345 P 474 PCT High frequency filter with blocking circuit coupling 5 The invention relates to a high frequency filter with blocking circuit coupling according to the preamble of Claim 1. High frequency filters are used in a broad range of 10 applications. For example, in digital mobile communications technology, the communication of the mobile subscriber with the base station is conducted via transmitting/receiving antennas 15 provided in the base station. In this case, it is desirable to use only one common antenna for the transmission and reception signals. Transmission and reception signals utilize in this regard 20 differing frequency ranges. The antenna used has to be suitable for transmitting and receiving in both frequency ranges. Separating the transmission and reception signals requires a suitable frequency filtering process which ensures, on the one hand, that the transmission signals 25 from the transmitter are forwarded only to the antenna (and not toward the receiver) and, on the other hand, that the reception signals from the antenna are forwarded only to the receiver. 30 For this purpose, use may be made of a pair of high frequency filters which both pass a specific frequency band, i.e. the frequency band desired in each case (band pass filters). However, use may also be made of a pair of high frequency filters which block a specific frequency band, i.e. the undesired frequency band in each case. These are known as band-stop filters. Also possible is the use of a pair of high frequency filters consisting of a 5 first filter, which passes frequencies below a frequency between the transmission and reception bands and blocks the ranges thereabove (low-pass filter), and a second filter, which blocks frequencies below this frequency between the transmission and reception bands and passes 10 the frequencies thereabove. This is then what is known as a high-pass filter. Further combinations of the aforementioned types of filters can be used. The transmission/reception bands are in this case 15 generally split up within the base station using duplex filters which have the aforementioned task of connecting the transmission/reception path to the common antenna with as little feedback as possible. The duplex filter consists in this regard of two interconnected band-passes, i.e. 20 what is known as the transmission band-pass (TX-band-pass) and the reception band-pass (RX-band-pass), separate terminals being provided for the reception branch, the transmission branch and the common connected antenna. 25 The band-passes used within the duplex filter therefore have, on the one hand, to display the selection necessary for interconnecting the transmission and reception band passes (TX/RX) (i.e. the necessary blocking attenuation) and should, on the other hand, in their respective pass 30 range attenuate the useful signals as little as possible. The band-pass structures used in duplex filters are constructed in the common mobile communications frequency ranges (for example, GSM/UMTS) predominantly as coaxial resonators. The construction and mode of operation of coaxial 5 resonators are known from the prior art, for example Ian Hunter, "Theory and Design of Microwave Filters", IEE Electromagnetic Waves Series, No. 48, 1. Microwave filters, page 197. 10 Filter theory describes band-passes (what are known as Cauer band-passes) which display in the blocking range transmission zero points (what are known as blocking poles). This type of filter is very frequently produced, in conjunction with coaxial resonators, by what is known 15 as cross-coupling. In this case, non-adjacent resonators (i.e. resonators not immediately following one another in the course of the signal) are capacitively or inductively cross-coupled within a band-pass structure in such a way that amplitude obliterations (transmission zero points or 20 blocking points), conditioned by signal splitting with subsequent phase-shifted combining, are produced in the transmission characteristic. A cross-coupling method of this type is described, for example, in IEEE Transactions on microwave theory and techniques, Vol. 51, No. 4, April 25 2003, "Cross-Coupling in Coaxial Cavity Filters - A Tutorial Overview", J. Brian Thomas, pages 1368 to 1376. However, what is known as cross-coupling has the drawback that the necessary cross-coupling requires resonators 30 which are not adjacent to one another, i.e. are not arranged one following the other. However, this requires filter topologies which mechanically facilitate this cross-coupling.
Owing to the limited number of suitable pairs of resonators for cross-coupling, only a limited number of blocking poles can therefore be generated. 5 A generic high frequency filter with blocking circuit coupling, in particular in the form of a duplex filter, is, in principle, to be taken as known from WO 2004/100305 Al. This is a high frequency filter comprising a plurality of resonators arranged between three terminals, i.e. 10 between a terminal for a transmission branch, a terminal for the reception branch and a terminal for the common antenna. According to the generic prior art, provision is made, for 15 improving a duplex filter of this type, for it to comprise at least one pair of resonators markedly cross-coupled to each other, the resonators markedly cross-coupled to each other oscillating, when coupled, at two differing coupling resonance frequencies which differ from the resonance 20 frequency which the two markedly cross-coupled resonators display, viewed in isolation, in the respective frequency range between the transmission and reception bands or to which the resonators are adapted. 25 Thus, it is known for example from WO 02/054527 A2 in the case of a coaxial high frequency filter to provide measures, whereby cross-coupling between two adjacent resonators can be adjusted differently. For this purpose, for example, coupling adjustment elements are provided on 30 a separating wall located between two internal conductors of two adjacent coaxial resonators, above which a coupling window is formed to the upper hollow conductor wall, and these coupling adjustment elements can be adjusted to alter the cross-coupling. A solution, which is in principle comparable in this 5 respect, is also known from JP 09-199906 A which describes the use of adjustment loops between two resonators to alter the coupling. According to US 4 216 448, the resonance behavior of the 10 filter is correspondingly adjusted on a coupling path, comprising four coaxial resonators, by screwing in screws (with axial lengthening of the internal conductor). In addition, in this publication, cross-coupling is provided between the first and the last resonator on the 15 transmission path (i.e not between two resonators lying adjacent one behind the other on the transmission path), namely by means of an additional coupling window and protrusions extending in the direction of this additional coupling window. 20 Finally, reference is also made to the high frequency filter according to US 5 389 903 A, which has T-shaped internal conductors with a cylindrical head which has a larger diameter than the internal conductor located under 25 it carrying the head. This cylindrical head can be moved eccentrically to the shaft of the internal conductor. The resonance frequency of each resonator can also be adjusted by screws similar to the aforementioned prior art. The resonators are cross-coupled one below the other 30 inductively or capacitively, in which the inductive or capacitive coupling can be adjusted primarily by changing the relationship with respect to the distance between the shafts and the distance between the heads of the internal conductors. In addition, adjustable screws are used for precision adjustment between the individual resonators. All these publications show, which has already been 5 referred to in the introduction, that it is definitely known to cross-couple individual coaxial resonators to each another and alter the cross-coupling of the resonators and adjust them differently. 10 However, if in particular when constructing band passes, blocking ranges with transmission zero points (so-called blocking poles) are to be generated, then up to now it has been necessary to cross-couple two resonators which are not immediately adjacent to each another (i.e resonators 15 not immediately following one another on a signal transmission path), capacitively or inductively to each other by means of additional measures in such a way that amplitude obliterations (transmission zero points or blocking points), conditioned by signal splitting with 20 subsequent phase-shifted combining, are produced in the transmission characteristic. This occurs within the scope of the so-called coupling method mentioned in the introduction. 25 The object of the present invention is, in contrast, to provide an improved high frequency filter having an improved passing and/or blocking effect for presettable frequencies or frequency ranges, wherein the filter topology should be as unrestricted as possible. 30 The invention is achieved in accordance with the features specified in Claim 1. Advantageous embodiments of the invention are recited in the sub-claims.
It has completely surprisingly been proved that it is possible to generate at least one blocking point (i.e at least one blocking pole) at a frequency which is less than + 50% away from the center frequency of a band filter, 5 namely by means of corresponding cross-coupling of two coaxial resonators provided immediately adjacent to each other on a signal path. According to the invention, the improvement over the prior 10 art is essentially achieved in that there is allocated to the high frequency filter, which comprises electromagnetically coupled coaxial resonators, with respect to at least one selected coaxial resonator pair, both coaxial resonators of which are located immediately 15 adjacent to each other in the transmission path, a specially preselectable and/or preadjustable coupling impedance which is configured in such a way that there is produced, based on the combination of capacitive and inductive coupling, a coupling impedance resonance at a 20 defined blocking frequency. As this type of cross-coupling generates a blocking point in the transmission behavior of the high frequency filter, it will also be referred to hereinafter as the blocking circuit coupling. 25 In other words, this blocking circuit coupling, i.e. the blocking frequency, may, within the scope of the invention, be laid in such a way that it is positioned, on the one hand, outside the pass range of the HF filter and, on the other hand, within the blocking range of an HF 30 filter. Preferably, the blocking frequency may be preset mainly by varying and/or adjusting and/or preselecting two variables substantially influencing or determining the blocking frequency. It is crucial in this regard that, in addition to a defined inductive resonator coupling, a defined capacitive coupling takes place. The defined inductive 5 coupling can in this case be adjusted, for example, via the distance between the resonators to be cross-coupled. The required capacitive coupling, on the other hand, can be produced, for example, by elongate extension on the upper side of the resonators to be cross-coupled. Even if 10 the variation of each of the two above-mentioned variables also exerts a certain influence on the respective other variable, presetting both the inductive and the capacitive coupling nevertheless allows the desired cross-coupling of adjacent resonators to be adjusted in varied form so that 15 the desired attenuation is outside the pass range of an HF filter and within a blocking range of an HF filter. Within the scope of the invention, the following fundamental advantages may therefore be achieved: 20 * Firstly, adjacent resonators can now be cross-coupled in such a way as to generate blocking points. This has the fundamental advantage that, in the production of high frequency filters, no such restrictions are placed on 25 the configuration of filter topologies, i.e. with regard to the arrangement of the coaxial resonators, as in the prior art. In the prior art there was the grave drawback that in the case of what is known as cross-coupling (i.e. the production thereof), the necessary cross 30 coupling could not be carried out between adjacent resonators. This then necessitated highly specific filter topologies in order to facilitate cross-coupling between two non-adjacent resonators.
As cross-coupling of two adjacent resonators is possible within the scope of the invention, this in principle also provides the option of cross-coupling any number of adjacent pairs of resonators. Within the scope of the 5 invention, for n-resonators, even n-1 blocking points can be generated, i.e. many more than in the case of conventional cross-coupling. * Within the scope of the invention, for corresponding 10 adjustment of the capacitive coupling, the distance between two internal conductors can be reduced in that said conductors are provided on their upper side with radial extensions. This can also be utilized to reduce the overall height of the filter. 15 The invention will be described hereinafter in greater detail with reference to the drawings, in which, specifically: 20 Fig. 1 is a schematic basic view of the schematic construction of a high frequency filter in the case of a duplex filter; Fig. 2 is a schematic plan view of a high frequency filter 25 with a signal path; Fig. 3 is an axial section along the line III-III in Fig. 2; 30 Fig. 4 is an equivalent circuit diagram with regard to the embodiment according to Figs. 2 and 3; Fig. 5 is a diagram for reproducing the passing and attenuation behavior of a band-pass filter according to the invention for a duplex filter; and 5 Figs. 6a to 6f are various views at differing adjustments of a coupling capacitance or coupling inductance. Fig. 1 is a schematic view of a high frequency filter in the form of a duplex filter 3, the HF filter 1 comprising 10 three terminals 5, 7 and 9, i.e. a terminal TX, RX and a terminal for the antenna port AP, so transmission signals originating from the transmission terminal 5 via a first signal path can be supplied to the antenna port AP (and from there to the common antenna) and, conversely, the 15 signals received by the antenna can be supplied to the reception terminal 7 via the antenna port AP (terminal 9). The duplex filter 3 comprises for this purpose in the two signal paths a corresponding band-pass filter 11 and 13 20 respectively, which display the necessary selection (i.e. blocking attenuation) to prevent any signals from passing from the transmission terminal into the reception branch. On the other hand, the pass ranges for the useful signals should be attenuated as little as possible. 25 In this regard, Fig. 2 is, by way of example, a schematic plan view (omitting an upper cover) of a high frequency filter 1 with a signal path 10 extending, for example, from a terminal 5 to a terminal 9, i.e. from a 30 transmission terminal to an antenna port terminal (i.e. only the one branch of a duplex filter) and thus comprises six coaxial resonators 15.
The coaxial resonators 15 are arranged in this case in a conductive housing 17 comprising a plurality of resonator chambers 19 where there extends in the illustrated embodiment, centrally or substantially in the central 5 region perpendicular to the housing base 17a, a respective conductive internal conductor 21 - as is apparent from the view according to Fig. 3 - which ends at a suitable distance below an electrically conductive housing lid 17b which can be placed onto the housing 17. 10 Each coaxial resonator 15 thus comprises a peripheral housing wall 17c, coupling openings 23, known as apertures, being provided in the respective housing wall 17c along the signal path, thus forming windows through 15 which the electromagnetic waves are able to spread. Provided in a known manner at the coaxial terminals 5 and 9 are internal conductors 5a and 9a respectively, which protrude into the associated resonator chambers 19 and 20 end, for example, in a conductive planar element 5b or 9b, via which, with the associated internal conductor in the respective coaxial resonators 15, as a result of the capacitance thus formed, the coupling-in of the electrical field at the terminal 5 and the corresponding decoupling 25 of the electrical field at the terminal 9 ensue (wherein for example, conversely, the signal received by the antenna is coupled into the associated resonator on the second signal path via the conductive planar element 9b and leads to a terminal 7 (not shown in detail in Fig. 2) 30 via a second signal path). Fig. 3 therefore shows merely a schematic cross section, by way of example, through a portion for the band-pass filter 11 provided for the transmission branch, wherein the signal path for the second reception branch, not shown in Fig. 3, of the duplex filter could be connected via the aperture gate 23a located on the left-hand side in Fig. 3. 5 With respect to the described signal path from the terminal 5 to the terminal 9, six coaxial resonators 15.1 to 15.6 are thus cross-coupled. Fig. 4 shows the corresponding equivalent circuit diagram, 10 with the signal path 10 from the terminal 5 to the terminal 9, the six resonators 11 being illustrated as parallel resonant circuits 111, of which one output is grounded and the opposing output is connected to the signal path 10 in the corresponding sequence. The parallel 15 resonant circuits 24 are in this case characterized in a known manner by a capacitor and an inductor. The stretches between the terminal points 25 of the individual parallel resonant circuits 24 can also be described by inductors 27, provided that these are conventional couplings between 20 coaxial resonators, i.e. not the cross-coupling according to the invention to be described hereinafter. Insofar as a coupling according to the invention is effective, the connection between two adjacent parallel resonant circuits cross-coupled in accordance with the invention can be 25 described not by an inductive coupling but rather by a blocking circuit coupling in the form of a parallel resonant circuit comprising a capacitor and an inductor, as shown in Fig. 4. Between the first and fifth coaxial resonator 15.1 and 15.4 respectively, there is 30 additionally formed, as in the prior art, a capacitive cross-coupling using a capacitor C (see Figs. 2 and Fig. 4).
This equivalent circuit diagram according to Fig. 4 shows that a cross-coupling according to the invention is formed between the first and the second coaxial resonators 15.1 and 15.2, between the second and third coaxial resonators 5 15.2 and 15.3, and the third and fourth coaxial resonators 15.3 and 15.4. In addition, also shown in the illustrated embodiment is a conventional cross-coupling according to the prior art between the first and the fifth coaxial resonators 15.1 and 15.5 which will also be considered 10 hereinafter. The side view according to Fig. 3 shows that for cross coupling the immediately adjacent resonators, i.e. the resonators immediately following one another in the course 15 of the signal, 15.1 and 15.2, the associated internal conductors, located in the connection direction thereof, are each provided with mutually facing, radially protruding internal conductor portions 21a. As a result, the clearance 121 between the two internal conductors 21 20 is adjusted in such a way as to produce a desired capacitive coupling. This capacitive coupling is illustrated, by way of example, between internal conductor portions 21a on the 25 two internal conductors 21 of the two first coaxial resonators 15.1 and 15.2 in Fig. 3, by correspondingly plotted E-field vectors 121' (Fig. 2). In addition, Fig. 3 shows an inductive cross-coupling 30 through an H-field line 121" for the two first resonators 15.1 and 15.2, too. Presetting the distance 321 between the two internal conductors 21 (without taking into account the aforementioned radially protruding internal conductor portions 21a) ultimately allows the inductive cross-coupling to be preselected and/or adjusted accordingly. 5 However, the inductive cross-coupling can also be pre adjusted or preselected differently using other alternative or supplementary, i.e. additional, measures. Very generally, it will be noted that the coupling capacitances and coupling inductances necessary for 10 adjusting the blocking circuit coupling can be adjusted using known coupling adjustment variations. The height and/or width of the coupling apertures (i.e. the through openings between two adjacent coaxial resonators), for example, can thus be adjusted differently so as to vary 15 the degree of coupling. Coupling pins, coupling loops or coupling webs can also be provided between the resonators. The coupling webs would, for example, extend at a partial height between two internal conductors, i.e. also extend parallel to the internal conductors (preferably 20 perpendicularly to the wall of the base) and thus be electrically connected to the base of the coupling resonators. The coupling loops can be electrically and mechanically connected in the manner of an inverted U-bolt between two internal conductors on the base. It is also 25 possible for a coupling loop to be positioned in the vertical orientation (i.e. so as to be located in a vertical plane) or in a plane slightly inclined thereto, via an axis of vertical rotation relative to the base, and thus to be able to be rotated in the circumferential 30 direction. The greater the surface area penetrated by the magnetic faces, the greater the coupling action. The aforementioned effects or parts thereof can also be used - 15 in combination in order accordingly to provide and implement the desired coupling adjustment possibilities. The above-mentioned adjustment possibilities are 5 represented, by way of example, with reference to Fig. 6a using electrically conductive coupling pins 301, one pin 301 provided in the upper lid 17b, for example, can be screwed in at a different position in terms of height, i.e. to a differing depth into the interior between two 10 resonators, for varying the coupling capacitance between two internal conductors 21, there also being arranged in Fig. 6a an electrically conductive coupling pin 302 which is screwed into the base 17a or positioned therein and the height and diameter of which contribute to varying the 15 coupling inductance. Fig. 6b shows in plan view that there is provided along the line connecting two adjacent internal conductors 21 a web which rises from the base and extends at this location at a partial height relative to the height of the internal conductors. This is what is 20 known as a coupling web 307. This coupling web 307 is in this case electrically connected to the base 17a of the housing 17 of the HF filter. According to a further embodiment, Fig. 6c shows in plan 25 view and Fig. 6d in vertical section that, for example, a first window opening 303 (coupling aperture) between the coaxial resonators 15.1 and 15.2 markedly decreases in size, whereas a further coupling aperture 303 between the resonators 15.2 and 15.3 markedly increases in size, so 30 the coupling aperture has in any case a greater width parallel to the face of the base or lid.
The embodiment according to Fig. 6e shows a coupling loop 305 for varying the coupling inductance which is positioned in the base in the manner of an inverted "U". Alternatively shown, with reference to Fig. 6f, is the use 5 of a coupling loop which can be rotated about its vertical axis 305', so the magnetic field strength penetrating the loop varies, thus allowing the coupling inductance to be varied and differingly adjusted. 10 Very generally, it will be noted that a broad range of possibilities for generating a desired capacitive, but also a desired inductive, coupling can be combined as desired; indeed, all of the above-mentioned variations can be applied cumulatively without limitation. 15 The differing configuration of the coaxial resonators 15 having the aforementioned differing resonator form allows the desired blocking point at a defined blocking frequency to be generated outside the pass range of an HF filter. It 20 is crucial in this regard that, in addition to the aforementioned defined inductive resonator coupling, a defined capacitive coupling is provided. The aforementioned inductive coupling can in this case, as stated, be adjusted by the distance 321 between the 25 resonators to be cross-coupled (position of the internal conductor 21 of the respective resonator), whereas the capacitive coupling is adjusted via the clearance 121 between two adjacent internal conductors 21 of two adjacent resonators, the size of which can be preset by 30 the clearance of the aforementioned elongate (radially protruding) internal conductor extensions 21.
In the embodiment shown, in addition to the cross-coupling between the first and second coaxial resonators 15.1 and 15.2, a further, directly adjoining cross-coupling is subsequently formed between the second and third coaxial 5 resonators 15.2 and 15.3. In contrast to the first internal conductor 21 of the first coaxial resonator 15.1, which is formed in cross section in the manner of an inverted L, the second 10 internal conductor of the second coaxial resonator 15.2 is in this case formed in the manner of a T, i.e. with a further, opposing internal conductor extension 21a generally protruding parallel to the base and thus transversely or radially to the internal conductor. Even 15 the third internal conductor 21, to be cross-coupled thereto, of the third coaxial resonator 15.3 could also be provided with a corresponding internal conductor extension 21a, the distance between these two adjacent internal conductor extensions 21a being very much greater than the 20 distance between the first and second coaxial resonators. In the example of the cross-coupling of the second and third coaxial resonator, there is also provided for this purpose an additional bridging member 221 which is held and positioned so as to be isolated from the housing. This 25 produces two spacer gaps 121a and 121b in which the electrical field vectors spread via air. The total distance, formed from the two individual distances 121a and 121b, produces that variable which is crucial for presetting the desired defined capacitive coupling. 30 Finally, the plan view according to Fig. 2 also shows that in this case, as a result of the further cross-coupling between the third and fourth coaxial resonators 15.3 and 15.4, the associated internal conductor 21 comprises not two internal conductor extensions opposing each other by 1800 (such as the second internal conductor 21 of the second coaxial resonator 15.2) but rather two internal 5 conductor extensions 21a facing each other at a 90' angle, i.e. in accordance with the signal path, angled by 90', of the electromagnetic waves. As may be seen from the plan view according to Fig. 2, the two internal conductors 21 are positioned closer to the second and third resonators 10 15.2 and 15.3. If the described signal path is, for example, the one signal path of a duplex filter, this may result in a band pass filter as represented in Fig. 4, at one or more 15 blocking frequencies f,, i.e. one or more so-called blocking poles. This transmission characteristic shows that, in accordance with the number of coaxial resonators cross-coupled within the scope of the invention, the plurality of blocking poles (blocking frequencies) can be 20 generated in such a way that these blocking frequencies are, for example, in the pass range (frequency range) of an adjacent, i.e. offset, band-pass filter. In the embodiment shown, a further cross-coupling 25 according to the invention may also be formed at any further desired point, i.e., for example, even between the fourth and fifth and/or the fifth and sixth coaxial resonators. In general, for n-coaxial resonators, five, i.e. n-1, coupling impedances can therefore be configured 30 in such a way that the communication of capacitive and inductive coupling results in a coupling impedance resonance at a respectively defined frequency f,, i.e. the type of cross-coupling results, in the transmission behavior of the high frequency filter, in a blocking point in the at least one frequency or the plurality of offset frequencies f,, fs2, fs3, etc. to fs,, which blocking point can be referred to as the blocking circuit coupling. 5 In the illustrated embodiment there is also formed a conventional cross-coupling which can additionally be provided in the HF filter according to the invention. This conventional cross-coupling is also illustrated in the 10 equivalent circuit diagram according to Fig. 4, i.e. via the path 131 connecting to the capacitor C provided therein. Shown for this purpose in Fig. 2 is, for example, a cross 15 coupling member 31 which acts between the first and fifth coaxial resonators and is conventionally formed by an electrically conductive coupling element which protrudes into the respective cavity in the associated resonator, is "bone-shaped" in side view and the enlarged opposing 20 closure of which at the associated internal conductor produces a capacitive cross-coupling in the respective coaxial resonator. This is also illustrated in the equivalent circuit diagram, i.e. by the capacitive cross coupling path 131. 25 Within the scope of the invention, on the other hand, there is provided a blocking circuit coupling 35 comprising an inductor connected in parallel and a capacitor, as is also shown in the equivalent circuit 30 diagram according to Fig. 4. In conclusion, reference will also be made to Fig. 5 showing a diagram concerning the band-pass behavior of a band-pass filter 11 for a transmission branch and of a band-pass filter 13 for a reception branch (in dotted lines) . This shows the plurality of blocking poles fRs, fTs as a function of the number of blocking circuit couplings 5 used. In Fig. 5, the increasing frequency F is shown on the x-axis and the attenuation D on the y-axis. For the one band-pass filter, it is also shown how the attenuation would progress if the coupling according to 10 the invention were not provided (dot-dash curve). In the band-pass filter according to the invention, the at least one blocking pole or the plurality of blocking poles can be laid so as to be located, for example, in a 15 frequency range, offset to the pass range of the respective band-pass, of an adjacent band-pass. Sufficient advantages according to the invention can, in any case, still be achieved if the one blocking pole or the plurality of blocking poles are arranged entirely, or at 20 least in part, so as to be located outside the actual pass range of the band-pass filter, in a frequency range which is less than ±50%, in particular less than ±40%, ±30%, ±20% and especially less than ±10% from the center frequency of the respective band-pass filter. 25 In the case of two band-pass pass frequency ranges offset relative to each other, as are used most frequently within a duplex filter, the advantages according to the invention can be achieved to a sufficient degree even if at least 30 one or more of the blocking poles (blocking points), viewed from a respective band-pass filter, is or are laid in such a way, by corresponding selection of suitable coupling capacitances and coupling inductances, that this at least one blocking pole is generated in a frequency range which is no further than five times the duplex separation (i.e. the center frequency separation of two adjacent band-passes) from the center frequency of the 5 respective band-pass. The blocking poles should therefore preferably be arranged, viewed from a band-pass filter, outside the pass range of the band-pass filter in such a way that the blocking poles come to lie no further than five times, in particular no further than four times, 10 three times, twice or one times the duplex separation (i.e. the center frequency separation of two adjacent band-passes). Regardless of this, one or more of the blocking poles can 15 obviously also be positioned in a frequency range of the adjacent band-pass. In conclusion, it will be noted that it can be ascertained, on appropriate selection of the inductive or 20 the capacitive coupling, whether the respective blocking pole is formed below a band-pass filter or above a band pass filter (i.e. at a lower frequency or higher frequency relative to the band-pass filter). This is achieved in that the coupling capacitance and the coupling inductance 25 of the blocking circuit coupling are selected in such a way that the resultant resonance frequency is, as required, either below or above the band-pass pass range.