JP2003309451A - Surface acoustic wave filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ladder type surface acoustic wave filter with a higher squareness ratio than that of a conventional filter by approaching a resonance frequency to an anti-resonance frequency. <P>SOLUTION: The ladder type surface acoustic wave filter configured by connecting surface acoustic wave resonators in series and/or in parallel comprises: comb-line electrodes; and one reflector or more, each comb-line electrode is provided with three electrode fingers or over within a pitch pi, each reflector comprises a plurality of gratings with a pitch pr equal to a half of the wavelength of a surface acoustic wave propagated through the reflector. The surface acoustic wave filter is characterized in that a distance L between a center position of a width of the grating closest to the comb-line electrode in the propagation direction of the surface acoustic wave and a center position of the width of a single electrode finger in the propagation direction of the surface acoustic wave is a multiple of (n/2+5/16) of a wavelength λ of the surface acoustic wave (n is a 0 or a positive integer) wherein the electrode finger existing closest to the reflector among the electrode fingers of the comb- line electrodes is assumed to be the single electrode finger. <P>COPYRIGHT: (C)2004,JPO

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 ladder type surface acoustic wave filter.

[0002]

2. Description of the Related Art A surface acoustic wave filter and a resonance circuit using a surface acoustic wave resonator can be provided in a small size and at a low cost. Therefore, the surface acoustic wave resonator is an indispensable component for downsizing communication devices such as today's mobile phones.

FIG. 17 shows a block diagram of a conventional general surface acoustic wave resonator. The surface acoustic wave resonator has a comb-shaped electrode 2 made of an aluminum alloy or the like and having a period corresponding to a desired frequency on a piezoelectric substrate 1, and a reflection that reflects a surface acoustic wave excited by the comb-shaped electrode 2. It consists of vessels 3-1 and 3-2. The electrode period pi of the comb-shaped electrode 2 is obtained from the surface acoustic wave velocity vi at the comb-shaped electrode portion of the substrate and the desired frequency fi as pi = vi / fi.

The surface acoustic wave resonator shown in FIG. 17 is a one-terminal pair resonator. One of the ends of the comb-shaped electrode 2 is an input electrode 2-1 to which an input signal is applied, and the other is the other. It becomes the output electrode 2-2 for taking out the output signal. Reflectors 3-1, 3-2
Are generally formed by a grating having periodicity. The grating can be formed by forming a groove on the piezoelectric substrate, but generally an aluminum alloy grating that can be formed at the same time as the comb-shaped electrode is used.

The period pr of the grating is obtained by 2 × pr = vr / fr from the surface acoustic wave velocity vr at the reflector portion and the desired frequency fr as in the comb electrode. Generally, when fi = fr, it is considered that vi and vr are almost equal, and it is often designed with pi = 2 × pr. here,
Twice the grating period pr is sometimes called the reflector period. The reflector in this case is also called a "half cycle reflector".

Generally, the comb electrode 2 is formed of a single electrode having two electrode fingers in the electrode period pi.
Similarly, the reflector has the same grating period pr as pi.
Since there are two grating electrode fingers 3-3 in twice as many as, the electrode was formed of the same single electrode as the comb-shaped electrode 2.

Here, the single electrode means that one electrode finger of the comb-shaped electrode comes out from the end portion of the input electrode 2-1 and one electrode finger comes out from the end portion of the output electrode 2-2. It means that fingers are arranged alternately. That is, the output electrode 2-2 must be placed between the two adjacent electrode fingers protruding from the end of the input electrode 2-1.
One electrode finger protruding from the end of is arranged. Also,
Each of the electrode fingers arranged alternately is referred to as a single electrode finger.

FIG. 18 is a block diagram of a conventional two-terminal pair resonator composed of a plurality of comb-shaped electrodes. Where 2-3, 2-4
Is a ground terminal. FIG. 19 shows the simplest electrical equivalent circuit of a one-terminal pair surface acoustic wave resonator formed on a piezoelectric substrate 1 such as quartz or LiTaO3. The surface acoustic wave resonator of one terminal pair is used by being electrically connected in series or in parallel as shown in FIGS.

In FIG. 19, R1 is a resistance, C0 and C1 are capacitances, L1 is an inductance, and Ti is an input electrode 2-1.
, And To is a terminal of the output electrode 2-2. Where R
1, C1, and L1 are values determined by the material of the piezoelectric substrate, and C
0 is a value that changes depending on the number of pairs of comb electrodes.

In the case of series connection as shown in FIG. 20 (a), a surface acoustic wave resonator of one terminal pair is provided between the input Ti and the output To as in the equivalent circuit shown in FIG. 20 (b). Rs are arranged in series. In the case of parallel connection as shown in FIG. 21 (a), as in the equivalent circuit shown in FIG. 21 (b), a one-terminal pair surface acoustic wave resonator is provided between the input Ti and the output To and the ground G. R is placed.

FIG. 22 shows a general bandpass characteristic when the surface acoustic wave resonators of one terminal pair are connected in series.
Here, the horizontal axis is frequency [Hz] and the vertical axis is attenuation [d
B]. According to this, the amount of attenuation is maximum at a certain frequency, which is called the anti-resonance frequency fas.

FIG. 23 shows impedance characteristics when the surface acoustic wave resonators of one terminal pair are connected in series. Here, the horizontal axis represents frequency and the vertical axis represents the absolute value (log value) of the impedance of the resonator. According to this, the resonance frequency frs where the impedance becomes the lowest in the lower frequency
Appears, and the anti-resonance frequency fas at which the impedance becomes maximum appears at the higher frequency, which means that it has a double resonance characteristic.

FIG. 24 shows a graph obtained by superposing FIGS. 22 and 23. In this figure, a portion that is the pass band of the ladder type filter and a portion that is the suppression region of the ladder type filter are shown. FIG. 25 shows the band pass characteristics when the surface acoustic wave resonators of one terminal pair are connected in parallel.
6 shows impedance characteristics when a surface acoustic wave resonator having a pair of terminals is connected in parallel. Here, the vertical axis of FIG. 25 shows the absolute value (log value) of the admittance.

In these figures, the frequency at which the amount of attenuation is minimum is the resonance frequency frp, the frequency at which the maximum admittance value is the resonance frequency frp, and the frequency at which the minimum admittance value is the antiresonance frequency fap. . Also in the case of this parallel connection, a double resonance characteristic having two resonance frequencies frp and fap is shown.

Such surface acoustic wave resonators are used alone or in combination as in a ladder type filter. FIG. 27 shows a configuration diagram of an embodiment of the ladder type filter. In the case of the ladder type filter, as shown in FIG. 27, the surface acoustic wave resonator (S ₁ ,
Connect several S ₂ , R ₁ , R ₂ ) in parallel and in series. At this time, the antiresonance frequency fap of the parallel resonators R ₁ and R ₂ and the series resonator S
The comb electrodes of each resonator are designed so that the resonance frequencies frs of ₁ and S ₂ are almost the same.

FIG. 28 shows a general pass band characteristic diagram of a ladder type filter. The ladder type filter is a bandpass filter that passes a specific frequency band. The characteristic values required for the bandpass filter are the passband width BW1 and the suppression region width BWa as shown in FIG.
tt1, BWatt2, suppression degree ATT1 and ATT2 in the suppression region.

Further, the bandwidths BW2 and BW at a constant attenuation amount
The ratio with W1 (BW1 / BW2) is called the squareness ratio, which is used as a characteristic value when a steep band characteristic is required. In general, the closer the squareness ratio is to 1, the better, and it is a high-square filter.

Further, the center frequency of the pass band of the filter is
If f0 is set, the values (BW1 / f0, BW2 / f0) obtained by standardizing BW1 and BW2 with the center frequency f0 are called the specific bandwidth. As shown in FIG. 24, the characteristic diagram of the series resonator shows that the anti-resonance frequency
The right part of fas is the suppression region of the ladder filter and corresponds to the BWatt2 part of FIG. Also, FIG.
The portion on the left side of the anti-resonance frequency frs of 4 in the vicinity of the resonance frequency frs and having a flat pass characteristic is the pass band of the ladder filter, and corresponds to the portion BW1 in FIG.

In the ladder filter, FIG. 24 and FIG.
2 and FIG. 25, the pass bandwidths BW1 and BW
W2 is substantially determined by the interval between the anti-resonance frequency fas of the series resonator and the resonance frequency frp of the parallel resonator. Also,
The resonance frequency frs and the anti-resonance frequency fas of the surface acoustic wave resonator are roughly determined by the material of the piezoelectric substrate 1. In particular, among the characteristics of the piezoelectric substrate material used, the electromechanical coupling coefficient substantially determines the bandwidth of the ladder type filter.

For example, AMPS (Advanced Mobile Ph
One service) uses a frequency of f0 = 836.5MHz, but the passband is 25MHz as a characteristic value of its specifications.
And the specific bandwidth is required to be about 3%. Such a wideband bandpass filter is realized only with a large electromechanical coupling coefficient such as 36 ° Y-cut X propagation LiTaO3.

[0021]

When a piezoelectric substrate having a large electromechanical coupling coefficient is used, the surface acoustic wave resonator has a large resonance / anti-resonance frequency interval, so that a wide band pass filter can be realized. Is. However, at the same time, the squareness ratio becomes worse. That is, as the ratio bandwidth increases, the squareness ratio tends to decrease. As described above, since the electromechanical coupling coefficient is a value peculiar to a substance, it is almost impossible to obtain an arbitrary value by determining the resonance and antiresonance frequency intervals by the piezoelectric substrate. Therefore, it is difficult to adjust the specific bandwidth and the squareness ratio to desired values.

The present invention has been made in consideration of the above circumstances, and has been devised by using the same piezoelectric substrate material as the conventional one by devising the structure of the comb-shaped electrode of the surface acoustic wave resonator. Even if it exists, it is an object to make the distance between the resonance frequency and the anti-resonance frequency of the surface acoustic wave resonator close to each other, and to realize a ladder type filter having a higher rectangular shape in the required specific bandwidth.

[0023]

SUMMARY OF THE INVENTION The present invention is a ladder type surface acoustic wave filter constituted by connecting surface acoustic wave resonators in series and / or in parallel, which is one of the surface acoustic wave resonators. , At least one surface acoustic wave resonator connected in parallel comprises a piezoelectric substrate and a plurality of electrode fingers formed on the piezoelectric substrate and having a period pi substantially equal to the wavelength of the surface acoustic wave to be excited. A comb-shaped electrode portion, and at least one disposed in the vicinity of the comb-shaped electrode portion and arranged to reflect the surface acoustic wave in a direction parallel to the traveling direction of the surface acoustic wave excited by the comb-shaped electrode portion. The above-mentioned reflector is provided, and the comb-shaped electrode portion has a period pi.
In which three or more electrode fingers are provided, and the reflector comprises a plurality of gratings having a period pr equal to half the wavelength of a surface acoustic wave traveling in the reflector, and further, among the gratings of the reflector, If it is assumed that the center position of the propagation direction of the surface acoustic wave of the grating closest to the comb-shaped electrode portion and the electrode finger closest to the reflector among the electrode fingers of the comb-shaped electrode are single electrode fingers. , The distance L from the center position of the width of the surface acoustic wave in the propagation direction of the single electrode finger is (n / 2 + 5/16) times (n = 0) times the wavelength λ of the surface acoustic wave excited by the comb-shaped electrode portion. Or a positive integer). A ladder type surface acoustic wave filter is provided.

Here, the electrode fingers of the comb-shaped electrode portion and the grating of the reflector may be made of electrodes of the same material and the same film thickness. Further, the cycle pr is the cycle
It may be half of pi. Further, the material of the electrode fingers of the comb-shaped electrode portion and the grating of the reflector is
Either aluminum or aluminum alloys can be used. Further, 42 ° Y-cut LiTaO ₃ may be used for the piezoelectric substrate. The grating of the reflector may be formed by a groove formed on the piezoelectric substrate in addition to the grating electrode formed by the electrode.

Further, according to the present invention, among the surface acoustic wave resonators, at least one or more surface acoustic wave resonators connected in series are formed on the piezoelectric substrate and the piezoelectric substrate, and the elastic surface to be excited. A comb-shaped electrode portion composed of a plurality of electrode fingers having a period pi that is substantially equal to the wavelength of the surface wave, and a comb-shaped electrode portion arranged in proximity to the comb-shaped electrode portion and parallel to the traveling direction of the surface acoustic wave excited by the comb-shaped electrode portion. At least one reflector arranged so as to reflect the surface acoustic wave in a direction, the comb-shaped electrode portion includes three or more electrode fingers in a period pi, and the reflector is , A plurality of gratings having a period pr equal to half the wavelength of the surface acoustic wave traveling through the reflector, and further propagating the surface acoustic wave of the grating closest to the comb-shaped electrode portion of the grating of the reflector. When the center position of the width in the direction and the electrode finger existing closest to the reflector among the electrode fingers of the comb-shaped electrode is assumed to be a single electrode finger, the width in the propagation direction of the surface acoustic wave of the single electrode finger. The distance L from the center position of
(N / of the wavelength λ of the surface acoustic wave excited by the comb electrode
The ladder-type surface acoustic wave filter according to claim 1, wherein the surface acoustic wave filter is 2 + 6/16) times (n is 0 or a positive integer).

Further, the present invention is a ladder-type surface acoustic wave filter constituted by connecting surface acoustic wave resonators in series and / or in parallel, wherein at least one surface acoustic wave resonator comprises: The present invention provides a surface acoustic wave filter including a surface acoustic wave resonator having the above structure. Further, the present invention provides a ladder-type surface acoustic wave filter in which, of the surface acoustic wave resonators, only the surface acoustic wave resonators connected in series have the above-described structure.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below based on the embodiments shown in the drawings. The present invention is not limited to this.

FIG. 17 shows a configuration diagram of a surface acoustic wave resonator of a one-terminal pair which has been conventionally used. In this conventional configuration, the number of electrode fingers existing within the period pi of the comb-shaped electrode is There are two. The period Pi of the comb electrode 2 and the period Pr of the reflectors 3-1 and 3-2 have a relationship of pi = 2 × pr when the comb electrode and the reflector are made of the same structure such as an Al thin film. Was designed to.

Further, of the grating electrodes constituting the reflectors 3-1 and 3-2, the one 3-4 closest to the comb-shaped electrode 2 and the center A in the propagation direction and the electrode fingers forming the comb-shaped electrode 2 Among them, the distance L to the portion B corresponding to the center of the electrode finger 2-3 closest to the reflector in the propagation direction is λ / 2 (where λ = pi) with respect to the wavelength λ of the surface acoustic wave that propagates. is there. The width of the electrode finger of the comb-shaped electrode and the grating electrode of the reflector is pi.
/ 4, and the distance between each electrode finger is also λ / 4.

<First Embodiment> FIG. 1 shows that in the present invention, three electrode fingers are present within the period pi of a comb-shaped electrode.
The block diagram of one Example of a terminal pair resonator is shown. Here, in the present invention, the period pr of the grating electrode of the reflector is set equal to half the wavelength λr of the surface acoustic wave that has propagated to the reflector after being excited by the comb electrode fingers (pr = λr / 2. ).

Generally, the wavelength λi of the surface acoustic wave excited by the comb electrode is designed to be equal to the period pi of the electrode fingers of the comb electrode (pi = λi). When the comb-shaped electrode and the reflector are made of the same material and have the same film thickness, the wavelength λi is equal to the wavelength λr of the surface acoustic wave traveling to the reflector (λi = λr). Therefore, when the comb electrode and the reflector are formed of the same structure such as an Al thin film, the period pi of the comb electrode and the period pr of the reflector have a relationship of pi = 2 × pr.

Further, among the grating electrodes constituting the reflector, the one 3-4 closest to the comb-shaped electrode, the center position A in the propagation direction of the surface acoustic wave, and the electrode finger forming the comb-shaped electrode are the most reflective. The distance L between the electrode finger 2-4 close to the vessel and the portion B corresponding to the center position of the width in the propagation direction of the surface acoustic wave when the electrode finger 2-4 is assumed to be a single electrode finger is excited. If the wavelength of the surface acoustic wave is λi,
λi / 2. Here, the period Pi of the comb-shaped electrode is designed to be equal to the wavelength λi of the excited surface acoustic wave, so λi
From = Pi, L = Pi / 2.

In FIG. 1, the width of the grating electrode 3-4 of the reflectors 3-1 and 3-2 is pi / 4, and the electrode fingers (2-4, 2-5, 2-6 of the comb-shaped electrode 2). , 2-7) is pi / 6. Comb-shaped electrode 2
In the period pi, the two electrode fingers 2-5 and 2-6 projecting upward from the lower electrode end and the two electrode fingers 2-projecting downward from the upper electrode end 2- There are portions corresponding to respective halves of 4 and 2-7, and eventually portions corresponding to three electrode fingers are present within the period pi of the comb-shaped electrode 2.

Further, in FIG. 1, for comparison, a comb-shaped electrode 2 composed of the conventional single electrode finger shown in FIG. 17 is shown in the lower part of the figure. Here, the electrode fingers shown by the dotted line in the upper comb-shaped electrode 2 in FIG. 1 correspond to the conventional single electrode fingers 2-3 and 2-3 'shown in the lower part.

<Second Embodiment> FIG. 2 shows the period of the comb electrodes.
The block diagram of one Example of a 1 terminal pair resonator in which four electrode fingers exist in pi is shown. Also in this embodiment, the period pr of the grating electrode of the reflector is set equal to half the wavelength λr of the surface acoustic wave traveling to the reflector (pr = λr / 2).

Further, the period pi of the comb electrode and the period of the reflector
pr has a relationship of pi = 2 × pr when the comb electrode and the reflector are made of the same structure such as an Al thin film. further,
Of the grating electrodes forming the reflector 3-2, the one closest to the comb-shaped electrode, the center position A in the propagation direction of the surface acoustic wave of 3-4, and the electrode finger forming the comb-shaped electrode 2 are the most reflectors. Regarding the electrode finger 2-8 close to 3-2, the distance L with the portion B corresponding to the center position in the propagation direction of the surface acoustic wave when the electrode finger 2-8 is assumed to be a single electrode finger is For the wavelength λ of the surface wave, λ / 2 (where λ = pi).

In FIG. 2, the width of the grating electrode 3-4 of the reflectors 3-1 and 3-2 is pi / 4, and the electrode fingers (2-8, 2-9, 2-10 of the comb-shaped electrode 2). , 2-11, 2-12) is pi / 8.
Further, in the comb-shaped electrode, four electrode fingers (2-9, 2-10, 2-11, 2-12) are present in the period pi.

In this embodiment, the center position B when the electrode finger is assumed to be a single electrode finger is set to two adjacent electrode fingers.
2-8, 2-9 coincide with the center position of the surface acoustic wave propagation direction. That is, in this embodiment, two adjacent electrode fingers 2-8 and 2-9, which are closest to the reflector 3-2, have the same width as the area occupied by the two electrode fingers. Assuming that one electrode finger is replaced, the center position of the width in the propagation direction of the surface acoustic wave of this one electrode finger and the propagation of the surface acoustic wave of two adjacent electrode fingers 2-8 and 2-9. Matches the center position of the direction.

In FIG. 2, the dotted line extending across the electrode fingers 2-8 and 2-9 is the electrode finger of the conventional example shown in FIG.
The position corresponding to 2-3 is shown, and electrode fingers 2-10, 2
The dotted line extending across -11 indicates the position corresponding to the electrode finger 2-3 'of the conventional example shown in FIG. The spacing between the grating electrodes of reflectors 3-1 and 3-2 is pi / 4.
And the distance between the electrode fingers of the comb-shaped electrode 2 is pi / 8.

By the way, when the electrode fingers of the comb-shaped electrodes and the grating electrodes of the reflector are formed of thin films of different materials, or even if the same materials have different thicknesses, surface acoustic waves excited by the comb-shaped electrodes are formed. The velocity vi of V and the velocity vr of the surface acoustic wave traveling to the reflector are slightly different (vi ≠ vr). That is, the wavelength λi of the surface acoustic wave excited by the comb electrode is slightly different from the wavelength λr of the surface acoustic wave traveling to the reflector. Where v = fλ, where v
Since i ≠ vr, if f is constant, then λi ≠ λr. Therefore, in the present invention, since λr = 2 × pr, when the material is different, the period pi of the comb electrode equal to the wavelength λi of the surface acoustic wave excited by the comb electrode and the period pr of the grating electrode are doubled. Is not the same as, but has a slightly different value (pi = λi ≠ λr = 2 × pr).

For example, when a comb electrode made of an Al thin film having a period pi = 4.6 μm and a film thickness of 300 nm is formed on a 42 ° rotated Y-cut X-propagation LT substrate, the resonance frequency f of the series resonator is 819.5 MHz.
Is. At this time, from vi = f × Pi, the velocity vi of the surface acoustic wave excited by the electrode fingers of the comb-shaped electrode is 3769.7 m / s.
On the other hand, when the grating electrode of the reflector is formed with an Al thin film of the same material as the comb-shaped electrode and a film thickness of 230 nm different from that of the comb-shaped electrode, the velocity vr of the surface acoustic wave traveling to the reflector in the reflector Is 3808.8m / s. Therefore, vi ≠ vr.

Further, the period pi is equal to the wavelength λi of the excited surface acoustic wave. That is, pi = λi = vr / f (= 4.6)
Is. On the other hand, from vr = f × λr, the wavelength λr of the surface acoustic wave in the reflector is approximately 4.6477 μm, which is different from the wavelength λi (λr ≠ λi).

As described above, according to the present invention, the period pr of the grating electrode of the reflector is half the wavelength λr of the surface acoustic wave traveling in the reflector, but the period pr = λr / 2. Therefore, the period pr should be about 2.3239 μm. Therefore, when the comb-shaped electrode and the electrode of the reflector have different thicknesses, pi = 2 × pr may not be obtained.

<Third Embodiment> FIG. 3 shows the period of the comb electrodes.
The block diagram of one Example of the 1 terminal pair resonator in which six electrode fingers exist in pi is shown. Period pi of comb electrode and period of reflector
pr is pi = 2 × pr when the comb electrode and the reflector are made of the same structure such as an Al thin film. Further, the one of the grating electrodes constituting the reflector 3-2 which is the closest to the comb-shaped electrode, the center position A in the propagation direction of the surface acoustic wave of 3-4 and the electrode finger forming the comb-shaped electrode 2 are the reflectors. Regarding the electrode fingers (2-13, 2-14, 2-15) close to 3-2, the electrode fingers (2-13, 2-13, 2-14, 2-15)
2-14, 2-15), the distance L to the portion B corresponding to the center position in the propagation direction of the surface acoustic wave when assuming one single electrode finger is relative to the wavelength λ of the propagating surface acoustic wave. , Λ / 2
(Where λ = pi).

In FIG. 3, the width of the grating electrode 3-4 of the reflectors 3-1 and 3-2 is pi / 4, and the width of the electrode fingers (2-13 to 2-20) of the comb-shaped electrode 2 is It is pi / 12. Reflectors 3-1, 3-
The spacing between the two grating electrodes is pi / 4, and the spacing between the electrode fingers of the comb-shaped electrode 2 is pi / 12. If the electrode finger is assumed to be a single electrode, the center position B is 3
It coincides with the center position of the two electrode fingers 2-13, 2-14, 2-15 in the propagation direction, and eventually coincides with the center position of the electrode finger 2-14.

At the center position B, the three adjacent electrode fingers 2-13, 2-14, 2-15 closest to the reflector are defined by one electrode finger having the same width as the occupied area. When replaced,
It can be said that this coincides with the center position of the width of the one electrode finger in the propagation direction of the surface acoustic wave.

In the comb-shaped electrode 2, five electrode fingers 2-15, 2-16, 2-17, 2-18, 2-19 and 2 are included in the period pi.
There are half of the two electrode fingers 2-14 and 2-20, and there are six electrode fingers in total. As shown in the lower part of FIG. 3, the center position of the electrode finger 2-3 and the center position of the electrode finger 2-14 of the conventional example match.

As described above, FIG. 1, FIG. 2 and FIG. 3 show the configuration diagrams of three embodiments of the surface acoustic wave resonator of the present invention.
By adopting such a configuration, the resonance frequency and the anti-resonance frequency of the surface acoustic wave resonator are made close to each other to have the same bandwidth as that of the conventional ladder type filter, but with a higher squareness ratio than the conventional one. You can get the ladder type filter you have.

<Specific Design Examples of the Present Invention> The occurrence of such effects will be described below with reference to some specific design examples and characteristic diagrams of the surface acoustic wave resonator. In the present invention, 3 is included in the period pi of the comb electrodes.
It suffices that there be more than three electrode fingers (n ≧ 3), and as described above, the number of electrode fingers is not limited to n = 3, 4, and 6.

FIG. 4 shows a bandpass characteristic diagram when the surface acoustic wave resonators are connected in series (see FIG. 20) in the present invention. In FIG. 4, a solid line is a characteristic graph in the case of FIG. 2 of the present invention in which four electrode fingers are provided in the period pi,
The broken line is a conventional characteristic graph in the case of FIG. 17 in which two electrode fingers are provided in the period pi.

Here, as the piezoelectric substrate, a LiTaO3 substrate corresponding to 36 ° Y-cut X propagation was used. The electrode film of the comb-shaped electrode is an Al-Cu alloy, the film thickness is 340 nm, the period pi of the comb-shaped electrode is pi = 4.6 μm when the number of electrode fingers in pi is 4, and the number of electrode fingers in pi is 4 In the case, pi = 4.656 μm. Period of grating reflector
The pr is pr = 2.3 μm when there are two electrode fingers in pi and pr = 2.328 μm when there are four electrode fingers in pi, and is made of an Al-Cu alloy with the same film thickness as the comb electrode. .

The center position A in the propagation direction of the grating electrode which is the closest to the comb-shaped electrode among the grating electrodes constituting the reflector.
With respect to the electrode finger closest to the reflector among the electrode fingers forming the comb-shaped electrode, the distance L from the position B of the portion corresponding to the center of the propagation direction when the electrode finger is assumed to be a single electrode finger is , Both were set to pi / 2. In FIG. 4, for easy comparison, the characteristic graph in the case of two electrode fingers in pi is shifted to the high frequency side by 50.3 MHz so that the anti-resonance frequencies fas of both the solid and broken characteristic graphs match. It was

In FIG. 4, the resonance frequency frs (F2 in the figure) of the present invention shown by the solid line is 880.73 MHz. Also, the conventional resonance frequency frs (F1 in the figure) shown by the broken line is 8
Although it is 19.43 MHz, in the graph of FIG.
Since it is shifted by MHZ, it is 869.73 MHZ. According to this figure, when the number of electrode fingers in the period pi is four, the excitation efficiency of the comb-shaped electrode decreases, the resonance frequency frs and the anti-resonance frequency fas approach each other, and the graph is larger than the case of two. It can be seen that the insertion loss decreases sharply, that is, the squareness is large. However, when the resonator having the characteristics shown in FIG. 4 is used as it is for the ladder type filter, ripples appear in the band portion, which is not preferable in terms of the band characteristics.

Therefore, position A and position B shown in FIG.
And the distance L (hereinafter referred to as the comb reflector distance or the AB distance) with respect to the wavelength λ of the propagating surface acoustic wave,
If (n / 2 + 3/8) λ and (n is 0 or a positive integer), the ripple is eliminated. This is because the side curve of the comb electrode and the pole of the side lobe of the resonance of the reflector are almost the same. Also, L = (n / 2 + 5/16)
Similarly, if λ, (n is 0 or a positive integer), there is no ripple.

FIG. 5 is a partial configuration diagram for explaining the positional relationship between the comb-shaped electrode and the reflector when n = 1 in the surface acoustic wave resonator of the present invention. Here, the distance between AB is L = (7/8) pi. Further, the electrode finger width of the comb-shaped electrode and its interval are pi / 8, and the electrode finger width of the reflector and its interval are pi / 4 (= pr / 2). The broken line is the position of the electrode finger in the case of the conventional single electrode as described above.

FIG. 6 is a partial block diagram for explaining the positional relationship between the comb-shaped electrode and the reflector when n = 0 in the present invention. Here, the distance between AB L = (3/8) p
i. Also, generally, the outermost electrode finger 2 of the comb-shaped electrode is
Since -30 is not related to the excitation of surface acoustic waves, removing it does not affect the band characteristics. In FIG. 7, the electrode fingers 2-30 are removed and the outermost electrode fingers are designated by reference numeral 2-31. Therefore, as shown in FIG. 7, the outermost electrode finger 2- of the comb-shaped electrode
Even if 30 is removed, a sharp band characteristic without ripple can be obtained as in FIGS. 6 and 7
In this case, since the AB distance L is shorter than that in FIG. 5, the width of the entire resonator can be reduced.

FIG. 8 shows a band pass characteristic diagram of the surface acoustic wave resonators connected in series when the AB distance is optimized.
Here, the solid line graph shows the case where there are four electrode fingers within the period pi of the comb-shaped electrode, and the AB distance L is L = (7/8) pi as shown in FIG. . As in the conventional case, the broken line graph shows the case where the number of electrode fingers in the period pi of the comb-shaped electrode is two.

The piezoelectric substrate is equivalent to 36 ° Y-cut X-propagation.
A LiTaO3 substrate was used. The electrode film of the comb-shaped electrode is an Al-Cu alloy, the film thickness is 340 nm, the period pi of the comb-shaped electrode is pi = 4.6 μm when the number of electrode fingers in pi is 4, and the number of electrode fingers in pi is 4 In the case, pi = 4.656 μm. The period pr of the grating electrode of the grating reflector is pr = 2.3 μm when there are two electrode fingers in pi, and pr = 2.328 when there are four electrode fingers in pi.
It was made of an Al-Cu alloy having a thickness of μm and the same thickness as the comb-shaped electrode. Further, for easy comparison, the characteristics in the case of two electrode fingers in pi are shifted to the high frequency side by 50.3 MHz so that the anti-resonance frequencies fas of both graphs match. According to this solid line graph, the bandpass characteristic is steeper than that of the conventional broken line graph, and it can be seen that almost no ripple appears even when compared with FIG.

FIG. 9 shows a bandpass characteristic diagram of the surface acoustic wave resonators connected in series when the AB distance L is (6/16) pi. FIG. 10 shows a bandpass characteristic diagram of surface acoustic wave resonators connected in series when the AB distance L is (5/16) pi. According to this, in both graphs, a bandpass characteristic that is steeper than in the past is obtained. However, comparing FIG. 9 with FIG. 10, when the resonator is directly connected, the ripple near the resonance frequency frs that defines the pass band of the ladder type filter becomes small because the AB distance L in FIG. (6/16) pi.

Further, in FIG. 11, the AB distance L is (6/16)
FIG. 12 shows the bandpass characteristics of the surface acoustic wave resonators connected in parallel when pi, and FIG. 12 shows the bandpass characteristics of the surface acoustic wave resonators connected in parallel when the AB distance L is (5/16) pi. The figure is shown. Comparing FIG. 11 and FIG. 12, when the resonators are connected in parallel, the ripple near the anti-resonance frequency fas that defines the pass band of the ladder type filter becomes small because the AB distance L in FIG. 5/16) pi.

Next, FIG. 13 shows a configuration diagram of an embodiment of a ladder type filter constructed by combining surface acoustic wave resonators of one terminal pair of the present invention in series and in parallel. Figure 1
3 corresponds to the conventional configuration of FIG. 27, but all the resonators S1 ', S2', R1 'and R2' have the surface acoustic wave resonator of the present invention shown in FIGS. The difference is the use of. In addition, although the case where the number of each of the series resonator and the parallel resonator is only two is illustrated, the number is not limited to this and may be three or more. The number of series resonators and parallel resonators may be two or more.

The piezoelectric substrate 1 is made of Li corresponding to 42 ° Y-cut X-propagation.
Using TaO3 substrate, the electrode film of all electrodes is 400nm thick
The Al-Cu alloy of is used. However, FIG. 13 shows a resonator having four electrode fingers in the period pi of the comb-shaped electrode.

FIG. 14 shows a bandpass characteristic diagram of the ladder type filter shown in FIG. Here, the solid line is the bandpass characteristic in the case where there are four electrode fingers within the period pi of the comb-shaped electrode according to the present invention (see FIG. 2), and the broken line is two electrode fingers within the conventional period pi. It is a band pass characteristic in the case of.

In the ladder-type filter, generally, the periods pis and pip of the resonators connected in series (series resonators) and the resonators connected in parallel (parallel resonators) are shifted so that the anti-parallel resonators have the opposite effect. The resonance frequency fap and the resonance frequency frs of the series resonator are substantially matched to form a pass band. At this time, the period difference between the parallel resonator and the series resonator is Δpi
= Pis-pip.

The embodiment shown in FIG. 14 is an 800 MHz band filter.
This is an example of designing the bandwidth at a loss of 3 dB to be 23 to 25 MHz. If there are two electrode fingers in pi, pip = 4.8μ
m, pis = 4.68 μm, Δpi = 0.12 μm, the grating reflector has a period of 1/2 of each comb-shaped electrode period, and when there are four electrode fingers in pi, pip = 4.94 μm, pis = 4.82μ
m, Δpi = 0.12 μm, the grating reflector has half the period of each comb electrode, and the distance between AB is 7 pis / 8 for the series resonator and 5 pip / 16 for the parallel resonator.

According to the solid line graph of FIG. 14, the insertion loss is
The bandwidth at 3 dB (3 dB bandwidth) was 25.25 MHz. The bandwidth when the insertion loss is 20 dB (20 dB bandwidth) is 37.75 dB,
The squareness ratio at this time was 0.67. In the conventional dashed line graph, the 3dB bandwidth is 23.25MHz, the 20dB bandwidth is 47.25MHz,
The squareness ratio was 0.49. According to the squareness ratio, it can be seen that steeper characteristics are obtained in the same bandwidth by using the resonator of the present invention in FIG. 2 than in the resonator having the conventional electrode structure.

In FIG. 15, in the present invention, the surface acoustic wave resonator of the present invention is used for the series resonators S1 'and S2' connected in series, and the parallel resonators R1 and R2 connected in parallel are the conventional resonators. FIG. 3 is a configuration diagram of an example of a ladder type filter using a surface acoustic wave resonator having the above configuration. In FIG. 15, the series resonators S1 ′ and S2 ′ have four electrode fingers in the period pi of the comb-shaped electrodes, and the parallel resonators R1 and R2 are
There are two electrode fingers in the period pi. Note that FIG.
Similar to 3, the number and the logarithm of the series resonator and the parallel resonator are not limited to those shown in this figure. The materials for the piezoelectric substrate and the electrode film are the same as those in FIG.

FIG. 16 shows a band pass characteristic diagram of the ladder type filter of the present invention shown in FIG. The solid line is Figure 1
6 is a graph of the ladder type filter of the present invention shown in FIG. 5, and a broken line is a graph in the case where there are two electrode fingers within the conventional period pi.

The parallel resonators R1 and R2 have four electrode fingers in pi, the period pip = 4.8 μm, and the grating reflector is
Series resonators S1 ', S2' with half the period of the comb electrodes
Has 4 electrode fingers in pi, pis = 4.75μm, Δpi = 0.05μ
m, the grating reflector has a period of 1/2 of the comb electrode period, and the distance L between AB is pis × 7/8 for the series resonator and pip × 1/2 for the parallel resonator as in the conventional case.

In the solid line graph of the present invention, the 3 dB bandwidth was 38 MHz, the 20 dB bandwidth was 50.50 MHz, and the squareness ratio was 0.75. On the other hand, in the graph of the broken line by the conventional electrode configuration, 38.75MHz, 20dB bandwidth is 56.25MHz, the squareness ratio is 0.69.
Is. Even in this case, compared to the conventional electrode configuration,
The squareness ratio becomes large, and steep bandpass characteristics are obtained.

[0071]

According to the present invention, since three or more electrode fingers constituting the comb-shaped electrode of the surface acoustic wave resonator are provided in one cycle of the comb-shaped electrode, the resonance frequency and the anti-resonance are provided. It is possible to obtain a bandpass characteristic close to the frequency.
Further, since the resonance frequency and the anti-resonance frequency can be made close to each other, in the ladder type surface acoustic wave filter configured by using the surface acoustic wave resonator of the present invention, a bandpass filter having a higher squareness ratio than the conventional one. Can be realized.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of a one-terminal pair surface acoustic wave resonator of the present invention.

FIG. 2 is a configuration diagram of an embodiment of a one-terminal pair surface acoustic wave resonator of the present invention.

FIG. 3 is a configuration diagram of an embodiment of a one-terminal pair surface acoustic wave resonator of the present invention.

FIG. 4 is a comparison diagram of bandpass characteristics of surface acoustic wave resonators connected in series.

FIG. 5 is a partial configuration diagram for explaining a positional relationship between a comb-shaped electrode and a reflector.

FIG. 6 is a partial configuration diagram for explaining a positional relationship between a comb electrode and a reflector.

FIG. 7 is a partial configuration diagram for explaining a positional relationship between a comb-shaped electrode and a reflector.

FIG. 8 is a bandpass characteristic diagram of the surface acoustic wave resonators connected in series according to the present invention.

FIG. 9 is a bandpass characteristic diagram of surface acoustic wave resonators connected in series according to the present invention.

FIG. 10 is a bandpass characteristic diagram of surface acoustic wave resonators connected in series according to the present invention.

FIG. 11 is a bandpass characteristic diagram of surface acoustic wave resonators connected in parallel according to the present invention.

FIG. 12 is a bandpass characteristic diagram of surface acoustic wave resonators connected in parallel according to the present invention.

FIG. 13 is a configuration diagram of an embodiment of a ladder type filter using the surface acoustic wave resonator of the present invention.

FIG. 14 is a pass band characteristic diagram of an example of a ladder type filter using the surface acoustic wave resonator of the present invention.

FIG. 15 is a configuration diagram of an embodiment of a ladder type filter using the surface acoustic wave resonator of the present invention as a series resonator.

FIG. 16 is a pass band characteristic diagram of an example of a ladder type filter using the surface acoustic wave resonator of the present invention as a series resonator.

FIG. 17 is a configuration diagram of a conventional one-terminal-pair surface acoustic wave resonator.

FIG. 18 is a configuration diagram of a conventional two-terminal-pair surface acoustic wave resonator.

FIG. 19 is an equivalent circuit of a conventional one-terminal pair surface acoustic wave resonator.

FIG. 20 shows a conventional one-terminal pair of surface acoustic wave resonators connected in series.

FIG. 21 is a conventional one-terminal pair of surface acoustic wave resonators connected in parallel.

FIG. 22 is a pass band characteristic diagram when the conventional 1-terminal pair surface acoustic wave resonators are connected in series.

FIG. 23 is an impedance characteristic diagram when a conventional one-terminal pair surface acoustic wave resonator is connected in series.

FIG. 24 is a graph showing a pass band and a suppression region of a conventional ladder type filter.

FIG. 25 is a pass band characteristic diagram when the conventional one-terminal pair surface acoustic wave resonators are connected in parallel.

FIG. 26 is an impedance characteristic diagram when a conventional one-terminal-pair surface acoustic wave resonator is connected in parallel.

FIG. 27 is a configuration diagram of a series / parallel connection of conventional ladder type filters.

FIG. 28 is a frequency characteristic diagram of a general ladder type filter.

[Explanation of symbols]

1 Piezoelectric substrate 2 comb electrodes 2-1 Input electrode 2-2 Output electrode 3-1 reflector 3-2 Reflector 3-3 Grating electrode finger 3-4 Grating electrode finger frs Resonant frequency of series resonator fas anti-resonance frequency of series resonator frp Resonant frequency of parallel resonator Fap anti-resonant frequency of parallel resonator F1 Resonant frequency frs of conventional resonator F2 Resonant frequency frs of the embodiment of the present invention Ti input terminal To output terminal S1 Series surface acoustic wave resonator S2 Series surface acoustic wave resonator S1 'Series surface acoustic wave resonator S2 'Series surface acoustic wave resonator R1 parallel surface acoustic wave resonator R2 parallel surface acoustic wave resonator R1 'Parallel surface acoustic wave resonator R2 'Parallel surface acoustic wave resonator Period of Pi comb electrode Pr reflector period

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Takashi Matsuda             4-1, Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa             No. 1 within Fujitsu Limited (72) Inventor Jun Tsutsumi             4-1, Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa             No. 1 within Fujitsu Limited (72) Inventor Shogo Inoue             4-1, Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa             No. 1 within Fujitsu Limited (72) Inventor Osamu Igata             4-1, Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa             No. 1 within Fujitsu Limited F term (reference) 5J097 AA15 AA18 BB01 CC05 DD07                       DD08 DD16 DD17 KK04

Claims (5)

[Claims] 1. A ladder-type surface acoustic wave filter configured by connecting surface acoustic wave resonators in series and / or in parallel, wherein at least one or more of the surface acoustic wave resonators are connected in parallel. Surface acoustic wave resonator
A comb-shaped electrode portion formed of a piezoelectric substrate, a plurality of electrode fingers formed on the piezoelectric substrate and having a period pi substantially equal to the wavelength of a surface acoustic wave to be excited, and a comb-shaped electrode portion arranged in proximity to the comb-shaped electrode portion At least one reflector arranged to reflect the surface acoustic wave in a direction parallel to the traveling direction of the surface acoustic wave excited by the electrode section. In which three or more electrode fingers are provided, and the reflector comprises a plurality of gratings having a period pr equal to half the wavelength of a surface acoustic wave traveling in the reflector, and further, among the gratings of the reflector, If it is assumed that the center position of the propagation direction of the surface acoustic wave of the grating closest to the comb-shaped electrode portion and the electrode finger closest to the reflector among the electrode fingers of the comb-shaped electrode are single electrode fingers. The distance L between the center position of the propagation direction of the width of the surface acoustic wave of the single-electrode fingers, the wavelength of the surface acoustic wave excited by the comb-shaped electrode portions λ (n / 2 + 5/1
6) A ladder-type surface acoustic wave filter, which is a multiple (n is 0 or a positive integer). 2. At least one surface acoustic wave resonator connected in series among the surface acoustic wave resonators is formed on a piezoelectric substrate and a piezoelectric substrate, and the wavelength of the surface acoustic wave to be excited. And a comb-shaped electrode portion composed of a plurality of electrode fingers having a period pi substantially equal to
And at least one reflector arranged so as to reflect the surface acoustic wave in a direction parallel to the traveling direction of the surface acoustic wave excited by the comb-shaped electrode portion, wherein the comb-shaped electrode portion comprises In the period pi, three or more electrode fingers are provided, and the reflector is composed of a plurality of gratings having a period pr equal to half the wavelength of the surface acoustic wave propagating in the reflector. It is assumed that the center position of the width of the surface acoustic wave in the propagation direction of the grating closest to the comb-shaped electrode portion and the electrode finger closest to the reflector among the electrode fingers of the comb-shaped electrode are single electrode fingers. In this case, the distance L from the center position of the width of the surface acoustic wave in the propagation direction of the single electrode finger is (n / 2 + 6/16) times (n / 2 + 6/16) times the wavelength λ of the surface acoustic wave excited by the comb-shaped electrode portion. Is 0 or positive Ladder type SAW filter as claimed in claim 1, wherein the a number). 3. The ladder-type surface acoustic wave filter according to claim 1, wherein the electrode fingers of the comb-shaped electrode portion and the grating of the reflector are made of electrodes having the same material and the same film thickness. . 4. The ladder type surface acoustic wave filter according to claim 3, wherein the period pr is half of the period pi. 5. The ladder type surface acoustic wave filter according to claim 3, wherein the material of the electrode fingers of the comb-shaped electrode portion and the grating of the reflector is either aluminum or aluminum alloy.