This application is based on and hereby claims priority to European Application No. 08104660 filed on Jul. 7, 2008, the contents of which are hereby incorporated by reference.
Described below is a filter for electronic signals and to a method for manufacturing it. The filters under consideration may be filters having a ceramic body with appropriate metallizations thereon such as ceramic monoblock filters.
Ceramic single mode monoblock filters are used in small and medium power base transmitting site (BTS) products for the reason of size and cost. Also the electrical performance is satisfactory, especially if the dimensions of the filter body are increased. This causes that the rectangular filter body starts passing through electromagnetic energy at higher resonance modes and begins to leak through power beyond a certain cut off frequency. The smaller the body is the higher the cut off frequency is. It is roughly relative to equation: 1/((εr)½×W×L). This leakage is caused by non-desired higher propagation modes in the ceramic material. That is similar to the propagation in a waveguide that is bigger than required to allow just the lowest order mode. The leaking energy is at the harmonics of the desired operating frequency. This would be tolerable if the second and third harmonic at least could be reduced sufficiently.
Surfaces of the body 10 including the inner surfaces of holes 17 may have a more or less continuous metallization thereon. It is to be noted that on one or more particular surfaces this more or less continuous metallization may not or only partially be provided, such as the first surface 11. Reference numeral 18 indicates conductors which may be metallizations and which are shown in
The holes 17 act as resonators. They have circular or elliptic cross-section. 101 and 102 define symmetry axes of the filter 1. 101 is a longitudinal axis defined by the mid-points of two holes 17 provided in the ceramic monoblock filter 1. It coincides with the symmetry axis of the body 10 itself in that it is the centerline between surfaces 13 and 14, so that the holes 18 are arranged symmetrically in block 10. The holes 17 extend perpendicularly with respect to the first surface 11. 102 is another symmetry axis. The two holes 17 are symmetrically positioned with respect to this axis 102 which is also the centerline between surfaces 15 and 16.
The body 10 and the overall filter 1 is a cuboid with three pairs of opposing surfaces (11 and 12, 13 and 14, 15 and 16), the surfaces being substantially flat/plane and rectangular to each other. The body 10 is formed of a ceramic material with a certain relative dielectric constant, which is again selected in view of electronic properties. Holes 17 together with their cladding 18 and the surrounding body 10 and its outside conductors 18 of the filter 1 serve as resonators, the resonating frequencies being adjusted by defining the geometrical dimensions of the body, by forming the coupling conductors 19 and appropriately selecting the dielectric constant εr. The thickness is the predominant parameter for defining the resonating frequency. The filter has a length L, a width W, and a height H. The holes extend in height direction.
The field of use of such ceramic monoblock filters is wireless communication. They are used both in mobile stations and in base stations. In mobile stations, size and cost are very relevant criteria. In base stations, quality and costs are relevant criteria. The holes 17 together with the coupling conductors 19 and the metallizations/conductors 18 serve as resonators, and through the resonating effect they provide filtering as desired. The design is such that desired propagation modes of electric fields and magnetic fields are supported as far as possible, whereas undesired propagation modes and frequencies and harmonics are suppressed as far as possible.
The manufacturing method is that first a cuboid as desired is pressed from powder of the material that is to form the body 10. After pressing, the body has a consistency similar to sugar cubes, i.e. it withstands some mechanical impact, but is destroyable. After pressing, the body is fired under a certain temperature profile over time. It may be exposed to temperatures higher than 1000 or 1500° C. for several hours. Through this firing, the powder particles do not melt, but are sintered together. After firing, the body is machined to the desired final external shape and the desired holes are drilled into the body 10, and thereafter the body is immersed into a bath of silver paint, what may be repeated several times. After drying, the paint-covered body is again fired for increasing conductivity of the conductor cladding on the walls. Finally, the conductors on the top surface (first surface 11) may be structured as required for the tuning of the filtering performance.
The disadvantage of the present ceramic monoblock filters is that, at a given dimensioning and external circuitry requirement, they show insufficient suppression of certain modes and frequencies, particularly, harmonics are insufficiently suppressed. For the reason that touching the filter dimensions leads to reduced performance, alternative methods, like additional filtering on the PCB 103 must be used. This is sufficiently good but causes additional loss and consumes PCB space. A receiver is protected by using higher power low noise amplifiers (LNA) and an additional small filter behind it. This consumes space and adds to costs. This is not so critical in big units but is getting increasingly important when the units get smaller like in medium range and active antenna products.
Another disadvantage of prior art circuitry is insufficient or undesired coupling of incoming and outgoing signals. The present coupling is made in voltage made or in current mode using one of the resonator rods as a coupling element. In voltage mode, the coupling conductor approaches a resonator hole 17, but is not in electrical contact with the conductor on the inner wall of the hole, whereas in current mode coupling the coupling conductor is in electrical contact with the conductor on the inner wall of the resonator hole.
The disadvantage of known couplings is that they are not optimized either in the sense of matching or in the sense of coupling efficiency or mode/frequency selectivity. The current mode coupling (
An aspect is to provide a filter for electronic signals and a manufacturing method therefor which are cheap and result in improved harmonics suppression.
Another aspect is to provide a filter for electronic signals having an improved coupling structure.
A filter for electronic signals has a dielectric body, at least two coupling structures for coupling in and coupling out electronic signals, and one or more conductors on surface portions of the body. An outer surface of the body has one or more indentations. The indentations may have rounded surface portions and their contours may follow a hole in the body of the filter. They may be provided in a pairwise manner and may be symmetrical. They may have an internal symmetry, and two or more of them may be symmetrical with respect to each other. Particularly, two opposing surfaces may have indentations, preferably symmetrical to each other, whereas at least another pair of opposing surfaces does not have indentations (except the holes/throughholes/resonator holes) and are substantially flat.
The indentations maintain the effective diameter of the resonators above a certain value and, thus, have little effect on the Q-value and the performance, but help to suppress harmonics, because the cut-off frequency for non-desired modes is roughly doubled so that particularly second and third harmonics are better suppressed.
According to the filters described below, cavities of resonators formed by resonator holes are separated by reducing the width in between the cavities so that one nevertheless can maintain their diameter and the effective filter width can be reduced roughly to half or even less. “Cavity” in this sense is the space between the resonator hole conductor and the outer wall conductor. It is filled by the material of the filter body. This reducing roughly doubles the cut off frequency for the non-desired modes to around the fourth harmonic, so that the difficult second and third harmonics would be covered.
Further, a filter for electronic signals, which may optionally be formed as mentioned above, includes a dielectric body, one or more conductors on surface portions of the body, at least one resonator hole extending from a first surface of the body into the body, and a coupling structure for coupling in and/or out an electromagnetic signal. On the first surface of the body the coupling structure includes a conductor for signal input and/or output from/towards external, and in relation to an end portion of the conductor a coupling hole extending from the first surface and/or from an opposing second surface into the filter body.
The coupling hole may have or—together with other components—provide no or one or more predetermined resonance frequencies. It may have or provide insignificant resonance frequencies (i.e., sufficiently remote in frequency space from the desired frequency band, or non-existent). The coupling hole may be a through-hole or may be a blind hole extending from the first surface where the coupling conductor is provided or extending from the second surface opposing the first surface.
The location and dimension can be selected to optimize coupling, incoming impedance and excited resonance mode. The coupling hole may be provided asymmetrically in the cavity, whereas the resonator hole/s is/are in the centre of the cavity and of the filter body. The structure may be such that the coupling element is shorted at the same end as the resonator is and it is fed with a stripline at the other end of the resonator. i.e. it can be manufactured with the same steps as the actual resonator.
These and other aspects and advantages will become more apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings of which:
Reference will now be made in detail to the preferred embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.
Generally speaking, features described in this specification shall be deemed combinable with each other, even if this is not said explicitly, to the extent that a combination is technically possible. Same reference numerals shall denote same components.
The body 10 of the filter 1 has a first surface 11 from which two holes 17, which may be resonator holes, extend into the body, preferably perpendicularly to the first surface 11, and preferably through the entire body down to the second (not visible) surface 12. A surface of the body 10, from which the holes do not extend, namely in
The indentation 20 has an innermost point 21, i.e., a surface point most remote from surface 14 from which the indentation starts. The indentation may in itself be symmetrical in a sense that left and right surface portions thereof, i.e. portions left and right of the innermost point 21, are symmetrical to each other with respect to a plane symbolized by dashed line 22. The symmetry plane may be rectangular to the plane defined by the axes of the holes 17. The innermost portion may, in cross section, be the vertex of an angle, the angle preferably being smaller than 120° or smaller than 90°.
The indentation 20 may be positioned to indent at a space between two resonator holes 17 for reducing the width between resonators. More particularly, the indentation may have the same symmetry plane as two holes 17 left and right thereof in lengthwise direction. Generally speaking, the most indented portion 21 may project on a mid portion of a connecting line between adjacent holes 17. The mid portion may be the middle of the connecting line plus/minus 30% of the line length.
Indentations may be provided symmetrically with respect to a plane defined by axes through adjacent holes 17. This symmetry is shown in
Turning back to
The dimensioning of the indentations 20 may optionally be such that the remaining body material between opposing indentations (20a and 20b in
The rounded contour may follow a circle or an ellipse. Likewise, the cross-section of the hole 17 may follow a circle or an ellipse. The mentioned circles or ellipses may be concentric. The rounded outer wall (corresponding to walls 13, 14, 15 and 16 in
In this specification, various shapes of indentations and holes are described. In a preferred embodiment, these shapes may be constant along the height direction of a filter. However, they may also be variable, and then the indications may apply to only a portion along the height of the filter or only to a cross-section at a particular height position.
The dimension G indicates the remaining width between an indentation 20a and the opposing surface which may again be an indentation or, if no indentation is provided, the opposing wall as indicated by the dashed lines in
The overall size of the filter may be set in relation to the desired operating frequency range of the filter. The operating frequency range may be a frequency or frequency range between 200 MHz and 10 GHz. It may be for mobile communication applications, particularly for base stations and stationary equipment thereof, and may be suitable for one or more of the frequency ranges required there.
The manufacturing method may be as follows:
First, a powder of the desired material is prepared. The powder may have an average grain size as desired. The material is selected also in view of its dielectric constant.
Thereafter, the powder is pressed into the desired shape of the filter body 10. This pressing may include the provision of holes 17 by having respective rods in the mold for pressing the powder. The mold for pressing the powder may already have the indentations 20. Likewise, however, at this stage, the walls may still be conventional, i.e. as shown in
Next, the pressed body is fired, i.e. heated up to a certain temperature and kept at a certain temperature profile over time. Time may be several hours (two or more hours), temperatures may exceed 1000° C. or 1200° C. Through this, the powder particles bake together as in sintering and become a solid body of high mechanical strength.
If not already provided in the pressed form, the so fired body would now be provided with the indentations 20, preferably after having cooled down. This may be done by mechanical treatment like grinding or using a diamond tool. A profiled wheel for grinding may be used. The mechanical treatment, however, may also be made when the fired body has already indentations. The mechanical treatment may then be for refining surfaces or bringing the wall geometry down to a finally defined shape.
Once the final geometry of the body, including the desired indentations 20 and holes 17, is reached, it will be covered with conductors, which may be metallizations in certain embodiments. This may be done by immersing the body into a silver paint bath and drying it. This may be done repeatedly.
For improving the conductivity the so obtained silver coating may again be fired for achieving the desired better conductivity of the conductors covering substantial parts of most of the surfaces. The surfaces (except one) may be covered by at least 50% or at least 70% thereof by conductors, these conductors are preferably interconnected and may be grounded in use. Metallization coverage may also be 100%. This also applies to the inner walls of the holes 17.
This covering process may also include the first (top) surface which, thereafter, would be structured as desired. Particularly, metallization must be removed by an appropriate process (e.g. etching), so that only the desired pattern remains. Particularly, the coupling conductors 19 must be formed. Further, as far as present, the connection to the wall metallizations must be interrupted. Bringing the desired structures onto the first surface may also be done by an appropriate printing technique, such as the silk screen method.
The coupling hole 41 may practically be non-resonant, or it may have a resonance frequency remote from the working frequency range of the filter, e.g. more than 2% or more than 5% of the nominal frequency away therefrom. The coupling impedance is tuned to the desired value at the operating frequency. The coupling conductor 42 serves to make electrical contact between outside circuitry to which it is connectable via its rim side end 42e. From there, it runs towards the coupling hole 41 and, there, may make voltage mode coupling as schematically indicated in
Coupling hole 41 may be a through-hole as shown in
In
Generally speaking, either voltage mode coupling or current mode coupling is combinable with any of the makes of coupling holes 41a or 41b as shown in
The coupling holes 41 can substantially be manufactured just as the resonator holes 17. Also providing their respective conductor on their surfaces can be made in the same way. Some extra steps need to be taken for
In the described filters, the coupling structure described with reference to
A description has been provided with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 358 F3d 870, 69 USPQ2d 1865 (Fed. Cir. 2004).
Number | Date | Country | Kind |
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EP08104660 | Jul 2008 | EP | regional |