Patent Application
20140132372
1. Field of the Invention
This invention relates to Radio Frequency Communication transceivers and in particular to RF filters with reduced intermodulation distortion characteristics.
2. Description of Related Art
A typical wireless communication system, such as cellular transceiver, includes uplink and downlink channels separated in frequency. Such communication systems use filters to route, combine, and/or separate signals at different frequencies, to prevent interfering with other channels or systems, and/or to prevent being interfered with by other channels or systems.
One type of filter used in such communication systems is constructed with coaxial cavity resonators, sometimes referred to as combline or interdigital resonators. These resonators typically consist of a metal outer conductor or cavity with a metal inner conductor. The inner conductor is electrically short circuited to the outer conductor at one end and open circuited at the other end. When an electromagnetic wave is coupled to this structure, the wave propagates along its length until it encounters the short circuit and is reflected back. This reflection causes a standing wave to be generated when the length of the inner conductor is approximately ¼ wave length long relative to the Frequency of the coupled wave. Shorter lengths can also be used by capacitively loading the open circuit end. This standing wave can then be further coupled to adjacent resonators, allowing waves at specific frequencies to propagate while rejecting waves at other frequencies.
However, coaxial cavity resonators can cause signal corruption. Signal corruption can occur when Intermodulation Distortion (IMD) generated by the uplink or downlink signals fall unintentionally into the downlink or uplink frequency band, respectively. IMD in filters can create the very interference they are supposed to be preventing.
As such there is a need to enhance the performance of such coaxial cavity resonators employed in wireless base stations and to specifically reduce or preferably eliminate intermodulation distortion.
As more spectrum is being allocated for wireless communications, the problem of intermodulation distortion has become more noticeable. A common construction of filters for wireless communication systems is machined metal housings using metal posts as combline or interdigital resonators. Current cost effective machining techniques are not accurate enough to produce these structures repeatedly so tuning elements are often employed to compensate for these inaccuracies. These tuning elements are often shaped as a threaded metal rod, with an arrangement for varying its length to achieve the desired filtering effect. Consequently, the contact area where the threads meet the housing is weak and/or intermittent. Current flows in these areas causing potential intermodulation distortion.
Intermodulation distortion is generated when two or more signals encounter non-linear elements during transmission. One source of non-linearity is weak and/or intermittent metal to metal contact in areas where current flows. As such, the tuning elements intended to fine tune the resonator filter can cause the very distortion that they intended to overcome. In accordance with one embodiment of the invention a coaxial cavity resonator filter is provided having a cylindrical hollow post. The post is configured to receive a frequency tuning element. The post includes a first opening and an inner wall, such as a cylindrical wall having a diameter that is larger than the diameter of the tuning element. The post further includes a flange that forms a second opening having a specified height and a diameter that is smaller than the diameter of the inner wall of the first opening.
An insulating support member is disposed within the post. The insulating support member is made of an insulating material such as Teflon® or a polyetherimide such as Ultem®, and it has a first head portion having a first diameter and a shoulder flange portion having a smaller diameter with a threaded internal wall. The shoulder flange portion of the insulating support is fitted within the second opening of the post. The insulating support is configured to receive a tuning element that can be screwed via its internal threaded portion. In an alternative embodiment, the tuning element includes an insulated threaded sleeve positioned at a desired portion along its length, and the second opening of the post is similarly threaded. As such, during operation the insulated threaded portion of the tuning element engages the threaded second opening and the length of the tuning element is adjusted to achieve a desired frequency response.
In accordance with yet another embodiment the insulated sleeve is moveable along the length of the tuning element to provide an optimum location for the tuning element along the hollow tube of the post.
In accordance with yet another embodiment the insulated sleeve is mounted in the cavity cover such that the tuning element is external to the resonator post. The length of the tuning element is adjusted to achieve the desired frequency response from the coaxial cavity resonator. In this configuration, the tuning element can also be used to adjust the coupling between adjacent resonators.
In accordance with various embodiments of the invention the following description and accompanying drawings describe the various features of the invention as claimed, wherein:
a and 2b illustrate a coaxial cavity resonator employed in the coaxial cavity resonator filter of
a and 3b illustrate an insulating support member according to one embodiment;
a-9d illustrate a coaxial cavity resonator filter having a solid post in accordance with one embodiment of the invention;
a-10b illustrate another filter having a solid post in accordance with an embodiment of the invention;
a-11d illustrate one embodiment of the invention with a hollow post;
a and 12d illustrate another embodiment of the invention with a hollow post; and
The coaxial cavity resonator filters discussed in relation to various embodiments of the invention are typically employed in wireless base stations, such as cellular communication base stations. A desired characteristic of such filters is to have low insertion losses in the passband frequency range of the transmitted or received signals, along with high attenuation in the stopband frequency range close to the passband frequency range.
a and 2b illustrate a coaxial cavity resonator 14 in accordance with one embodiment of the invention.
a illustrates an insulating support member 30 in accordance with one embodiment of the invention. Insulating support member 30 is made of an insulating material such as Ultem® or Teflon®, and is configured to fit within coaxial cavity resonator 14 illustrated in
Furthermore, the inside diameter of shoulder portion 34 is threaded so as to accommodate the turning of a tuning element configured to pass through insulating support 30 as will be explained in more detail below. In accordance with one embodiment of the invention, the diameter of head portion 32 is about 8 mm. For this embodiment, the length of the shoulder portion is about 10 mm and the length of the head portion is about 2.5 mm providing an overall length of 12.5 mm for the insulated support member.
The insulated support member is configured to fit within the coaxial cavity resonator, such as 14 illustrated in
Once insulated support 30 is placed within the coaxial cavity resonator as described above, a tuning element 80 illustrated in
In accordance with another embodiment of the invention, instead of using insulating support 30, tuning element 80 is fitted with a threaded insulating sleeve. As such
It is appreciated by those skilled in the art that, depending on frequency characteristics requirements, sometimes a single coaxial cavity resonator is employed and other times two or more coaxial cavity resonators are coupled together by employing an arrangement where a coupling tuning element is used to achieve the desired filter characteristics. In accordance with one embodiment of the present invention,
As such,
In accordance with other embodiments, the intermodulation distortion effect can be substantially reduced in a variety of cavity resonator structures. For example,
In accordance with one embodiment of the invention,
a and 10b illustrate another embodiment of filter 160. In this embodiment an insulator member such as a plug 180 is inserted within opening 160. Insulator member 180 has a double flange configuration, such that when inserted into the opening, the upper flange engages against the upper surface of cover 166 and the lower flange engages against the bottom surface of the cover. In accordance with one embodiment, the inside wall portion of the plug is threaded so as to allow tuning element 80 to move along the inside surface of the plug until the desired frequency response is achieved.
a through 11d show another embodiment where the resonator post 190 is hollow, allowing the tuning element 80 to engage with the cavity resonator from its bottom side.
a and 12b show yet another embodiment where the resonator post is also hollow, allowing the tuning element 80 to engage with the cavity resonator from the bottom side. As illustrated in
The intermodulation distortion effect is substantially eliminated by using the various embodiments of the present invention as described above. For example,
As such, in accordance with various embodiments of the present invention, an arrangement for insulating the tuning element of a coaxial cavity resonator from the remaining portions of the structure provides a substantial reduction in intermodulation distortion.
While only certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes or equivalents will now occur to those skilled in the art. It is therefore, to be understood that this application is intended to cover all such modifications and changes that fall within the true spirit of the invention.
