JPS6165615A - Surface acoustic wave filter device - Google Patents

Abstract

PURPOSE:To suppress a dent in a band or a fall at edges of the band in a frequency characteristic due to variance in power supply conductance or load conductance by connecting two surface acoustic wave filters in parallel so that the overall conductance viewed from a parallel connection terminal is not fluctuated rapidly in the band thereby not incurring deterioration in insertion loss. CONSTITUTION:The 1st surface acoustic wave filter F1 consists of the 1st read screen transducer (IDT)4 and the 2nd IDT5 arranged opposite to the 1st IDT. Further, the 2nd surface acoustic wave filter F2 consists of the 3rd IDT6 and the 4th IDT7. The conductance of the 1st IDT4 and the 4th IDT7 and the conductance of the 2nd IDT5 and the 3rd IDT6 have a frequency characteristic to be compensated with each other, and since the 1st IDT4 and the 3rd IDT6 are connected in parallel, a voltage applied between input terminals 8 and 8' is almost constant within the pass band regardless of a power conductance Gg. Further, a voltage generated between output terminals 9 and 9' has a desired voltage versus frequency characteristic not affected by the quantity of a load conductance Gl.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a surface acoustic wave filter device with improved frequency characteristics.

[Technical background of the invention and its problems] Conventionally, as shown in FIG. An IDT 2 (referred to as an IDT) 2 and a second IDT 3 disposed opposite the first IDT 2 are commonly used.

In such a surface acoustic wave filter, a normal IDT is used as the first IDT, and an apodized IDT is used as the second IDT.

As shown in Figure 4(a), the conductance of a normal IDT has the form (Sin The conductance of the avoidized IDT is shaped as shown in Figure 2(b) to compensate for the conductance of the normal IDT within the passband. Therefore, by using these IDTs in combination,
A surface acoustic wave filter having a passage characteristic as shown in C) is constructed.

However, when a surface acoustic wave filter with such conductance is operated by connecting it to an external circuit with a power supply conductance Gg and a load conductance Gλ from the perspective of the surface acoustic wave filter, in the frequency range where the conductance of the IDT is high, the power supply conductance or There is a problem in that the voltage approaches the load conductance, causing a drop in the voltage applied to the IDT and causing level fluctuations, causing a dent in the frequency characteristic band or a drop at the edge of the band.

In order to avoid such problems, attempts have been made to increase the power supply conductance or load conductance to reduce the above effects, but this method has the disadvantage of increasing the insertion loss of the surface acoustic wave filter. Ta.

[Purpose of the Invention] The present invention has been made to solve these conventional problems, and it is possible to eliminate dents in the frequency characteristic of the frequency characteristic due to fluctuations in power supply conductance or load conductance, or at the edge of the band, without deteriorating insertion loss. It is an object of the present invention to provide a surface acoustic wave filter device that suppresses dropout.

[Summary of the Invention] In order to achieve the above-mentioned object, the surface acoustic wave filter device of the present invention includes a first interdigital electrode for mutually converting electricity and surface waves and the first interdigital electrode on a piezoelectric substrate. a first filter consisting of a second interdigital electrode disposed facing the third interdigital electrode; a fourth interdigital interdigital electrode disposed opposite to the third interdigital electrode; In the surface acoustic wave filter, the first interdigital electrode and the third interdigital electrode are electrically connected in parallel, and the second interdigital electrode The electrode and the fourth interdigital electrode are electrically connected in parallel, and the first interdigital electrode and the fourth interdigital electrode are set to have substantially the same conductance within a pass band. , and the second interdigital electrode and the third interdigital electrode are also set to have substantially the same conductance within a passband.

The surface acoustic wave filter device of the present invention has a configuration in which the overall conductance seen from the parallel connection terminals does not fluctuate rapidly within the band, so that it does not change rapidly within the band due to fluctuations in power supply conductance or load conductance. The dent or obi 1! It is possible to suppress the fall of the i-end.

[Embodiments of the Invention] Examples of the surface acoustic wave filter device of the present invention will be described below with reference to the drawings.

In FIG. 1, a first surface acoustic wave filter F1 includes a first interdigital transducer (hereinafter referred to as IDT) 4 for electrical-to-surface wave conversion and a first IDT on a piezoelectric substrate.
and a second IDT 5 disposed opposite to the second IDT 5. Further, the second surface acoustic wave filter F2 has almost the same structure as the surface acoustic wave filter F1, that is, the third I
DT6 and a fourth IDT arranged opposite to this third IDT.
IDT7. Here, the first t
DT4 and the fourth 1DT7 are normalized DTs with the same structure, and the second IDT5 and the third DT6 are weighted IDTs of the avocized form with the same structure. The IDT 4 and the third IDT 6 are electrically connected in parallel, and the second IDT 5 and the fourth IDT 7 are also electrically connected in parallel.

Next, with reference to FIG. 2, the effects of the surface acoustic wave filter device shown in FIG. 1 will be explained.

Since the surface acoustic wave filter of this embodiment is configured as described above, the conductance of the first IDT 4 and the fourth IDT 7 changes the frequency by
When taken along the axis, it has a frequency characteristic of the form (Sin It has a frequency characteristic of Also, since the first 1DT4 and the third IDT6 are connected in parallel, 8-
When looking at the surface acoustic wave filter device side from line 8', the input conductance exhibits almost constant frequency characteristics within the passband as shown in FIG. 4(C). For the same reason, when looking at the surface acoustic wave filter device side from line 9-9' in the figure, the output conductance also no longer exhibits a nearly constant frequency characteristic within the passband, as shown in Figure 2 (C). .

Therefore, the output conductance G at input terminal 8-8'
When the surface acoustic wave filter device is operated by connecting a power supply having Gffl to the output terminals 9-9' and connecting a load circuit having an input conductance Gffl to the output terminals 9-9', the input conductance of the surface acoustic wave filter device is as shown in Fig. 4 ( As shown in C), it has an almost constant value (3s) within the passband,
The voltage applied to input terminals 8-8' is the power supply conductance G
Regardless of the magnitude of g, it remains approximately constant within the passband.

Similarly, the output conductance of the surface acoustic wave filter device is approximately constant 1aGs within the passband as shown in Fig. 2<C>, so the voltage generated at the output terminals 9-9' is determined by the magnitude of the load conductance Gβ. It has desired voltage frequency characteristics that are not affected by

These effects are caused by the first surface acoustic wave filter F1 and the second surface acoustic wave filter F1.
The same result can be obtained even if the surface acoustic wave filter F2 is formed on the same piezoelectric substrate or on different piezoelectric substrates. In addition, the first surface acoustic wave filter F
When the piezoelectric substrates of the first and second surface acoustic wave filters F2 are formed on separate substrates, the same effect can be obtained even if these piezoelectric substrates are made of the same material or cut material. be able to.

In addition, in the above embodiment, the case where two surface acoustic wave filters are formed on the piezoelectric substrate was explained.
The present invention is not limited to such examples,
When the surface acoustic wave filter device is configured by connecting three or more surface acoustic wave filters in parallel, the same effect can be obtained by making at least one set of the surface acoustic wave filters have the configuration of the present invention. I can play with it.

[Effects of the Invention] As explained above, in the surface acoustic wave filter device of the present invention, two surface acoustic wave filters are connected in parallel so that the overall conductance seen from the parallel connection terminal side does not fluctuate rapidly within the band. Therefore, it is possible to suppress indentations in the frequency characteristic band or drop at the edge of the band due to fluctuations in power supply conductance or load conductance, without causing deterioration in insertion loss.

[Brief explanation of drawings]

FIG. 1 is a plan view showing the configuration of a surface acoustic wave filter device according to an embodiment of the present invention, FIG. 2 is a characteristic diagram of its conductance, and FIG. 3 is a plan view showing the configuration of a conventional surface acoustic wave filter device. , FIG. 4 is a characteristic diagram of its conductance.

Claims (3)

[Claims] (1) A first filter comprising, on a piezoelectric substrate, a first interdigital electrode that mutually converts electricity and surface waves, and a second interdigital electrode disposed opposite to the first interdigital electrode. and a second filter consisting of a third interdigital electrode and a fourth interdigital electrode disposed opposite to the third interdigital electrode. The first interdigital electrode and the third interdigital electrode are electrically connected in parallel, the second interdigital electrode and the fourth interdigital electrode are electrically connected in parallel, and further the The first interdigital electrode and the fourth interdigital electrode are set to have substantially the same conductance within a passband, and the second interdigital electrode and the third interdigital electrode are set to have substantially the same conductance within a pass band.
A surface acoustic wave filter device characterized in that the interdigital interdigital electrodes are set to have substantially the same conductance within a pass band. (2) The elastic surface filter device according to claim 1, wherein the first filter and the second filter are provided on the same piezoelectric substrate. (3) The sum of the conductance of the first interdigital electrode and the conductance of the third interdigital electrode is set to be approximately constant within the passband. 2. The surface acoustic wave filter device according to item 2.