
The pitch between these electrode fingers is set to λ / 2.
The width of the electrode fingers of the positive electrode, negative electrode, and floating electrode is λ /
Set to 12. Each electrode finger of the floating electrodes 22 and 23 is deviated in the main propagation direction of the surface acoustic wave by a distance of λ / 12 from an intermediate position between the electrode finger of the positive electrode and the electrode finger of the negative electrode to which the electrode fingers are adjacent. Rank The second reflector 7 is arranged so as to be separated from the receiving side converter 3 by an approximately integral multiple of λ / 2. The second reflector 7 has electrode fingers 7a to 7f having a width λ / 4, and these electrode fingers have an array pitch equal to the array pitch of the electrode fingers of the output side transducer 4, that is, the propagation direction of the surface acoustic wave. Are sequentially formed with a distance of λ / 2. As described above, the second reflector 7 has a high reflection characteristic with respect to the surface acoustic wave incident along the propagation direction of the surface acoustic wave indicated by the arrow a, similarly to the first reflector, so that the output Surface acoustic waves that have passed through without being picked up by the side converter 7 can be strongly reflected toward the first receiving side converter 3. The number of electrode fingers of each reflector can be appropriately set based on the characteristics of the converter and the substrate. For example, when the number of pairs of converters is large, the number of electrode fingers of the reflector can be reduced to If the number of pairs of reflectors is small, it is preferable to increase the number of electrode fingers of the reflector.

In this example, the second filter stage has the same configuration as the first filter stage. Therefore, the second transmitter-side converter 4 and the second receiver-side converter 5 of the second filter stage have the same structure as the first transmitter-side converter 2 and the first receiver-side converter 3 of the first filter stage, respectively. , The third and fourth reflectors 8 and 9 of the second filter stage have the same structure as the reflectors 6 and 7 of the first filter stage. As shown in FIG. 1, the positive electrode 11 of the transmitting side converter 2 of the first filter stage is connected to the signal input terminal 5
0 and the negative electrode 10 is grounded. The negative electrode 20 of the receiving side converter 3 of the first filter stage is grounded, and the positive electrode 21 is the positive electrode 31 of the transmitting side converter 4 of the second filter stage via the conductor pattern 51 formed on the crystal substrate 1. Connect to. The negative electrode 30 of this transmitter 4 is grounded. Further, the negative electrode 40 of the receiving side converter 5 of the second filter stage is grounded and the positive electrode 41 is connected to the signal output terminal 52. It is preferable to combine a predetermined inductance with the conductor pattern 51 for impedance matching. Further, in this example, the first
Guard electrodes 53 and 54 are formed to prevent electromagnetic coupling between the second filter stage and the second filter stage. These guard electrodes 53 and 54 extend between the first-stage converter and the second-stage converter, and also between the transmitting-side converter and the receiving-side converter, respectively. And is formed along the conductor pattern 51.

The electrical signal to be filtered is the signal input terminal 5
It is supplied to the first transmitting side converter through 0 and the ground terminal. The surface acoustic wave excited by the first transmitting-side converter 2 propagates in the direction of arrow a, is picked up by the first receiving-side converter 3, and is converted into an electric signal. This electric signal is supplied to the positive electrode 31 of the second transmitting-side converter 4, the elastic wave excited by the second transmitting-side converter 4 propagates in the direction of arrow c, and the second receiving-side converter is generated. It is picked up by 5 and converted into an electric signal and taken out from the signal output terminal 52. Therefore, the signal to be filtered will be filtered twice by the two serially connected filter stages.

Next, the insertion loss of the transmitter and receiver converters
Will be described. In a comparative experiment, shown in FIGS.
Only the first filter stage of the surface acoustic wave filter device is required.
And the configuration without the reflector from the first filter stage
And a surface acoustic wave filter device having a known structure shown in FIG.
Was used. The transducer of FIG. 4 is the same as the transducer of FIG.
Floating with the electrode fingers 11a and 11b of the negative electrode in the transducer
Open the electrode width of λ / 12 between the electrode fingers 12b and 13b.
Disposing the floating floating electrodes 60 and 61 respectively, the short-circuit floating
It is a mixture of both a floating electrode and an open floating electrode.
It For other matters, both set the same conditions
did. Both of these prototypes have a center frequency f _O= 150 MHz
The result of the characteristic evaluation test is shown in FIG. Figure
5, the horizontal axis represents frequency and the vertical axis represents attenuation.
As is apparent from FIG. 5, the elasticity table according to the present invention shown in FIG.
The insertion loss of the surface wave filter device is 5.0 dB.
The insertion loss of the mixed type filter device shown in Fig. 4 is 9.8.
dB, and it is possible to reduce the insertion loss by about 4.8 dB.
Came. From this experimental result, water with a small electromechanical coupling coefficient
In case of crystal substrate, mix short-circuit type floating electrode and open type floating electrode.
Only short-circuited floating electrodes were placed rather than the existing structure
It is clear that the structural form can further reduce insertion loss
Is. The reason for this is considered as follows. Crystal base
In the case of a plate, the sign of the reflection coefficient of the floating electrode is the short-circuit type floating
The electrode and the open type floating electrode are the same. Therefore,
Mix both short-circuited floating electrodes and open-type floating electrodes
And in the direction from the transmitter transducer to the receiver transducer
The reflected waves cancel each other out, which is a unidirectional problem.
As a result, the insertion loss increases. others
Therefore, in the present invention, in the case of using a quartz substrate, a short-circuit type floating electrode
Alternatively, only one of the open type floating electrodes is used.

Next, the electrode width of each electrode (width in the propagation direction of the surface acoustic wave of the electrode finger) will be described. Possible causes of the insertion loss are a loss due to the propagation of surface acoustic waves, a loss due to electric resistance at the electrodes, and a return loss due to mismatch of an electric circuit. Of these, the return loss can be improved by performing electrical matching in an electric circuit. Further, the loss due to the propagation of the surface acoustic wave is unique to the surface acoustic wave filter device. Therefore, the present inventor examined the relationship between the insertion loss and the electrical resistance of the electrode. The electric resistance of the electrodes is closely related to the electrode widths of the positive and negative electrodes, and it is expected that the smaller the electrode width, the larger the electric resistance and the larger the insertion loss. Further, regarding the relationship between the electrode width and the reflection efficiency and the excitation efficiency, it is expected that the reflection efficiency and the excitation efficiency will decrease if the electrode width is made thin. FIG. 6 is a graph showing the experimental results of the relationship between the electrode width and the insertion loss. In this experiment, the aperture length of the surface acoustic wave filter device having only the first filter stage of the surface acoustic wave filter device having the configuration shown in FIGS. (Lengths in which the electrode fingers of the positive electrode and the negative electrode overlap each other in the traveling direction of the surface acoustic wave) Two types of filter devices of 40λ and 100λ were prepared. And from 0.5 × λ / 12 to 1.3 × λ / 12 for a filter device with an aperture length of 100λ
A filter device in which the electrode width increases every 0.1 × λ / 12 was prototyped and the characteristics were evaluated. For a filter device with an aperture length of 40λ, from 0.6 × λ / 12 to 1.3 × λ / 12, 0.1 × λ /
A filter device in which the electrode width increases every 12 was prototyped and its characteristics were evaluated. In FIG. 6, the horizontal axis represents the electrode width (× λ / 1
2) and the vertical axis shows the amount of attenuation (dB). The solid line shows data for an aperture length of 40λ, and the broken line shows data for 100λ.
As can be seen from FIG. 6, an element with an aperture length of 40λ and 1
The insertion loss decreases as the electrode width increases for the 00λ element. Since the practical standard of insertion loss is 6 dB or less, the electrode width must be 0.8 × λ / 12 or more to satisfy this standard.

On the other hand, if the electrode width is increased, the GDT may increase and the waveform distortion in frequency characteristics may increase. Therefore, the relationship between the electrode width and GDT was examined. FIG. 7 shows the experimental results of the relationship between the electrode width (× λ / 12) and GDT (μsec). The horizontal axis represents the electrode width (× λ / 12), and the vertical axis represents the GDT (μsec). The GDT also increases as the electrode width increases. The practical user specification standard of GDT is 1.0 microsecond or less. Therefore, from the result of FIG. 7, in order to satisfy the practical standard of GDT, the electrode width is 1.3 × λ /
Must be set to 12 or less.

From the results of examination of these insertion loss and GDT, in the surface acoustic wave filter device in which the asymmetric unidirectional transducer is formed on the quartz substrate, the GDT increases as the electrode width increases, and the electrode width narrows. There is a characteristic that insertion loss deteriorates. From the results of this experiment, considering the user specification standard, the electrode width d is 0.8 ×
It is preferable to set so as to satisfy λ / 12 ≦ d ≦ 1.3 × λ / 12. Therefore, even when a plurality of filter stages are connected in series as in the present invention, it is preferable to set the electrode widths of the positive and negative electrodes and the floating electrode within the above range from the viewpoint of insertion loss and GDT.

FIG. 8 is a schematic plan view showing another modification of the surface acoustic wave filter device according to the present invention. The same members as those used in FIG. 1 will be described with the same reference numerals. In this example, the logarithm of the transmitter side converter and the logarithm of the receiver side converter are made different. In this way, by appropriately differentiating the logarithm of the transmitter side converter and the logarithm of the receiver side converter, for example, by setting the logarithm to 4: 1, it is possible to improve the overall structure as compared with the one-stage transversal type filter device. Logarithm as 1
Even if it is reduced to / 2, it is possible to realize a filter device having the same pass bandwidth. Further, in this example, one guard electrode 55 is used to generate undesired electromagnetic coupling between the first filter stage and the second filter stage and between the transmitter side converter and the receiver side converter. Prevent. In this example, the first
The receiving-side converter and the second transmitting-side converter can be interconnected via a bonding wire, a gold ribbon, or a conductor pattern formed on a package accommodating the substrate.

FIG. 9 shows the structure of a converter suitable when a LiNbO ₃ substrate is used as the piezoelectric substrate. In the case of a LiNbO ₃ substrate, the entire structure can be the structural form shown in FIG. 1 or FIG. 8 similarly to the crystal substrate. On the other hand, the LiNbO ₃ substrate is
Since the reflection coefficient of the open floating electrode has the opposite polarity to the reflection coefficient of the short floating electrode, unlike the quartz substrate, it is necessary to use both the open floating electrode and the short floating electrode as the floating electrode. Therefore, as shown in FIG. 9, the open type floating electrodes 70a, 70d are arranged with a center distance of λ / 6 so as to be adjacent to the electrode fingers of the positive electrode 11 and the electrode plate of the negative electrode 10. The converter is used.

10 to 13 are diagrams showing modified examples of the reflector. The width and arrangement pitch of the electrode fingers of the reflector are not limited to the above-mentioned examples, and various reflectors having a characteristic of reflecting an incident surface acoustic wave in a direction opposite to the incident direction can be used. The form can be used.

The present invention is not limited to the above-described embodiments, and various modifications and changes can be made. For example, in the above-mentioned embodiments, the quartz substrate is used as the piezoelectric substrate, but the present invention can be applied to a substrate material such as Li ₂ B ₄ O ₇ having an electromechanical coupling coefficient as small as that of quartz. it can.
Further, in the embodiment using the quartz substrate, only the short-circuit type floating electrode is used as the floating electrode, but in the present invention, since the reflector is provided to improve the efficiency, only the open type floating electrode is used as the floating electrode. An electrode structure can also be used.

Further, the present invention can be applied not only to a narrow band filter but also to a surface acoustic wave filter device having various bandwidths. For example, by appropriately setting the logarithm of the converter according to the pass bandwidth, for example, an image Wideband filter for circuits,
The present invention can be applied to a narrow band filter for mobile communication systems and further to a filter device for digital communication where phase characteristics are important.

Furthermore, the number of filter stages is not limited to two, but three.
It is also possible to adopt a form in which multiple stages are connected in series such as stages or four stages.

Further, in the above-mentioned embodiment, the width of the electrode finger of the reflector is set to λ / 4, but it may be set to a thickness other than λ / 4. However, when the thickness is other than λ / 4, the reflectance is slightly reduced, and therefore it is preferable to increase the number of electrode fingers.

[0025]

As described above, according to the present invention, a plurality of transmitting side converters and a plurality of receiving side converters are used to form a plurality of filter stages, and these filter stages are connected in series and each filter is connected. A reflector is arranged adjacent to the stage transducer, and the surface acoustic wave excited from the transmitter side transducer in the opposite direction to the receiver side transducer is reflected toward the transmitter side transducer, and the receiver side transducer is Since the transmitted surface acoustic wave is reflected toward the receiving side transducer, the out-of-band attenuation characteristic is further improved and the insertion loss is reduced. As a result, the surface acoustic wave excellent in the loss characteristic and the out-of-band attenuation characteristic is obtained. A filter device can be realized.

[Brief description of drawings]

FIG. 1 is a schematic plan view showing an example of a surface acoustic wave filter device according to the present invention.

FIG. 2 is a plan view showing a detailed configuration of a transmitter converter and a reflector.

FIG. 3 is a plan view showing a detailed configuration of a receiving side converter and a reflector.

FIG. 4 is a plan view showing a configuration of a surface acoustic wave filter device used in a comparative experiment.

FIG. 5 is a graph showing the results of comparative experiments on the insertion loss characteristics of converters.

FIG. 6 is a graph showing the relationship between the electrode width of the converter and the insertion loss.

FIG. 7 is a graph showing the relationship between the electrode width of the converter and GDT.

FIG. 8 is a schematic plan view showing a modified example of the surface acoustic wave filter device according to the present invention.

FIG. 9 is a diagram showing a configuration of a converter suitable for a LiNbO ₃ substrate.

FIG. 10 is a diagram showing a modification of the reflector.

FIG. 11 is a diagram showing a modification of the reflector.

FIG. 12 is a diagram showing a modification of the reflector.

FIG. 13 is a diagram showing a modification of the reflector.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Piezoelectric substrate 2 1st transmitting side converter 3 1st receiving side converter 4 2nd transmitting side converter 5 2nd receiving side converter 6,7,8,9 Reflector 10, 20, 30 , 40 Negative electrode (positive electrode) 11, 21, 31, 41 Positive electrode (negative electrode) 50 Signal input terminal 51 Conductor pattern 52 Signal output terminal 53, 54, 55 Guard electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasufumi Horio 2-74, Shirasuna-cho, Mizuho-ku, Nagoya, Aichi 202 Nabisinai Shirasuna 202 (72) Masatsugu Oshima 3-150, Omoteyama, Tenpaku-ku, Nagoya, Aichi

Claims (7)

[Claims] 1. A piezoelectric substrate, and N (N is an integer of 2 or more) filter stages formed on the piezoelectric substrate and connected in series with each other, each filter stage being a transmitter transducer. And a receiving side converter, and a reflector arranged so as to be adjacent to at least one of the transmitting side converter or the receiving side converter, and these transmitting side converter and receiving side converter are The interdigital positive electrode and the negative electrode are arranged between the positive electrode and the negative electrode, and each electrode finger is displaced from the intermediate position between the adjacent positive electrode electrode electrode and the negative electrode electrode finger. And a floating electrode disposed at a position where the transmitter side transducer of each filter stage excites a surface acoustic wave excited in a direction opposite to the corresponding receiver side transducer by the reflector. It is reflected toward the transducer and excited by the transmitter transducer A surface acoustic wave filter device, characterized in that the surface acoustic wave passing through the corresponding receiving side transducer is reflected by the reflector toward the receiving side transducer. 2. The surface acoustic wave filter according to claim 1, wherein a guard electrode is arranged between the filter stages so that adjacent transducers are not electromagnetically coupled to each other. apparatus. 3. A conductor pattern formed on a piezoelectric substrate to electrically connect the positive electrode of the receiving side converter of the preceding filter stage and the positive electrode of the transmitting side converter of the succeeding filter stage, and this conductor The surface acoustic wave filter device according to claim 1, wherein a guard electrode is formed along at least a part of the pattern. 4. The reflector according to claim 1, wherein the reflector has a plurality of electrode fingers formed at an arrangement pitch of λ / 2, where λ is a wavelength of the surface acoustic wave. The surface acoustic wave filter device according to claim 1. 5. The surface acoustic wave according to claim 1, wherein the number of electrode pairs of the transmitter transducer and the number of electrode pairs of the receiver transducer are different from each other. Filter device. 6. The piezoelectric substrate is made of crystal or a piezoelectric material having an electromechanical coupling coefficient similar to that of crystal, and each electrode finger of the positive electrode and the negative electrode of the transmitter-side and receiver-side converters comprises: When the wavelength of the fundamental surface acoustic wave is λ, they are alternately formed along the propagation direction of the surface acoustic wave with a center-to-center distance of λ / 2, and each electrode finger of the floating electrode of the transmitter-side converter is From the intermediate position between the electrode finger of the positive electrode and the electrode finger of the negative electrode adjacent to these electrode fingers to the front side in the propagation direction of the surface acoustic wave by λ / 12.
The electrode fingers of the floating electrode of the receiving-side transducer, which are spaced apart by a distance of, from the intermediate position between the electrode finger of the positive electrode and the electrode finger of the negative electrode adjacent to these electrode fingers are elastic surfaces. The surface acoustic wave filter device according to any one of claims 1 to 6, wherein the surface acoustic wave filter devices are located apart from each other in a wave propagation direction by a distance of λ / 12. 7. The width d in the propagation direction of the surface acoustic wave of each electrode finger of the positive electrode, the negative electrode and the floating electrode satisfies the formula: 0.8 × λ / 12 ≦ d ≦ 1.3 × λ / 12 The surface acoustic wave filter device according to claim 6, wherein the surface acoustic wave filter device is set as follows.