DE10234686B4 - filter circuit - Google Patents

Abstract

Filter circuit for converting a balanced / unbalanced signal into an unbalanced / balanced signal, with
first resonators (R _s1 , R _s2 ; R _s21 , R _s22 ) having a fixed first resonant frequency;
second resonators (R _p1 , R _p2 , R _p21 , R _p22 ) having a second resonance frequency shifted from the first resonance;
an input / output (404, 406) for providing an unbalanced signal, a terminal (406) of which is connected to a reference potential terminal;
an output / input (410, 412) for providing a balanced signal;
a first stage (I) having at least a first resonator and at least one second resonator, the first stage (I) being connected to the input / output (404, 406);
a second stage (II) having at least a first resonator and at least one second resonator, the second stage (II) being connected to the output / input (410, 412); and
a transmitter element (424) connected between the first stage (I) and the second stage (II) and coupling them together,
where the ...

Description

The The present invention relates to a filter circuit, in particular to a filter circuit for converting unbalanced signals in symmetrical signals; and in particular to a filter circuit, which BAW resonators (BAW = Bulk Acoustic Wave). Further The present invention relates to a filter circuit with a plurality of BAW resonators, which is a transformation of impedance levels between an input port and an output port of the filter circuit.

Resonator-based RF filters, such as BAW filters, have two basic topologies, which are based on the 1 and 2 be explained in more detail.

The first topology (see 1 ) is the so-called "ladder filter." The ladder filter 100 includes an entrance gate 102 with a first input terminal 104 and a second input terminal 106 , Furthermore, the filter includes 100 an exit gate 108 with a first output terminal 110 and a second output terminal 112 , At the first input connection 104 of the entrance gate 102 an input signal IN is received, and at the first output port 110 the exit gate 108 an output signal OUT is output. At the in 1 shown filter 100 are between the first input terminal 104 and the first output terminal 110 serially connected three series resonators R _s1 , R _s2 and R _s3 . Furthermore, four parallel _resonators R _p1 to R _{p4 are} provided. The first parallel _resonator R _p1 is parallel to the input gate 102 connected. The second parallel _resonator R _p2 is connected in parallel to the first series resonator R _s1 . The third parallel _resonator R _p3 is connected in parallel to the second series resonator R _s2 . The fourth parallel _resonator R _p4 is parallel to the output _gate 108 and connected in parallel to the third series resonator R _s3 . The second input terminal 106 and the second output port 112 are with a reference potential 114 , z. B. mass connected. The parallel _resonators R _p1 to R _p4 are also connected to the reference potential. At the in 1 The conventional filter shown is a ladder filter with 3.5 stages (the three series _resonators R _s1 to R _s3 and the four parallel _resonators R _p1 to R _p4 ). In the ladder filter 100 It is a filter circuit with a single input ON and a single output OFF. The ladder filter is a filter for unbalanced signals.

In 2 An 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 the description of the 2 be similar or same components, already based on the 1 have been described, provided with the same reference numerals.

The lattice filter 120 receives a balanced input signal IN at the first input terminal 104 and at the second input terminal 106 of the entrance gate 102 , and outputs a balanced output signal OUT at the output gate 108 at the connections 110 and 112 out. Between the first input connection 104 and the first output terminal 110 a series connection of two series resonators R _s1 and R _{s2 is} provided. Likewise, between the second input terminal 106 and the second output terminal 112 a series circuit of two series resonators R _s3 and R _s4 provided. Between the first input connection 104 and a node located between the series resonators R _s3 and R _s4 122 a first parallel _resonator R _{p1 is} connected. Further, between the second input terminal 106 and a second node disposed between the first series resonator R _s1 and the second series resonator R _s2 124 a second parallel _resonator R _p2 connected. Further, between the second nodes 124 and the second output terminal 112 a third parallel _resonator R _{p3 is} connected, and between the first node 122 and the first output terminal 110 a fourth parallel _resonator R _{p4 is} connected. This in 2 illustrated filters 120 is completely differential, ie, both input ports 102 and 110 are balanced.

Regarding the basis of the 1 and 2 It is noted that the series resonators and parallel resonators are preferably BAW resonators, with the series resonators R _s1 through R _{s4 made} at a predetermined resonant frequency. The parallel _resonators R _p1 to R _p4 are substantially identical in frequency characteristics to the series resonators, but are detuned in resonance frequency as compared with the resonance frequencies of the series resonators so as to achieve the desired filtering effect. It should be noted that in the ladder filter 100 used series resonators and parallel resonators from those in the Lattice filter 120 used series resonators and parallel resonators differ, especially in filter circuits with substantially the same filter characteristics, but different topology.

As can be seen from the above discussion of 1 and 2 results exists in the ladder filter 100 only the ability to receive an unbalanced input signal, and a correspondingly unbalanced output signal give. The lattice filter also makes this possible 120 merely receiving a balanced input signal and outputting a balanced output signal.

It However, there are applications that require a Transformation / conversion of an unbalanced input signal to perform in a balanced output signal. There are also applications where alternatively or additionally to the conversion from the unbalanced signal into a symmetric one Signal at the inputs and outputs different gate impedances exist, which are also handled Need to become.

The conventional Method to perform a corresponding conversion / transformation consists It is to provide an extra component, which as Symmetrierbauglied (Balun) is called. The Symmetrierbauglied can either a magnetic transformer (magnetic transformer), an LC circuit or a stripline structure on a printed circuit board. For surface acoustic wave filters can be an acoustic balancing function without additional Components are implemented, which, however, reduces the behavior of the Overall filter is significantly deteriorated. Furthermore, this leads Symmetrizing function that these filters against electrostatic Discharges are very sensitive, and further the skills drastically limited in the handling of services.

in the Only a few approaches have been suggested so far in connection with BAW filters been a transformation / transformation of unbalanced signals to perform in symmetrical signals.

An example of converting unbalanced signals into balanced signals is that 1 and 2 to combine known filter circuits, ie to the output of the ladder filter 100 the lattice filter 120 to join. This approach has significant disadvantages for practical applications of such a filter. The arrangement just described is particularly disadvantageous in that it can be used only for "floating" (floating) differential loads, so no HF leakage current is allowed to ground.

The DE 695 16 561 T2 relates to a symmetrical surface acoustic wave filter in a single-stage or two-stage design, the two-stage design being a series connection of two filter stages.

The US-A-2,173,894 describes a variable bandwidth filter using quartz resonators. The filter comprises a first stage and a second stage, which are coupled together via a transformer. The two filter stages each comprise quartz crystals, which are bridged by capacitors. The quartz crystals have different properties. The lattice arms of the two filter stages each have different resonator circuits.

Steinbuch, K .; Rupprecht, W. describe in "Nachrichtentechnik - ein introductory presentation", Berlin [u. a].: Springer-Verlag 1973, pp. 99 to 102, ISBN: 3-540-06083-9 Transformations of Filter Circuits.

The US-A-5,949,299 describes a multilayer balanced-to-unbalanced transformer with center connection.

outgoing from this prior art, the present invention is the Task is to provide an improved filter circuit, which is easy to produce and simple Way the conversion between symmetric and unbalanced Signals possible.

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

According to the present Invention will be the problems set forth above with reference to the known approaches solved, da according to the invention to a "mixed structure" of the filter circuit regarding the filter topologies used in the different Steps is omitted. Unlike in the known approaches will no mix of ladder filter topology and lattice filter topology used. According to the invention takes place of which a first symmetrical stage and a second symmetrical stage Stage, each having the same first resonators and use the same second resonators, with the "equality" in the stages used resonators on the resonance frequency characteristics of the same Respectively.

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 detail above, which are then used in both stages. The same resonators are used in both filter stages, the parallel resonators each being compared by the same amount the series resonators are detuned.

The Filter circuit is preferably through the first symmetric stage, the second symmetrical stage and the transformer element are formed, these elements are all formed on a common chip / substrate are.

According to one preferred embodiment The present invention uses the filter circuit as well as a first stage as well as a second stage a balanced bridge circuit (Lattice filter topology), so that the benefits of lattice filter topology also in the input stage (first symmetric stage) of the filter circuit can be exploited.

According to one another preferred embodiment, the individual resonators are BAW resonators whose resonant frequency essentially by the layer sequence deposited on the chip of lower electrode, piezoelectric material and upper electrode is fixed. In this embodiment is the filter circuit according to the invention particularly advantageous because here due to the use of only two Resonator types in different stages a simple production guaranteed is.

According to one another embodiment of the present invention is the transmitter element configured such that a winding ratio thereof is selected such that an impedance transformation between an input impedance level and an output impedance level. Again, in terms of the resonant properties in the two stages identical BAW resonators are used, which only in the resonator on differ from the chip, since the resonator surface inversely proportional to the impedance. As stated above, the resonance property is of the BAW resonator essentially by the layer sequence consisting of lower electrode, piezoelectric material and upper electrode determined (choice of material, thickness of individual layers, etc.), is essentially independent from a surface of the resonator on the chip.

According to one in turn another embodiment According to the present invention, the transformer element can be designed as a differential transformer be to either a differential transformation exclusively in the input stage or both in the input stage and in the output stage perform. The advantage of using a differential transformer is that here the original Lattice filter configuration replaced with a simplified filter configuration can be, in which case in particular a plurality of resonators with a predetermined impedance by a lower number of Resonators with a higher Impedance can be replaced, so here's the area needed per level can be significantly reduced.

The Filter circuit according to the invention is preferably used to convert an unbalanced signal into a symmetric signal or to convert a balanced one Signals used in an unbalanced signal, with an additional Impedance transformation feasible is. According to one other embodiment can the filter circuit according to the invention also for symmetrical input signals and balanced output signals with a big one Common mode error can be used.

preferred Further developments of the present application are defined in the subclaims.

following With reference to the accompanying drawings preferred embodiments of the closer to the present application explained. Show it:

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

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

3 an example of a filter circuit with a balanced input stage and with a balanced output stage;

4 electrically equivalent bridge circuits;

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

6 A second embodiment of the filter circuit according to the invention with a differential transformation in both the symmetrical input stage and in the balanced output stage.

Based on 3 An example of a filter circuit will be described in more detail below. This example describes a filter circuit for converting an unbalanced input signal to a balanced output signal, the filter circuit having two balanced stages connected via a magnetic transformer (magneto) transformer) are coupled together.

The filter circuit is in its entirety by the reference numeral 400 provided, and includes an entrance gate 402 which has a first input terminal 404 and a second input terminal 406 having. Furthermore, the filter circuit comprises 400 an exit gate 408 which has a first output terminal 410 and a second output terminal 412 having. An unbalanced input signal IN is at the first input terminal 404 of the entrance gate 402 on, and the second input terminal 406 of the entrance gate 402 is with a reference potential 414 , z. B. mass connected. At the exit gate 408 the balanced output signal is OFF at the output terminals 410 and 412 at. The filter circuit 400 thus comprises a first stage I and a second stage II, wherein both the first stage I and the second stage II are in the example shown symmetrical stages, which are designed as a lattice filter topology. The first stage I includes in addition to the input terminals 404 and 406 a first output terminal 416 and a second output terminal 418 , Stage II includes in addition to the two output terminals 410 and 412 a first input terminal 420 and a second input terminal 422 ,

Stage I comprises two series _resonators R _s11 and R _s12 . Furthermore, two parallel _resonators R _p11 and R _{p12 are} provided. The series resonators and the parallel resonators are made substantially the same, but the parallel resonators are detuned with respect to the series resonators, so their resonance frequency are shifted by a predetermined amount of the resonant frequency of the series resonators. At the in 3 the example shown is the first series _resonator R _s11 between the first input _terminal 104 and the first output terminal 416 the stage I switched. The second series _resonator R _s12 is between the second input _terminal 406 and the second output terminal 418 the stage I switched. The first parallel _resonator R _p11 is between the input _terminal 404 and the second output terminal 418 the stage I switched. The second parallel _resonator R _p12 is between the second input terminal 406 and the first output terminal of stage I connected.

Similarly to stage I, stage II also contains two series _resonators R _s21 and R _s22 , as well as two parallel _resonators R _p21 and R _p22 . The first stage II series _resonator R _s11 is between the second input _terminal 422 Stage II and the first output terminal 410 connected. The second series _resonator R _s22 is between the first input _terminal 420 Stage II and the second output terminal 412 connected. The first parallel _resonator R _p21 is between the first stage II input terminal and the first output terminal 410 connected. The second parallel _resonator R _p22 is between the second input _terminal 422 the stage I and the second output terminal 412 connected.

The Series resonators in the first stage I and the series resonators in the second stage II are substantially identical, at least in terms of the frequency response of the same. Likewise, the parallel resonators the two stages I and II, at least in terms of frequency response identical.

The resonators of the filter circuit 400 can be formed by BAW resonators, and in such an example, the series resonators of the two filter stages I and II are formed by identical layer sequences consisting of lower electrode, piezoelectric material and upper electrode, the identity in this case on the layer sequence, the materials used and thicknesses etc. of the individual layers. The same applies to the parallel resonators, which have only due to slight differences in the individual parameters just mentioned a detuning in terms of their resonance frequency compared to the series resonators. The parallel resonators in the two stages are also identical.

Between the first stage I and the second stage II is a magnetic transformer 424 arranged. The magnetic transformer 424 comprises on the input stage side a primary winding 426 and on the output stage side, a secondary winding 428 , The primary winding 426 is between the first and second output terminals 416 . 418 the input stage I switched, and the secondary winding 428 of the transformer 424 is connected between the first input terminal and the second input terminal of the output stage II, so that a coupling of the first stage I and the second stage II is effected. In an alternative example, which is in 3 is shown, includes the secondary winding 428 a center connection 430 , with the reference potential, z. B. mass, is connected. As a result, the secondary winding in a first secondary winding 428a and a second secondary winding 428b divided.

In the 3 shown filter circuit 400 is preferably formed on a common chip, so that the resonators of the first stage, the resonators of the second stage, the transformer and the required connection lines are formed together on this chip.

At the in 3 shown example of a filter circuit are two lattice stages I and II via the on-chip transformer (on-chip Übertra ger) 424 coupled together. The first stage I comprises the primary winding 426 of the transformer 424 For this reason, this lattice stage I operates properly and has the performance advantages normally encountered with ladder filters in this topology Insertion losses, as well as in a wider passband.

The transformer 424 almost completely blocks the common-mode signals and produces a symmetrical signal at its secondary turn 428 , As mentioned, a center connection 430 be provided, which is connected to the secondary winding, whereby the effects of parasitic stray capacitances are reduced.

The second lattice level II is used to control the steepness of the transition from the passband to the stopband to improve and at the same time a additional Stop-band attenuation to obtain.

Regarding the basis of the 3 As further described, each of the two stages I and II could be further modified to create notches in a region between the passband and the stopband of the filter, achieved by the use of slightly different areas for the series resonators and the parallel resonators can be.

According to another example, it is possible with the basis of the 3 described filter circuit to perform an impedance transformation. When an input impedance and an output impedance at the input port 402 and at the exit gate 408 are different, can by means of the filter circuit 400 a corresponding impedance transformation using the transformer 424 be performed by the transformer is designed such that it has a suitable winding ratio for the 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 in the respective stages are also different.

Although the filter circuit according to 3 is suitable for the impedance conversion of different input and output impedances with a small difference, problems can occur 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 replace the in 3 structure described below with reference to 5 or on the basis of 6 To use the described circuit arrangement: Using the in 4 The equivalent circuit diagrams for a balanced 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 are shown in FIG 3 illustrated first stage I, which is a balanced bridge circuit, in a differential bridge circuit according to the right figure in 4 transformed.

The resulting structure is in 5 shown here, with reference to the 3 elements described are provided with the same reference numerals. How to compare with 3 can be seen, the second stage II has remained essentially unchanged, only the level I, which in 3 was shown as a symmetrical bridge circuit was, accordingly 4 , replaced by an equivalent circuit, which includes a differential transformer (differential transformer). The transformer 424 now includes a first primary winding on the input stage side 426a and a second primary winding 426b , The input stage I comprises only one series resonator R _s1 and only one parallel _resonator R _p1 , wherein the series resonator R _s1 between the input terminal 404 of the input gate and a first terminal of the first primary winding 426a is switched. The parallel _resonator R _p1 is between the input _terminal 404 of the input port and a first port of the second primary winding 426b connected. The second input terminal 406 of the entrance gate is connected to a second terminal of the first primary winding 426a and a second terminal of the second primary winding 426b connected.

As 5 combined with 3 and 4 can be seen in the in 5 illustrated embodiment made use of the fact that any symmetrical bridge structure can be replaced by an equivalent circuit comprising a differential transformer, as shown in FIG 4 is shown. After the transformer (transformer) in any case for the conversion according to 3 is required, the replacement of the transformer in accordance with 2 through a differential transformer no additional effort.

Using the in 4 It can be shown that the two resonators R _p11 and R _{p12 can be replaced in parallel} with the respective impedance Z in _parallel , and the two resonators R _s11 and R _s12 with the respective impedance Z can be _serially replaced by only one resonator R _s1 and R _p1 . wherein the parallel resonator R _p1 twice the impedance (Z _{pa rallel twice) as compared with the parallel resonators from stage I in} 3 having. The series resonator R _s1 also has an impedance of two times Z _serial , that is twice the impedance of one of the series resonators as in 3 , Since the impedance of a BAW resonator is inversely proportional to its resonator surface, can be achieved by the embodiment according to 5 the total area of the stage I resonators versus the total area of the stage I resonators as shown in FIG 3 are shown to reduce the factor 4. In the 5 The illustrated configuration is particularly suitable for such filter circuits where there is a large difference between the input impedance and the output impedance as compared with the filter circuit of FIG 3 For example, 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 port or gate of the filter, that is, in the configuration as shown in FIG 5 is shown, provided that 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 opposite 3 essentially unchanged.

Based on 6 In the following, a further embodiment of the present invention will be explained in more detail, in which, in addition to that described in FIG 5 In the embodiment shown, the second stage II, which was still designed as a symmetrical bridge structure, was replaced by a corresponding equivalent circuit with a differential transformer. In 6 are those elements which have already been described in the preceding figures, provided with the same reference numerals.

As can be seen, corresponds to the stage I according to the embodiment of 6 Stage I, which is based on the 5 has been described. In the stage II were similar, similar to the above with reference to the 5 with respect to stage I, which _replaces two series _resonators R _s21 and R _s22 with a series _resonator R _s2 having twice the impedance. Likewise, the parallel _resonators R _p21 and R _{p22 were} replaced by a parallel _resonator R _p2 with twice the impedance. Similar to Stage I, the transformer became 424 now also output stage side designed such that the same a first secondary winding 428a and a second secondary winding 428b includes. The parallel _resonator R _p2 is connected between a first terminal of the first secondary winding 428a and between the first output port 410 the output gate 408 connected. The series resonator Rs2 is connected between a first terminal of the second secondary winding 428b and the first output terminal 410 connected. The second output terminal 412 is connected to a second terminal of the first secondary winding 428a and a second terminal of the second secondary winding 428b connected.

In the 6 shown circuit leads with respect to both gates, the entrance gate 402 and the exit gate 408 a differential transformation. In both stages I and II results as compared with the circuit according to 3 This is particularly interesting for filters operating at frequencies below 1.7 GHz, or for filters whose impedances are much lower than 50 ohms, since in these cases the chip area normally is is dominated by the resonator size.

The basis of the 3 . 5 and 6 described filter circuits include an input stage I, which receives an unbalanced input signal, and an output stage II, which outputs a balanced output signal. However, it is noted at this point that the in 3 . 5 and 6 just as well work when the input signal is a balanced signal or a partially balanced signal with a large common mode error. In this case, it is sufficient that in 3 . 5 and 6 shown connection of the first port 406 of the entrance gate 402 to solve with the reference potential (ground) and instead connect the second signal line there.

Regarding From the above description, it should be noted that the described inputs and outputs in principle are interchangeable. This means that the direction of the signal flow is reversed can be. Therefore, all structures are suitable for both, an asymmetrical Signal source and a balanced load or unbalanced Load and a balanced signal source. The position the center taps of the transformer windings can for changed an inverse operating mode become.

Of the Advantage of the present invention is that these, im Contrary to the prior art, a miniaturized magnetic Transformer as an intermediate element between symmetrical filter stages which blocks common mode signals. According to one Another aspect of the present invention is at least one Stage performed a differential transformation, resulting in significant space savings in the arrangement of elements on a chip surface, and at the same time a simplified impedance transformation can be performed.

The present application has been described above with reference to preferred embodiments in which BAW resonators have been used. However, the present application is not limited to the use of BAW resonators, but can generally be applied to filter circuits be applied to a plurality of resonators, in which case in particular SAW resonators, line resonators, resonators of concentrated components, etc. come into consideration.

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

100

Ladder filter

102

front gate

104

first input port

106

second input port

108

output gate

110

first output port

112

second output port

114

reference potential

120

Lattice filter

122

node

124

node

400

filter circuit

402

front gate

404

first input port

406

second input port

408

output gate

410

first output port

412

second output port

414

reference potential

416

first Output connection of stage I

418

second Output connection of stage II

420

first Input connection of stage II

422

second Input connection of stage II

424

exchangers

426

primary

426a

first primary

426b

second primary

428

secondary winding

428a

first secondary winding

428b

second secondary winding

430

center connection

I

first symmetrical stage

II

second symmetrical stage

ONE

input

OUT

output

R _s1 , R _s2 , R _s3 , R _s4

Series resonators

R _p1 , R _p2 , R _p3 , R _p4

Parallel resonators

R _s11 , R _s12 , R _s21 , R _s22

Series resonators

R _p11 , R _p12 , R _p21 , R _p22

Parallel resonators

Claims (7)

Filter circuit for converting a balanced / unbalanced signal into an unbalanced / balanced signal, _comprising first resonators (R _s1 , R _s2 , R _s21 , R _s22 ) having a fixed first resonant frequency; second resonators (R _p1 , R _p2 , R _p21 , R _p22 ) having a second resonance frequency shifted from the first resonance; an input / output ( 404 . 406 ) for providing an unbalanced signal, wherein a terminal ( 406 ) thereof is connected to a reference potential terminal; an output / input ( 410 . 412 ) for providing a balanced signal; a first stage (I) with at least one first resonator and with at least one second resonator, the first stage (I) being connected to the input / output ( 404 . 406 ) connected is; a second stage (II) having at least one first resonator and at least one second resonator, the second stage (II) being connected to the output / input (II) 410 . 412 ) connected is; and a transformer element ( 424 ) which is connected between the first stage (I) and the second stage (II) and couples the same to one another, wherein the transformer element ( 424 ) a first primary winding ( 426a ) and a second primary winding ( 426b ) in order to effect a differential transformation in the first stage (I), the first stage (I) having an input port with a first terminal (I) 404 ) and with a second connection ( 406 ), wherein between the first terminal ( 404 ) of the input gate and a first terminal of the first primary winding ( 426a ) a first resonator (R _s1 ) is connected, wherein between the first terminal ( 404 ) of the input gate and a first terminal of the second primary winding ( 426b ) a second resonator (R _p1 ) is connected, wherein the second terminal ( 406 ) of the input port is connected to a node connected to a second terminal of the first primary winding ( 426a ) and with a second terminal of the second primary winding ( 426b ) connected is. Filtering circuit according to Claim 1, in which the second stage (II) comprises a balanced bridge circuit with a first gate ( 420 . 422 ) and with a second goal ( 408 . 410 . 412 ) as the output / input is. Filtering circuit according to Claim 2, in which, in the second balanced bridge circuit (II), a first resonator (R _s22 ) is connected between a first terminal (R ₂ ). 420 ) of the first port and a first port ( 412 ) of the second gate, a first resonator (R _s21 ) between a second terminal ( 422 ) of the first port and a second port ( 410 ) of the second port, a second resonator (R _p21 ) between the first port ( 420 ) of the first port and the second port ( 410 ) of the second port, and a second resonator (R _p21 ) between the second port ( 422 ) of the first gate and the first port ( 412 ) of the second gate is switched. Filter circuit according to Claim 1, in which the transformer element ( 424 ) a first secondary winding ( 428a ) and a second secondary winding ( 428b ) in order to effect a differential transformation in the second stage (II), the second stage (II) being an output gate (II) 408 ) with a first connection ( 410 ) and with a second connection ( 412 ), wherein between a first terminal of the first secondary winding ( 428a ) and the first connection ( 410 ) of the exit gate ( 408 ) a first resonator (R _p2 ) is connected, wherein between a first terminal of the second secondary winding ( 428b ) and the first connection ( 410 ) of the exit gate ( 408 ) a second resonator (R _s2 ) is connected, wherein a node connected to a second terminal of the first secondary winding ( 428a ) and with a second terminal of the second secondary winding ( 428b ) is connected to the second port ( 412 ) of the exit gate ( 408 ) connected is. Filter circuit according to one of Claims 1 to 3, in which the transformer element ( 424 ) has a center terminal connected to a reference potential. A filter circuit according to any one of claims 1 to 5, wherein an input port of the filter circuit has a first impedance, and an output port ( 408 ) of the filter circuit has a second impedance that is different from the first impedance, a winding ratio of the transformer element ( 424 ) in order to effect an impedance transformation from the first impedance to the second impedance, the first resonators (R _s1 ) and the second resonators (R _p1 ) in the first stage (I) have a first area corresponding to the first impedance, and the first resonators (R _s21 , R _s22 ) and the second resonators (R _p21 , R _p22 ) in the second stage (II) have a second area corresponding to the second impedance, the first area and the second area being different. Filter circuit according to one of claims 1 to 6, wherein the first stage (I), the second stage (II) and the transformer element ( 424 ) are formed on a common chip.