JPS6243363B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention eliminates unnecessary vibrations in a composite energy trapping filter consisting of a plurality of filter elements provided on the same piezoelectric substrate and a part that electrically couples these filter elements. The purpose is to improve the guaranteed attenuation cutoff characteristics of the stopband.

One way to increase the guaranteed attenuation of energy trapping filters and achieve good cutoff characteristics is to use a tandem configuration method in which energy trapping filters are electrically connected in parallel by a capacitance, or It is generally known that there is a method of realizing the coupling portion using a thick film resistive element or the like.

However, in a configuration in which multiple energy trapping filter elements are arranged on the same piezoelectric substrate, the overall size of the filter becomes smaller, and the number of processes such as electrical connections is reduced, so a low-cost filter can be expected. From this point of view, for the reasons described below, it is significantly more efficient than a filter in which one energy trapping filter is formed on one piezoelectric substrate and the filter elements and coupling parts made of a large number of piezoelectric substrates are electrically connected in cascade. It gets worse.

In other words, in a structure in which multiple filter elements are arranged on the same piezoelectric substrate, unnecessary elastic vibrations are generated from the filter elements and propagate directly to the output side, resulting in a decrease in the guaranteed attenuation amount and the steepness of the cutoff characteristics. It causes a lot of things.

Therefore, in a type of filter having a composite filter structure formed on a single piezoelectric plate, it is necessary to sufficiently suppress the propagation of unnecessary elastic waves generated from the filter element.

FIG. 1 is a front view of a conventional piezoelectric filter in which an energy trapping filter and a coupling capacitance are simultaneously realized on the same piezoelectric substrate, and FIG. 2 is a rear view thereof. In the figure, 11 is a piezoelectric substrate,
A dual mode filter 1 is mounted on this piezoelectric substrate 11.
2 and 13, and an electrode 14 for realizing coupling capacitance. However, in this state, it is impossible to suppress the propagation of unnecessary elastic waves generated from the filter element.

On the other hand, as a measure to suppress the propagation of unnecessary elastic waves, conventionally, a slit or a hole has been formed in the filter substrate in the middle of the propagation path of the elastic waves.

However, the size of the filter substrate is about 7 mm x 7 mm at most, taking an energy trapping filter that uses a 10.7 MHz thickness longitudinal mode as an example.
Forming slits or holes in the substrate requires a very sophisticated microfabrication technique, and there are also drawbacks such as the possibility of cracks being formed in the substrate.

In order to eliminate the above-mentioned drawbacks, the present invention has the following features:
The propagation of elastic waves is suppressed by forming a multi-branched comb-shaped conductive film in the middle of the filter element, and will be explained below with reference to the drawings.

FIG. 3 is a front view of a piezoelectric filter showing an embodiment of the present invention, and FIG. 4 is a rear view thereof.

In the figure, 31 is a piezoelectric substrate, on which are formed dual mode filters 32, 33 and an electrode 34 that serves as a coupling capacitance. A comb-shaped conductive film 36 of the invention is provided.

Further, FIG. 5 is a cross-sectional view of a comb-shaped conductive film portion of the present invention, where 51 indicates a piezoelectric substrate and a hatched portion 52 indicates a conductive film.

That is, by forming a comb-shaped conductive film on the piezoelectric substrate, the difference in acoustic impedance between the exposed portion of the piezoelectric substrate and the portion on which the conductive film 52 is formed in FIG. The reflection of the propagating wave that reaches directly and the potential of the conductive film portion are all the same everywhere, making it possible to electrically block the propagation of the wave and significantly improving the stopband characteristics of the filter. can.

As an example of the present invention, thickness longitudinal mode is used 10.7M
The Hz tandem configuration energy trapping filter will be explained. As shown in FIGS. 1 and 2, this filter has a structure consisting of two dual mode filters and a coupling capacitance section, and its operational attenuation characteristics are shown in FIG. 6. The solid line in FIG. 6 is the characteristic when the comb-shaped conductive film of the present invention shown in FIGS. 3 and 4 is formed, and the dotted line is the characteristic of the conventional type shown in FIG. , which shows the characteristics of the structure of FIG.

As is clear from the above embodiments, if a filter is manufactured without taking any measures at the intermediate portion of the filter element, the characteristic of a steep rise in the stopband and the guaranteed attenuation amount will be reduced.

On the other hand, according to the present invention, it is possible to obtain good filter characteristics, and contributes to providing a high-performance piezoelectric filter.

[Brief explanation of the drawing]

FIG. 1 is a front view of a conventional piezoelectric filter, FIG. 2 is a rear view thereof, FIG. 3 is a front view showing an embodiment of the present invention, and FIG. 4 is a rear view thereof. In the figure, 31 is a piezoelectric substrate, 32 and 33 are dual mode filters, 34 is an electrode, and 35 and 36 are comb-shaped conductive films of the present invention. FIG. 5 shows a cross-sectional view of the comb-shaped conductive film portion of the present invention, where 51 is a piezoelectric substrate and 52 is a conductive film. FIG. 6 shows the operating attenuation characteristics of the tandem configuration energy trapping filter, where the dotted line is the characteristic of the conventional type and the solid line is the characteristic of the filter according to the present invention.

Claims (1)

[Claims] 1 Two energy trapping filter elements provided on the same piezoelectric substrate, electrodes formed on the front and back surfaces of the substrate to serve as coupling capacitors, and electrodes on the front and back surfaces of the substrate at an intermediate portion between the two energy trapping filter elements. A piezoelectric filter comprising a comb-shaped conductive film branched into a plurality of branches.