JPS6243364B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a ladder-type piezoelectric filter designed for imaging parameters, and an object of the present invention is to expand the permissible value of the capacitance ratio due to manufacturing variations in piezoelectric vibrators. Conventionally, when designing a ladder-type piezoelectric filter as shown in Figure 1, (here, the value of the parallel capacitance of A ₁ and A _n is 1/2 of the value of the parallel capacitance of A ₂ ...A _n-1. ) The inverted L-shaped basic half-interval circuit shown in Figure 2 is considered as a pure reactance circuit with a dynamic resistance value of zero, and the resonant frequency of piezoelectric vibrator A and the anti-resonant frequency of piezoelectric vibrator B are expressed as Match the impedance curves of the piezoelectric vibrators shown in the figure at _p . Let Z _A be the impedance of piezoelectric vibrator A, and Z _B be that of piezoelectric vibrator B. The cutoff frequency is the frequency where Z _A = -Z _B , and the terminal impedance is
Matching is performed using the image impedance value at _p . Further, a desired amount of attenuation is obtained by connecting basic half-interval circuits in multiple stages under the above frequency settings. On the other hand, in order to obtain a high guaranteed attenuation amount, it is necessary to increase the parallel capacitance ratio C _OB /C _OA , and when this is achieved using a vibrator, the following method is used. In Fig. 1, the type A resonator has a capacitance ratio γ _A determined from the circuit design, and in order to realize a small capacitance, for example, a fourth
As shown in the figure, it has a thick-walled, fully polarized partial electrode structure. The B-type vibrator similarly has a predetermined capacitance ratio γ _B , and in order to achieve a large capacity, it has a thin-walled, half-polarized electrode structure, for example, as shown in Figure 5. There is. Furthermore, from a circuit design standpoint, the capacitance ratio of type A and type B resonators unilaterally affects the cutoff frequency of the filter, so in order to satisfy the desired filter standards, it is necessary to It is necessary to suppress the variation in the capacitance ratio to an extremely small value. However, due to variations in the piezoelectric material itself, variations in the dimensions of the partial electrodes in the B type vibrator shown in Fig. 1, which form a partial electrode structure by a method such as etching, and the polarization in the A type vibrator. The capacity ratio varies greatly depending on the degree of damage. Therefore, it is extremely difficult to minimize variations in the capacitance ratio of vibrators during manufacturing. Furthermore, if the resonator is incorporated into the filter by simply matching the resonant frequency of the A-type resonator and the anti-resonant frequency of the B-type resonator without taking into account the variation in capacitance ratio, ripples will occur in the passband. or cause the specification in the attenuation range to be violated. On the other hand, according to the present invention, the range of usable capacitance ratio values is significantly increased by taking into account variations in the capacitance ratio of the resonators during manufacturing and incorporating a combination of resonators with different capacitance ratios into a filter. It can be enlarged beautifully. In other words, in Figure 1, _p
Set the average value of the capacitance ratios γ _A1 , γ _A2 , ... γ _An of the oscillators of A ₁ , A ₂ , ...A _n whose frequencies have been adjusted to be the best value in the circuit design, and similarly
Capacity ratio of B ₁ , B ₂ , ..., B _o γ _B1 , γ _B2 , ... γ _B
The average value of _o is set to be the best value in circuit design. In order to avoid a sudden change in the cutoff frequency between adjacent fundamental sections, the magnitude of the capacitance ratio for the A-type oscillators A ₁ , A ₂ , . . . A _n is set to γ _A1 < γ _A2 < . ＜γ _An
or arrange in the order of γ _A1 > γ _A2 >...> γ _An , and similarly B type oscillators B ₁ , B ₂ ,...
...B _o is also arranged in the order of γ _B1 < γ _B2 <...< γ _Bo , or in the order of γ _B1 > γ _B2 >...> γ _Bo . However, without following the present invention, the positions of the vibrators A ₁ and A _n at both ends are fixed, and the positions of the vibrators A 1 and A n at _{both ends} are fixed, and
If the oscillators A _n-1 and _the oscillators B ₁ , . This is due to the sudden change in cutoff frequency between adjacent fundamental sections. Next, as an example of the present invention, an 11-element ladder-type filter (configuration where m = 6n = 5 in Fig. 1) is constructed from square plate-shaped PZT piezoelectric ceramics with a center frequency of 420 KHz.
I will explain about it. Table 1 shows the values of the resonant frequency and anti-resonant frequency, parallel capacitance, and mechanical Qm of the vibrators A ₁ to _{A 6} and B ₁ to _{B 5} in FIG. 1.

[Table] From Table 1, it can be seen that the A type vibrators are arranged gradually from large to small capacitance ratios. The B type vibrator also has a similar arrangement. The passband operation attenuation characteristic at this time is shown by a solid line in FIG. Furthermore, the dotted lines in the same figure indicate that the arrangement of the transducers A ₁ and A ₆ is the same, but the transducers A ₂ to _{A 5} are arranged in series, and the transducers B ₁ to _{B 4} are arranged in parallel.
2 shows the passband characteristics when the arrangement is done in a completely random manner not according to the present invention. (Terminal resistance is 2KΩ for both). Further, the dashed line in the same figure indicates the design characteristics (calculated values) of the image parameter filter configured with the vibrator having the various constants shown in Table 2.

[Table] As is clear from the above examples, if vibrators with different capacitance ratios are combined 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, the passband is not disturbed and characteristics equivalent to the design characteristics of the image parameter filter shown by the dashed line in FIG. 6 can be obtained. Furthermore, variations in the capacitance ratio of each vibrator can be tolerated to a considerable extent. Therefore, 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 capacitance ratio of the vibrator caused by manufacturing, and it is possible to supply 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 is a thin-walled large-capacity piezoelectric vibrator arranged in parallel. Figure 2 is an inverted L-shaped basic half-section equivalent circuit of a ladder-type piezoelectric filter, where C _OA and C _OB are parallel capacitances, and L _A and L _B
is dynamic inductance, 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 in the image parameter design method, where Z _A and Z _B are the reactance characteristics of oscillators A and B, respectively, ₁₀₀ and ₂₀₀ are the pole frequencies, ₁ ,
₂ indicates the cutoff frequency, _{and p} indicates 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. Figure 6 shows the passband operational attenuation characteristics of a ladder-type piezoelectric filter, the solid line is the operational attenuation characteristics of the filter according to the construction method of the present invention, and the dotted line is the operational attenuation characteristics of the filter obtained by the construction method of the present invention. This is the motion attenuation characteristic when the arrangement is random, and the dash-dotted line shows the calculated value of the motion attenuation characteristic using the image parameter design method.

Claims (1)

[Claims] 1 Imaging parameter design In a ladder-type piezoelectric filter, the transducers arranged in series are sequentially arranged from the input side.
A ₁ , A ₂ , ...A _n , their capacity ratios are y _A1 ,
y _A2 , ...y _An , and the oscillators arranged in parallel
B ₁ , B ₂ , ... B _o , their capacity ratios are y _B1 ,
y _B2 ,...y _Bo , and when the capacitance ratio is set to y _A1 > y _A2 >... > y _An for the transducers arranged in series, y _B1 > for the transducers arranged in parallel.
If y _B2 >...y _Bo , or if y _A1 < y _A2 <... < y _An for the transducers arranged in series, then y _B1 < y _B2 <... for the transducers arranged in parallel.
A ladder-type piezoelectric filter characterized in that it is configured as ...<y _Bo .