WO2003071677A1

WO2003071677A1 - Filter circuit

Info

Publication number: WO2003071677A1
Application number: PCT/EP2003/001917
Authority: WO
Inventors: Martin Handtmann; Hans-Jörg TIMME; Robert Aigner
Original assignee: Infineon Technologies Ag
Priority date: 2002-02-25
Filing date: 2003-02-25
Publication date: 2003-08-28

Abstract

The invention relates to a filter circuit (400) which comprises first resonators (Rs11, Rs12, Rs21, Rs22) having a defined first resonance frequency and second resonators (Rp11, Rp12, Rp21, Rp22) having a second resonance frequency that is offset with respect to the first resonance frequency. The filter circuit further comprises a first symmetrical stage (I) having at least one first resonator and at least one second resonator, and a second symmetrical stage (II) having at least one first resonator and at least one second resonator. A transmission element (424) is connected between the first symmetrical stage (I) and the second symmetrical stage (II) and couples the said stages each other.

Description

description

filter circuit

The present invention relates to a filter circuit, in particular to a filter circuit for converting asymmetrical signals into symmetrical signals, and here in particular to a filter circuit which comprises BAW resonators (BAW = Bulk Acoustic Ave = acoustic bulk wave). Furthermore, the present invention relates to a filter circuit with a plurality of BAW resonators, which enables a transformation of impedance levels between an input port and an output port of the filter circuit.

RF filters based on resonators, such as BAW filters, have two basic topologies, which are explained in more detail with reference to FIGS. 1 and 2.

The first topology (see FIG. 1) is the so-called “ladder filter ^λ (ladder filter). The ladder filter 100 comprises an input port 102 with a first input connection 104 and a second input connection 106. Furthermore, the filter 100 comprises an output port 108 with a first output connection 110 and a second output connection 112. A is connected to the first input connection 104 of the input port 102 Received input signal ON, and an output signal OFF is output at the first output terminal 110 of the output gate 108. In the filter 100 shown in FIG. 1, three series resonators R _sl , R _s2 and R _{s3 are} connected in series between the first input connection 104 and the first output connection 110. Four parallel _resonators R _pX to R _{p4 are also} provided. The first parallel resonator R _pl is connected in parallel to the input gate 102. The second parallel _resonator R _p2 is connected in parallel to the first series resonator R _s χ. The third parallel _resonator R _p3 is connected in parallel to the second series resonator R _s2 . The fourth parallel _resonator is R _p4 connected in parallel to the output gate 108 and in parallel to the third series resonator R _s3 . The second input connection 106 and the second output connection 112 are connected to a reference potential 114, e.g. B. ground connected. The parallel resonators R _p χ to R _p4 are also connected to the reference potential. The conventional filter shown in FIG. 1 is a ladder filter with 3.5 stages (the three series resonators R _si to R _s3 and the four parallel _resonators R _p ι to R _p ). The ladder filter 100 is a filter circuit with a single input ON and a single output OFF. The ladder filter is a filter for unbalanced signals.

An exemplary embodiment of a known lattice filter (bridge filter) with two stages (four series resonators and four parallel resonators) is explained in more detail below in FIG. 2. In the description of FIG. 2, similar or identical components that have already been described with reference to FIG. 1 are provided with the same reference symbols.

The lattice filter 120 receives a symmetrical input signal IN at the first input connection 104 and at the second input connection 106 of the input port 102, and outputs a symmetrical output signal OUT at the output port 108 at the connections 110 and 112. A series connection of two series resonators R _s ι and R _{s2 is} provided between the first input connection 104 and the first output connection 110. Likewise, a series connection of two series resonators R _s3 and R _{s4 is} provided between the second input connection 106 and the second output connection 112. A first parallel resonator R _p ι is connected between the first input connection 104 and a node 122 arranged between the series resonators R _s3 and R _s4 . Furthermore, a second parallel _resonator R _{p2 is} connected between the second input connection 106 and a second node 124 arranged between the first series resonator R _s ι and the second series resonator R _s2 . Furthermore, 124 is between the second nodes and a second parallel _resonator R _{p3 is} connected to the second output connection 112, and a fourth parallel _resonator R _{p4 is} connected between the first node 122 and the first output connection 110. The filter 120 shown in FIG. 2 is completely differential, ie both input gates 102 and 110 are symmetrical (balanced).

With regard to the filter described with reference to FIGS. 1 and 2, it is pointed out that the series resonators and parallel resonators are preferably BAW resonators, the series resonators R _s χ to R _{s4 being produced} with a predetermined resonance frequency. The parallel _resonators R _p ι to R _p4 are essentially identical to the series _resonators with regard to the frequency _properties , but are detuned with regard to the resonance frequency compared to the resonance frequencies of the series resonators in order to achieve the desired filter effect. It should be noted that the series resonators and parallel resonators used in the ladder filter 100 differ from the series resonators and parallel resonators used in the Lattice filter 120, in particular in filter circuits with essentially the same filter characteristics but different topology.

As can be seen from the above discussion of FIGS. 1 and 2, the ladder filter 100 only has the possibility of receiving an unbalanced input signal and outputting a correspondingly unbalanced output signal. Likewise, the lattice filter 120 only enables the reception of a symmetrical input signal and the output of a symmetrical output signal.

However, there are applications where it is necessary to transform / convert an unbalanced input signal to a balanced output signal. There are also applications in which, alternatively or in addition to the conversion of the unbalanced signal Different gate impedances exist in a symmetrical signal at the inputs and outputs, which must also be handled.

The conventional method for performing a corresponding conversion / transformation is to provide an extra component, which is referred to as a balun. The balun can be either a magnetic transmitter (magnetic transformer), an LC circuit, or a stripline structure on a printed circuit board. In the case of acoustic surface wave filters, an acoustic balancing function can be implemented without additional components, but this considerably deteriorates the behavior of the overall filter. Furthermore, this balancing function means that these filters are very sensitive to electrostatic discharges and, furthermore, the ability to handle power is drastically limited.

Only few approaches have so far been proposed in connection with BAW filters for converting / transforming asymmetrical signals into symmetrical signals.

An example of the conversion of asymmetrical signals into symmetrical signals is to combine the filter circuits known from FIGS. 1 and 2, that is, to connect the lattice filter 120 to the output of the ladder filter 100. This approach has considerable disadvantages for practical applications of such a filter. The arrangement just described is particularly disadvantageous in that it can only be used for “floating” differential loads, ie no HF leakage current to ground is permitted.

Proceeding from this prior art, the present invention is based on the object of providing an improved filter switching to create, which is easy to manufacture.

This object is achieved by a filter circuit according to claim 1.

The present invention provides a filter circuit with

first resonators with a fixed first resonance frequency;

second resonators with a second resonance frequency shifted with respect to the first resonance frequency;

a first symmetrical stage with at least one first resonator and with at least one second resonator;

a second symmetrical stage with at least one first resonator and with at least one second resonator; and

a transmitter element, which is connected between the first symmetrical stage and the second symmetrical stage and couples them together.

According to the present invention, the problems set out above on the basis of the known approaches are solved, since, according to the invention, a “mixed structure” of the filter circuit with regard to the filter topologies used in the different stages is dispensed with. According to the invention, a first symmetrical stage and a second symmetrical stage are used instead, which respectively use the same first resonators and the same second resonators, the “equality” of the resonators used in the stages referring to the Obtain resonance frequency properties of the same. The advantage of the present invention is that such a filter circuit can be produced in a simple manner, since it is no longer necessary to produce three or four different resonator types with different resonance frequencies on one chip. It is only necessary to produce two different types of resonators, namely the parallel resonators and series resonators already described in more detail above, which are then used in both stages. The same resonators are used in both filter stages, the parallel resonators being detuned by the same amount in relation to the series resonators.

The filter circuit is preferably formed by the first symmetrical stage, the second symmetrical stage and the transmitter element, these elements all being formed on a common chip / substrate.

According to a preferred exemplary embodiment of the present invention, the filter circuit uses a symmetrical bridge circuit (lattice filter topology) both as the first stage and as the second stage, so that the advantages of the latice filter topology also in the input stage (first symmetrical stage) Filter circuit can be used.

According to a further preferred exemplary embodiment, the individual resonators are BAW resonators, the resonance frequency of which is essentially determined by the layer sequence of lower electrode, piezoelectric material and upper electrode deposited on the chip. In this exemplary embodiment, the filter circuit according to the invention is particularly advantageous, since simple manufacture is ensured here because only two types of resonator are used in the different stages. According to a further exemplary embodiment of the present invention, the transmitter element is designed such that a winding ratio of the same is selected in order to carry out an impedance transformation between an input impedance level and an output impedance level. Here too, identical BAW resonators can be used with regard to the resonance properties in the two stages, which differ only in the resonator area on the chip, since the resonator area is inversely proportional to the impedance. As already explained above, the resonance property of the BAW resonator is essentially determined by the layer sequence consisting of the lower electrode, piezoelectric material and upper electrode (choice of material, thickness of the individual layers, etc.), so it is essentially independent of an area of the resonator on the chip.

According to yet another exemplary embodiment of the present invention, the transformer element can be designed as a differential transformer in order to carry out either a differential transformation exclusively in the input stage or both in the input stage and in the output stage. The advantage of using a differential transformer is that the original lattice filter configuration can be replaced by a simplified filter configuration, in which case in particular a plurality of resonators with a predetermined impedance are replaced by a lower number of resonators with a higher impedance can be reduced so that the space requirement per level can be significantly reduced.

The filter circuit according to the invention is preferably used for converting an asymmetrical signal into a symmetrical signal or for converting a symmetrical signal into an asymmetrical signal, an impedance transformation also being able to be carried out. According to another exemplary embodiment, the filter circuit according to the invention can also be used for balanced input signals and balanced outputs. can be used with a large common mode error.

Preferred developments of the present application are defined in the subclaims.

Preferred exemplary embodiments of the present application are explained in more detail below with reference to the accompanying drawings. Show it:

1 shows a known ladder filter with 3.5 stages and three series resonators and four parallel resonators;

2 shows a known lattice filter with two stages and four series resonators and four parallel resonators;

3 shows a first exemplary embodiment of the filter circuit according to the invention with a symmetrical input stage and with a symmetrical output stage;

Fig. 4 electrically equivalent bridge circuits;

5 shows a second exemplary embodiment of the filter circuit according to the invention with a differential transformation in the symmetrical input stage; and

6 shows a third exemplary embodiment of the filter circuit according to the invention with a differential transformation both in the symmetrical input stage and in the symmetrical output stage.

A preferred exemplary embodiment of the filter circuit according to the invention is described in more detail below with reference to FIG. 3. This exemplary embodiment describes a filter circuit in order to convert an asymmetrical input signal into a symmetrical output signal, the filter circuit having two balanced stages, which via a magnetic transformer (magnetic transformer) are coupled together.

The filter circuit according to the invention is provided in its entirety with reference numeral 400 and comprises an input port 402 which has a first input connection 404 and a second input connection 406. The filter circuit 400 further comprises an output port 408, which has a first output connection 410 and a second output connection 412. An asymmetrical input signal ON is present at the first input connection 404 of the input gate 402, and the second input connection 406 of the input gate 402 is connected to a reference potential 412, e.g. B. ground connected. At the output gate 408, the symmetrical output signal AUS is present at the output connections 410 and 412. The filter circuit 400 thus comprises a first stage I and a second stage II, both the first stage I and the second stage II in the exemplary embodiment shown being symmetrical stages which are designed as a lattice filter topology. The first stage I includes a first output port 416 and a second output port 418 in addition to the input ports 404 and 406. The stage II includes a first input port 420 and a second input port 422 in addition to the two output ports 410 and 412.

Stage I comprises two series resonators R _s u and R _s ι ₂ . Furthermore, two parallel resonators R _p n and R _p ι _{2 are} provided. The series resonators and the parallel resonators are produced essentially identically, but the parallel resonators are detuned from the series resonators, that is to say their resonance frequency is shifted by a predetermined amount from the resonance frequency of the series resonators. In the _exemplary embodiment shown in FIG. 3, the first series _resonator R _{sll is connected} between the first input connection 104 and the first output connection 416 of stage I. The second series resonator R _s χ ₂ is between the second input connection 406 and the second output connection 418 Stage I switched. The first parallel resonator R _p n is connected between the input connection 404 and the second output connection 418 of stage I. The second parallel resonator R _p ι ₂ is connected between the second input connection 406 and the first output connection of stage I.

Similar to stage I, stage II also contains two series _resonators R _s2 ι and R _s22 , and two parallel _resonators R _p2 ι and R _p22 . The first series resonator R _s n of stage II is connected between the second input terminal 422 of stage II and the first output terminal 410. The second series _resonator R _s22 is connected between the first input connection 420 of stage II and the second output connection 412. The first parallel _resonator R _p2 ι is connected between the first input connection of stage II and the first output connection 410. The second parallel _resonator R _p22 is connected between the second input connection 422 of stage I and the second output connection 412.

The series resonators in the first stage I and the series resonators in the second stage II are essentially identical in construction, at least with regard to their frequency behavior. The parallel resonators of the two stages I and II are also structurally identical, at least with regard to the frequency response.

In a preferred exemplary embodiment, the resonators of the filter circuit 400 are formed by BAW resonators, and in such an exemplary embodiment the series resonators of the two filter stages I and II are identical

Layer sequences, consisting of lower electrode, piezoelectric material and upper electrode, the identity relating to the layer sequence, the materials used and thicknesses etc. of the individual layers. The same applies to the parallel resonators, which only detune their resonance due to slight differences in the individual parameters just mentioned. frequency compared to the series resonators. The parallel resonators in the two stages are also identical.

A magnetic transmitter 424 is arranged between the first stage I and the second stage II. The magnetic transmitter 424 comprises a primary winding 426 on the input stage side and a secondary winding 428 on the output stage side. The primary winding 426 is connected between the first and the second output connection 416, 418 of the input stage I, and the secondary winding 428 of the transmitter 424 is connected between the first input connection and the second input connection of the output stage II is switched, so that a coupling of the first stage I and the second stage II is effected. In an alternative embodiment, which is shown in FIG. 3, the secondary winding 428 comprises a center connection 430, which is connected to the reference potential, e.g. B. ground, is connected. As a result, the secondary winding is divided into a first secondary winding 428a and a second secondary winding 428b.

The filter circuit 400 shown in FIG. 3 is preferably formed on a common chip, so that the resonators of the first stage, the resonators of the second stage, the transmitter and the necessary connecting lines are formed together on this chip.

In the exemplary embodiment of the filter circuit according to the invention shown in FIG. 3, two lattice stages I and II are coupled to one another via the on-chip transformer (on-chip transformer) 424. The first stage I includes the primary winding 426 of the transformer 424 as a load, which is practically "floating". For this reason, this lattice stage I works properly and has the behavioral advantages that are normally encountered with this topology compared to ladder filters The improvements consist of improved barrier band attenuation, lower insertion losses and a wider passband. The transmitter 424 almost completely blocks the common mode signals and generates a symmetrical signal at its secondary winding 428. As mentioned, a center connection 430 can be provided which is connected to the secondary winding, thereby reducing the effects of parasitic stray capacitances.

The second Lattice stage II is used to improve the steepness of the transition from the pass band to the stop band and to obtain additional stop band damping at the same time.

With regard to the exemplary embodiment described with reference to FIG. 3, it is pointed out that each of the two stages I and II could also be modified in order to produce notches in a region between the pass band and the stop band of the filter, which is done by using easily different areas for the series resonators and the parallel resonators can be achieved.

According to a further exemplary embodiment, it is possible to carry out an impedance transformation using the filter circuit described with reference to FIG. 3. If an input impedance and an output impedance at the input gate 402 and at the output gate 408 are different, a corresponding impedance transformation can be carried out by means of the filter circuit 400 using the transmitter 424, in that the transmitter is designed such that it has a suitable winding ratio for the has primary winding and the secondary winding. As a consequence of the different impedances in the two stages I and II, the areas of the resonators used are also different in the respective stages.

3 for the impedance conversion of different input and output impedances is suitable with a small difference, problems can arise with large impedance differences, since it is generally difficult and problematic to arrange very small and very large resonators on the same chip. In this situation, it is advisable to use the circuit arrangement described below with reference to FIG. 5 or with reference to FIG. 6 instead of the structure shown in FIG. 3.

Using the equivalent circuit diagrams shown in Fig. 4 for a symmetrical bridge circuit (left

Figure), a hybrid circuit (middle figure) and a differential bridge circuit (right figure), which are well known in the field of electrical engineering, the first stage I shown in FIG. 3, which is a symmetrical bridge circuit, becomes a differential bridge circuit according to the right figure in Fig. 4 converted.

The resulting structure is shown in FIG. 5, elements already described here with reference to FIG. 3 being provided with the same reference numerals. As can be seen from a comparison with FIG. 3, the second stage II has remained essentially unchanged, only stage I, which was represented in FIG. 3 as a symmetrical bridge circuit, was, according to FIG. 4, by an equivalent circuit - sets, which includes a differential transformer (differential transformer). The transformer 424 now comprises a first primary winding 426a and a second primary winding 426b on the input stage side. The input stage I comprises only a series resonator R _s ι and only a parallel resonator R _p ι, the series resonator R _s ι being connected between the input connection 404 of the input gate and a first connection of the first primary winding 426a. The parallel resonator R _pi is connected between the input connection 404 of the input gate and a first connection of the second primary winding 426b. The second input port 406 of the entrance gate is with a second port of the first Primary winding 426a and connected to a second connection of the second primary winding 426b.

5 in conjunction with FIGS. 3 and 4, use is made of the fact that any symmetrical bridge structure can be replaced by an equivalent circuit comprising a differential transformer in the exemplary embodiment shown in FIG. 5, as shown in FIG. 4. After the transformer (transformer) for the conversion according to

3 is necessary, the replacement of the transformer according to FIG. 2 by a differential transformer means no additional effort.

Using the equivalences shown in FIG. 4, it can be shown that the two resonators R _p n and R _p ι ₂ with the respective impedance Z _para n _el , and the two resonators R _s ιι and R _s ι ₂ with the respective impedance Z _ser ieiι by only one resonator R _si and R _p ι may be replaced, wherein the parallel resonator R _p ι twice the impedance (twice Z _pa - _ra ii _e i) compared with the parallel resonators from stage I in Figure 3 has.. The series resonator R _s ι also has an impedance of two times

So double the impedance of one of the series resonators as in FIG. 3. Since the impedance of a BAW resonator is inversely proportional to its resonator area, the total area of the resonators of stage I can be compared to the total area of the resonators of FIG Stage I, as shown in Fig. 3, reduce by a factor of 4. The configuration shown in FIG. 5 is particularly suitable for those filter circuits in which there is a great difference between the input impedance and the output impedance. 3, the area ratio between the largest resonator and the smallest resonator on the chip is improved by a factor of about 2. It is advantageous to provide the differential transformer at the low-impedance connection or gate of the filter, that is in the configuration as shown in FIG. 5, provided the level of the input impedance is lower than the level of the output impedance at the output port 408. The resonance behavior of the resonators is essentially unchanged compared to FIG. 3.

A further exemplary embodiment of the present invention is explained in more detail below with reference to FIG. 6, in which, in addition to the exemplary embodiment shown in FIG. 5, the second stage II, which was still designed as a symmetrical bridge structure, by means of a corresponding equivalent circuit with a differential Transformer was replaced. 6, those elements which have already been described in the previous figures are provided with the same reference symbols.

As can be seen, stage I according to the exemplary embodiment from FIG. 6 corresponds to stage I, which was described with reference to FIG. 5. In stage II, similar to that described _above with reference to FIG. 5 in relation to stage I, the two series _resonators R _s2 ι and R _{s22 were} replaced by a series _resonator R _s2 with twice the impedance. The parallel _resonators R _p2i and R _{p22 were also} replaced by a parallel _resonator R _p2 with twice the impedance. Similar to stage I, the transformer 424 has now also been designed on the output stage side in such a way that it comprises a first secondary winding 428a and a second secondary winding 428b. The parallel resonator R _p2 is connected between a first connection of the first secondary winding 428a and between the first output connection 410 of the output gate 408. The series resonator R _s2 is connected between a first connection of the second secondary winding 428b and the first output connection 410. The second output connection 412 is connected to a second connection of the first secondary winding 428a and to a second connection of the second secondary winding 428b. The circuit shown in FIG. 6 performs a differential transformation with respect to both gates, the input gate 402 and the output gate 408. In both stages I and II, compared to the circuit according to FIG. 3, there is a saving in resonator area by a factor of about 4. This is of particular interest for filters which operate at frequencies below 1.7 GHz, or for Filters whose impedances are much lower than 50 ohms, since in these cases the chip area is usually dominated by the resonator size.

The filter circuits described with reference to FIGS. 3, 5 and 6 comprise an input stage I, which receives an asymmetrical input signal, and an output stage II, which outputs a symmetrical output signal. However, it is pointed out at this point that the filter circuits shown in FIGS. 3, 5 and 6 work just as well if the input signal is a balanced signal or a partially balanced signal with a large common mode error. In this case, it is sufficient to disconnect the connection of the first connection 406 of the input gate 402 shown in FIGS. 3, 5 and 6 to the reference potential (ground) and instead to connect the second signal line there.

With regard to the above description, it is pointed out that the inputs and outputs described are basically interchangeable. This means that the direction of the signal flow can be reversed. Therefore, all structures are suitable for both; to use an unbalanced signal source and a balanced load or an unbalanced load and a balanced signal source. The position of the center taps of the transformer windings can be changed for an inverted operating mode.

The advantage of the present invention is that, in contrast to the prior art, it has a miniaturized magnetic transformer as an intermediate stage element between symmetrical filter stages, whereby common mode signals are blocked. According to a further aspect of the present invention, a differential transformation is carried out at least in one stage, which results in considerable space savings in the arrangement of the elements on a chip surface, and at the same time a simplified impedance transformation can be carried out.

The present application was described above on the basis of preferred exemplary embodiments in which BAW resonators were used. However, the present application is not limited to the use of BAW resonators, but can generally be applied to filter circuits with a plurality of resonators, SAW resonators, line resonators, resonators made of concentrated components, etc. in particular also being considered here.

The filter circuits described above each comprised two stages. However, the present invention is not limited to this configuration. In addition to the stages described, one or more stages can be provided on the input side and / or output side in the filter circuits according to the invention. These further stages can be coupled to the remaining stages by simply connecting them and / or via transmitters.

reference numeral

100 ladder filters

102 entrance gate

104 first input port

106 second input connection

108 exit gate

110 first output connector

112 second output connection

114 reference potential

120 lattice filters

122 knots

124 knots

400 filter circuit

402 entrance gate

404 first input port

406 second input connection

408 exit gate

410 first output connector

412 second output connector

414 reference potential

416 first stage I output connector

418 second stage II output connector

420 first level II input connector

422 second stage II input connector

424 transformers

426 primary winding

426a first primary winding

426b second primary winding

428 secondary winding

428a first secondary winding

428b second secondary winding

430 center connection

I first symmetrical stage

II second symmetrical stage

ON input signal

OFF output signal R-slr s ₂ r s3? Rs4 series resonators Rpl? Rp2f p3? Rp4 parallel resonators

Rsii R _s ι ₂ , Rs2i / Rs2 ₂ series resonators Rpii Rpi2r Rp2i / Rp22 parallel resonators

Claims

claims

1. Filter circuit, with

first resonators (R _si , R _s2 ; R _s ιι, R _s i2, R _S 2i, R _s 22) with a fixed first resonance frequency;

second resonators (R _p ι, R _p2 ; R _p n, R _p ι ₂ , R _p2 ι, R _p22 ) with a second resonance frequency shifted with respect to the first resonance;

a first symmetrical stage (I) with at least one first resonator and with at least one second resonator;

a second symmetrical stage (II) with at least one first resonator and with at least one second resonator; and

a transmitter element (424) which is connected between the first symmetrical stage (I) and the second symmetrical stage (II) and couples them to one another.

2. Filter circuit according to claim 1, wherein the first symmetrical stage (I) is a symmetrical bridge circuit with a first port (402, 404, 406) and with a second port (416,

418), and in which the second symmetrical stage (II) is a symmetrical bridge circuit with a first gate (420, 422) and with a second gate (408, 410, 412), the transmitter element (424) between the second gate (416, 418) of the first symmetrical bridge circuit (I) and the first gate (420, 422) of the second symmetrical bridge circuit (II) is connected.

3. Filter circuit according to claim 2, in which in the first symmetrical bridge circuit (I) and in the second symmetrical bridge circuit (II) in each case a first resonator (R _s ιι, R _s22 ) between a first connection (404, 420) of the first gate and a first connection (416, 410) of the second gate, a first resonator (R _s ι ₂ , R _S 2i) between a second connection (406, 422) of the first gate and a second connection (418, 412) of the second gate, a second resonator (R _p ιι, R _p2 ι) between the first connection (404, 420) of the first gate and the second connection (418, 412) of the second gate, and a second resonator (R _p ι ₂ , R _p2 ι) between the second connection (406, 422) of the first gate and the first connection (416, 410) of the second gate.

4. Filter circuit according to claim 2 or 3, in which a symmetrical signal can be applied to the first gate (402) of the first symmetrical stage (I), and in which to the second gate

(408) of the second symmetrical stage (II) a symmetrical signal can be applied, being at the first symmetrical stage

(I) an input signal can be applied or an output signal can be tapped, and being at the second symmetrical stage

(II) the output signal can be tapped or the input signal can be applied.

5. A filter circuit according to claim 2 or 3, in which an asymmetrical signal can be applied to the first gate (402) of the first symmetrical stage (I), the asymmetrical signal being able to be applied to a first connection (404) of the first gate, and wherein a second connection (406) of the first gate is connected to a reference potential, and in which a symmetrical signal can be applied to the second gate (408) of the second symmetrical stage (II), one at the first symmetrical stage (I) Input signal can be applied or an output signal can be tapped, and the output signal can be tapped at the second symmetrical stage (II) or the input signal can be applied.

6. Filter circuit according to one of claims 1 to 5, wherein the transmitter element (424) has a center connection (430) which is connected to a reference potential.

7. Filter circuit according to one of claims 1 to 6, in which

an input port (402) of the filter circuit has a first impedance, and an output port (408) of the filter circuit has a second impedance, which differs from the first impedance,

the transformer element (424) comprises a first winding (426) and a second winding (428), the first winding (424) being assigned to the first symmetrical stage (I), the first symmetrical stage (I) being the input gate (402) The second winding (428) is assigned to the second symmetrical stage (II), the second symmetrical stage (II) has the output gate (408), and a winding ratio between the first winding (426) and the second winding ( 428) is set to effect an impedance transformation from the first impedance to the second impedance,

the first resonators (R _s ιι, Rsi ₂ ) and the second resonators (R _p ι, R _p ι ₂ ) in the first symmetrical stage (I) have a first surface corresponding to the first impedance, and

the first resonators (R _S 2i _/ Rs2 ₂ ) and the second resonators (R _p2 ι, R _p22 ) in the second symmetrical stage (II) have a second surface corresponding to the second impedance, the first surface and the second surface being different ,

8. Filter circuit according to claim 7, wherein the transmission element (424) has a first primary winding (426a) and a second

Primary winding (426b) in order to effect a differential transformation in the first symmetrical stage (I), the first symmetrical stage (I) having an input gate (402) with a first connection (404) and with a second connection ( 406), a first resonator (R _s ι) being located between the first connection (404) of the input gate (402) and a first connection of the first primary winding (426a). is switched, a second resonator (R _p ι) being connected between the first connection (404) of the input gate (402) and a first connection of the second primary winding (426b), the second connection (406) of the input gate (402) being connected is connected to a node which is connected to a second connection of the first primary winding (426a) and to a second connection of the second primary winding (426b).

The filter circuit of claim 8, wherein the transmitter element (424) has a first secondary winding (428a) and a second secondary winding (428b) to effect a differential transformation in the second symmetrical stage (II), the second symmetrical stage (II) has an output gate (408) with a first connection (410) and with a second connection (412), wherein between a first connection of the first secondary winding (428a) and the first connection (410) of the output gate (408) a first resonator (R _s2 ) is connected, a second resonator (R _p2 ) being connected between a first connection of the second secondary winding (428b) and the second connection (412) of the output gate (408), a node connected to a second connection of the first secondary winding (428a) and to a second connection of the second secondary winding (428b) is connected to the second connection (412) of the output gate (408).

10. Filter circuit according to one of claims 1 to 10, wherein the first symmetrical stage (I), the second symmetrical stage (II) and the transmitter element (424) are formed on a common chip.