JP2748009B2 - Surface acoustic wave resonator filter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bandpass filter using a surface acoustic wave resonator in which an interdigital electrode and a grating reflector are formed on a piezoelectric substrate.

[Related Art] In recent years, a surface acoustic wave device has been used as a front-end filter for wireless communication using the VHF band and the UHF band,
It has been proposed to reduce the size and weight of a wireless device. The electrical characteristics required for the front-end filter include low insertion loss, low in-band ripple, low out-of-band spurious levels, high out-of-band attenuation, and the number of channels required for communication systems. Therefore, it is necessary to have an appropriate bandwidth, and to satisfy all of these conditions.

Conventional surface acoustic wave filters are mainly classified into two types. One is a transversal device that has a large fractional bandwidth but a large insertion loss, and the other is a transversal device with a fractional bandwidth of 0.
Although it is as narrow as about 05%, it is a resonator type device with small insertion loss. Therefore, in order to use a conventional surface acoustic wave device as a front-end filter, it is necessary to improve the characteristics of each device, and it is desired to provide a device having both advantages.

On the other hand, in order to use the surface acoustic wave device in the UHF band, the termination impedance is usually selected to be 50Ω. As a method for obtaining such a low impedance and low insertion loss surface acoustic wave filter, LiTaO ₃ and a quartz substrate are used, and one interdigital electrode for input and another interdigital electrode for output are used. A proposal has been made to form a surface acoustic wave resonator by using a symmetric primary mode and an antisymmetric secondary mode of an energy confinement mode, and to configure a vertical dual mode bandpass filter. (JP-A-61-285814).

[Problems to be Solved by the Invention] However, in this vertical dual mode bandpass filter, when quartz is used as the piezoelectric substrate, the specific bandwidth is 0.3 even if the normalized electrode film thickness is increased to 4%. %, And there is a problem that the in-band ripple is large. In addition, lithium tantalate (Li
It is known that when TaO ₃ ) is used, even if the normalized electrode film thickness is increased to 4%, the specific bandwidth can be achieved only up to 0.47%, and the in-band ripple is large.

Therefore, the conventional surface acoustic wave device has a fractional bandwidth of
At 0.3% or more, the in-band ripple was small, the out-of-band spurious level was low, and the filter could not be put to practical use as a front-end filter that required a large amount of out-of-band attenuation.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide a surface acoustic wave resonator filter having a wide bandwidth, a small insertion loss, a small in-band ripple, and a low input impedance. It is in.

[Means for Solving the Problems] In order to achieve the above object, the present invention disposes three or more interdigital electrodes close to each other along the direction of surface acoustic wave propagation on a piezoelectric substrate, and In a surface acoustic wave resonator filter in which one is connected to an input terminal, the remaining interdigital electrodes are connected to an output terminal, and at least one set of reflectors are arranged on both outer sides of the interdigital electrodes, The total logarithm N ₁ + N ₂ of the digital electrodes should be 40 to 150 pairs, and the ratio N ₁ / N ₂ of the total logarithm N ₂ of the input interdigital electrode to the total logarithm N ₁ of the output interdigital electrode is 1.4 or higher 2.
4 or less, and the ratio between the maximum cross width W of the electrode finger of the interdigital electrode and the surface wave wavelength λ of the passing center frequency.
W / λ was set in the range of 50 or more and 180 or less. When a plurality of input or output interdigital electrodes are provided, their logarithms N ₂ and N ₁ indicate the logarithm of the sum of the input or output interdigital electrodes.

In other words, the present inventors have examined in detail each condition required when using a surface acoustic wave resonator filter for a wireless communication front-end filter, and found that the above numerical range is appropriate. Came to the conclusion. Therefore,
The process of the examination will be described.

In order to use a surface acoustic wave resonator filter as a front-end filter, it is necessary to satisfy more electrical characteristics than before, as described above. Therefore, the degree of freedom in design was first increased by increasing the number of interdigital electrodes.

Next, it was considered to reduce the input impedance of the surface acoustic wave resonator filter. The terminal impedance is a constant value with respect to the frequency, but the image impedance of the surface acoustic wave resonator filter is complicated and fluctuates within the band, and cannot be perfectly matched. As an approximation, if the broken line connecting the midpoints of the vibration peaks (peaks and valleys) matches the terminal impedance (50Ω), it is considered that the matching has been achieved. Therefore, to lower the input impedance,
What is necessary is just to make the bent line close to a straight line with a slope of zero, and to lower the value. A general technique for this is to increase the cross width of the electrode fingers. In this case, the imaging impedance of the surface acoustic wave resonator filter decreases. However, if the crossing width is too large, the resistance of the electrode finger itself increases and the insertion loss increases. Further, the chip size becomes large, and it is impossible to achieve downsizing, which is one of the features of the surface acoustic wave device. On the other hand, the imaging impedance of the surface acoustic wave resonator filter decreases even if the number of input interdigital electrode pairs is increased. However, when the value exceeds a certain value, the reflection of the surface acoustic wave by the interdigital electrode itself increases, and the energy does not reach the reflector, and the performance is the same as that of the multi-pair interdigital surface acoustic wave device. In this case, it is not preferable because the filter becomes a narrow band, a sufficient bandwidth cannot be obtained, and the element becomes large.

Furthermore, when a practical value is assumed as the terminal impedance, matching cannot be achieved even if the imaging impedance is too small, so that there is not only an upper limit but also a lower limit in the intersection width and the number of input interdigital electrode pairs.

Next, a method for reducing the in-band ripple was studied. Although in-band ripple is often not discussed, it is a very important characteristic in practical use.

An effective method for designing the in-band characteristics is to divert the method for designing a crystal monolithic filter using a bulk wave. The newest design method of a multi-electrode pair monolithic filter using a high-coupling piezoelectric plate using bulk waves is as follows. First, the impedance Za of the symmetric confinement mode with respect to the symmetry plane at the center of the filter is added to the series arm, and the antisymmetric Is converted to a symmetrical lattice circuit connected to the lattice arm. At this time, the image impedance Zim of the filter is Given by Next, while the number of symmetric modes is p and the number of antisymmetric modes is q, the ith resonance frequency and antiresonance frequency of the symmetric mode are faRi and faAi, and the jth resonance frequency and Anti-resonance frequency fb
Assuming that Rj and fbAj, Za and Zb can be approximated by the following equations.

Equation _{(2), fa A i =} fb R i (i = 1,2, ‥‥ p) ...... (4) fa R j + 1 = fb A j (j = 1,2 in (3), ‥‥ q- 1) The frequency adjustment of (5) is performed. Bandwidth in this case is (fb _R q-fa _R1) or becomes _{(fa R p-fa R1)} . This method is applied to the design of a surface acoustic wave device.
Three or more interdigital electrodes are arranged close to each other, some of them are connected to input terminals, and the remaining interdigital electrodes are connected to output terminals. When the surface acoustic wave resonator is cascaded in multiple stages symmetrically with respect to the connection surface, the electric characteristics near the pass band can be expressed by a symmetric lattice circuit. The resonance frequency and anti-resonance frequency of the input impedance Za when the output terminal of the surface acoustic wave resonator cut by the line of symmetry is short-circuited and the input impedance Zb when the output terminal is opened are determined. The resonance frequency and the anti-resonance frequency only need to satisfy Expressions (4) and (5), but it is not always possible to set them so as to completely satisfy them. Therefore, complicated in-band ripples occur due to the difference between the imaging impedance and the termination impedance. As a practical matter, it is important to set the resonance frequency and the anti-resonance frequency so that the in-band ripple falls within an allowable value.

The most effective means for this purpose are the logarithmic ratio of the input / output interdigital electrodes and the total logarithm of the interdigital electrodes. By the way, the specific bandwidth becomes narrower as the total logarithm N _T (N ₁ + N ₂ ) of the interdigital electrodes and the logarithmic ratio N ₁ / N _{2 of the} input / output interdigital electrodes become larger. Therefore, there are upper limits on these values. On the other hand, when these values are small, the in-band ripple becomes large. Therefore, there must be lower bounds on these values in order to satisfy the in-band ripple tolerance.

Based on the above considerations, the present inventors have calculated the total logarithm of the interdigital electrodes, the ratio of each logarithm of the input and output interdigital electrodes, and the ratio W / λ of the maximum cross width and wavelength of the electrode finger ( Hereinafter, an experiment was conducted to determine the upper and lower limits of the standardized intersection width), and the above-described numerical range was determined.

[Operation] According to the above-described means, since a filter is configured by providing three or more interdigital electrodes, the degree of freedom of design is increased, and the design is facilitated to obtain desired filter characteristics. Since the total logarithm and the log ratio of the electrodes are selected within a predetermined range, the in-band ripple can be reduced, and the maximum cross width of the interdigital electrode and the logarithm of the input interdigital electrode can be set within the predetermined range. Since the selection is made, the input impedance can be reduced, the insertion loss can be reduced, and the matching between the imaging impedance and the terminal impedance can be easily achieved, and the bandwidth of the filter can be widened.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a conceptual diagram showing a configuration example of a surface acoustic wave resonator filter according to the present invention, in which an input interdigital electrode 2a is formed on the surface of a piezoelectric substrate 1 and both sides are excited. Output interdigital electrodes 3a, 3a 'are arranged adjacent to each other along the propagation direction of the surface acoustic wave,
Further, grating reflectors 4a, 4a 'are arranged on both sides of the output interdigital electrodes 3a, 3a'. The interdigital electrodes 2a, 3a, 3a 'and the reflector
4a, 4a 'and electrodes 2b, 3b, 3b' and reflectors 4b,
4b 'is formed into a two-stage structure, and the electrodes 3a and 3b and 3a'
3b 'is cascaded. Furthermore, a pair of comb-shaped electrodes constituting the upper interdigital electrode 2a are each connected to an input terminal.
IN and one of the interdigital electrodes 3a, 3a 'are connected to the ground point, and a pair of interdigital electrodes constituting the lower interdigital electrode 2b are connected to the output terminal OUT and the ground point, respectively. , And each interdigital electrode 3b,
One electrode of 3b 'is connected to the ground point.

The terminal impedances 5, 5 'are connected between the input / output terminals. The terminating impedance
5,5 'is a 50Ω resistor that is widely used in the UHF band.
As the piezoelectric substrate 1, lithium tetraborate (Li
₂ B ₄ O ₇ ) A 45 ° rotation X plate Z propagation substrate was used. The surface wave velocity in this case is 3440 m / sec. Input / output interdigital electrodes 2, 3, 3 'and reflectors 4, 4' are arranged so that surface acoustic waves propagate in the Z-axis direction on the surface of the substrate 1. The electrode was an aluminum layer. However, Li ₂ B ₄ O ₇ substrate is Al
The electrodes were formed by a lift-off method to dissolve in the etching solution. The reflectors of the reflectors 4, 4 'were metal strips made of an aluminum layer. The number of the reflectors of the grating reflector was set to 100 as a number capable of sufficiently reflecting the surface acoustic wave in consideration of the surface acoustic wave reflectance per Al strip. The maximum intersection width is
Later, it is determined that the imaging impedance of the filter matches 50Ω, but initially, the normalized cross width W / λ is set to 150 by preliminary examination. Weighting for transverse mode suppression is performed by general cosine apodization.

In the surface acoustic wave resonator having the above configuration, the normalized Al film thickness
When H / λ is set to 0.01, the logarithmic ratio N ₁ / N ₂ of the input / output interdigital electrodes is fixed to 2 and the total number of interdigital electrodes _NT is changed from 40 pairs to 160 pairs. The input impedance Za when the output terminal of the first surface acoustic wave resonator is short-circuited and the input impedance when the output terminal is open
The respective resonance frequency and antiresonance frequency of Zb were obtained. The result is shown in FIG. The frequency here is shown as a normalized frequency normalized by the center frequency of the grating reflector. FIG. 3 shows the frequency response when the total logarithm of the interdigital electrodes is 100 and the logarithmic ratio N ₁ / N _{2 of} the input / output interdigital electrodes is 2. From these figures
It can be seen that the ripple increases where the interval between the resonance frequency fr and the antiresonance frequency fa of Za and Zb is large. Note that the symbol r ₁ indicates a zero-order logarithm, r ₂ indicates a symmetric second-order, a ₁ indicates an anti-symmetric zero-order, and a ₂ indicates an anti-symmetric second-order frequency.

Next, when the total logarithm _{NT of the} interdigital electrodes is fixed at 116 pairs, and the logarithmic ratio of the input / output interdigital electrodes is changed from 0.8 to 3.0, the respective resonance frequencies fr and antiresonance frequencies of Za and Zb are obtained. asked for fa. The result is shown in FIG. As is clear from FIG. 4, by increasing the logarithmic ratio N ₁ / N ₂ of the input / output interdigital electrode, the interval between the resonance frequency and the anti-resonance frequency of each of the input impedances Za and Zb can be reduced. Recognize. Also,
FIG. 5 shows the frequency response when the logarithmic ratio of the input / output interdigital electrodes is 1.5. From this figure, it can be seen that the ripple on the high frequency side in the band is large.

Therefore, in order to widen the filter characteristics and reduce the in-band ripple, the total logarithm of the interdigital electrodes is required.
With less N _T, it is preferable that the ripple caused by it to compensate at input-output interdigital logarithmic ratio of the electrode N ₁ / N _2. However, there is a limit to the amount that can be compensated by the logarithmic ratio of the interdigital electrode, and the normalized Al film thickness H / λ ≒
In the case of 0.01, the total logarithm of the interdigital electrodes needs to be 80 or more in order to reduce the input impedance. On the other hand, as is clear from FIG.
To achieve 3% or more, the total logarithm of the interdigital electrodes must be 1
Must be less than 50 pairs. At this time, the logarithmic ratio N ₁ / N _{2 of the} interdigital electrodes required to guarantee the ripple
Is 1.4 to 2.4.

When the normalized Al film thickness H / λ = 0.04, for the same reason, the total logarithm of the interdigital electrodes is 40 or more and 100 or less, and the logarithmic ratio of the interdigital electrodes is preferably 1.4 to 2.4. .

Next, the relationship between the maximum intersection width and the image impedance is examined. Total logarithm of the interdigital electrode and 116 pairs, shown in Figure 6 the variation of the image impedance of the in-band with respect to the change of the maximum cross width when the log ratio N ₁ / N ₂ of the input-output interdigital electrode and 2.0 . As is apparent from FIG. 6, when the normalized Al film thickness H / λ ≒ 0.01, the specific intersection width W / λ is set to 80.
From 180 to 180, the image impedance is 20
It takes a value of 100100Ω and is almost matched to the termination impedance of 50Ω. Normalized Al film thickness H /
When λ = 0.04, if the normalized cross width W / λ is set in the range of 50 to 90, the image impedance in the band becomes 20 to 100.
Take the value of Ω.

In order to experimentally confirm that the setting range is effective, two types of devices under the following conditions were created.
That is, the first device is an interdigital electrode 2a, 2b
50.5 pairs of interdigital electrodes 3a, 3a ', 3b, 3
The logarithm of b 'was 41.5 pairs, and the output interdigital electrodes were arranged close to both sides of the input interdigital electrodes at a distance of 2.25 μm. Further, grating reflectors composed of 80 metal strips were arranged on both outer sides of the output interdigital electrode with a spacing of 2.25 μm. The maximum intersection width was 1320 μm, and the weight for the interdigital electrodes was cosine apodized. The Al film thickness of the interdigital electrode and the grating reflector was set to 1000 mm in consideration of electrode finger resistance and variation in center frequency at the time of manufacturing. The manner of connecting each electrode to the input / output terminals is the same as in FIG. FIG. 7 shows the frequency response of this device. Electrical characteristics include center frequency of 380MHz, fractional bandwidth of 0.53%, insertion loss of 2dB, in-band ripple of 1dB, out-of-band attenuation of 60dB, and low-frequency out-of-band spurious -50dB, which are practically sufficient. doing.

The second device has 38.5 pairs of interdigital electrodes 2a and 2b and 40.5 pairs of interdigital electrodes 3a, 3a ', 3b and 3b'.

Furthermore, the maximum cross width of the interdigital electrode is 2300μ.
m, weighting was performed by linear apodization, and the thickness of the interdigital electrode and the strip was 1540 °.

FIG. 8 shows the frequency characteristics of the second device. 8th
From the figure, it was found that the electrical characteristics were as follows: center frequency 254.5 MHz, fractional bandwidth 0.6%, insertion loss 1.5 dB, in-band ripple 1 dB, out-of-band spurious -35 dB, which were sufficient for practical use. .

By the way, in the above embodiment, since the 45 ° rotation X-plate Z-propagation Li ₂ B ₄ O ₇ was used as the piezoelectric substrate, the normalized Al film thickness H / λ was used.
Was set to 0.01 and 0.04.
Considering the surface acoustic wave reflectance per strip, it is 0.
Corresponds to 01 to 0.1. Therefore, when using another substrate material, the normalized Al film thickness may be set so that the surface acoustic wave reflectance per Al strip of the grating reflector corresponds to 0.01 to 0.1.

In the experiment, an element with a center frequency of 380 MHz and an element with a frequency of 254.5 MHz were created.As a feature of the surface acoustic wave device, the center frequency can be freely changed by changing the line width and period of the interdigital electrode and grating reflector. Can be. Therefore, the present invention is not limited by the center frequency of the surface acoustic wave device.

In both the first device and the second device, the period of the grating on the reflector side was adjusted to the wavelength of the surface acoustic wave.

Further, in the above embodiment, weighting is performed by cosine apodization or linear apodization for lateral mode suppression. Other transverse mode suppression methods include cosine squared apodization and modified cosine apodization. In the case of using the weighting method, it was confirmed that the same result as in the case of using cosine apodization was obtained by increasing or decreasing the number of interdigital electrode pairs.

Further, when actually manufacturing the surface acoustic wave device of the present invention in which required parameters are set in the above-described range, for example, a shield electrode having an appropriate width is provided between input and output interdigital electrodes, and this is provided. It is needless to say that it is desirable that the grounding or the electrode finger at the end of the input / output interdigital electrode be grounded to prevent a direct wave between the input / output interdigital electrodes.

On the other hand, as the material of the piezoelectric substrate, lithium tetraborate with little variation in characteristics such as temperature characteristics and electromechanical coupling coefficient is currently optimal. However, it goes without saying that the principle of the present invention can also be applied to a composite substrate or the like in which a piezoelectric substance is attached to all piezoelectric materials or glass.

In the above description, only the use of surface acoustic waves has been described.However, the present invention is not necessarily limited to this.For example, slip waves, love waves, SSBW, SH waves, Alternatively, the present invention can be applied to Bluestein Courier, Shimizunami and the like. That is, it has already been proved that the resonator having the interdigital electrode excites each of the above-mentioned waves, and it is possible to confine the vibration energy under the electrode and to generate a multi-mode vibration. It is.

The method of connecting the electrodes to the input / output terminals is not limited to the above embodiment. For example, the interdigital electrodes 2a, 3b, 3
b ′ is an input terminal pair and the interdigital electrodes 2b and 3a, 3
a ′ may be used as an output terminal pair. However, it is possible to reduce the in-band ripple by adopting a connection method in which the combination of the input side electrode and the output side electrode is symmetric.

[Effects of the Invention] As described above, according to the present invention, three or more interdigital electrodes are arranged close to each other along the propagation direction of surface acoustic waves on a piezoelectric substrate, and at least one of the interdigital electrodes is used as an input terminal. Connected, the remaining interdigital electrodes are connected to the output terminals, and at least one set of reflectors is arranged on both outer sides of the interdigital electrodes. ₁ + N ₂ was 150 pairs less than 40 pairs, and the ratio of the total log N ₁ of total log N ₂ and the output interdigital electrodes of the input interdigital electrodes, as well as 1.4 to 2.4, an interdigital electrode The ratio W / λ between the maximum cross width W of the electrode finger and the surface wave wavelength λ of the passing center frequency is 50 or more.
Since the filter is set to a range of 180 or less, the filter is configured by providing three or more interdigital electrodes, so that the degree of freedom of design increases, and the design becomes easy to obtain desired filter characteristics. Since the total logarithm and log ratio of the interdigital electrodes are selected within a predetermined range, the in-band ripple can be reduced, and the maximum cross width of the interdigital electrodes and the number of input interdigital electrodes can be set to a predetermined value. Since the selection is made within the range, the input impedance can be reduced, the insertion loss can be reduced, and the image impedance and the termination impedance can be easily matched, so that the filter bandwidth can be widened. There is. Also,
The filter having the above configuration has an advantage that the out-of-band spurious level is low and the out-of-band attenuation is large.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a configuration example of a surface acoustic resonator filter according to the present invention. FIG. 2 is a diagram showing an input impedance Za when the output terminal is short-circuited and a case where the output terminal is open in the surface elastic resonator. FIG. 3 is a diagram showing the relationship between the total logarithm and the respective resonance frequency and antiresonance frequency of the input impedance Zb. FIG. 3 is a diagram showing the frequency response when the total logarithm of the interdigital electrode is 100 pairs and the termination impedance is 50Ω. FIG. 4 shows the respective resonance frequencies and antiresonance frequencies of the input impedances Za and Zb when the total logarithm of the interdigital electrodes is fixed and the logarithmic ratio of the interdigital electrodes is changed. FIG. FIG. 6 shows the frequency response when the log ratio of the electrodes is 2.0. FIG. 6 shows the maximum logarithm of the interdigital electrodes and the log ratio of the interdigital electrodes fixed. FIGS. 7 and 8 show the frequency response of a device obtained by setting each parameter specifically within the range proposed by the present invention, showing a change in image impedance when the intersection width is changed. FIG. 1. Piezoelectric substrate, 2a, 2b, 3a, 3b ... interdigital electrode, 4a, 4b ... reflector.

Claims (2)

(57) [Claims] 1. Three interdigital electrodes made of an aluminum film having a thickness h are arranged close to each other along the direction of propagation of surface acoustic waves on a rotating X-plate Z-propagating lithium tetraborate substrate, and one set of interdigital electrodes is provided on both outer sides thereof. Are formed in a direction perpendicular to the surface acoustic wave propagation direction, of which the interdigital electrode at the center of the first electrode structure is used as an input terminal and the electrode structure of the second electrode structure is used. The center interdigital electrodes are connected to the output terminals, respectively.
The interdigital electrodes on both sides of the electrode structure and the interdigital electrodes on both sides of the second electrode structure are connected so as to be symmetrical with respect to the connection surface, and the electrical equivalent circuit can be represented by a symmetric lattice circuit. Using the resonance frequency and the anti-resonance frequency of the input impedance Za at the input terminal when the cut ends of the interdigital electrodes on both sides are short-circuited at the connection surface and the input impedance Zb when open, respectively In the energy trapping type surface acoustic wave resonator filter forming the band, when the logarithm of each central interdigital electrode in the first and second electrode structures is N2, and the sum of the logarithms of the interdigital electrodes on both sides is N1, The total logarithm N1 + N2 of the interdigital electrodes is 40 to 150 pairs, and the ratio N1 / N2 of the logarithm N1 to N2 is 1.4 to 2.4. As well as a lower, maximum cross width W of the electrode fingers of the interdigital electrode
And the ratio W / λ between the pass center frequency and the surface wave wavelength λ
0 or less, and the ratio h / λ between the thickness h of the aluminum film and the surface wave wavelength λ of the pass center frequency is set in the range of 0.01 to 0.04 so as to form a pass band having a relative bandwidth of 0.3% or more. A surface acoustic wave resonator filter characterized in that: 2. The surface acoustic wave resonator filter according to claim 1, wherein a 45-degree rotation X-plate Z-propagating lithium tetraborate substrate is used.