JP3595034B2 - Triple mode piezoelectric filter and frequency adjustment method thereof - Google Patents

Description

[0001]
The present invention relates to an improved method of adjusting the frequency adjustment of the multimode piezoelectric filter and its, in particular to a frequency adjustment method may fit the bandwidth and band ripple triple mode filter to desired characteristics.
[0002]
[Prior art]
Conventionally, two pairs of electrodes are disposed close to each other on both sides of a piezoelectric plate to excite two frequencies by acoustic coupling between the electrodes, and a double band having twice the frequency between the two frequencies as a pass band. Mode monolithic crystal filters (hereinafter referred to as MCFs) are well known. The principle of the MCF will be described with reference to the drawings. 4A and 4B are electrode structure diagrams showing an example of an MCF, wherein FIG. 4A shows an electrode structure on one surface and FIG. 4B shows an electrode structure on the other surface. As shown in FIG. 4A, two electrodes 2 and 3 are formed on a piezoelectric plate 1 by depositing gold (Au), silver (Ag), aluminum (Al), or the like by vapor deposition or the like. The input / output electrodes 6 and 7 are extended, and two electrodes 2 'and 3' are formed on the other surface corresponding to the two electrodes (a) as shown in FIG. In addition, when both are formed close to each other, two vibrations are strongly excited. Of these, vibration having a low frequency and symmetrical vibration displacement is called a symmetric mode (frequency f1), and vibration asymmetrical at a high frequency is called an antisymmetric mode (frequency f2). FIG. 4C is an electrical equivalent circuit diagram viewed from the input / output terminals 6 and 7 and the common terminal 5 in which the back electrodes 2 ′ and 3 ′ are connected by lead wiring as shown in FIG. 4B. As shown in the figure, a ladder circuit is formed, and if properly terminated, a filter is formed.
[0003]
The center frequency of the filter is determined by the addition of the mass of the electrodes 2, 2 ', 3 and 3' if the thickness of the piezoelectric plate 1 and the electrode dimensions are constant, and the frequency difference df = f2-f1 which determines the bandwidth is shown in FIG. ) And (e), it is determined by the frequency reduction amount of the electrode and the electrode gap g. Here, the amount of frequency decrease refers to a difference between the frequency of the piezoelectric substrate and the frequency when the frequency is shifted by adding an electrode material to the substrate. That is, when the amount of decrease in the frequency of the electrode increases or the electrode gap increases, the coupling between the two modes decreases, and the frequency difference df decreases. On the other hand, if a mass is added to the gap between the electrodes, the coupling between the two modes increases, and the frequency difference df can be widened. When a filter is constituted by this double mode resonator, about twice df becomes the filter bandwidth, which corresponds to a second-order filter. The frequency arrangement is calculated based on the required specifications, and the impedance Z0 is set. Since the terminal impedance Z0 is determined by the product of the bandwidth and the inductance of the equivalent circuit, the motional inductance L1 is calculated from the bandwidth and the terminal impedance. The inductance L1 and the capacitance C0 are determined by the thickness of the substrate and the electrode area. With this, all the parameters required for the dual mode resonator are prepared. After that, the dual mode resonator is adjusted by the frequency drop amount of each electrode so as to match the calculated frequency arrangement by using the method of FIG. 4D or the above-described method of adding mass between the electrodes. I do.
[0004]
On the other hand, 4, 6, 8, and 10th-order (pole) filters in which several second-order filters (referred to as two-pole MCFs) are connected in cascade have been commercialized. Research and development on high-order MCFs in which three or more electrode pairs are arranged on one piezoelectric plate have been performed for a long time. In the example shown in FIGS. 5A and 5B, a case will be described in which a 35 ° rotation thickness-slip vibration (AT plate) having good frequency temperature characteristics among the rotation Y plates is used as the piezoelectric plate. The electrodes 2-2 ', 3-3' and 4-4 'are formed on the quartz plate 1 by a method such as vapor deposition using a metal such as gold (Au), silver (Ag), aluminum (Al) or the like as an electrode material. Generally, the gaps g1 and g2 between the electrodes are formed to be equal, and there are many designs in which the area of each electrode is made equal and the inductance is the same. Generally, the shape of each electrode is determined by the impedance of the filter, suppression of spuriousness of the filter, and required filter dimensions.
[0005]
When three pairs of electrodes are arranged close to each other and electrically excited as shown in FIG. 5, in a resonance state, wave coupling is induced between the electrodes, and three resonance frequencies are strongly generated. According to the energy confinement theory, it is known that the vibration displacement becomes cosine-shaped in the electrode portion and exponentially attenuates in the portion without the electrode. Therefore, also in the case of the three-pole MCF, the acoustic coupling can be controlled by the electrode gap or the amount of frequency reduction. When g1 and g2 are widened, the coupling is weak, that is, the frequency interval becomes narrow, and the frequency reduction of the electrode becomes large. The coupling is then small, ie the frequency spacing is small. Further, in an anisotropic piezoelectric material such as quartz, the elastic constant varies depending on the cutting orientation, so that the coupling coefficient, that is, the frequency interval varies depending on the crystal axis and the electrode arrangement. In the case of the AT plate, the configuration in which the electrodes are arranged along the Z 'axis has a small coupling coefficient, and the configuration in which the electrodes are arranged along the X axis has the maximum coupling coefficient.
[0006]
FIG. 5A is referred to as an A surface, and FIG. 5B is referred to as a B surface, and the lead wiring of each of the electrodes 2, 3, and 4 on the A surface is fixed to an external hermetic terminal with a conductive adhesive or the like. The electrodes 2 ', 3', and 4 'on the B side can be shared by the lead wiring 5' as shown in FIG. 5B, and the number of terminals can be reduced. Alternatively, the electrodes may be connected to external terminals independently of each other, but the phase relationship differs only depending on the connection method, and a short circuit is generally used unless the phase characteristics are more important than the amplitude characteristics. In the configuration in which the B-side electrode is short-circuited by the lead wire portion, when adjusting by lowering the frequency by a method such as vapor deposition, there is no occurrence of a defect such as a short-circuit between the electrodes, and the practicability is high. When the lead wire 5 'is connected to the lead wire 5 of the electrode 3 on the A side with a conductive adhesive, the three-pole MCF can be constituted by three external terminals.
[0007]
FIG. 5C shows a schematic diagram of a cross section, and shows an electrode pair sandwiching the piezoelectric substrate and its symbol. When the three-pole MCF resonator is electrically excited, the energy of the wave is trapped under the electrode, and the vibration displacement attenuates exponentially in the periphery without the electrode, and as a result, the three modes resonate vigorously. Of these three modes, S-0 is a mode in which the vibration displacement is symmetric and the frequency (f1) is low, A-0 is a mode in which the vibration displacement is an antisymmetric mode and the frequency (f2) is the second, and a frequency is a symmetric displacement and the frequency is (f2). The mode with the highest (f3) is called S-1. The fact that these vibrations have a displacement distribution as shown in FIG. 5D has been studied in detail by X-ray topography and other techniques. It is also well known that four terminals connecting terminals 5 and 5 'with terminals 6 and 7 as input / output terminals are represented by an equivalent circuit of FIG. It is self-evident that the filter can be constructed by appropriately terminating the circuit.
[0008]
The small multi-electrode MCF was developed as an intermediate frequency filter of a wireless device, and was partially put to practical use.
However, in the above-mentioned multi-electrode MCF, one large piezoelectric substrate was still expensive, the method of adjusting the frequency of the multi-electrode MCF was complicated and took too much time, and the yield was poor because the adjustment method was difficult. There was a problem. For these reasons, it became expensive and the multi-electrode MCF was not used in the general radio field. Accordingly, a technique for adjusting the frequency of a high-order MCF in which three or more electrode pairs are arranged on both sides of a single piezoelectric plate has not been studied yet. For example, taking a three-pole MCF as an example, the frequency interval between f1 and f2 can be adjusted by a method similar to the frequency adjustment method of the two-pole MCF, but at this time, the frequency is shifted to a frequency f3 of a mode which is not originally desired to be shifted. This result three resonance frequencies of the acoustic coupling, it is impossible to deal independently any one mode for a wave to be excited. If the frequency of the excited three waves does not match the designed frequency, the filter characteristics will cause ripples in the band and the pass band will not be symmetric. As described above, it is extremely difficult to adjust and match all three frequencies f1, f2, and f3 to desired frequencies, and it has not been widely used in intermediate frequency filters for general wireless devices that emphasize economic efficiency.
[0009]
The present invention has been made to solve the frequency adjustment method of each mode, which is a problem of the conventional high-order MCF, and is a frequency capable of easily and economically adjusting the frequency of a three-pole MCF resonator. An object of the present invention is to provide a triple mode piezoelectric filter having a symmetrical pass band with little ripple by an adjustment method and fine adjustment .
[0010]
Summary of the Invention
In order to achieve the above object, according to the present invention, when the three-wave resonance frequency of a three-pole MCF having three pairs of electrodes arranged on both sides of a piezoelectric plate is set to f1, f2, and f3 from a low frequency, Of the center electrode and adjust the frequency difference (f3-f2) while keeping the frequency difference (f3-f1) substantially constant, or adjust the mass of the electrodes at both ends of the three electrodes (f3 −f1) is kept almost constant, and the frequency difference (f2−f1) is adjusted to manufacture a three-pole MCF resonator.
[0011]
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, the present invention will be described in detail with reference to the illustrated embodiments. The electrode configuration on the surface A of the three-pole MCF used in the following examples is assumed to be the same as that shown in FIG. 5A, but only the electrode configuration on the surface B which is important for the description of the present invention is shown in FIG. a). That is, three pairs of electrodes 2-2 ', 3-3', and 4-4 'are formed on both surfaces of the piezoelectric plate 1 as shown in FIG. As the electrode material, for example, gold (Au), silver (Ag), aluminum (Al), or a mixture thereof with copper is used, and is formed by a method such as vapor deposition or sputtering. As described above, the electrodes 2, 3, and 4 have no change in the amplitude characteristic of the filter even when connected to the common lead 5 except for the phase relationship between the terminals of the finally formed 3-pole MCF resonator. is there. Of course, it is possible to construct a filter in which the lead wires of each electrode are connected to independent external terminals and focus on the phase characteristics. However, here, the simplest case in which the lead wires of each electrode on the B side are collected in a common part will be described. .
[0012]
When the mass of the electrodes at both ends of the three electrodes arranged side by side as shown by hatching in FIG. 1A is added to the mass by a method such as vapor deposition to lower the frequency, the frequency arrangement shown in FIG. It shifts as shown in FIG. In this graph, both ends of a four-terminal network having terminals 6 and 7 as input / output terminals and terminal 5 as a ground terminal are terminated with a resistance (for example, 50 ohms) sufficiently lower than the impedance of the filter. When the frequency is changed by connecting a signal generator (SG) to the output terminal, the signal level appearing between the output terminals 7 and 5 is recorded. Referring to FIG. 1C with reference to FIG. 1B, the frequency shift amount between the frequency f1 ′ and the frequency f3 ′ is substantially the same, and the frequency difference (f3′−f1 ′) is substantially constant, but the frequency f2 Only the amount of change in 'increases, and the frequency difference (f2'-f1 ') decreases. This indicates that when the mass of the electrodes at both ends is changed, the vibration displacement having the largest frequency change is the antisymmetric mode A-0 (frequency f2). That is, when the same mass is added to the vibrating portion of the vibrating body, the frequency change amount becomes maximum when the position is the maximum portion of the vibration displacement. Conversely, when the mass of the electrodes at both ends is reduced, the frequency difference (f2'-f1 ') increases while (f3'-f1') remains almost constant.
[0013]
Next, when the frequency is changed by adding mass to the center electrode among the three electrodes arranged close to each other as shown by diagonal lines in FIG. It has been experimentally confirmed that the symmetric mode is S-0 (frequency f1), the next is the symmetric mode S-1 (frequency f3), and the mode with low sensitivity is the antisymmetric mode A-0 (f2). Therefore, when the mass is added to the center electrode of the three electrodes of the three-pole MCF to reduce the frequency, the frequency variation of the antisymmetric mode A-0 is small, so that the frequency difference (f3′−f1 ′) is kept almost constant. The frequency difference (f3'-f2 ') can be reduced while keeping the same. Conversely, if the center electrode mass is removed to increase the frequency, the frequency difference (f3'-f2 ') can be increased.
[0014]
It should be noted that even if the mass of any of the three electrodes of the three-pole MCF is changed, it affects the entire frequency to some extent, so a method of lowering the entire frequency without disturbing the frequency arrangement is necessary. However, if the mass is added to the entire three electrodes using a method such as vapor deposition, the three frequencies move in parallel, so that method may be used. By using the above-described two frequency adjustment methods of the present invention and the method of moving the entire frequency in parallel, it is possible to match the three frequencies of the actual filter to the frequency arrangement of the designed three-pole MCF.
[0015]
FIG. 3 is a diagram showing a frequency arrangement and passband characteristics of a three-pole MCF. When manufactured based on the design, the frequency arrangement shown in FIG. Here, the center frequency was 58.125 MHz, the pass band width was 16 kHz (3 dB), the area of the electrode was 1.2 square mm in consideration of spurious, and the electrode gaps g1 and g2 were calculated from the bandwidth and the amount of frequency drop. In this state, the passband characteristic becomes as shown in FIG. 3 (b), ripples occur in the band, and the band is not symmetric. Therefore, when the frequency arrangement of the three-pole MCF is made to match the frequency arrangement of the calculated values as shown in FIG. 3C, the passband characteristics of the filter are as shown in FIG. A filter having a characteristic whose bandwidth is also symmetric is obtained. As can be seen, in order to minimize the in-band ripple of the 3-pole MCF and to make the bandwidth symmetrical with respect to the center frequency, adjust the three frequency arrays to match the values calculated by the calculation. Is very important from these graphs.
[0016]
As described above, the present invention has described the method of adjusting the frequency of a three-pole MCF using a quartz AT plate. However, the present invention is not limited to this, and the present invention is also applicable to a three-pole MCF using another piezoelectric material. Of course, it can be applied. For example, LiTaO3, LiNbO3, piezoelectric ceramic, or the like may be used. As for the frequency adjustment, the electrode for adjusting the mass has been described on the surface B, but the effect of the present invention is exactly the same even if the mass of the electrode on the surface A is adjusted. Although the vapor deposition method has been described as an example of the mass adjustment method, a sputtering method or an electron beam method for thinly shaving an electrode may be used. Although the shape of the piezoelectric plate is exemplified by a circular plate, a rectangular plate may be used, and the electrode formed on the piezoelectric plate is not necessarily required to be rectangular, and the effect of the present invention is affected even if it is elliptical. Not.
[0017]
【The invention's effect】
As described above, according to the present invention, it is possible to finely control the three frequencies of the three-pole MCF, and particularly to freely control the frequency f2 of the anti-symmetric mode A-0, thereby adjusting the frequency f2 to a desired frequency arrangement. A triple mode piezoelectric filter having characteristics can be obtained, and a filter having one order higher than that of a two-pole MCF can be economically manufactured in almost the same shape, and the effect is extremely large.
[Brief description of the drawings]
1 (a), 1 (b) and 1 (c) are a plan view and a frequency shift chart showing an embodiment of the present invention.
2 (a), 2 (b) and 2 (c) are a plan view and a frequency shift chart showing another embodiment of the present invention.
3 (a), (b), (c) and (d) are frequency arrangements according to the present invention and filter characteristics thereof.
FIGS. 4 (a), (b), (c), (d), and (e) are schematic diagrams of electrodes, circuit diagrams, frequency reduction amounts, and the relationship between electrode gaps and frequency intervals df for explaining a two-pole MCF. Relationship diagram.
FIG. 5 is a schematic diagram of an electrode, a sectional view, a vibration displacement, and an electric equivalent circuit showing the principle of a three-pole MCF.
[Explanation of symbols]
1 piezoelectric substrate 2, 2 ', 3, 3', 4, 4 '... electrodes 5, 5', 6, 7 ... lead electrode df ... resonance frequency f2, difference f1, f1 f2, f3 ... resonance frequencies f1 ', f2', f3 '... adjusted resonance frequencies g, g1, g2 ... gaps between electrodes S-0, S-1 ... symmetry mode A-0 ... Antisymmetric modes

Claims (1)

In a triple mode piezoelectric filter formed by arranging three pairs of electrodes on both sides of a piezoelectric plate, when the resonance frequencies of three waves excited by the electrodes are set to f1, f2, and f3 in ascending order, the two ends of the electrodes A frequency adjustment method for a triple mode piezoelectric resonator, characterized in that the frequency difference (f2-f1) is arbitrarily adjusted while the frequency difference (f3-f1) is kept substantially constant by adjusting the mass of the electrodes of the set. .

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