JP2002141776A - Ladder filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ladder filter with less number of components to attain downsizing that causes no migration in parallel resonators even when a high voltage is applied to its output. SOLUTION: The ladder filter in which a ladder circuit is configured by mounting series resonators 10-12 and parallel resonators 13-15 on a substrate 20 on which input, output and earth pattern electrodes are formed, and a cap 30 is adhered to the substrate 20 so as to cover the resonators 10-15, and a DC-cut capacitor 19 is connected in series halfway the output electrode 22 of the substrate 20.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ladder filter having a ladder circuit formed by connecting a series resonator and a parallel resonator in a ladder manner.

[0002]

2. Description of the Related Art Conventionally, a ladder-type filter has been widely used as an intermediate frequency filter for AM or a second IF filter for communication equipment. A ladder-type filter is a ladder circuit configured by connecting one or more series resonators and a parallel resonator in a ladder form, and generally includes a resonance frequency of the series resonator and an anti-resonance frequency of the parallel resonator. To match.

As an exterior structure of a ladder-type filter, for example, a series resonator, a parallel resonator, and a terminal plate are housed in a case, and a lead portion of the terminal plate is connected to the case as disclosed in Japanese Patent Application Laid-Open No. 6-310977. And a case in which the opening of the case is sealed with a resin.
No. 0668, a structure in which a series resonator and a parallel resonator are mounted on a substrate on which pattern electrodes for input, output and ground are formed, and a cap is bonded to the substrate so as to cover these resonators There are things.

[0004]

By the way, in the ladder type filter, the capacity of the parallel resonator needs to be several tens of times the capacity of the series resonator in order to secure the attenuation. Therefore, a ceramic resonator having a smaller thickness than a series resonator is used as the parallel resonator. However, when the ladder-type filter is used in a high-humidity environment and a high DC voltage acts on the output side, the electrode material (eg, silver) of the parallel resonator causes migration, and the electrodes of the parallel resonator cause a short circuit. Failure may occur.

In order to prevent migration from occurring in the parallel resonators even when a high voltage acts on the output side in a high humidity environment, a DC cut capacitor is externally provided at the output end of the ladder filter. I have. However, DC
When a cut capacitor is externally connected to the output terminal of the ladder-type filter, not only does the number of components increase, but also the time and effort required for external mounting are increased, and the circuit becomes large.

Accordingly, an object of the present invention is to provide a ladder-type filter which does not cause migration in a parallel resonator even when a high voltage acts on the output side, and has a small number of components and can be miniaturized.

[0007]

In order to achieve the above object, an invention according to claim 1 is a ladder-type filter comprising a ladder circuit formed by connecting a series resonator and a parallel resonator in a ladder type. At the output terminal of the ladder circuit
A ladder-type filter is provided in which cut capacitors are connected in series and this DC cut capacitor is built in together with the series resonator and the parallel resonator.

When a high DC voltage is applied to the output terminal in a high humidity environment, the electrode material of the parallel resonator may cause migration. However, a DC cut capacitor is connected in series to the output terminal of the ladder circuit. Therefore, it is possible to prevent a high DC voltage from being applied to the parallel resonator. Therefore, a short circuit of the parallel resonator can be prevented, and a highly reliable ladder-type filter can be obtained. As a DC cut capacitor, for example, in the case of a ladder type filter for 450 kHz,
A capacitor having a capacitance of about 0.001 μF or more is used.
The DC cut capacitor blocks a high DC voltage applied to the parallel resonator, but needs to be set to a capacitance value that does not block the AC signal itself. The DC cut capacitor is built into the ladder filter, so when the ladder filter is connected to the circuit, the number of circuit components can be reduced and the complexity of externally connecting the DC cut capacitor can be eliminated. Can be reduced in size.

The ladder-type filter has an exterior structure in which a series resonator, a parallel resonator, a terminal plate, and the like are housed in a box-shaped case, an opening of the case is sealed with a resin, and a series resonance is mounted on a substrate. There is a method in which a resonator and a parallel resonator are mounted, and a cap is placed on the resonator. Claim 2 adopts the latter structure, wherein the series resonator and the parallel resonator are mounted on a substrate on which pattern electrodes for input, output, and ground are formed, and are covered so as to cover these resonators. A cap is bonded to a substrate, and a DC cut capacitor is provided on the substrate. In this case, since the DC cut capacitor is provided on the substrate, the ladder-type filter can be made compact, and the connection of the DC cut capacitor is simple.

It is preferable that the DC cut capacitor is integrally formed by baking or laminating the substrate on the substrate. In this case, since the DC cut capacitor is formed integrally with the substrate, a smaller ladder filter can be configured. In particular, when the substrate is formed of a multilayer substrate and a DC cut capacitor is laminated inside the substrate, a highly reliable ladder filter can be realized without the DC cut capacitor being short-circuited with another resonator. .

The series resonator and the parallel resonator may be piezoelectric resonators utilizing spread vibration as in the prior art, but may be a columnar base having a piezoelectric layer and internal electrodes laminated in the longitudinal direction. 2 formed so as to be alternately connected to the internal electrodes on one surface or two opposing surfaces of the base.
And two external electrodes, and the piezoelectric layer may be polarized in the longitudinal direction of the base. In the case of a laminated piezoelectric resonator using such a length vibration mode, the polarization direction,
The use of the piezoelectric longitudinal effect in which the direction of the electric field and the direction of vibration are the same. Can be. In the case of a ladder filter using a laminated piezoelectric resonator, the thickness of the piezoelectric layer of the piezoelectric resonator used as a parallel resonator is extremely thin, and when a high voltage is applied in a high humidity environment, the electrode material is Silver causes migration and is likely to cause a short circuit. Therefore, by connecting the DC cut capacitor of the present invention to the output terminal, such a problem can be surely solved.

[0012]

FIG. 1 is a circuit diagram showing an example in which a ladder filter according to the present invention is used as a 2nd IF filter for a communication device or the like. In the figure, 1 is a ladder-type filter according to the present invention, and 2 is an amplifier circuit. The ladder-type filter 1 has three series resonators 10, 11, 12 and three parallel resonators 13, 14, 15 connected in a ladder type. Input signal is input. Output terminal 22 is amplifying circuit 2
And the ground terminal 23 is connected to the ground (GND). 6 is an IF amplifier 5
This is a matching resistor connected between the input of the IGBT and the ground.

The ladder filter 1 of the present invention has a DC cut capacitor 19 built in at its output end. Therefore, the DC voltage applied from the IF amplifier 5 is cut,
It is possible to prevent a DC voltage from being applied to the parallel resonator 15. Even when the ladder-type filter 1 is used in a high-humidity environment, migration of the electrode material of the parallel resonator 15 can be prevented, and short-circuit failure can be prevented. It is desirable to set the capacitance value of the DC cut capacitor according to the following relationship. R ≧ 1 / 2πf · C R: resistance value of resistor 6 (Ω) f: center frequency (Hz) C: capacitance value of DC cut capacitor (pF)

FIGS. 2 and 3 show a specific structure of the ladder-type filter 1. FIG. This ladder type filter has an insulating substrate 2
0, three series resonators 10, 11, 12 and three parallel resonators 13, 14, 15 are mounted side by side in parallel, and a cap 30 is sealed on the substrate 20 so as to cover the entire resonator. It is what I wore.

The series resonators 10, 11, 12 of this embodiment
Are the same piezoelectric ceramic resonators utilizing length vibration, and include a rectangular parallelepiped base 10a as shown in FIG. The base 10a is formed by alternately stacking two types of piezoelectric layers 10b and 10c made of a piezoelectric ceramic material as shown in FIG. It is a thing. As shown by an arrow P in FIG. 4, the piezoelectric layers 10b and 10c on both sides of one internal electrode 10d are polarized in the longitudinal direction of the base 10a such that the polarization axes are opposite to each other. However, both ends of the base 10a may or may not be polarized. On the lower surface where the internal electrode 10d is exposed (shown upside down in FIGS. 4 and 5), two external electrodes 10e and 10f are formed parallel to the longitudinal direction, and every other internal electrode 10d is provided. Connected to external electrodes 10e and 10f. In this embodiment, the external electrodes 10e, 1
Between 0f, a groove 10g extending in the length direction of the base 10a is formed, but the groove 10g is not always required. At the center (node portion) in the length direction of the external electrodes 10e and 10f, convex conductive supports 10h and 10i are provided. The conductive supports 10h and 10i of this embodiment
Is obtained by applying a conductive paste at a predetermined thickness on the external electrodes 10e and 10f and curing the conductive paste. In addition,
The conductive supports 10h and 10i can be omitted by sharing with a conductive adhesive used for mounting on the substrate 20 described later.

In the series resonators 10, 11, and 12, external electrodes 10e and 10f are used as input and output electrodes. At this time, the electric field is applied between the adjacent internal electrodes 10d in portions other than the both ends of the base 10a, so that the base 10a becomes piezoelectrically active. Since no electrodes are formed on both end faces of 10a, no electric field is applied and the electrodes become piezoelectrically inactive. Therefore, by applying a signal between the external electrodes 10e and 10f, an AC electric field in the longitudinal direction of the base 10a is applied to each of the piezoelectric layers 10b and 10c of the active portion, and the expansion and contraction driving force is applied to the piezoelectric layers 10b and 10c. Then, the basic vibration in the length vibration mode is excited in the base 10a as a whole. Although inactive portions are provided at both ends of the base 10a, the inactive portions are not essential, and the entire base 10a may be the active portion.

The parallel resonators 13, 14, and 15 are also laminated resonators using the length vibration having substantially the same structure as the series resonators 10, 11, and 12. Series resonators 10, 1
1 and 12 are generally different from the parallel resonators 13 and 1.
The thicknesses of the piezoelectric layers 10b and 10c of the piezoelectric layers 10b and 10c are smaller in the layers 4 and 15 than the piezoelectric layers 10b and 10c stacked with the internal electrode 10d interposed therebetween.
0c is a large number of layers. In this way, the inter-terminal capacitance obtained between the external electrodes 10e and 10f is approximately 20 to 30 times larger than that of the series resonator.

The substrate 20 is a rectangular insulating thin plate made of alumina ceramic, glass ceramic, glass epoxy resin, heat-resistant resin, or the like, and has an input surface as shown in FIG. The pattern electrode 21, the output pattern electrode 22, the ground pattern electrode 23, and the intermediate electrodes 24 and 25 are formed by a known method such as sputtering, vapor deposition, and printing. Each pattern electrode 21
The external connection parts 23 to 23 extend to the rear surface side via the side edge of the substrate 20. The conductive supports 10h and 10i of the first-stage series resonator 10 are connected to the land 21a of the input electrode 21 and one land 24a of the intermediate electrode 24 by using a conductive adhesive or solder. Fixed. Similarly, the land portion 24a of the intermediate electrode 24 and the ground electrode 2
The parallel resonator 13 is connected and fixed to one of the land portions 23a. The other land 2 of the intermediate electrode 24
4b and one land 25a of the intermediate electrode 25, the series resonator 11 is straddled and fixed, and the land 25a of the intermediate electrode 25 and the other land 2 of the ground electrode 23 are connected.
A parallel resonator 14 is straddled and fixed to 3b.
Further, the parallel resonator 15 is connected and fixed to the land 23 b of the ground electrode 23 and the land 22 a of the output electrode 22, and the land 22 a of the output electrode 22 and the other land of the intermediate electrode 25 are fixed. 25b has a series resonator 12
Is straddled and fixed. Thus, the ladder circuit as shown in FIG. 1 is configured. In the series resonators 10 to 12 and the parallel resonators 13 to 15, the central portions (node portions) in the length direction have conductive supports 10h and 10i.
, And both ends are supported with a gap from the substrate 20, so that the longitudinal vibration is not hindered.

The middle of the output electrode 22 of the substrate 20 is divided into two as shown in FIG. 3E, and a DC cut capacitor 19 is connected between these electrodes 22b and 22c. In this embodiment, the DC cut capacitor 19
Is a multilayer ceramic capacitor, but a capacitor obtained by applying and baking a dielectric paste may be used. D
The C-cut capacitor 19 is also housed in the cap 30.

On the surface of the substrate 20, an insulating layer 26 covering the pattern electrodes 21 to 25 formed on the periphery of the substrate 20 is formed.
(See FIG. 3B) is formed in a frame shape.
An opening of the cap 30 that covers the resonators 10 to 15 is adhered on the 6 by an adhesive (not shown). Therefore, the periphery of the resonators 10 to 15 is sealed, and the input and output electrodes 21 and 22 and the intermediate electrodes 24 and 25 and the cap 30 are electrically insulated. Although the cap 30 in this embodiment is formed by pressing a metal plate, it may be a resin cap or a ceramic cap. In this embodiment, the insulating layer 2 covering the ground electrode 23
A window 26a is formed in a part of 6 and the cap 30 and the ground electrode 23a are electrically connected through the window 26a. Therefore, a ladder-type filter having a shield structure can be obtained. In addition, a resonator having an external electrode formed on one surface is mounted in a plane on the substrate 20 using a conductive adhesive or the like, and is covered with the cap 30, so that a ladder-type filter having a low-profile structure can be obtained. This has the effect that it can be performed.

In the case of the ladder filter of the above embodiment, the DC cut capacitor 19 is connected on the substrate 20 and in the middle of the output electrode 22, so that the ladder filter has a built-in DC cut capacitor. Become. Therefore, when this ladder filter is connected to an external circuit,
The DC voltage applied to the parallel resonator 15 can be cut without externally attaching a DC cut capacitor, and the parallel resonator 15 can be protected.

FIG. 6 shows a second embodiment of the ladder-type filter according to the present invention. This ladder type filter is also used for the substrate 20.
Equipped with three series resonators and three parallel resonators,
It is sealed with a cap 30, and the same parts as those in the first embodiment are denoted by the same reference numerals, and redundant description will be omitted.

In this embodiment, a multilayer substrate made of a dielectric material is used as the substrate 20, and the DC cut capacitor 19 is integrally formed in the substrate 20. That is,
As shown in FIG. 6B, the internal electrodes 19a and 19b alternately laminated inside the substrate 20 are alternately connected to two conduction holes 19c and 19d having electrodes embedded therein. Output electrode 22 with holes 19c and 19d separated
b, 22c. In this case, since the DC cut capacitor 19 is formed integrally with the substrate 20, the DC cut capacitor 19 is
A highly reliable ladder-type filter can be configured without short-circuiting with 0 to 15 or the cap 30 or falling off from the substrate 20.

The series resonator and the parallel resonator used in the present invention are not limited to the laminated resonator using the length vibration as shown in FIGS. 4 and 5
A length vibration element having a structure different from that of the first embodiment may be used, or a conventional expansion vibration element may be used. The ladder-type filter of the present invention is not limited to a structure in which a series resonator and a parallel resonator are mounted on a substrate as shown in FIGS. It may have a structure in which the resonator, the parallel resonator and the terminal plate are accommodated, and the opening of the case is sealed with resin. Further, the ladder-type filter of the present invention is not limited to one using three series resonators and three parallel resonators, but one or two series resonators and one or two parallel resonators. And a resonator using four or more series resonators and four or more parallel resonators.

[0025]

As is apparent from the above description, according to the present invention, since a DC cut capacitor is connected in series to the output terminal of the ladder circuit, a high DC voltage is prevented from being applied to the parallel resonator on the output side. As a result, a short circuit failure of the parallel resonator can be prevented, and a highly reliable ladder filter can be obtained. Since the DC cut capacitor is built in the ladder filter together with the series resonator and the parallel resonator, when the ladder filter is connected to an external circuit,
The number of circuit components can be reduced, and the complexity due to externally attaching a DC cut capacitor can be eliminated, and the external circuit can be downsized.

[Brief description of the drawings]

FIG. 1 shows a ladder-type filter according to the present invention in a communication device 2;
It is a circuit diagram of the example used as an ndIF filter.

2A and 2B are external views of an example of a ladder filter according to the present invention, wherein FIG. 2A is a plan view, FIG. 2B is a front view, and FIG.
Is a side view, and (d) is a bottom view.

FIG. 3 shows an internal structure of the ladder-type filter of FIG. 2,
2A is a cross-sectional view taken along line XX of FIG. 2, FIG. 2B is a plan view showing a state where a cap is removed, FIG. 2C is a cross-sectional view taken along line YY, FIG. FIG. 3 is a partially enlarged sectional view taken along line AA.

FIG. 4 is a perspective view of an example of a series resonator.

FIG. 5 is a front view of a piezoelectric layer used in the series resonator of FIG.

6A is a plan view of a substrate according to a second embodiment of the present invention, and FIG. 6B is an enlarged cross-sectional view taken along the line BB.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ladder type filter 10-12 Series resonator 13-15 Parallel resonator 19 DC cut capacitor 20 Substrate 21 Input pattern electrode 22 Output pattern electrode 23 Ground pattern electrode 30 Cap

Claims (3)

[Claims] 1. A ladder filter comprising a ladder circuit formed by connecting a series resonator and a parallel resonator in a ladder circuit, wherein a DC cut capacitor is connected in series to an output terminal of the ladder circuit. A ladder-type filter including a cut capacitor built therein together with the series resonator and the parallel resonator. 2. The series resonator and the parallel resonator are mounted on a substrate on which pattern electrodes for input, output and ground are formed, and a cap is bonded to the substrate so as to cover these resonators. The ladder filter according to claim 1, wherein a DC cut capacitor is provided on the substrate. 3. The ladder filter according to claim 2, wherein the DC cut capacitor is formed integrally by baking or laminating the DC cut capacitor on a substrate.