JPH0718163Y2 - Dielectric filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a dielectric filter, and in particular, a plurality of coaxial resonators are formed in one dielectric block and a circuit board is provided on the open end side thereof. , A dielectric filter.

[Background technology]

The applicant of the present invention has previously proposed this type of dielectric filter in, for example, Japanese Patent Application No. 61-254452.

As shown in FIG. 2, this dielectric filter is composed of a plurality of coaxial resonance elements formed in one dielectric block 1. A dielectric filter having such a configuration is already known, and a detailed description thereof will be omitted here.

In this background art, a circuit board 2 to which a predetermined coaxial resonance element is connected is provided on the open end side of the dielectric block 1 for the purpose of facilitating adjustment of filter characteristics by trimming, forming an attenuation pole, or the like. Is placed, and the metal case 3 is covered from above, and the metal case 3 is connected to the outer conductor (earth).

[Problems to be Solved by the Invention] In the dielectric filter of the background art, as shown in FIG.
Since the circuit board 2 and the metal case 3 are closely fixed to each other, the hot side pattern on the circuit board 2 and the metal case 3 are fixed.
A stray capacitance is created between and. Since such stray capacitance greatly changes depending on the state and degree of contact between the circuit board 2 and the case 3, the resonance frequency changes.
Therefore, the entire filter characteristic changes. It takes time to eliminate the influence of such stray capacitance and adjust the filter characteristics, and therefore, there is a problem that mass productivity is not good.

Therefore, a main object of the present invention is to provide a dielectric filter that can suppress the influence of stray capacitance as much as possible and improve mass productivity.

[Means for solving problems]

Briefly, the present invention relates to a dielectric filter in which a plurality of coaxial resonance elements are formed by an inner conductor formed in a plurality of through holes and a common outer conductor in one dielectric block. In a dielectric filter including a circuit board provided on the open surface side of the circuit board and a metal case that covers the circuit board and is connected to an outer conductor, a low dielectric layer for separating a constant distance between the circuit board and the metal case. A dielectric filter is characterized in that a spacer made of a dielectric having a dielectric constant is provided.

[Action]

A spacer formed of a dielectric material having a low dielectric constant is interposed between the circuit board and the case to form a constant space therebetween, thereby reducing the stray capacitance.

[Effect of device]

According to this invention, the distance between the circuit board and the circuit board can be made constant by the spacer made of a dielectric material having a low dielectric constant, so that the above-mentioned stray capacitance can be made small and stable. Therefore, since the fluctuation of the resonance frequency of the coaxial resonator is small and stable, the filter characteristics can be adjusted easily and easily, and improvement in mass productivity can be expected.

The above-mentioned objects, other objects, features and advantages of the present invention will become more apparent from the detailed description of the embodiments below with reference to the drawings.

(Embodiment) FIG. 1 is a sectional view showing an embodiment of the present invention.
The dielectric filter 10 includes a dielectric block 12 having a substantially rectangular parallelepiped shape and made of, for example, titanium oxide-based ceramic,
A plurality of through holes 14 are vertically formed in the dielectric block 12. An inner conductor 16 made of a metal film is formed on each inner wall of the through hole 14. An outer conductor 18 made of a metal film is formed on the outer surface of the dielectric block 12 except for the open end, that is, the bottom surface in FIG. The outer conductor 18 is electrically connected to each inner conductor 16 on the short-circuit end side. In this manner, the dielectric block 12, the inner conductors 16 and the common outer conductor 18 form a λ / 4 coaxial resonator for each through hole 14.

Between each through hole 14, a coupling hole 20 penetrating in the length direction of the dielectric block 12 is formed. No conductor is formed on the inner surface of the coupling hole 20, so that adjacent ones of the above-described coaxial resonators are electromagnetically coupled by the coupling hole 20,
It exhibits filter characteristics as a whole. However, such a dielectric filter itself is already known, and a further description will be omitted here.

In this embodiment, the dielectric bushings 22 are fitted into the respective through holes 14 from the open end side of the dielectric block 12 where the outer conductor 18 is not formed. On each dielectric bushing 22, metal pins 26a to 26d are provided so as to project from the lower surface of the flange portion 24 at their centers.
On the lower surface of the dielectric bushing 22, a circuit board made of an insulating material such as alumina or Teflon (trade name) glass.
28 is stuck. The distance between the circuit board 28 and the open end surface of the dielectric block 12 is set by the height of the flange portion 24 of the dielectric bushing 22.

Although not shown, an appropriate conductive pattern is formed on the circuit board 28. As desired, by changing the filter characteristics, forming the attenuation pole, etc. by connecting or not connecting any coaxial resonator, that is, the metal pins 26b and / or 26c to its conductive pattern, as required. I do.

Note that FIG. 1 is merely for the purpose of illustration, and the specific filter characteristics are determined by the shape of the above-described conductive pattern, the number and position of coaxial resonators connected to the conductive pattern, the reactance value, and the like. However, since their specific configurations are not important for the present invention, further description will be omitted.

The metal pin 26a coupled to the first-stage coaxial resonator and the metal pin 26d coupled to the last-stage coaxial resonator are not connected to the circuit board 28 (but may be connected as necessary). , Further downwardly formed. The metal pin 26a serves as an input terminal and the metal pin 26d serves as an output terminal.

Spacers 30 are fitted into the metal pins 26a and 26d, respectively. The spacer 30 is made of, for example, a dielectric having a low dielectric constant, such as a synthetic resin, and has a hole 26 formed at the center thereof and is formed into a substantially disc shape having a constant thickness (height). Metal pins 26a and 26d are inserted into the holes, respectively.

Below the spacers 30 is also a metal cover or case.
32 is provided, which is electrically connected and mechanically fixed to the outer conductor 18, whereby the open end side is shielded. It should be noted that the spacer 30 is interposed between the metal case 32 and the circuit board 28, so that a certain distance or more is provided between them.

By forming the spaces in this way, the hot side pattern (not shown) formed on the circuit board 28 and the metal case are formed.
The stray capacitance between 32 and can be reduced.

If the stray capacitance is Cs and the coaxial resonator constituting the dielectric filter 10 is a λ / 4 resonator, the resonance angular frequency ω has a characteristic impedance Z, an effective dielectric constant ε _r ,
When the length of the resonator is l and the speed of light is c, Becomes

On the other hand, when there is no influence of the stray capacitance Cs, the resonance frequency f _o
Is Becomes Here, substituting an example of numerical values, the characteristic impedance Z = 10Ω and the effective dielectric constant ε _r = 90.
And when there is no influence of the stray capacitances Cs length l = 9 mm of the resonator and (2), the resonance frequency f _o is, f _o = 88
It becomes 7.8MHz.

Next, assuming that a stray capacitance Cs of 1 pF exists, the resonance frequency f is 848 MHz from the equation (1) in the above example.
Becomes That is, the stray capacitance Cs of 1 pF lowers the resonance frequency by about 30 MHz. Therefore, the stray capacitance Cs
If is fluctuated by 10%, the resonance frequency f will fluctuate by 3 MHz. With this, the required filter characteristics cannot be obtained.

However, as in the present invention, if the space between the circuit board 28 (the hot side pattern thereof) and the metal case 32 is provided by the spacer 30 at a certain distance or more, the stray capacitance Cs
Can be reduced to, for example, about 0.1 pF. Therefore,
The change in the resonance frequency f due to this is as small as about 3 MHz, and even if the stray capacitance Cs changes by 10%, the change in the resonance frequency f is only about 300 kHz, and therefore has little effect on the filter characteristics.

The material of the spacer 30 described above may be any material as long as it is an insulator, and its shape is not limited to a cylinder, but a cylindrical body (a cylinder,
Square tube), and the number thereof may be one or more.

In addition, the spacer 30 is a dielectric bushing 22 in the exemplary embodiment.
However, they may be integrally formed if possible.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing an embodiment of the present invention. FIG. 2 is a schematic sectional view showing an example of a dielectric filter which is the background of the present invention. In the figure, 12 is a dielectric block, 14 is a through hole, 16 is an inner conductor, 18 is an outer conductor, 20 is a dielectric bushing, 28 is a circuit board, 30 is a spacer, and 32 is a metal case.

Claims (1)

[Scope of utility model registration request] 1. A dielectric filter in which a plurality of coaxial resonant elements are formed in one dielectric block by an inner conductor formed in a plurality of through holes and a common outer conductor, the dielectric block comprising: In a dielectric filter including a circuit board provided on an open surface side and a metal case that covers the circuit board and is connected to the outer conductor, a constant space is provided between the circuit board and the metal case. A dielectric filter, characterized in that spacers made of a dielectric material having a low dielectric constant are provided.