JPH0334715A - Surface acoustic wave resonator and filter - Google Patents

Abstract

PURPOSE:To reduce spurious level by other than the low frequency band by setting a distance between an interdigital electrode and a reflector to a specific range in a surface acoustic wave resonator utilizing the zero-order and 2nd order resonance modes. CONSTITUTION:An input interdigital electrode 2a is formed to the surface of a piezoelectric substrate 1 and output interdigital electrodes 3a, 3a' are arranged adjacent to each other along the propagation direction of a surface acoustic wave stimulated at both outsides. Then grating reflectors 4a, 4a' are arranged to both sides of the electrodes 3a, 3a'. In the surface acoustic wave element utilizing the zero-order and 2nd order resonance modes through the constitution above, the distance LIR between the said interdigital electrode and the reflector is set in a range of equation I. Thus, the out-band spurious level is suppressed to be -20dB or below. It is considered that this is caused by the mutual interference between a standing wave between input and output interdigital electrodes and a standing wave between reflectors.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a surface acoustic wave resonator formed by forming interdigital electrodes and a grating reflector on a piezoelectric substrate, and particularly relates to a surface acoustic wave resonator that is formed by forming interdigital electrodes and a grating reflector on a piezoelectric substrate. It relates to techniques that can be effectively used to reduce levels.

[Prior Art] In recent years, it has been proposed to use a surface acoustic wave device as a front-end filter for a wireless communication device using the VHF band and UHF band to reduce the size and weight of the wireless communication device. The electrical characteristics required for a front-end filter include low insertion loss, low in-band ripple, low out-of-band spurious level, large out-of-band attenuation, and ensuring the number of channels required for the communication system. All of these conditions must be met.

Conventional surface acoustic wave filters are mainly classified into two types. One is a transversal device with a wide fractional bandwidth but large insertion loss, and the other is a transversal device with a fractional bandwidth of 0.
．． It is a resonator type device with a narrow insertion loss of about 0.05%, but a small insertion loss. Therefore, in order to use conventional surface acoustic wave devices as front-end filters, it is necessary to improve the characteristics of each type, and a device that combines the advantages of both properties has been desired.

On the other hand, in order to use a surface acoustic wave device in the UHF band, the termination impedance is usually selected to be 50Ω. In order to obtain such a surface acoustic wave filter with low impedance and small insertion loss, a surface acoustic wave resonator is constructed using one interdigital electrode for input and another interdigital electrode for output. , a vertical dual-mode band-pass filter has been proposed that utilizes two energy-confined modes: a symmetric zero-order mode and an anti-symmetric first-order mode (Japanese Patent Laid-Open No. 61-285
No. 814).

[Problems to be Solved by the Invention] However, in this vertical dual-mode bandpass filter, when crystal is used as the piezoelectric substrate, the fractional bandwidth is 0 even if the normalized electrode film thickness is increased to 4%. There are problems in that it can only achieve up to .3% and the in-band ripple is large. In addition, lithium tantalate (L) is used as a piezoelectric substrate.
When using iTa 03), even if the normalized electrode film thickness is increased to 4%, only a fractional band @0.47% can be achieved, and the in-band ripple is large.

Moreover, in the prior invention, no consideration is given to the out-of-band spurious level, which is one of the important electrical characteristics when used as a front-end filter.

Therefore, conventional surface acoustic wave devices cannot be put to practical use as front-end filters that require a fractional bandwidth of 0.3% or more, small in-band ripples, low out-of-band spurious levels, and large out-of-band attenuation. I couldn't.

The present invention was made to solve the above problems, and its objectives are to provide a wide bandwidth, low insertion loss, and
It is an object of the present invention to provide a surface acoustic wave resonator filter with small in-band ripples and a low out-of-band spurious level.

[Means for Solving the Problems] The present inventors have developed a method for reducing the out-of-band spurious level, which is one of the requirements when using a surface acoustic wave resonator filter as a front-end filter for a wireless communication device. was studied in detail.

For practical filter performance, it is necessary to keep the out-of-band spurious level to -20 dB or less.

However, in a surface acoustic wave resonator filter as shown in Figure 1, which synthesizes band characteristics using zero- and second-order energy confinement modes, the out-of-band spurious closest to the passband is the fourth-order energy confinement mode. It is.

FIG. 2 shows the edge distance 1lII between the interdigital electrode and the reflector shown in FIG. 1(b) in the above filter.
It shows the frequency characteristics when L x R is 0.244λ (129 μm).

Note that the logarithm of the input interdigital electrodes in this case is 5.
0.5 pairs, the number of output interdigital electrodes is 4 on one side.
There are 1.5 pairs, and the distance between the edges of the interdigital electrodes is 2.2 μm.

A filter having such parameters had an extremely large out-of-band spurious level of 110 dB.

Therefore, the present inventors studied each parameter that determines the element structure of a surface acoustic wave resonator filter.

As a result, it was found that the spurious of the fourth-order energy trapping mode varies greatly depending on the distance LIR between the interdigital electrode and the reflector. FIG. 3 shows the relationship between the distance ILIR between the interdigital electrode and the reflector and the out-of-band spurious level. From the figure, in order to suppress the out-of-band spurious level to below -20 dB, the LIR should be set to 0.
It can be seen that it is necessary to set the value to 21λ or more and 0.23λ or less.

This invention was made based on the above knowledge, and includes a 45° ± 5a rotating X plate Li, a B4O7 substrate Z direction ±
When 5° is the surface acoustic wave propagation direction, 3
of these interdigital electrodes are arranged in close proximity, the central interdigital electrode of these interdigital electrodes is connected to the input terminal, the remaining interdigital electrodes are connected in parallel to the output terminal, and the interdigital electrodes At least one set of reflectors is placed on both sides of the
In a surface acoustic wave resonator that utilizes next- and second-order resonance modes, the distance between the interdigital electrode and the reflector @
It is proposed to set the L engineering R in the range of (where n is O or a positive integer, and λ is the surface acoustic wave wavelength of the center frequency).

However, since the distance between the electrodes varies somewhat due to process variations due to lithography technology, it may be defined by the distance between the centers of the electrode fingers.5 In that case, the width of the electrode fingers is 0.25λ, so Center distance is 0.46
It is sufficient to set the value to be greater than or equal to λ and less than or equal to 0.48λ.

Note that here, λ is the surface acoustic wave wavelength of the passing center frequency, and L
IR is the distance between the edge of the electrode finger of the interdigital electrode and the edge of the electrode finger of the reflector, as shown in FIG. 1(b). Moreover, the reason for setting +n/2 is that even if the LIR increases by an integer multiple of a half wavelength, the relationship between the phase of the standing wave and the interdigital electrode and reflector does not change, so it is possible to suppress the spurious level in the same way. It is.

[Operation] The reason why the out-of-band spurious level can be suppressed to below -20 dB by the above means is considered to be due to mutual interference between the standing waves between the input and output interdigital electrodes and the standing waves between the reflectors.

[Example] Hereinafter, an example 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 filter using a surface acoustic wave resonator according to the present invention, in which an input interdigital electrode 2a is formed on the surface of a piezoelectric substrate 1, and an The output interdigital electrodes 3a and 3a' are arranged adjacent to each other along the propagation direction of the excited surface acoustic waves. and output interdigital electrode 3a,
Grating reflectors 4a, 4a' are provided on both sides of 3a'.
It is arranged. In addition, the interdigital electrode 2
a, 3a, 3a' and reflectors 4a, 4a', and electrodes 2b, 3b, 3b' and reflectors 4b, 4 of the same configuration.
b' is formed to have a two-stage structure, and electrodes 3a and 3b and 3a' and 3b' are connected in cascade.

Further, a pair of comb-shaped electrodes constituting the upper stage interdigital electrode 2a are connected to the input terminal IN and the ground point, and one electrode of each interdigital electrode 3a, 3a' is connected to the ground point, and the lower stage interdigital electrode 2a is connected to the ground point. Digital electrode 2
A pair of interdigital electrodes 3b and 3b' are respectively connected to the output terminal OUT and the ground point, and each interdigital electrode 3b, 3b'
Connect one electrode to the ground point. Terminal impedances 5 and 5' are connected between the input and output terminals, respectively. Note that this terminal impedance 5, 5' is UHF
A 50Ω resistor, which is often used in bands, may be used.

In this example, as the piezoelectric substrate 1, a lithium tetraborate (L x z B 40□) 45° rotated X plate Z propagation substrate was used. In this case, the sound speed of the surface acoustic wave is approximately 34
The speed is 40m/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 l. The electrodes were made of aluminum. However, since the Li, B, and 07 substrates dissolve in the AQ etching solution, the electrodes were formed by the lift-off method.The gratings of the 0 reflectors 4 and 4'' were metal strips made of aluminum.
Considering the surface acoustic wave reflectance per AQ strip, the number of strips that can sufficiently reflect surface acoustic waves is 100.
I selected a book. Further, the maximum crossing width W was set to 170λ.

Weighting for transverse mode suppression was performed by linear apodization in this example.

In the surface acoustic wave resonator having the above configuration, two devices having the following parameters were created.

The first device is an interdigital electrode 2a.

The logarithm of 2b is 46.5 pairs, and the logarithms of interdigital electrodes 3a, 3a', 3b, and 3b' on both sides are respectively,
36.5 pairs and interdigital electrodes 2a
, 2b, 3a, 3a', 3b, and 3b' (that is, the sum of the width of the aluminum electrode fingers and the interval between the electrode fingers) is 4.45 μm, and the interdigital electrodes are arranged on both sides of the interdigital electrodes 2a and 2b. Electrodes 3a, 3a
' and 3b.

3b' were placed close to each other with a distance of 2.2 μm. However, this distance is the distance between the edges of the interdigital electrodes. Furthermore, the interdigital electrode 3a
, 3a', 3b, and 3b', reflectors 4 and 4' each consisting of 100 metal strips are placed on both sides of 1.95
They were installed at a distance of μm (LX R) from each other. reflector 4
, 4' was set at a pitch of 4.5 μm, and the AQ film thickness of the interdigital electrodes and reflector was set at 1000 in consideration of electrode finger resistance and center frequency variation during manufacturing. Note that the width of each electrode finger of the interdigital electrode and the reflector is one half of the pitch.

The surface acoustic wave wavelength λ at the center frequency of this device is 9.0 pm. Therefore, the LIR is 01217λ.

FIG. 4 shows the frequency response of this device. From the same figure, the electrical characteristics are a center frequency of 380 MHz and a fractional bandwidth of 0.
5%, insertion loss 2 dB, in-band ripple 1 dB, out-of-band attenuation 60 dB, low frequency side out-of-band spurious -33 dB
It can be seen that the properties are sufficient for practical use.

The second device is an interdigital electrode 2a.

The logarithm of 2b is 50.5 pairs, the interdigital electrode 3a,
The number of logarithms of 3a', 3b, and 3b' is 36.5 pairs, the pitch of the interdigital electrodes is 4845 μm, the pitch of 1 reflector is 4.5 μm, the distance between the interdigital electrodes is 2.25 μm between the edges, and the pitch of the interdigital electrodes is 4845 μm. The distance LIR between the reflectors was set to 2.04 μm (0,227λ).

FIG. 5 shows the frequency characteristics of the second device.

From Figure 5, the center frequency is 380MHz as an electrical characteristic.
, a fractional bandwidth @0.5%, an insertion loss of 2 dB, an in-band ripple of 1 dB, an out-of-band attenuation of 60 dB, and a low-frequency out-of-band spurious of 50 dB. It can be seen that the characteristics are sufficient for practical use.

The third device is an interdigital electrode 2a.

The logarithm of 2b is 48.5 pairs, the interdigital electrode 3a,
The logarithm of 3a', 3b, and 3b' is 34.5 pairs, the pitch of the interdigital electrodes is 13.3 μm, and the distance MLXR between the interdigital electrodes and the reflector is 2.80 μm (
0,209λ). The electrical characteristics of this third device are: center frequency 250 MHz, fractional bandwidth 90.5%, insertion loss 1.6 dB, in-band ripple 1.2 dB, out-of-band attenuation 60 dB, and low-frequency side out-of-band spurious -36 dB.
It showed an excellent value.

In the above description, the structure shown in FIG. 1, in which surface acoustic wave resonators having the same structure are connected in cascade in two stages, has been considered. However, it is possible to suppress the spurious level on the low frequency side in a surface acoustic wave resonator with only one stage by setting the distance LX, R between the interdigital electrode and the reflector to a range of 0.20λ to 0.23λ. It's obvious.

Incidentally, in the above embodiment, a 45-degree rotated
It goes without saying that the present invention can also be applied to composite substrates in which a piezoelectric substance is attached to aO, all other piezoelectric materials, or glass. Further, when Li2B,07 is used as the piezoelectric substrate, the cut surface may be within the range of the X plate rotated by 45°±5°, and the propagation direction may also be within the range of ±5° from the Z direction.

Furthermore, in the experiment, a device with a center frequency of 380 MHz was created, but as a feature of surface acoustic wave devices, the center frequency can be freely changed by changing the line width and period of the interdigital electrodes and the brating reflector. Therefore, the present invention is not limited by the center frequency of the surface acoustic wave device. In addition, in the above-mentioned embodiments, even-numbered resonance modes are used, and the fourth-order mode is suppressed as a higher-order mode. However, by changing the symmetry of the arrangement of the interdigital electrodes, odd-numbered resonance modes, such as the first, third, and fifth-order resonance modes, can be used to create higher-order modes such as the fifth-order mode. The present invention can also be used in the case of suppression.

Note that in both the first device and the second device, the period of the grating reflector was adjusted to 1/2 of the wavelength of the surface acoustic wave at the center frequency.

Furthermore, in the above embodiment, in order to suppress the transverse mode,
Weighting was performed using linear apodization, but other transverse mode suppression methods include cosine apodization, cosine squared apodization, and modified cosine apodization.
These weightings may be performed using a method.

In addition, when actually manufacturing the surface acoustic wave device of the present invention in which the necessary parameters are set within the above-mentioned ranges, for example, a shield electrode having an appropriate width is provided between the input and output interdigital electrodes. It goes without saying that it is desirable to prevent direct waves between the input and output interdigital electrodes by grounding or by grounding the electrode fingers at the ends of the input and output interdigital electrodes.

In addition, in the above explanation, only the use of surface acoustic waves has been explained, but it is not necessarily limited to this.
The same applies to Love waves, 5SBWs, SIi waves, leaky waves, Bluestein-Fourier waves, Shimizu waves, etc.

In other words, it has already been proven that a resonator with interdigital electrodes can also excite each of the waves mentioned above, and it is possible to confine the vibrational energy under the electrodes, and there are conditions for generating multimode vibrations. It is.

Note that the method of connecting the electrodes to the input/output terminals (IN and 0UT) is not limited to the above embodiment; for example, the interdigital electrodes 3b, 3b' may be connected to the input terminal IN, and the interdigital electrodes 3a 23a' may be output. Terminal OUT
Alternatively, the interdigital electrodes 2a and 2b may be connected to each other. 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 symmetrical.

[Effects of the Invention] As explained above, the present invention provides three interdigital electrodes on a 45@±5° rotating X-plate Li2B4O7 substrate, when the Z direction ±5° is the surface acoustic wave propagation direction. of these interdigital electrodes, the central interdigital electrode 1! Connect the pole to the input terminal,
In a surface acoustic wave resonator in which the remaining interdigital electrodes are connected to an output terminal in parallel, at least one set of reflectors is arranged on both sides of the interdigital electrodes, and the zero-order and second-order resonance modes are utilized. The distance LzR between the interdigital electrode and the reflector (where n
is O or a positive integer, and λ is the surface acoustic wave wavelength of the center frequency), so there is an effect that the out-of-band spurious level on the low frequency side can be reduced. Moreover, the surface elastic resonator filter having the above configuration has the advantage of having a wide bandwidth, low insertion loss, and low in-band ripple.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing a configuration example of a surface acoustic wave resonator filter to which the present invention is applied. Of these, FIG. 1(,) is an overall view, and FIG. 1(b) is an interdigital 1! An enlarged sectional view illustrating the distance LIR between the pole and the reflector, Figure 2 is a diagram showing the frequency response in a conventional surface acoustic wave resonator, and Figure 3 is the distance itlL between the interdigital electrode and the reflector.
A diagram showing the relationship between zg/λ and the out-of-band spurious level. Figures 4 and 5 are diagrams showing the frequency response of the device obtained by specifically setting each parameter within the range proposed by the present invention. It is. 1...Piezoelectric substrate, 2a, 2b, 3a, 3b...
Interdigital electrodes, 4a, 4b...reflector. Figure 1(a)

Claims (3)

[Claims] (1) A plurality of interdigital electrodes are arranged close to each other along the surface acoustic wave propagation direction on a piezoelectric substrate, at least one of these interdigital electrodes is connected to an input terminal, and the remaining interdigital electrodes are connected to an input terminal. In a surface acoustic wave resonator that is connected to an output terminal and utilizes a plurality of resonance modes by disposing at least one set of reflectors on both sides of the interdigital electrode, the distance L_I_R between the interdigital electrode and the reflector is , (0.20+n/2)λ≦L_I_R≦(0.23+n
/2) A surface acoustic wave resonator characterized in that the setting range is λ (where n is 0 or a positive integer, and λ is the surface acoustic wave wavelength of the center frequency). (2) When the Z direction ±5° of the 45° ± 5° rotating X plate Li_2B_4O_7 substrate is the surface acoustic wave propagation direction, three interdigital electrodes are placed close to each other on the substrate,
Among these interdigital electrodes, the central interdigital electrode is connected to the input terminal, the remaining interdigital electrodes are connected in parallel to the output terminal, and at least one set of reflectors is provided on both outer sides of the interdigital electrodes. In a surface acoustic wave resonator that utilizes zero-order and second-order resonance modes, the distance L_I_R between the interdigital electrode and the reflector is (0.20+n/2)λ≦L_I_R≦(0.23+n
/2) A surface acoustic wave resonator characterized in that the setting range is λ (where n is 0 or a positive integer, and λ is the surface acoustic wave wavelength of the center frequency). (3) A surface acoustic wave resonator filter comprising a plurality of surface acoustic wave resonators according to item 2 connected in series.