WO2001011707A1

WO2001011707A1 - A controlled performance dielectric ceramic filter

Info

Publication number: WO2001011707A1
Application number: PCT/IB2000/001167
Authority: WO
Inventors: Nakamura Hiroshi; Masahiko Kitajima; Nishimura Kosuke
Original assignee: Ube Electronics, Ltd.
Priority date: 1999-08-06
Filing date: 2000-08-03
Publication date: 2001-02-15
Also published as: KR20010080045A; BR0007030A; TW523962B; CN1320287A

Abstract

A ceramic block filter with a series of transmission holes spaced a distance from one another, and a trap hole wherein the filter has a transmission hole diameter to filter height ratio of 0.16- 0.2: 1 and a ratio of transmission hole diameter to distance between transmission holes of 0.45-0.5 : 1. The trap hole is spaced a distance of one and one half to twice the distance between said at least two transmission holes. The zero point of this filter is controlled without effecting critical parameters, such as filter dimensions, insertion loss and attenuation, by adjusting the ratio of transmission hole diameter to filter height and the transmission hole diameter to distance between the transmission holes.

Description

A CONTROLLED PERFORMANCE DIELECTRIC CERAMIC FILTER

CROSS-REFERENCE TO PROVISIONAL APPLICATION

The present invention claims the benefit of U.S. Provisional Application No. 60/147.677, filed on August 6, 1999.

FIELD OF THE INVENTION

This invention relates to high performance ceramic block filters. More specifically, the invention relates to controlling the zero point in a filter response so as to improve attenuation.

BACKGROUND OF THE INVENTION

Filters pass alternating currents at some frequencies (referred to as the band pass frequencies) while attenuating or blocking currents at other frequencies. In the development of filters to specified requirements, focus is often emphasized on critical parameters such as filter dimensions, insertion loss (loss of energy by the filter in a circuit) and attenuation (reduction in signal amplitude). When designing a filter with these parameters, however, a filter^'s zero point, i.e., the lowest point in the filter response, may not be realized.

The filters generally comprise a ceramic body and a coaxial hole bored through its length forming 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. A filter is formed when multiple resonators are combined. 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 6A-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 6B. 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 6C and 6D.

The capacitive coupling can be regulated in Figure 6B 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 6C has a greater diminishing effect on the capacitive coupling of the block filter 101, than the broken line M' of Figure 6D. 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 transmission 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 transmission holes a distance greater than the spacing between transmission holes so as to avoid mutual interference between the transmission holes and traps. As shown in Figure 3, whereas transmission holes 31 are separated from each other a distance D, a distance of 2D is placed between trap 33 and the transmission 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 10mm. Traditionally, the traps will be spaced from 1.5D to 2D from the transmission holes. Notwithstanding all of the considerations that go into designing a filter with a desired performance, at times the filter response is not as sharp as one would desire. Sharpness is another way of referring to the distance between the high side of pass band and the zero point. The closer these two points are on the graph the sharper the response. Referring to Figure 4. the zero point A does not produce as sharp a filter response as the zero point B. It is desirable to improve the attenuation of the filter whose response includes the zero point A, so that the zero point moves to B.

SUMMARY OF THE INVENTION

Accordingly, it is desirable to control the zero point in a high performance dielectric ceramic filter without effecting any of the other critical parameters of the filter performance. This invention provides a simple and effective method to control the zero point without effecting the above-mentioned critical parameters. The present invention seeks to increase sharpness of the filter response, that is, the distance between the high side of pass band and the zero point. As described below, the closer the zero point is to the high side of pass band, the sharper the response. Control of the zero point is critical to 1) improve attenuation of frequencies outside a desired pass band, 2) to increase the margin against requested characteristics (such as filter dimensions, insertion loss and attenuation) and 3) to improve the productivity of the filter by increasing sharpness of the filter response. The present invention controls the zero point using a dielectric ceramic filter herein described below. In accordance with the present invention, this is accomplished by selecting a proper ratio of transmission hole diameter (d) to filter height (T) and a proper ratio of transmission hole diameter (d) to distance between transmission holes (D).

One specific embodiment of the present invention is a new design for a filter where the , ,„„.-

WO 01/11707 zero point response is controlled by adjusting the ratio of transmission hole diameter to filter height between the values of 0.16-0.2 : 1 and by adjusting the ratio of transmission hole diameter to distance between transmission holes between the values of 0.45 - 0.5 : 1.

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 is a graph of a filter response illustrating the effect the zero point has on the sharpness of the filter response.

Figure 5 is a plain view of the top surface of a dielectric ceramic filter designed in accordance with the present invention.

Figure 6A illustrates the open surface of a ceramic block plated with silver on all other surfaces.

Figure 6B illustrates the ceramic block of Figure 6A, but with a printed pattern on the open face surface.

Figure 6C illustrates the ceramic filter of Figure 6B with a second printed pattern.

Figure 6D illustrates the ceramic figure of Figure 6C with a third printed pattern. DETAILED DESCRIPTION OF THE INVENTION

The present invention describes a high performance dielectric ceramic filter, wherein the zero point response can be controlled without effecting any other critical parameters of the filter performance. The filter comprises 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; at least three holes extending through said block of dielectric material from said top surface to said bottom surface, wherein at least one of said at least three holes which is relatively closer to one of said side- walls connecting said top surface to said bottom surface along said height of said block, is a trap hole and at least two adjacent holes of said at least three holes are transmission holes, said transmission holes spaced one from the other a distance D along the width of the block, and wherein said filter has a ratio of transmission hole diameter to filter height of 0.16 - 0.2 : 1 and a ratio of transmission hole diameter to distance between transmission holes of 0.45 - 0.5 : 1. Control of the zero point response in the filter is determined by selecting a proper ratio of transmission hole diameter (d) to filter height (T) and the ratio of transmission hole diameter (d) to distance between transmission holes (D).

Control of the zero point in a high performance dielectric ceramic filter in accordance with the present invention occurs without effecting any of the other critical parameters of the filter performance by selecting the proper ratios of transmission hole diameter to filter height and transmission hole diameter to distance between transmission holes. Both ratios are simultaneously present in the filter. The distance between the trap holes are not necessarily equally space from another. Where the transmission holes are not equally spaced, proper spacing between the transmission holes is determined by an average value. Referring to Figure 5 to facilitate this discussion, the filter^'s height is shown with the variable T. Variable D represents the spacing or distance between transmission holes 51. Variable d represents the diameter of transmission holes 51. In accordance with the present invention the transmission hole diameter (d) to filter height (T) , (d : T) should be 0.16-0.2 : 1 and the ratio of transmission hole diameter (d) to spacing (D) between the transmission holes 51 (d : D) should be 0.45-0.5 : 1. By maintaining these ratios, the changes are so slight so as not to effect the performance specifications of the filter, other than to improve attenuation. As a result, the productivity of the filter is improved and there is an increased margin against the desired performance specifications.

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 manufacture of block filters is known in the art, including the process of laying the conductive material on the dielectric. As stated above, copper or silver are usually the conductive materials 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.

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; at least three holes extending through said block of dielectric material from said top surface to said bottom surface, wherein at least one of said at least three holes which is relatively closer to one of said side-walls connecting said top surface to said bottom surface along said height of said block, is a trap hole and at least two adjacent holes of said at least three holes are transmission holes, said transmission holes spaced one from the other a distance D along the width of the block; wherein said filter has a ratio of transmission hole diameter to filter height of 0.16 - 0.2 : 1 and a ratio of transmission hole diameter to distance between transmission holes, D, of0.45 - 0.5 : l; and conductive material substantially covering said bottom surface, side-wall surfaces and inner surfaces of said at least three holes and layered on said top surface in a geometrical pattern.

2. The filter of claim 1 wherein said trap is spaced a distance of one and one half to twice the distance between said at least two transmission holes.

3. The filter of claim 1 wherein said block is ceramic.

4. The filter of claim 1 wherein said block has a dielectric constant between 20 and 150.

5. The filter of claim 1 wherein said conductive material is selected from the group comprising copper or silver.