WO2001011709A1

WO2001011709A1 - Dielectric ceramic filter

Info

Publication number: WO2001011709A1
Application number: PCT/IB2000/001224
Authority: WO
Inventors: Masahiko Kitajima; Kosuke Nishimura; Hiroshi Nakamura
Original assignee: Ube Electronics, Ltd.
Priority date: 1999-08-06
Filing date: 2000-08-03
Publication date: 2001-02-15
Also published as: TW466797B

Abstract

A filter is created where the impedance of the filter is adjusted to a desired value by adjusting the shape and dimension of one of the transmission lines that is printed on the top surface of the filter, as well as adjusting the shunt capacity of the filter. In one preferred embodiment of the present invention a transmission line from the input/output pad extends from one lateral side of the filter, across part of the top surface, partially encircling at least one hole of the transmission poles, at least partially encircling the trap hole and nearly reaching the edge between the top surface and opposing lateral surface. The shunt capacity of the filter is adjusted by adjusting the gap between the leading edge of the input/output pad on the top surface of the filter and the edge of the filter where the top surface meets the opposing lateral surface.

Description

DIELECTRIC CERAMIC FILTER

CROSS-REFERENCE TO PROVISIONAL APPLICATION

The present invention claims the benefit of U.S. Provisional Application No. 60/147,674,

filed on August 6, 1999.

FIELD OF THE INVENTION

This invention relates generally to high performance ceramic block filters. More

specifically the invention relates to a given filter where said filter^'s impedance can be matched to a

desired specification.

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 8A-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

8B. 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 8C and 8D.

The capacitive coupling can be regulated in Figure 8B 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 8C has a greater diminishing effect on the capacitive coupling of

the block filter 101. than the broken line M^' of Figure 8D.

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

Patent Nos. 4.896,124, 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 and a duplexer needs 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 through 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 an input signal at the

desirable frequencies while the traps remove the input signal 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 D, a distance of 2D is placed between

trap 33 and the hole nearest to trap 33. The precise distance D is one of design choice for

achieving a specified performance. D typically falls within a preferred range of 1 to 10 mm.

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

Conventionally the holes 31 and traps 33 in a ceramic filter are positioned along a straight

line. This design together with the spacing requirements addressed above limits the extent to

which a filter may be reduced in size. Specifically, the performance characteristics of a given

filter are a function of its width, length, number of holes and diameter of holes. The usual length is 2 to 20 mm. The width of a filter is a function of the number of holes in the filter. Typically, the

width of the block filter ranges from 2 to 70 mm.

While the traps play a pivotal role in the performance of a filter they also impact on the

impedance of the filter. Conventionally, circuit boards are rated for 50 ohms impedance. Thus a

filter which is intended to be used with a 50 ohm circuit board must also have an impedance of 50

ohms, or risk poor performance. However, the experience in the art is that traps on a filter make it

more difficult to design a given filter with a specific impedance rating. Accordingly, it is desirable

to create a filter whose impedance can be adjusted without otherwise effecting the performance of

the filter.

SUMMARY OF THE INVENTION

A filter is created where the impedance of the filter is adjusted to a desired value by

adjusting the shape and dimension of one of the transmission lines that is printed on the top

surface of the filter, as well as adjusting the shunt capacity of the filter. In one preferred

embodiment of the present invention a transmission line extends from one lateral side of the filter,

across part of the top surface, partially encircling at least one hole of the transmission poles, at

least partially encircling the trap hole, and nearly reaching the edge between the top surface and

the opposing lateral surface. The shunt capacity of the filter is adjusted by adjusting the gap

between the leading edge of the transmission line from the input/output pad on the top surface of

the filter and the edge of the filter where the top surface meets the opposing lateral surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates the increased sharpness of the 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.

Figure 4 illustrates the present invention with one trap resonator.

Figure 5 shows the equivalent electrical circuit of the filter of Figure 4.

Figure 6 is close up view of a specified portion of the filter of Figure 4.

Figure 7 is a close up view of a specified portion of Figure 6.

Figure 8 A illustrates the open face surface of a ceramic block plated with silver on all other

surfaces.

Figure 8B illustrates the ceramic block of Figure 8 A, but with a printed pattern on the open

face surface.

Figure 8C illustrates the ceramic filter of figure 8B with a second printed pattern.

Figure 8D illustrates the ceramic filter of Figure 8C with a third printed pattern.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figure 4, in accordance with the present invention the impedance of a

dielectric ceramic filter is relatively easily controlled by adjusting the shape and dimension of the

transmission line 41 from the input/output pad and the shunt capacity, labeled CS in Figure 5. The

shunt capacity is adjusted by changing the gap between the edge of the transmission line 41 from the input/output pad and the edge of the filter, depicted in Figure 7. The gap shown in Figure 7

can be viewed in relation to filter 40 with reference to the circled portion 61 in Figure 6, where

Figure 6 is the circled portion 45 in Figure 4. The transmission line 41 from the input/output pad

is located between the trap hole 43 and the next adjacent hole of a transmission pole 49. Holes 46,

47, 48 and 49 are all transmission poles of the filter. Finding the appropriate shape and CS for a

given filter is easily accomplished without undue experimentation, for one skilled in the art. The

transmission line 41 from the input/output pad on the lateral surface is placed between the trap

hole and the hole of transmission pole adjacent to the trap hole on the top surface. The leading

edge of this transmission line 41 from the input/output pad has a small gap to the edge between the

top surface and the opposing lateral surface. (See Figures 4, 6 and 7).

The shape of the transmission line 41 from the input/output pad is determined so as to have

an adequate capacitance between transmission line and the trap hole, and the hole of transmission

pole.

The gap between the leading edge of the transmission line 41 from the input/output pad on

the top surface of the filter and the edge of the filter where the top surface meets the opposing

lateral surface is between the values of 0.2 - 2.0 mm and the ratio of the gap to filter height is

between the values of 0.03: 1 - 0.2: 1. (See Figure 7).

As for the remainder of the construction of the filter, it may be designed as any dielectric

ceramic filter. The choice of dielectric being 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. Similarly, the process of laying the conductive material on the dielectric is well known in

the art. As stated above, copper or silver are usually the conductive material of choice. The

conductive material generally covers the substantially all of the bottom and sidewalls 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 known in the art.

Lastly, filters made in accordance with the present invention may be simplex (a single

filter) or duplexer (the combination of two filters such as a transmitter filter and a receiver filter).

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 and two opposing side-walls connecting said top surface to said bottom surface

along the height of said block, said top surface having printed thereon a pattern of

conductive material, said pattern including a transmission line extending from the

input/output pad placed on a first one of said side walls along the width of block, across

said top surface toward an edge of said block formed by said top surface and a second one

of said side walls along the width of said block;

at least three holes extending through said block of dielectric material from said top surface to said bottom surface, at least one of said three holes being a trap hole;

conductive material substantially covering said bottom surface, side-wall surfaces

and inner surfaces of said at least three holes; said filter whose impedance can be adjusted

by adjusting the distance between a leading edge of said transmission line extending from

the input/output pad and said edge formed by said top surface and said second one of said

side walls along the width of said block.

2. The filter of claim 1 wherein said transmission line extending from said

input/output pad is at least partially encircling at least one hole of the transmission

poles and at least partially encircling the trap hole

3. The filter of claims 1 and 2 where the distance between the leading edge of said transmission line on the top surface of the filter and the edge of the filter where

the top surface meets the opposing lateral surface is between the values of 0.2 -

2.0 mm and the ratio of the gap to filter height is between the values of 0.03: 1 - 0.2: 1.