Dielectric filter and RF apparatus employing thereof

Details Dielectric filter and RF apparatus employing thereof Dielectric filter and RF apparatus employing thereof

1999-11-08

2001-09-25

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Coupling networks

Wave filters including long line elements

C333S202000, C333S222000, C333S134000

Reexamination Certificate

active

06294969

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of dielectric filters employed in a range of radio communications apparatuses and broadcasting equipment in the several hundred MHz frequency bands.
BACKGROUND OF THE INVENTION
Today, RF apparatuses used in mobile communications and broadcasting are rapidly becoming smaller and lighter. Coaxial resonators made of dielectric materials with high dielectric constant and low loss are extensively used as filters in RF apparatuses, which are required to be small and light. Such dielectric coaxial resonators are also made smaller by designing resonator shapes, for example, to change the characteristic impedance of the line stepwise, as well as using dielectric materials with large specific inductive capacity.
Next, a conventional dielectric filter is described.
FIG. 7
is a cutaway sectional view of a conventional dielectric filter. As shown in
FIG. 7
, through holes
2
A and
2
B are created on a rectangular dielectric block
1
, and the inside of the through holes
2
A and
2
B is metallized with inside conductors
4
A and
4
B. The periphery of the dielectric block
1
is metallized with an outside conductor
5
. The inside conductors
4
A and
4
B are connected to the outside conductor
5
through one of openings in through holes
2
A and
2
B, respectively. An I/O electrode
7
A is created by providing an isolated electrode on a part of the outside conductor
5
. The I/O electrode
7
A is electromagnetically coupled with the inside conductor
4
A, and is connected to an external circuit. Another I/O electrode
7
B (not shown in
FIG. 7
) is provided on a cut part, opposing the I/O electrode
7
A. In the above configuration, a resonator is formed in the through holes
2
A and
2
B, and the dielectric filter shown in
FIG. 7
operates as a two-step filter.
If the diameter of a through hole is stepped to configure a coaxial resonator with a larger hole diameter at the open-circuit end than that at the short-circuit end where the inside conductor and outside conductor are connected, capacitance for the outside conductor
5
is added to the line comprising the inside conductors
4
A and
4
B, enabling the shortening of the resonator length. In other words, the characteristic impedance of the resonance line formed by inside conductors
4
A and
4
B is stepped. By making the characteristic impedance at the open-circuit end lower than that at the short-circuit end, the resonator length can be made shorter than that of resonators with fixed characteristic impedance, thus allowing the overall size of the filter to be reduced.
However, in the conventional dielectric filter shown in
FIG. 7
, the resonator length can only be reduced to about half the size of a resonator with fixed characteristic impedance. Accordingly, no further reduction in size is feasible. At present, the conventional dielectric filter shown in
FIG. 7
can be made several millimeters square for the 800 MHz band by using high dielectric material. This type of dielectric filter is often used in the RF section of mobile phones using this frequency band. For other RF apparatuses using lower frequency bands than 800 MHz, which require larger dielectric filters, helical filters are commonly employed instead of dielectric filter to reduce size. Since dielectric filters are inexpensive and easy to manufacture, and have several specific advantages such as low loss and high power resistance, a reduction in size would allow them to be employed in low-frequency band apparatuses.
The present invention aims to solve the problems described above and provide a small, light, and low-loss dielectric filter, compared to conventional ones, which are easily manufacturable and are particularly used at low frequency bands from VHF to UHF.
SUMMARY OF THE INVENTION
A dielectric filter of the present invention comprises a dielectric block; plural parallel through holes created in the dielectric block; at least one groove surrounding an opening of the through hole at the first end, one end of two ends in which one of them is at least open; an in-groove conductor made by forming a conductor inside the groove; an inside conductor made by forming conductor inside each of the through hole; an outside conductor made by covering the periphery of the dielectric block with a conductor; and an I/O electrode connected to an external circuit and electromagnetically coupled with the inside conductor. The outside conductor and inside conductor are connected at a second end at which each of the through hole is open, and the in-groove conductor and inside conductor are connected at the opening of the through hole surrounded with the groove. The opening is made inside the first end of the dielectric block.
With the above configuration, the length of a resonator formed by the inside conductor may be significantly reduced, enabling to achieve smaller filter, as a whole, compared to a conventional configuration.
In the dielectric filter of the present invention, the groove provided around the opening of the through hole forms a line with one short-circuit end, and this line is loaded in series to a line resonator formed by the inside conductor. In other words, the line formed by the groove has shorter wavelength than the quarter wavelength. Accordingly, an inductance element is loaded in series, and impedance of the line formed at the open-circuit end is reduced to add large capacitance, enabling to significantly reduce resonance frequency. In other words, inductance and capacitance may be increased with a fixed resonator length. If the resonator frequency is fixed, the resonator length can be significantly shortened, enabling to drastically reduce the size of the entire filter. Furthermore, since the resonance line formed of the inside conductor and in-groove conductor formed in the through hole and groove is created inside the outside conductor, spreading of the electric field to outside of the outside conductor can be prevented. High no-load Q for the resonator can be assured, enabling to configure a low-loss filter.
By reducing the size of the resonator as described above, multiple resonance frequencies are differed from an odd-numbered multiple of the fundamental frequency. Accordingly, harmonic of the fundamental frequency may be suppressed when the dielectric filter of the present invention is applied to an output filter of non-linear circuits such as power amplifiers.
Still more, the dielectric block with through holes and grooves can be integrally molded. Since the connection of the inside conductor and in-groove conductor is provided inside the open-circuit end, the filter may be formed by integrally molding dielectric ceramics into the shape of the dielectric filter of the present invention using molds. The entire face of the dielectric ceramics is coated with a metal film, and the end on which the groove is formed is ground to create the open-circuit end. Then the I/O electrode is formed. With these processes, the dielectric filter of the present invention can be easily manufactured, which is suitable for mass production.
In the dielectric filter of the present invention, the groove is formed concentric to the through hole or parallel to the periphery of the dielectric block. Concentric grooves facilitate its molding and realize rigid structure. Grooves parallel to the periphery of the dielectric block achieve further larger capacitance to the open-circuit end. This enables to further shorten the resonator length, and thus further reduce the size of the filter.
Furthermore, plural grooves are created around the opening of the through hole in the dielectric filter of the present invention. This enables to load further larger inductance in series to the line resonator formed by inside conductor. Thus, the resonator length may be further reduced, and accordingly the size of the filter is further reduced.
The groove in the dielectric filter of the present invention may be tapered. This enables to create a deeper groove, thus further reducing the resonator length. This also prevents peelin

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