Radio frequency filter

FIELD OF THE INVENTION

0001The present invention relates to a Radio Frequency (RF) filter.

DESCRIPTION OF THE RELETED ART

0002A filter is an essential part of an RF system, for passing or rejecting a predetermined frequency band. Filters are classified into Band Pass Filters (BPFs), Band Rejection Filters (BRFs), High Pass Filters (HPFs), Low Pass Filters (LPFs), etc. Such filters are designed so as to satisfy frequency pass or rejection characteristics required for the RF system. Once the filters are installed, their characteristics are difficult to change unless they are replaced. For example, when a BRF is to be reinstalled in an RF system equipped with a BPF, the BPF should be removed and then replaced with the BRF. The filter replacement may lead to communication disconnection and increase cost.

0003Meanwhile, the BRF is designed by connecting resonators using a 50-ohm (Ω) line. In this case, since no coupling occurs between resonators, a frequency band equal to or wider than a predetermined bandwidth cannot be rejected. In addition, as the number of resonators increases, the total length of the 50-ohm line also increases, thereby increasing path loss. Another shortcoming of the BRF is that a low or high frequency band is selectively rejected with respect to a specific frequency and thus it is difficult to improve band edge characteristics. <patcit id="pcit0001" dnum="WO9515018A"><text>WO 95/15018</text></patcit> refers to a hybrid notch filter. A hybrid notch filter comprises an impedance inverter network connected across a two port filter network. In its simplest form, the impedance inverter network comprises impedance inverters of substantially <maths id="math0001" num=""><math display="inline"><mrow><msqrt><mrow><mn>2</mn></mrow></msqrt></mrow></math><img file="EP2498331B1_D0001.tif" /></maths> characteristic admittance connected between the input and output ports across which the two port filter network is connected, and impedance inverters of substantially unity characteristic admittance interconnecting the <maths id="math0002" num=""><math display="inline"><mrow><msqrt><mrow><mn>2</mn></mrow></msqrt></mrow></math><img file="EP2498331B1_D0002.tif" /></maths> characteristic admittance impedance inverters at their respective ends. The two port filter network may be a plurality of serially connected resonators and is connected across the output ports of the impedance inverter network. The hybrid notch filter may be formed into a switched hybrid notch filter by providing a switch between an adjacent pair of resonators in the filter network.

DESCRIPTION OF THE INVENTION

TECHNICAL OBJECT OF THE INVENTION

0004An aspect of embodiments of the present invention is to address at least the problems and/or disadvantages and to provide at least the advantages described below.

0005It is the object of the present invention to provide an improved RF filter for decreasing path loss.

0006This object is solved by the subject matter of the independent claim.

0007Preferred embodiments are defined by the dependent claims.

0008Another aspect of embodiments of the present invention provides an RF filter for improving band edge characteristics.

0009Another aspect of embodiments of the present invention provides an RF filter for inverting a signal characteristic easily.

0010Another aspect of embodiments of the present invention provides an RF filter for rejecting a wide frequency band by increasing coupling between resonators.

0011A further aspect of embodiments of the present invention provides an RF filter for facilitating design of notch characteristics.

TECHINCAL MEAN TO ACHIEVE THE OBJECT

0012In accordance with an aspect of embodiments of the present invention, there is provided an RF filter in which a coupler receives an input signal through a first port, divides the input signal, outputs the divided signals through second and third ports, combines signals received through the second and third ports according to phases of the signals, and outputs the combined signal through the first port or as an output signal of the RF filter through a fourth port, a first filter unit has a first port connected to the second port of the coupler and a second port connected to the third port of the coupler, for having a predetermined frequency filtering characteristic, and first and second switch units dynamically establish a signal path according to an external switching control signal so that input and output ports of the RF filter are connected to the first filter unit directly or indirectly via the coupler, a signal passes only though the coupler, or a signal bypasses the first filter unit and the coupler.

EFFECT OF THE INVENTION

0013As is apparent from the above description of the embodiments of the present invention, because the present invention provides a new filter by combining a filter with a hybrid coupler, it can reduce the path loss of a signal, improve band edge characteristics, easily invert signal characteristics, reject a wide frequency band by increasing coupling between resonators, and facilitate design of notch characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

0014<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> schematically illustrates the structure of a chameleon filter;</li><li><figref idref="f0002 f0003 f0004 f0005">FIGs. 2A to 2D</figref> schematically illustrate the structure of a chameleon filter according to an embodiment of the present invention;</li><li><figref idref="f0006">FIG. 3</figref> schematically illustrates the structure of a chameleon filter;</li><li><figref idref="f0006">FIG. 4</figref> schematically illustrates the structure of a chameleon filter; and</li><li><figref idref="f0007">FIGs. 5A and 5B</figref> are diagrams illustrating the waveforms of signals in a general filter and a chameleon filter.</li></ul>

DETAILED DESCRIPTION OF EMBODIMENTS

0015Now, a preferred embodiment of the present invention will be described with reference to the attached drawings. While specific details such as components are described in the following description, they are given to help comprehensive understanding of the present invention. Therefore, it is clearly to be understood to those skilled in the art that changes or modifications can be made to the present invention within the scope of the present invention.

0016The following description is given of a so-called chameleon filter which is configured by combining a hybrid coupler with a filter and thus has new characteristics modified from original characteristics. For example, a Band Pass Filter (BPF) and a hybrid coupler may be combined into a chameleon Band Rejection Filter (BRF) and a Low Pass Filter (LPF) and a hybrid coupler may be combined into a chameleon HPF, or vice versa in the present invention.

0017<figref idref="f0001">FIG. 1</figref> illustrates the structure of a chameleon filter.

0018Referring to <figref idref="f0001">FIG. 1</figref>, a 3-dB hybrid coupler 110 is connected to a filter unit 120 in the chameleon filter. That is, the filter is configured so that a first port of the hybrid coupler 110 serves as an input port of the chameleon filter, a fourth port of the hybrid coupler 110 serves as an output port of the chameleon filter, a second port of the hybrid coupler 110 is connected to a first port (e.g. an input port) of the filter unit 120, and a third port of the hybrid coupler 110 is connected to a second port (e.g. an output port) of the filter unit 120. The filter unit 120 may be configured with various types of filters such as a BPF, BRF, High Pass Filter (HPF), LPF, etc. If the filter unit 120 is configured with a BPF, the chameleon filter has BRF characteristics.

0019Characteristics of the filter unit 120 will be described with reference to <figref idref="f0007">FIGs. 5A and 5B</figref>, when the filter unit 120 is configured with a BPF. <figref idref="f0007">FIG. 5A</figref> illustrates the waveforms of a signal S₂₁ passed through the general BPF and a signal S₁₁ returned from the general BPF. <figref idref="f0007">FIG. 5B</figref> illustrates the waveforms of signals in the chameleon filter in which the hybrid coupler 110 is combined with the filter unit 120 configured with a BPF in an RF system. Compared to <figref idref="f0007">FIG. 5A</figref>, the phases of the passed signal S₂₁ and the returned signal S₁₁ are inverted in <figref idref="f0007">FIG. 5B</figref>. Therefore, although the filter unit 120 is configured with a BPF in the chameleon filter, its overall characteristics are BRF characteristics.

0020More specifically, a hybrid coupler generally functions to equally divide signal power with a phase difference of 90 degrees. For example, when a signal is input to the first port of the hybrid coupler 110, the power of the signal is equally divided into two parts with a 90-degree phase difference and then provided to the first and second ports of the filter unit 120 through the second port (0 degrees) and the third port (-90 degrees) of the hybrid coupler 110. Signals in the pass band of the filter unit 120 pass through the filter unit 120 and are fed back to the opposite ports, that is, the third and second ports of the hybrid coupler 110. Then the signals are combined due to the phase difference and output to the first port of the hybrid coupler 110, with no signal output to the fourth port of the hybrid coupler 110. Meanwhile, signals outside the pass band of the filter unit 120 do not pass through the filter unit 120, return from the first and second ports of the filter unit 120, and then are fed back to the second and third ports of the hybrid coupler 110, respectively. These signals are combined due to the phase difference and output to the fourth port of the hybrid coupler 110.

0021In conclusion, a signal of the pass band of the filter unit 120 in a signal input to the input port of the chameleon filter (i.e. the first port of the hybrid coupler) is reflected to the input port of the chameleon filter, whereas a signal outside the pass band of the filter unit 120 in the input signal is output to the output port of the chameleon filter (i.e. the fourth port of the hybrid coupler) in the whole filter structure.

0022Parts that divide/combine the power of signals according to their phase difference include a hybrid ring, a branchline directional coupler, a 3-dB directional coupler, a magic T, etc. If such a part is used instead of the hybrid coupler 110, it may be configured so as to adjust the phase of a signal using a phase shifter additionally. For example, if a magic T substitutes for the hybrid coupler 110, a phase shifter is provided on a connection path between a second port (or third port) of the magic T and the filter unit, to thereby shift the phase of a passed signal by 90 degrees.

0023If the chameleon filter having the above configuration is designed to operate as a BRF on the whole, that is, in a 'hybrid coupler+BPF' structure, it is more useful. That is, a typical BRF is designed by connecting resonators with one another via a 50-ohm (Ω) line. This structure requires a relatively large size and is complex. In addition, since much coupling does not occur between resonators, the BRF has limitations in rejecting a frequency band having a predetermined or wider bandwidth. In contrast, the chameleon filter is relatively easily implemented and facilitates realization of a BRF using a BPF structure that can process a wide frequency band. The chameleon filter can remarkably reduce path loss that may be caused as the total length of the 50-ohm line increases.

0024Further, the 50-ohm line should be lengthened or shortened with respect to a corresponding frequency in order to achieve notch characteristics in the general BRF. Thus it is difficult to achieve notch characteristics with the general BRF. However, the chameleon filter realizes notch characteristics in the BPF and thus implements the BRF on the whole using the notch characteristics. Hence, the notch characteristics of the BRF are easily achieved and skirt characteristics can be improved.

0025Similarly, the chameleon filter can realize an HPF structure on the whole using an LPF structure. In this case, an HPF which is relatively difficult to fabricate can be easily implemented.

0026<figref idref="f0002 f0003 f0004 f0005">FIGs. 2A to 2D</figref> illustrate the structure of a chameleon filter according to an embodiment of the present invention. Referring to <figref idref="f0002 f0003 f0004 f0005">FIGs. 2A to 2D</figref>, the chameleon filter according to an embodiment of the present invention includes a hybrid coupler 210, a filter unit 220, and first and second switch units 240 and 230. The hybrid coupler 210 and the filter unit 220 may be identical to their counterparts in the embodiment illustrated in <figref idref="f0001">FIG. 1</figref>. The first and second switch units 240 and 230 each may have a combination of general single on/off switches, or a combination of a Double Pole Double Throw (DPDT) switch and a general single on/off switch, which establishes a signal connection path dynamically according to an external switching control signal so that the input and output ports of the chameleon filter are connected directly to the filter unit (<figref idref="f0002">FIG. 2A</figref>), the input and output ports of the chameleon filter are connected to the filter unit indirectly via the hybrid coupler 210 (<figref idref="f0003">FIG. 2B</figref>), a signal passes through the hybrid coupler 210 (<figref idref="f0004">FIG. 2C</figref>), and a signal bypasses the hybrid coupler 210 and the filter unit 220 (<figref idref="f0005">FIG. 2D</figref>). Hereinbelow, with reference to <figref idref="f0002">FIGs. 2A</figref> and <figref idref="f0003">2B</figref>, the structure and operation of the filter unit 220 will be described in detail, in the case where the filter unit 220 is configured with a BPF.

0027<figref idref="f0002">FIG. 2A</figref> illustrates the structure of a chameleon filter according to the present invention, when the chameleon filter operates as a BPF and <figref idref="f0003">FIG. 2B</figref> illustrates the structure of a chameleon filter according to the present invention, when the chameleon filter operates as a BRF.

0028Referring to <figref idref="f0002">FIG. 2A</figref>, in the case where the chameleon filter of the present invention operates as a BPF, when a signal is input to an input port of the first switch unit 240, the signal is output to an output port through the filter unit 220 configured with a BPF and the second switch unit 230. Therefore, there is no signal input and output to and from the hybrid coupler 210 and the chameleon filter of the present invention is used to pass a predetermined frequency band.

0029Referring to <figref idref="f0003">FIG. 2B</figref>, in the case where the chameleon filter of the present invention operates as a BRF, the first and second switch units 240 and 230 switch in a different manner from in <figref idref="f0002">FIG. 2A</figref>, such that a path is established to pass input and output signals through the hybrid coupler 210.

0030When a signal is input to the input port of the first switch unit 240, the signal is provided to a first port of the hybrid coupler 210, equally divided into two signals each having a half power with a phase difference of 90 degrees, and then output through second and third ports of the hybrid coupler 210. Signals of the pass band of the filter unit 220 in the divided two signals pass through the filter unit 220 and the first and second switches 240 and 230, whereas signals outside the pass band of the filter unit 220 in the divided two signals are reflected from the filter unit 220 and fed back to the second and third ports of the hybrid coupler 210. Signals of the pass band of the filter unit 220 in these input signals are output through the fourth port of the hybrid coupler 210. The signals output from the fourth port of the hybrid coupler 210 are output to the output port via the second switch 230.

0031Referring to <figref idref="f0004">FIG. 2C</figref>, an input signal is input only to the hybrid coupler 210. In this structure, the input signal bypasses the filter unit 220.

0032More specifically, a signal input to the first port of the hybrid coupler 210 is divided into two signals each having an equal half power, with a phase difference of 90 degrees. The divided signals are output through the second and third ports of the hybrid coupler 210, fully reflected from the first and second switches 240 and 230, and then fed back to the second and third ports of the hybrid coupler 210. These signals are combined due to the phase difference and then output through the fourth port of the hybrid coupler 210. The signal output from the fourth port of the hybrid coupler 210 is just output through the second switch 230.

0033The structure illustrated in <figref idref="f0004">FIG. 2C</figref> simply outputs an input signal without any signal processing. Thus the input signal bypasses the filter unit 220 without being filtered.

0034Like the structure illustrated in <figref idref="f0004">FIG. 2C</figref>, a signal bypasses the filter unit 220. A signal input to the input port of the first switch 240 bypasses the filter unit 220 or the hybrid coupler 210 in a bypass path and is output to the output port via the second switch 230.

0035As illustrated in <figref idref="f0004">FIGs. 2C</figref> and <figref idref="f0005">2D</figref>, the filter of the present invention may be appropriate for a wireless communication system in which filter is not required in an initial use environment and then required in a subsequent use environment.

0036For example, it may occur that a mobile communication BS system should return a part of a used frequency band on a service provider basis. For example, a specific service provider may use 800 to 825MHz in a current year and then may use only 810 to 825MHz in the next year. That is, the service provider should return the frequency band from 800MHz to below 810MHz.

0037In this case, the service provider should install a BRF that rejects the frequency band from 800MHz to below 810MHz or a BPF that passes only 810 to 825MHz in every BS. It is almost impossible to simultaneously install the filters in all BSs distributed nationwide. Therefore, the filters are sequentially installed in the BSs over a relative long period (e.g. 6 months) before the next year.

0038However, BSs in which filters are installed early cannot service the frequency band from 800MHz to below 810MHz. In this case, use of the filter according to the present invention enables signal bypassing despite the preliminary filter installation in the BS. Thus an entire serviceable band can be used in the current year. In the next year, signals pass through the filters through switching control of the filters in the BSs so that the service band of all BSs may be changed to 810 to 825MHz almost simultaneously.

0039While <figref idref="f0002 f0003 f0004 f0005">FIGs. 2A to 2D</figref> have been described in the context of the filter unit 220 being a BPF, it is clearly understood that the BPF may be replaced with another filter such as a BRF, an HPF, an LPF, etc.

0040<figref idref="f0006">FIG. 3</figref> illustrates the structure of a chameleon filter.

0041Referring to <figref idref="f0006">FIG. 3</figref>, the chameleon filter includes a hybrid coupler 310 and a filter unit 320. While the filter units may be configured with a single filter, the filter unit 320 includes at least two identical filters, that is, first and second filters 3210 and 3220. The filter unit 320 further includes a phase shifter 3230. More specifically, the chameleon filter is configured such that a first port of the hybrid coupler 310 serves as an input port of the chameleon filter, a fourth port of the hybrid coupler 310 serves as an output port of the chameleon filter, a second port of the hybrid coupler 310 is connected to a first port (e.g. an input port) of the first filter 3210, and a third port of the hybrid coupler 310 is connected to a first port (e.g. an input port) of the second filter 3220. In addition, a second port (e.g. an output port) of the first filter 3210 is opened and a second port (e.g. an output port) of the second filter 3220 is connected to an end of the phase shifter 3230. The other end of the phase shifter 3230 is opened.

0042A signal input to the hybrid coupler 310 through the first port is equally divided into two signals each having a half power, with a phase difference of 90 degrees and output through the second and third ports of the hybrid coupler 310. Signals of the pass bands of the first and second filters 3210 and 3220 in the output signals are input to the first and second filters 3210 and 3220 and signals outside the pass bands of the first and second filters 3210 and 3220 are reflected from the first and second filters 3210 and 3220. As described before, the first and second filters 3210 and 3220 may be the same types of filters, such as BPFs, BRFs, HPFs, LPFs, etc.

0043Signals outside the pass bands of the first and second filter 3210 and 3220 are reflected from the first and second filter 3210 and 3220, fed back to the second and third ports of the hybrid coupler 310, and then output through the fourth port of the hybrid coupler 310.

0044Meanwhile, since the second port of the second filter 3220 is connected to the phase shifter 3230, the phase variation of a signal of the pass band passed through the second filter 3220 can be adjusted dynamically. Hence, the signal in the pass band of the second filter 3220 is phase-shifted in the phase shifter 3230, reflected from the open port of the phase shifter 3230, and fed back to the third port of the hybrid coupler 310 through the second filter 3220. The signal in the pass band of the first filter 3210 is reflected from the open port of the first filter 3210 and fed back to the second port of the hybrid coupler 310. If the signal whose phase variation was not adjusted (the signal input from the first filter 3210) and the signal whose variation was adjusted (the signal input from the second filter 3220) are input to the hybrid coupler 310 through the second and third ports of the hybrid coupler 310, respectively, the phase difference between the signals is not accurately 90 degrees. Consequently, the signals are divided into the first and fourth ports of the hybrid coupler 310 according to the phase difference.

0045It can be concluded that parts of signals in the pass bands of the first and second filters 3210 and 3220 are leaked to the fourth port of the hybrid coupler 310 according to the phase variation adjusted by the phase shifter 3230 in the chameleon filter, illustrated in <figref idref="f0006">FIG. 3</figref>. For example, if the first and second filters 3210 and 3220 are BPFs, the chameleon filter operates as a BRF on the whole and the loss bandwidth (dB) of a signal in a rejection band of the chameleon filter is changed according to a phase variation controlled by the phase shifter 3230. This chameleon filter is applicable to a system in which the rejected amount of a signal in a rejection band needs to be adjusted.

0046<figref idref="f0006">FIG. 4</figref> illustrates the structure of a chameleon filter.

0047Referring to <figref idref="f0006">FIG. 4</figref>, the chameleon filter includes a first filter unit 420, a hybrid coupler 410, and a second filter unit 430 and has a low signal loss equal to or smaller than a predetermined threshold. The chameleon filter is similar to the chameleon filter described before, except that the second filter unit 430 is further installed between a fourth port of the hybrid coupler 410 and an output port of the chameleon filter. The number of resonators may differ for the first and second filter units 420 and 430. For instance, the second filter unit 430 may be configured with a 6-stage BPF and the first filter unit 420 may be configured with a 4-stage BPF. In this case, the hybrid coupler 410 and the first filter unit 420 in combination may create BRF characteristics. That is, the chameleon filter is a combination of a BPF structure and a BRF structure. If the rejection band of the BRF structure is positioned at one band edge of the BPF, this structure may have less signal loss and similar or improved characteristics of one band edge, for example, compared to a 10-stage BPF structure.

0048It is clear also in <figref idref="f0006">FIG. 4</figref> that the first and second filter units 420 and 430 may be replaced with various filters other than BPFs, such as BRFs, HPFs, LPFs, etc. While it has been described in <figref idref="f0001 f0002 f0003 f0004 f0005 f0006">FIGs. 1 to 4</figref> that the chameleon filters are configured with filters (i.e. indicated by reference numerals 120, 220, 320, 330, 410 and 430), other mobile communication parts may be used instead of the filters 120, 220, 320, 330, 410 and 430, to thereby achieve a similar function to the function of a chameleon filter.

0049While embodiments have been shown and described with reference to the drawings, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the inventive concept as defined by the appended claims.