JPS6217413B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a ladder-type piezoelectric filter composed of two types of vibrators with different parallel capacitances _.
The _frequency difference △ (= _a −
The purpose is to significantly expand the permissible range of _r ). Conventionally, when designing a ladder-type piezoelectric filter as shown in Fig. 1, the resonance frequency and piezoelectric vibration of piezoelectric vibrator A are considered as a pure reactance circuit with a dynamic resistance value of zero in the basic half-section circuit shown in Fig. 2. The antiresonance frequency of element B is made to match at ₀ of the impedance curve of the piezoelectric vibrator shown in FIG. If the impedance of piezoelectric vibrator A is Z _A and that of B is Z _B , the frequency where Z _A + Z _B = 0 is the cutoff frequency, and the terminal impedance is
Matching is performed from the image impedance value at ₀ . Further, the desired amount of attenuation is obtained by connecting basic half-interval circuits in multiple stages under the above frequency settings. However, with this configuration, at least three types of oscillators with different parallel capacitances are required, so generally the value of the parallel capacitance of the oscillator in the center of the filter and the oscillators arranged at the input and output terminals is Similarly, it is composed of two types of oscillators. On the other hand, in order to obtain a high guaranteed attenuation amount, it is necessary to increase the parallel capacitance ratio C _0B /C _0A , and when this is achieved using a vibrator, the following method is used. In Figure 1, the A type resonator has a Δ value determined from the circuit design, and in order to achieve a small capacity, it has a thick and completely polarized part as shown in Figure 4. It has an electrode structure. The B-type vibrator has a predetermined capacitance ratio and, in order to realize a large capacity, has a thin-walled, half-polarized electrode structure as shown in FIG. Furthermore, due to circuit design, the △ values of type A and type B resonators unilaterally affect the cutoff frequency of the filter, so in order to satisfy the desired filter standard, it is necessary to The variation in Δ values must be kept to a minimum. However, in the type A vibrator shown in FIG. 1, the Δ value changes greatly due to variations in the piezoelectric material itself, whether the partial electrode structure is formed by a method such as etching, or variations in the size and position of the partial electrodes. Furthermore, it is inevitable that the B type vibrator will have large variations in the Δ value depending on the degree of polarization. Therefore, it is extremely difficult to minimize the variation in the Δ value of the vibrator during manufacturing. On the other hand, if the resonant frequency of the A-type resonator and the anti-resonant frequency of the B-type resonator are simply arranged randomly without taking into account the variation in the Δ value, ripples may occur in the passband. , there is a high risk of breaking the standard in the attenuation range. In contrast, according to the present invention, the range of usable Δ values is significantly expanded by combining vibrators with different Δ values into a filter, taking into account the variation in Δ values of manufactured transducers. can do. That is, frequency matching was performed at ₀ in FIG. The average value of the △ values of the oscillators of A ₁ , A ₂ , ..., A _n is set to be the best value in the circuit design, and the △ values of B ₁ , B ₂ , ..., B _o are similarly set. Set the average value to be the best value for the circuit design. In order to avoid sudden changes in the cutoff frequency between adjacent basic sections, the transducer at the input/output end is always placed with the maximum Δ value, and the transducer at the other end is
When A-type vibrators placed in series are arranged so that the △ value gradually increases from the input side, B-type vibrators placed in parallel are similarly arranged so that the △ value gradually increases from the input side. do. Alternatively, if the A-type vibrators are arranged so that the Δ value gradually decreases from the input side, the B-type vibrators are similarly arranged so that the Δ value gradually decreases. . However, without following the present invention, if the above-mentioned oscillators are randomly assembled into a filter, ignoring the order of the A-type oscillators and the B-type oscillators, the pass band will be affected. A ripple is generated. This is due to the sudden change in cutoff frequency between adjacent fundamental sections. Next, as an example of the present invention, the center frequency is 516KHz.
An 11-element ladder filter constructed from disc-shaped PZT-based piezoelectric ceramic resonators will be explained below. (In Figure 1, m = 6, n = 5 configuration) In Figure 1, the resonant frequency _r and anti-resonant frequency _a of the vibrators A ₁ to _{A 6} and B ₁ to _{B 5} , and the resonance anti-resonant frequency difference △, Parallel capacitance C ₀ and mechanical Q _n values are the first
Shown in the table.

Table 1 It is clear from Table 1 that this arrangement is in accordance with the invention. The passband operation attenuation characteristic at this time is shown by a solid line in FIG. Furthermore, the dotted line in the figure indicates that the transducers shown in Table 1 are used, but the arrangement of the transducers arranged in series and the transducers arranged in parallel is completely random according to the present invention. The terminal impedance is
It is 2.0KΩ. As is clear from the above embodiments, if oscillators with different Δ values are installed at random, the passband characteristics of the filter will be disturbed and large ripples will occur. On the other hand, if the arrangement according to the present invention is adopted, good characteristics can be obtained without disturbing the passband, and variations in the Δ value of each vibrator can be tolerated to a considerable extent. Note that in a filter whose passband characteristics are relatively lenient in its specifications, a large effect can be obtained by simply incorporating a vibrator with the largest Δ value in the input/output end vibrator. In other words, according to the present invention, it is possible to incorporate a vibrator that could not be incorporated into a filter conventionally due to variations in the Δ value of the vibrator caused by manufacturing, thereby making it possible to build a low-cost, high-performance filter. Its industrial value is enormous.

[Brief explanation of the drawing]

FIG. 1 shows an electrical connection diagram of a ladder-type piezoelectric filter, where A ₁ , A ₂ , A ₃ , ..., A _n are thick-walled small-capacity piezoelectric vibrators arranged in series, B ₁ , B ₂ ,..., B _o
are thin-walled, large-capacity piezoelectric vibrators arranged in parallel. Figure 2 shows the basic interval equivalent circuit of a ladder-type piezoelectric filter, where C _0A and C _0B are parallel capacitances, L _A and L _B are dynamic inductances, C _A and C _B are dynamic capacitances, and R
_A and R _B are dynamic resistances. Figure 3 shows the reactance characteristics of the oscillators, where Z _A and Z _B are the reactance characteristics of oscillators A and B, respectively, _1∞ and _2∞ are the pole frequencies, ₁ and ₂ are the cutoff frequencies, and ₀ is the center frequency. FIG. 4 shows a thick-walled, small-capacity piezoelectric vibrator made up of partial electrodes, and FIG. 5 shows a thin-walled, large-capacity piezoelectric vibrator made up of full-surface electrodes. FIG. 6 shows the passband operation attenuation characteristics of a ladder-type piezoelectric ceramic filter, the solid line is the filter characteristic according to the construction method of the present invention, and the dotted line is the arrangement method using piezoelectric vibrators that obtained the characteristics shown in the solid line. This is the characteristic when is made random.

Claims (1)

[Claims] 1 Regarding parallel capacitance, in a ladder-type piezoelectric ceramic filter composed of two different types of oscillators,
The transducers at the input/output end are always placed with the highest △ value, and the transducers other than the input/output end are arranged so that the △ value gradually increases from the input end for the transducers placed in series. In this case, if the △ value is gradually increased for the vibrators placed in parallel, or if the △ value is gradually decreased from the input side for the vibrators placed in series, the vibrations placed in parallel. A method for configuring a ladder-type piezoelectric ceramic filter, which is characterized in that the Δ value is gradually decreased from the input side in the same way for the elements.