JP5179290B2 - Surface acoustic wave filter - Google Patents

Description

  The present invention relates to a surface acoustic wave filter using a resonator-type unidirectional electrode, and more particularly to a surface acoustic wave filter capable of improving group delay characteristics.

  SAW (surface acoustic wave) filters, which are surface acoustic wave filters, are widely used in the communication field because of their high performance, miniaturization, mass productivity, etc. As one of them, a resonator-type unidirectional electrode is used. There is something used. The structure of this SAW filter will be briefly described with reference to FIG. 1 which is an embodiment of the present invention. An input side electrode 3A and an output side electrode 3B each made of a resonator type unidirectional electrode are formed on a piezoelectric substrate 21. The shield electrodes 23 are formed side by side in the SAW propagation direction. The input side electrode 3A and the output side electrode 3B are composed of bus bars 31a and 31b, cross-finger electrodes 32a and 32b for exciting the SAW, and dummy electrodes 33a and 33b that do not contribute to the excitation.

  A SAW filter using a resonator-type unidirectional electrode has strong internal reflection, and a reflective surface for confining the SAW energy exists in the SAW propagation direction (busbar extension direction). It exists in the center (excitation center) of the electrode 32a, 32b. The SAW filter utilizes multiple reflections between the reflecting surfaces so that the SAW can be propagated from the input side electrode 3A to the output side electrode 3B. The widths 32a and 32b are appropriately adjusted.

  By the way, in a communication device used in a base station recently, a communication module can be configured so that high-quality communication can be performed up to the end of the passband for reasons such as suppressing deterioration in modulation accuracy on the terminal side. It has been requested. More specifically, this requirement is required to satisfy not only good amplitude characteristics from end to end of the passband but also group delay characteristics. The group delay characteristic refers to a maximum time and a minimum time in the pass band with respect to a delay time from when a frequency signal is input to the input side electrode 3A until a signal corresponding to the frequency signal is output from the output side electrode 3B. It is the characteristic which shows the relationship between the difference (group delay deviation) and a zone | band. FIG. 9 described in an embodiment described later shows an example of the group delay characteristic together with the amplitude characteristic. The left vertical axis indicates the signal intensity, and the right vertical axis indicates the delay time. In FIG. 9, f0 is the center frequency of the passband. The delay time changes according to the frequency, and it looks like a wave is hitting the frequency axis. The difference between the maximum time and the minimum time of the delay time is small in the band excluding the end in the pass band. That is, the change width of the delay time wave is small as seen from the frequency axis.

  However, since the delay time is considerably longer at the end of the pass band than at the center band, the group delay deviation is large in the pass band including the end. When the propagation distance of the surface acoustic wave between the input side electrode 3A and the output side electrode 3B is D, the present inventor determines the propagation distance D as a dimension in the propagation direction of the SAW filter (both ends in the propagation direction for the electrode group region). It was found that D / L standardized by L) and the quality of the group delay deviation in the passband including the end portion are related. The propagation distance D (mm) has a relationship of T = D / V where V (m / s) is the propagation velocity of the surface wave and T (μs) is the delay time for the frequency signal of the center frequency f0 in the passband. . If the piezoelectric substrate is a quartz substrate, V is 3150 m / s (seconds).

  Conventionally, since the ratio D / L has not been set appropriately, it has been found that the group delay deviation in the pass band including the end portion is large. The reason for this is that the ratio D / L has been set to about 0.68 in the prior art because of the demand for fast propagation of surface acoustic waves in a flat group delay deviation bandwidth.

  By the way, if you want to obtain good group delay characteristics up to the end of the required passband, you can consider a method to ensure a margin if you widen the passband on the filter side. Attenuation in the vicinity of the band cannot be obtained sufficiently, and the specifications of the communication device are not satisfied.

  On the other hand, as a SAW filter that solves such a problem, in Patent Document 1, a resonator-type unidirectional electrode on an input side and an output side is stacked on each other via an insulating film on a piezoelectric substrate, and a SAW propagation distance is obtained. A SAW filter that improves the group delay time by shortening the frequency is described. Also, in Patent Document 2, the resonator-type unidirectional electrodes on the input side and the output side are arranged obliquely, and the mutual excitation centers are described. Describes a SAW filter that improves the group delay time by changing the distance of based on the SAW frequency.

Japanese Patent Laid-Open No. 01-319314 JP 2002-057551 A (paragraph 0016)

  The present invention has been made under such circumstances, and an object thereof is to provide a SAW filter capable of obtaining a good group delay characteristic up to the end of the passband.

A resonator-type unidirectional electrode configured by providing a pair of bus bars facing each other on a piezoelectric substrate and a plurality of electrode fingers extending in a comb shape so as to intersect with each bus bar. In the surface acoustic wave filter that constitutes the electrode and has a shield electrode provided between the input side electrode and the output side electrode,
The delay time from when the frequency signal of the center frequency of the pass band is input to the input side electrode until the signal corresponding to the frequency signal is output from the output side electrode is T (μs), and the propagation speed of the surface acoustic wave is Assuming V (m / s), the propagation distance D (mm) of the surface acoustic wave from the input electrode to the output electrode represented by T × V is set to the output electrode from the end of the input electrode opposite to the shield electrode. A surface acoustic wave filter characterized in that D / L normalized by a dimension L (mm) up to the end opposite to the shield electrode is 0.52 <D / L <0.65.

  In the present invention, the propagation distance D from the excitation center of the input-side electrode to the excitation center of the output-side electrode, which is determined by the delay time T and the propagation speed of the surface acoustic wave on the piezoelectric substrate, is propagated through the electrode arrangement region in the SAW filter. Distance between both ends in direction (dimension from the end of the input electrode opposite to the shield electrode to the end of the output electrode opposite to the shield electrode) L / D / L value normalized by L Since (0.52 <D / L <0.65) is set, it is possible to obtain favorable characteristics for the amplitude characteristics and the group delay time characteristics up to the end of the passband.

  The SAW filter 2 according to the embodiment of the present invention is manufactured for a GSM base station having a relative bandwidth of 0.18% using an ST cut quartz wafer as the piezoelectric substrate 21 as shown in FIG. It is comprised so that it may have. An input side electrode 3A and an output side electrode 3B each made of a resonator type unidirectional electrode are provided on the piezoelectric substrate 21, and a shield electrode 23 is formed between the input side electrode 3A and the output side electrode 3B. ing.

  The input side electrode 3A and the output side electrode 3B have a pair of bus bars 31a and 31b extending in parallel along the direction in which the SAW propagates on the piezoelectric substrate 21, and crossing from one bus bar 31a. The finger electrode 32a extends toward the other bus bar 31b, and the cross finger electrode 32b extends from the other bus bar 31b toward the one bus bar 31a. These cross finger electrodes 32a and 32b correspond to IDT (Inter Digital Transducer).

  Specifically, the cross finger electrodes 32a and 32b are not alternately arranged at equal intervals, and one or a plurality of cross finger electrodes 32a are sandwiched between the cross finger electrodes 32b and one Alternatively, a plurality of cross finger electrodes 32b are sandwiched between the cross finger electrodes 32a. A dummy electrode 33a extending from the bus bar 31b is provided on the extended line of the cross finger electrode 32a, and a dummy electrode 33b extending from the bus bar 31a is provided on the extended line of the cross finger electrode 32b. The role of these dummy electrodes 33a and 33b is to charge positively or negatively when a high-order wave in the transverse mode propagates, thereby neutralizing the charges excited by the cross-finger electrodes 32a and 32b, thereby providing a bus bar 31a, It has a role of suppressing the energy of electric charge leaking near 31b.

  The number, width, and arrangement interval of the cross-finger electrodes 32a, 32b of the input side electrode 3A and the output side electrode 3B are the center part in the SAW propagation direction in the input side electrode 3A and the center part in the SAW propagation direction in the output side electrode 3B. The SAW is configured to be reflected multiple times and strongly propagated (in one direction) from the input side electrode 3A side to the output side electrode 3B side. Further, the ratio W / W0 of the intersection width (W) between the cross finger electrode 32a extending from each bus bar 31a and the cross finger electrode 32b extending from the bus bar 31b and the opening length (W0) between the bus bars 31a and 31b is For example, 0.85.

  The shield electrode 23 includes bus bars 24a and 24b parallel to the bus bars 31a and 31b, and electrode fingers 25 connecting the bus bars 24a and 24b, and electromagnetic waves between the input side electrode 3A and the output side electrode 3B. It has the role of suppressing out-of-band floor levels by suppressing static and electrostatic coupling.

  The excitation center A1 of the input side electrode 3A is located near the middle of the SAW propagation direction in the input side electrode 3A, and the excitation center B1 of the output side electrode 3B is located near the middle of the SAW propagation direction in the output side electrode 3B. ing. The distance between the excitation center A1 of the input-side electrode 3A and the excitation center B1 of the output-side electrode 3B corresponds to the SAW propagation distance D. The propagation distance D is defined as SAW propagation speed V and SAW delay time. Assuming T, there is a relationship of T = D / V. For example, when a quartz substrate is used as the piezoelectric substrate 21, the propagation velocity V is 3150 m / s, and when the delay time T = 3.0 μs at the center frequency f0 of the pass band, the propagation distance D is 9.45 mm. It becomes.

  The dimension from the end A2 of the input side electrode 3A opposite to the shield electrode 23 to the end B2 of the output side electrode 3B opposite to the shield electrode 23, that is, the dimension between both ends of the SAW filter is L (mm). Then, the D / L obtained by standardizing the SAW propagation distance D (mm) at the center frequency f0 of the passband by the L (mm) is set to 0.52 <D / L <0.65.

  In the SAW filter configured as described above, when a frequency signal is applied to the input side electrode 3A, the cross finger electrode 32b of the input side electrode 3A is excited, and SAW is generated on the piezoelectric substrate 21. This SAW propagates on the piezoelectric substrate 21, is converted into a frequency signal by the output side electrode 3B, and an output signal is taken out with a delay time T from the time when it is applied to the input side electrode 3A.

  As described above, according to the SAW filter of this embodiment, the D / L obtained by normalizing the propagation distance D (mm) by the L (mm) is set to 0.52 <D / L <0.65, which will be described later. As is clear from the test results, good characteristics can be obtained for the amplitude characteristic and the group delay time characteristic up to the end of the passband.

  The test results supporting the effects of the present invention will be described below. In this test, the electrode period is λ and the electrode thickness is H. For example, the L is 1180λ, the film thickness H / λ normalized by the electrode period λ is 1.7%, and the specific bandwidth (passband) This is a measurement result when a quartz substrate is used as the piezoelectric substrate 21 under the condition that the frequency difference fd is divided by the center frequency f0) W is 0.32%.

  FIG. 2 shows the insertion loss on the vertical axis and D / L on the horizontal axis, and the insertion loss here indicates the minimum value of attenuation. As can be seen from FIG. 2, the insertion loss generally increases as D / L increases. The insertion loss is 7.0 dB or less because D / L is in the range of 0.67 or less. I can read.

  FIG. 3 shows the amplitude deviation on the vertical axis and D / L on the horizontal axis. The amplitude deviation indicates the difference between the maximum value and the minimum value of the insertion loss in the pass band. The smaller the amplitude deviation, the better the flatness of the pass band. From FIG. 3, it can be seen that the amplitude deviation generally decreases with an increase in D / L, and that the amplitude deviation in the passband is 1.7 dB or less is D / L is 0.52 or more.

  FIG. 4 shows the group delay deviation on the vertical axis and D / L on the horizontal axis. The group delay deviation is a difference between the maximum value and the minimum value of the delay time within the evaluation band in the specific bandwidth. Here, as the evaluation band, three of the first evaluation band, the second evaluation band, and the third evaluation band are set, and the first evaluation band is compared with the specific bandwidth W itself, and the second evaluation band is compared. A band of 0.875 with respect to the bandwidth W and a third evaluation band of 0.75 with respect to the specific bandwidth W were set. The center frequencies of the second evaluation bandwidth and the third evaluation bandwidth are the same as the center frequency of the specific bandwidth W. From FIG. 4, it can be seen that, in the first evaluation band (W × 0.75), the group delay deviation is 0.7 μs or less because D / L is 0.52 or more and 0.65 or less. Further, in the second evaluation band (W × 0.875), it can be read that the group delay deviation is 1.0 μs or less because D / L is 0.45 or more and 0.65 or less. Furthermore, in the third evaluation band (W × 1.0), it is understood that the group delay deviation is 1.6 μs or less when D / L is 0.52 or more and 0.65 or less. Therefore, it can be seen that the group delay deviation is low when D / L is in the vicinity of 0.52 <D / L <0.65.

FIG. 5 shows the specific bandwidth on the vertical axis and D / L on the horizontal axis. From FIG. 5, it can be seen that the specific bandwidth is substantially constant regardless of the change in the value of D / L. From this, it can be seen that the group delay characteristic is not affected by the fluctuation of the specific bandwidth.
Based on the above test results, the region where good group delay characteristics can be obtained is when D / L is in the range of 0.52 <D / L <0.65 or less.

  Next, amplitude characteristics and group delay characteristics of the SAW filter of the present invention having D / L of 0.57 are shown in FIG. The horizontal axis of FIG. 6 is a normalized frequency obtained by normalizing each frequency f (f / f0) with the center frequency of the passband being “1.0”. In the figure, reference numeral 41 denotes an amplitude characteristic, and 42 denotes a group delay characteristic. FIG. 6 shows that the delay time at the end of the pass band is large and the difference from the delay time at the center of the pass band is small. The insertion loss was 6.2 dB, the main delay time (delay time at the center frequency f0) was 2.9 μs, and the amplitude deviation was 1.4 dB. The group delay deviation is 0.55 μs for the first evaluation band (W × 0.75), 0.70 μs for the second evaluation band (W × 0.875), and the third evaluation band. When (W × 1.0), it was 1.28 μs. FIG. 7 shows the time response characteristics of FIG. 6. The maximum value can be read as 2.9 μs, and it can be seen that the main delay time is 2.9 μs. From the above, the insertion loss is almost the same as the conventional value, and a good amplitude characteristic is maintained. Further, the group delay time has little variation between the end portion and the center portion of the passband, and it can be said that the group delay characteristic is good.

  Next, amplitude characteristics and group delay characteristics of a conventional SAW filter having a D / L value of 0.68 are shown in FIG. It can be seen from FIG. 8 that the delay time is small at the end of the pass band and the difference from the delay time at the center is large. The insertion loss was 7.1 dB, the main delay time was 3.8 μs, and the amplitude deviation was 1.3 dB. The group delay deviation is 0.89 μs for the first evaluation band (W × 0.75), 1.15 μs for the second evaluation band (W × 0.875), and the third evaluation band. In the case of (W × 1.0), it was 2.01 μs. From the above, it can be seen that the group delay characteristic is bad at the end of the pass band, the insertion loss is large, and the group delay time is degraded when the evaluation band is increased.

  Next, a case where the value of D / L is smaller than the range of the present invention, for example, a case where the value of D / L is 0.33 will be described with reference to FIG. From this figure, it can be seen that the delay time is small at the end of the passband, and the difference from the delay time at the center is large as in FIG. 8, and the insertion loss is 5.3 dB, the main delay time. Was 1.8 μs, and the amplitude deviation was 2.8 dB. From the above, it can be said that although the insertion loss is small, it can be said that it is good, but since the value of the amplitude deviation is large, a flat in-band characteristic is not obtained.

  According to the embodiment of the present invention described above, the propagation distance from the excitation center A1 of the input side electrode 3A to the excitation center B1 of the output side electrode 3B determined by the delay time T and the propagation speed of the surface acoustic wave on the piezoelectric substrate. The value of D / L normalized by the distance L between the propagation direction ends of the electrode arrangement region in the SAW filter is set to an appropriate range of 0.52 <D / L <0.65. Good characteristics can be obtained for the amplitude characteristic and the group delay time characteristic up to the end of the passband.

It is a block diagram of the SAW filter which concerns on embodiment of this invention. It is the graph which showed the change of the insertion loss based on the change of D / L. It is the graph which showed the change of the amplitude deviation based on the change of D / L. It is the graph which showed the change of the group delay deviation based on the change of D / L. It is the graph which showed the change of the specific bandwidth based on the change of D / L. It is the graph which showed the amplitude characteristic and group delay characteristic which concern on embodiment of this invention. It is the graph which showed the time response characteristic which concerns on embodiment of this invention. It is the graph which showed the conventional amplitude characteristic and group delay characteristic. It is the graph which showed the conventional amplitude characteristic and group delay characteristic.

Explanation of symbols

21 Piezoelectric substrate 23 Shield electrode 3A Input side electrode 3B Output side electrode 32a, 32b Cross finger electrode D Propagation distance L Propagation direction dimension

Claims (1)

A resonator-type unidirectional electrode configured by providing a pair of bus bars facing each other on a piezoelectric substrate and a plurality of electrode fingers extending in a comb shape so as to intersect with each bus bar. In the surface acoustic wave filter that constitutes the electrode and has a shield electrode provided between the input side electrode and the output side electrode,
The delay time from when the frequency signal of the center frequency of the pass band is input to the input side electrode until the signal corresponding to the frequency signal is output from the output side electrode is T (μs), and the propagation speed of the surface acoustic wave is Assuming V (m / s), the propagation distance D (mm) of the surface acoustic wave from the input electrode to the output electrode represented by T × V is set to the output electrode from the end of the input electrode opposite to the shield electrode. A surface acoustic wave filter characterized in that D / L normalized by a dimension L (mm) up to the end opposite to the shield electrode is 0.52 <D / L <0.65.

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