Wave transmission lines and networks – Coupling networks – Wave filters including long line elements
Reexamination Certificate
1998-12-14
2001-03-06
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Wave filters including long line elements
C333S203000, C333S207000, C333S223000
Reexamination Certificate
active
06198363
ABSTRACT:
The invention relates to a filter comprising a shell construction of conductive material with at least one section and at least one resonator of conductive material in said at least one section for forming at least one resonance circuit, in which filter the resonator comprises as its extreme ends a base and a second end, the base being fastened to the shell construction and the second end being directed elsewhere towards the shell construction at a distance therefrom, the resonator comprising a means which directs its surface towards the shell construction and increases the cross-sectional area of the resonator to increase the capacitance between the resonator and the shell construction, the filter further comprising a frequency tuning element of conductive material for tuning the resonance frequency of the resonator of the resonance circuit.
The invention also relates to a tuning element, particularly a frequency tuning element, for tuning the resonance frequency of a resonance circuit formed by a filter section and a resonator disposed in the section.
Radio frequency filters, such as resonator filters, are used to implement high-frequency circuits in base stations of mobile telephone networks, for example. One way is to use radio frequency filters as interface circuits and filtering circuits in transmitters or receivers in base stations, for example.
Resonator filters comprising a shell construction, i.e. a body, are of various types including e.g. coaxial resonator filters and L-C filters. The present solution is particularly related to coaxial resonators. In addition, for instance a helix resonator and a cavity resonator are also known. All these resonator types comprise a metal shell construction. In coaxial resonators, for example, the shell construction envelops a conductor which is positioned in the middle of the cavity of the shell and is called a resonator or resonator rod. In helix resonators, the resonator wire is wound as a spiral coil. A cavity resonator only comprises a cavity.
As the size of devices comprising filters decreases, resonators also have to be made small. To reduce the space required by a resonator, a helix coil is used where the same operational length fits into a shorter space, since in a helix resonator the resonator is formed as a coil. However, helix coils are difficult to manufacture, a further drawback being the difficulty to fasten to a helix coil a coupling element or other such projection, needed to adjust the coupling between two resonance circuits. A further problem in helix resonators is the difficulty to support them and carry out temperature compensation. A conventional resonator is a quarter wave long.
In a coaxial resonator, the resonator is usually a straight rod which is fastened only to the bottom of the resonator. Such a resonator is long and consequently takes up much space. A U-shaped coaxial resonator type is also known, i.e. one that comprises a turning point. Such a construction allows a smaller size, but its manufacture is problematic particularly because the initial section of the resonator has to be fastened and the end section supported to different surfaces, which significantly complicates the manufacture and assembly of the filter. To be able to save space by employing shorter resonators when straight coaxial resonators are used, without, however, changing the frequency range, it is known to use at one end of the resonator, such as the free end or in the area thereof, a conductive plate having a large surface area as compared with the resonator rod. Said plate increases the cross-sectional area of the resonator, and the increased area causes the capacitance between one end of the resonator, such as the free end, and the shell construction to increase, and the frequency range shows a tendency to a lower frequency thus compensating for the tendency of the frequency range of a shorter resonator to a higher frequency. Accordingly, a quarter-wave electric length is achieved although the physical length is clearly shorter.
The element increasing the cross-sectional area of a resonator, such as a conductive plate, can be thought to operate as one electrode of the capacitance, the cover of the shell constituting the other electrode. The surface area of the element increasing the cross-sectional area of the resonator increases the capacitance.
The present invention is particularly applicable to a filter using a conductive plate or other such construction which increases the cross-sectional area of the resonator end.
The operative frequency, i.e. resonance frequency, of the resonance circuit formed by a resonator and a section is tuned to make the resonance circuit operate in the desired manner, whereby the resonance circuit alone, or in practice, however, the integral formed by several resonance circuits, implements a desired frequency response, which e.g. in a band-pass filter is the pass band, the signals inside of which the filter lets through. The pass band can be e.g. a 25-MHz frequency band, employed in TDMA-based base stations of the GSM system at the frequency range from 925 to 960 MHz, within which the 200-kHz single channels of the GSM system are located.
Tuning the resonance frequency of the resonance circuit of the filter is known to be achieved by changing the distance between the free end of the resonator and the grounded shell by means of a frequency tuning element. As the distance decreases, the capacitance between the free end of the resonator and the shell increases and the resonance frequency decreases, whereas, as the distance increases, the capacitance decreases and the resonance frequency increases.
A known solution for tuning the resonance frequency of a resonance circuit is a tuner bolt in the filter cover, the distance of which from the free end of the resonator in the section below the cover can be tuned by rotating the bolt. Said solution is not optimal, since it requires extra constructions on the outer surface of the shell. Another problem is that the tuner bolt requires a thick cover, or one which is thickened at least at some point to enable threads to be provided in the cover for the frequency tuner bolt, or, alternatively, a threaded nut-type part to be fastened to the cover. The cover has to be thick because it also has to be rigid for the distance of the frequency tuning element from the resonator not to change after tuning and cause the capacitance and, consequently, the resonance frequency, to change in an undesired manner.
In another known solution the resonance frequency is tuned by means of a bendable strip-like tuning projection. Said solution is also problematic because for the tuning to remain unchanged it also requires a thick cover, but an easily bendable tuning projection is difficult to implement in a thick cover.
It is an object of the present invention to provide a new kind of filter and frequency tuning element which eliminate the problems of known solutions.
This object is achieved by the filter of the invention, which is characterized by the frequency tuning element for tuning the resonance frequency of the resonance circuit and the means fastened to the resonator for increasing the cross-sectional area of the resonator forming an integral whole, the frequency tuning element being a projection projecting from the means for increasing the cross-sectional area, the resonance frequency of the resonance circuit being tuned by adjusting the distance of said projection to the shell construction.
The frequency tuning element of the invention, in turn, is characterized by forming, together with a means fastened to the resonator for increasing the cross-sectional area of the resonator, an integral whole and being a projection projecting from the means for increasing the cross-sectional area.
The solution of the invention provides several advantages. The invention enables a highly integrated integral construction in which the frequency tuning element is formed in the same piece which is used for forming the plate-like or other such means for increasing the cross-sectional area of the
Henriksson Mika
Juntunen Pauli
Kotanen Anssi
Vuoppola Esa
ADC Telecommunications OY
Ladas & Parry
Nguyen Patricia T.
Pascal Robert
LandOfFree
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