WO2001011711A1

WO2001011711A1 - Dielectric filter with a transmission line

Info

Publication number: WO2001011711A1
Application number: PCT/IB2000/001336
Authority: WO
Inventors: Masahiko Kitajima; Yoshiaki Fujiyama; Kosuke Nishimura
Original assignee: Ube Electronics, Ltd.
Priority date: 1999-08-06
Filing date: 2000-08-03
Publication date: 2001-02-15

Abstract

A reduced size ceramic block filter has at least two holes extending through the filter from its top surface to its bottom surface and a transmission line printed on the top surface between the holes and the top surface edge of the filter that joins with a side wall running along the width of the filter. Conductive material covers substantially all of the bottom surface, the side wall surfaces and the inner surface of the holes. Trap holes are not needed as their function is replaced by the interaction of the transmission poles and the transmission line.

Description

DIELECTRIC FILTER WITH A TRANSMISSION LINE

CROSS REFERENCE TO PROVISIONAL APPLICATION

The present invention claims the benefit of U.S. Provisional Application No. 60/147.678.

filed on August 6, 1999.

FIELD OF THE INVENTION

This invention relates to ceramic block filters with high performance in a small package.

More specifically, the present invention relates to a new design for a high performance dielectric

ceramic filter that is smaller than conventional filters with comparable performance

specifications.

BACKGROUND OF THE INVENTION

A ceramic body with a coaxial hole bored through its length forms a resonator that

resonates at a specific frequency determined by the length of the hole and the effective dielectric

constant of the ceramic material. The holes are typically circular, or elliptical. Combining

multiple resonators may form a dielectric ceramic filter. The holes in a filter must pass through

the entire block, from the top surface to the bottom surface. This means that the depth of a hole

is the exact same length as the axial length of a filter. The axial length of a filter is selected based

on the desired frequency and specified dielectric constant of ceramic.

The ceramic block functions as a filter because the resonators are inductively coupled

and/or capacitively coupled between every two adjacent resonators. These couplings are formed by the electrode pattern designed on the top surface of the ceramic block, plated with a

conductive material such as silver or copper. More specifically and with reference to Figures

9A-D, a ceramic block 101 is shown with two holes 103 and 105. All surfaces, except for the

front open face 107 through which the two holes 103 and 105 extend, are plated with silver. Due to the size of the holes, their proximity and the conductive coating, the two holes 103 and 105 are

inductively and capacitively coupled to each other. However, block 101 will not perform as a

filter because these couplings cancel each other out.

To form a filter, a pattern of conductive material is printed on face 107. as shown in

Figure 9B. In this embodiment the patterns A and A' enhance the capacitive coupling between

holes 103 and 105. While the capacitive coupling is enhanced, the inductive coupling remains

substantially unaffected. This is because inductive coupling is mostly a function of the hole

diameter, shape and spacing between holes. These parameters are the same in Figures 9C and

9D.

The capacitive coupling can be regulated in Figure 9B by adjusting parameters L and G. By decreasing G or increasing L, the capacitive coupling is strengthened. The capacitive

coupling can also be weakened such that the inductive coupling is stronger, by printing line M on

open face 107. The simple line M in Figure 9C has a greater diminishing effect on the capacitive

coupling of the block filter 101, than the broken line M" of Figure 9D.

Ceramic filters are well known in the art and are generally described for example in U.S.

Patent Nos. 4,742.562; and 4,692,726, both of which are hereby incorporated by reference as if

fully set forth herein. With respect to its performance, it is known in the art that the band pass characteristics of

a dielectric ceramic filter are sharpened as the number of holes bored in the ceramic block are

increased. The number of holes required depends on the desirable attenuation properties of the

filter. Typically a simplex filter requires at least two holes while a duplexer (having a transmitter

filter and a receiver filter) requires more than three holes. This is illustrated in Figure 1 where

graph 10 represents the filter response with fewer holes than graphs 12 and 14. It is apparent that

graph 14 which is the response of the filter with the most holes, is the sharpest of the three

responses shown. Referring to Figure 2, it can be seen that the band pass characteristic of a

particular dielectric ceramic filter is also sharpened with the use of trap holes bored into the

ceramic block. Solid line graph 21 represents the response of a filter without a high end trap.

Dashed line graph 23 represents the response of the same filter with a high end trap.

Trap holes, or traps as they are commonly referred to are resonators which resonate at a

frequency different from the primary filter holes, commonly referred to simply as holes. They

are designed to resonate at the undesirable frequencies. Thus, the holes transmit signals at the desirable frequencies while the traps remove signals at the undesirable frequencies, whether low

end or high end. In this manner the characteristic of the filter is defined, i.e. high pass, low pass,

or band pass.

The traps are spaced from holes a distance greater than the spacing between holes so as to

avoid mutual interference between the holes and traps. As shown in Figure 3. whereas holes 31

are separated from each other a distance equal to D, a distance of 2D is placed between trap 33

and the hole nearest to trap 33. The precise distance being a design choice for achieving a specified performance. D Typically falls within a preferred range of 1 to 10mm. Traditionally.

the traps will be spaced from 1.5D to 2D from the holes.

It should be clear, that the need for trap holes with their requisite spacing requirements in

a filter adds a significant constraint to the degree to which the filter can be made smaller.

SUMMARY OF THE INVENTION

A ceramic block filter for a given performance includes at least two transmission poles

and a transmission line printed on the top surface of the block filter. The transmission line is

printed between the transmission poles and one of the long edges of the block filter. The

interaction of the transmission line with the transmission poles results in a transmission zero.

Transmission zero is the mathematical concept that describes the response characteristics of a

filter, where a given filter decreases the frequency of an incoming signal to the filter and passes

the signal of another frequency. With proper design of the geometry of the transmission lines,

the resulting transmission zero can be made equivalent to one or more trap holes found in a

conventional ceramic block filter. Accordingly, use of the transmission line in accordance with

the present invention allows for the elimination of the trap holes of a conventional filter designed

for the same performance, resulting in a smaller filter.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates the increased sharpness of the bandpass response of a dielectric

ceramic filter as the number of holes in the filter increase.

Figure 2 illustrates the effectiveness of traps in removing high end frequencies.

Figure 3 is representative of the spacing between holes and hole and trap on a conventional ceramic block filter. Holes are represented by 33 while the trap is represented by 31.

Figure 4 depicts the dielectric ceramic block filter of the present invention with a

transmission line printed on the top surface of the filter.

Figure 5 is the equivalent electrical circuit of the filter shown in Figure 4.

Figure 6 is a plot of three different filter responses: no trap; one trap; and no trap with a

transmission line.

Figure 7 is a plot of two filter responses showing the improved performance of the filter of the present invention over a conventional filter without a trap.

Figure 8A depicts a conventional filter with a low end trap requiring a ceramic block with

a relatively large width.

Figure 8B depicts one embodiment of the present invention without a low end trap and

having a ceramic block with a relatively small width.

Figure 8C depicts another embodiment of the present invention without low end trap and

having a ceramic block with a relatively small width.

Figure 9A illustrates the open face surface of a ceramic block plated with silver on all

other surfaces.

Figure 9B illustrates the ceramic block of Figure 9 A, but with a printed pattern on the

open face surface.

Figure 9C illustrates the ceramic filter of figure 9B with a second printed pattern.

Figure 9D illustrates the ceramic filter of Figure 9C with a third printed pattern. DETAILED DESCRIPTION OF THE INVENTION

Referring to Figure 4. the dielectric ceramic filter designed in accordance with the present invention has no low end trap. Rather the filter has four transmission poles 42, electrode pads 41

as in any conventional dielectric ceramic filter, and a transmission line 43 printed on the top

surface, having width (w)and height (h).

Values for w = 3 - 15mm.

Values for h = 0.1 -2.0mm.

The transmission line 43. is located on a surface of the filter such that it is located

between the edge of the transmission poles 42 and an edge of the filter. The distance between the

edge of the transmission poles 42 to the transmission line 43 is 0.5 - 2.0 mm. The distance

between the transmission line 43 and aforementioned filter edge, which the transmission line 43

is placed between that and the edge of the transmission poles is 0.5 - 2.0 mm.

Values for the transmission pole's 42 holes diameter are 0.3 - 2.0 mm

The distance between transmission pole 42 holes is/are 0.5 - 5.0 mm.

The manufacture of block filters is known in the art, including the process of laying the

conductive material on the dielectric. Filters made in accordance with the present invention may

be simplex (a single filter) or duplex (the combination of two filters such as a transmitter filter

and a receiver filter). As stated above, the usual conductive materials of choice are copper or

silver. The conductive material generally covers substantially all of the bottom and side walls of

the ceramic block. This is accomplished by one of several known methods. These include dipping, spraying or printing a copper or silver paste onto the dielectric and firing the coated

dielectric. Other methods include electrolytic plating or electroless plating, also processes well

known in the art. As with other dielectric filters the choice of dielectric is one of design. In one advantageous embodiment of the present invention, the dielectric is ceramic and has an effective

dielectric constant between 20 and 150.

The equivalent electrical circuit for the filter of Figure 4 is shown in Figure 5. From

Figure 5 it can be seen that the interaction between transmission line 43 and the four

transmission poles 42 result in four capacitors 45 a-d. Electrode pads 41 are also described in

Figure 5. The transmission line 43 of Figure 4 is represented as inductor 44 characteristic on the

circuit diagram of Figure 5. The interaction between the inductor 44 characteristic of

transmission line 43 and the four capacitors 45 a-d result in a transmission zero. Traps are

conventionally used to create a transmission zero, however, they are no longer necessary where a

transmission line 43 is used as it is in the present invention. Thus, the elimination of traps allows

the present invention to be physically smaller in size than conventional filters with trap holes and

similar performance capabilities. (See Figure 6). Further, the present invention also results in

an improved performance over conventional filters without trap holes. (See Figure 6 and 7).

The transmission line 43 is preferably formed with a silver paste on the top surface of the

filter and its frequency response is defined by/= l/(LC)^{l 2}where L is the inductance of the

transmission line 43 and C is the combined capacitance of the capacitors 45 a-d.

Referring to Figure 6, three graphs are plotted showing the response of three different

filters designed to provide similar performance. The filter response graph 63 is clearly in need of

attenuation, as the undesirable low end frequencies have a significant presence at the output of the filter. Graph 63 is the response of a four transmission pole filter without any traps. In

contrast, the filter response is considerably improved for graph 62. Graph 62 is the response of a

filter with the same four transmission poles and one trap.

Finally, graph 61 (solid line curve) represents the response of a filter in accordance with

the present invention in which the same four transmission poles are present, but without any

traps. However, a transmission line as described above was printed on the top surface of the

filter. Clearly, the response of the filter of the present invention is equivalent to that of the filter

with the trap. Yet, the filter with the transmission line has a significantly smaller package, as

shown below.

Similarly, the enhanced performance that is realized with the filter of the present

invention, as compared with a conventional filter without a trap, can be seen from Figure 7. Graph 71 (solid line graph) depicts the response of the filter of the present invention, while graph

72 depicts the response of a conventional filter without any traps.

By replacing the low end trap with a transmission line the size of the filter is reduced

considerably. This is readily apparent by comparing the top plan views of the filters shown in

Figures 8A and 8B. In Figure 8A a conventional filter with a low end trap 81 is shown. This

filter is clearly wider than the filter shown in Figure 8B which has no low end trap, only

transmission line 82, which has no bearing on the width of the filter. The parameters in

determining the design of a transmission line include width of the transmission line, height of the

transmission line, and distance of the transmission line from the edge of the filter and the

distance of the transmission lines between it and the transmission pole holes. The foregoing merely illustrates the principles of the present invention. Those skilled in

the art will be able to devise various modifications, which although not explicitly described or

shown herein, embody the principles of the invention and are thus within its spirit and scope.

Claims

WHAT IS CLAIMED IS

1. A filter, comprising:

a block of dielectric material having a top surface, a bottom surface, two opposing side walls connecting said top surface to said bottom surface along the width of said

block of dielectric material and two side walls connecting said top surface to said bottom

surface along the height of said block;

a plurality of holes extending through said block from said top surface to said

bottom surface, each of said holes having the characteristic of a transmission pole;

conductive material covering substantially all of the bottom surface, said side wall

surfaces and the inner surface of said holes; and

at least one transmission line printed on said top surface of said block between

said plurality of holes and an edge of said block of dielectric material formed by said top

surface and one of said side walls along the width of said block of dielectric material.

2. The filter of claim 1 wherein said plurality of holes comprise at least three

transmission poles.

3. The filter of claim 1 wherein said transmission line has a height (h) of 0.1 -2.00

mm and a width (w) of 3 - 15 mm.

4. The filter of claim 2 wherein said transmission line has a height (h) of 0.1 -2.00

mm and a width (w) of 3 - 15 mm.

5. The filter of claim 1 wherein said transmission line is located a distance of 0.5 -

2.00 mm from the edge of said plurality of holes and a distance of 0.5 - 2.00 mm of said

edge of said block of dielectric material, so that said transmission line is between said plurality of holes and said edge of said block.

6. The filter of claim 2 wherein said transmission line is a distance of 0.5 -2.00 mm from the edge of said plurality of holes and a distance of 0.5 - 2.00 mm of said edge of

said block of dielectric material, so that said transmission line is between said plurality of

holes and said edge of said block.

7. The filter of claim 3 wherein said transmission line is a distance of 0.5 -2.00 mm

from the edge of said plurality of holes and a distance of 0.5 - 2.00 mm of said edge of

holes and said edge of said block.

8. The filter of claim 4 wherein said transmission line is a distance of 0.5 -2.00 mm

holes and said edge of said block.

9. The filter of claim 1 wherein the diameter of said holes is 0.3 -2.0 mm and the

distance between said holes is 0.5 -5.0 mm.

10. The filter of claim 2 wherein the diameter of said holes is 0.3 -2.0 mm and the

distance between said holes is 0.5 -5.0 mm.

11. The filter of claim 3 wherein the diameter of said holes is 0.3 -2.0 mm and the

distance between said holes is 0.5 -5.0 mm.

12. The filter of claim 4 wherein the diameter of said holes is 0.3 -2.0 mm and the

distance between said holes is 0.5 -5.0 mm.

13. The filter of claim 5 wherein the diameter of said holes is 0.3 -2.0 mm and the distance between said holes is 0.5 -5.0 mm.

14. The filter of claim 6 wherein the diameter of said holes is 0.3 -2.0 mm and the

distance between said holes is 0.5 -5.0 mm.

15. The filter of claim 7 wherein the diameter of said holes is 0.3 -2.0 mm and the

distance between said holes is 0.5 -5.0 mm.

16. The filter of claim 8 wherein the diameter of said holes is 0.3 -2.0 mm and the

distance between said holes is 0.5 -5.0 mm.