WO2004066495A1

WO2004066495A1 - Circuit arrangement providing impedance transformation

Info

Publication number: WO2004066495A1
Application number: PCT/IB2003/006336
Authority: WO
Inventors: Hendrik K. J. Ten Dolle; Robert F. Milsom
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-01-20
Filing date: 2003-12-10
Publication date: 2004-08-05
Also published as: AU2003292481A1

Abstract

The present invention relates to a circuit arrangement (10) which is applicable for communication devices for instance handheld GPS or personal communication units. Accordingly, a circuit arrangement (10) comprising a certain combination of at least one resonator lattice circuit (20a, 20b) with resonator elements (22-1, 22-2, 24-1, 24-2, 26-1, 26-2, 28-1, 28-2) that are preferably BAW resonator elements. This at least one resonator lattice circuit (20a, 20b) is combined with at least one reactance circuit element (31, 32) which can be an inductance and capacitance element. The circuit arrangement (10) provides impedance transformation between different impedance levels at its input port (1) and its output port (2). Moreover, the circuit arrangement (10) according to the invention provides a frequency selective behavior which can be made according to a first embodiment being a broad band or according to a second embodiment being a narrow band with respect to the application needs. Thus, the invention is applicable in modern communication units since communication standards like UMTS can be achieved with a small amount of circuit elements. Moreover, according to the signal guiding, implementation in unbalanced-in to balanced-out applications is possible.

Description

Circuit arrangement providing impedance transformation

The present invention relates to a circuit arrangement according to the preamble of claim 1.

More specifically, the invention relates to a circuit arrangement for providing impedance transformation between an input port and an output port. The development of mobile telecommunications continues towards ever smaller and increasingly complicated handheld units. This leads to increasing requirements on the miniaturization of the components and structures used in the mobile communication means. This concerns radio frequency circuits, which despite the increasing miniaturization should be able to withstand considerable power levels and also always frequency selective characteristics are necessary. Due to use of high frequencies in the range of GHz special circuit elements for building circuit structures are required and high frequency related concerns have to be dealt with.

For example, receive band filters for modern telecommunication standards, like UMTS, need steep transition from stopband to passband since Tx and Rx are closely separated. For instance, extended GSM (EGSM) is the standard for European second generation 1 GHz mobile communication. The Rx and Tx bands are centered at 942.5 and 897.5 MHz, respectively. Both of these have a bandwidth of 35 MHz, resulting in fractional bandwidth of 3.71% and 3.9% for the Rx and Tx, respectively. Moreover, some newer applications, for example, GPS or TV up conversion filter require even smaller bandwidths. Accordingly, it is known to use mechanical resonator characteristics in filter circuits for electrical signals. These resonators can be divided into two classes that are derived from the utilized kind of mechanical vibration. In a first case, surface acoustic vibration modes of a solid surface are utilized, in which modes the vibration is confined to the surface of the solid, decaying quickly away from the surface. In other words, a surface acoustic wave is travelling on the surface of the solid material, wherein the mechanical or acoustic waves, respectively, are coupled in and out via applicable formed electrical connections that cause a frequency selective behavior. Due to the used surface acoustic waves such elements are called Surface Acoustic Wave (SAW) filters or SAW resonators. A SAW resonator typically comprises a piezoelectric solid and two interdigitated structures as electrodes. Various circuits as filters or oscillators containing resonator elements are produced with SAW resonators, which have the advantage of very small size, but unfortunately a weakness in withstanding high power levels.

In the second case, a mechanical vibration of a bulk material is used which is sandwiched between at least two electrodes for electrical connection. Typically the bulk material is a single piezoelectric layer (piezo) disposed between the two electrodes. When alternating electrical potential is applied across the metal-piezo-metal sandwich, the entire bulk material expands and contracts, creating the mechanical vibration. This vibration is in the bulk of the material, as opposed to being confined to the surface, as is the case for SAWs. Therefore, such elements are called Bulk Acoustic Wave (BAW) resonators. BAW resonators are often employed in bandpass filters having various topologies. Further known BAW resonator elements are thin film bulk acoustic resonators, so called FB ARs, which are created using a thin film semiconductor process to build the metal-piezo-metal sandwich in air in contrast to the afore-mentioned BAWs, which are usually solidly mounted to a substrate.

The electrical behavior of a SAW or BAW resonator is quite accurately characterized by the equivalent circuit, which is shown in the accompanying Fig. 2. In this there is a branch comprising a series combination of an equivalent inductance Ls, an equivalent capacitance Cs, and an equivalent resistance Rs. Ls and Cs are the motional inductance and capacitance respectively and Rs represents the acoustic losses of the resonator. These series elements are connected in parallel to a capacitance Cp that follows from the dielectric properties of the piezoelectric material. Therefore, each SAW or BAW resonator comprises two characteristic resonance frequencies, which is a series resonance frequency and a parallel resonance frequency. The first is mostly called resonance frequency f_R and the second is also known as anti-resonance frequency f_A.

Circuits comprising BAW or SAW elements in general are better understood in view of above-introduced element equivalent circuit. The series resonance of the individual resonator element is caused by the equivalent inductance Ls and the equivalent capacitance Cs. At frequencies that are lower than the series resonance frequency, the impedance of the resonator element is capacitive. At frequencies higher than the series resonance frequency of the resonator element, but which are lower than the parallel resonance frequency of the resonator element, caused by the parallel capacitance Cp, the impedance of the resonator element is inductive. Also, at higher frequencies than the parallel resonance frequency impedance of the resonator element is again capacitive. As to the impedance characteristic of the resonator element with respect to signal frequency, at the (series) resonance frequency f_R of the resonator element, the impedance of the resonator element is low, i.e. in an ideal case, where there are no losses in the element, the resonator element functions like a short circuit. At the parallel or anti- resonance frequency f_A, respectively, the impedance of the resonator element is high, i.e. in an ideal case without losses the impedance is infinite and the device resembles an open circuit at the anti-resonance frequency. Therefore, the resonance- and anti-resonance frequencies (f and f_A) are important design parameters in filter design. The resonance and anti-resonance frequencies are determined by process parameters like the thickness of the piezoelectric layer of each resonator element and/or the amount of massloading.

A known circuit topology for filters is the BAW lattice circuit, which circuit topology is also called balanced bridge design. Such a BAW lattice circuit has a stopband when all branches have approximately equal impedance and a passband when one branch type, i.e. the series arm or the lattice arm, respectively, behaves inductive and the other capacitive. Fig. 4 shows the impedance characteristics of two different BAW resonator elements, BAW-1 and BAW-2, usually used in filter design. BAW-1 and BAW-2 are made such, as anti-resonance frequency f _\ of BAW-1 is substantially equal to resonance frequency f_R2 of BAW-2. Thus, it can be seen that with such two types of BAW resonators according to the afore-mentioned circuit topologies BAW resonator filters can be constructed, which have a passband approximately corresponding to the difference Δf between the lowest resonance frequency, here fϊ , and the highest anti-resonance frequency, here f_A2. BAW series and lattice resonator elements may be exchanged provided series or horizontal resonators are of one type and lattice or diagonal resonators are of the other type. The bandwidth, i.e. the passband, of the thus created filter corresponds approximately to the difference between the highest anti-resonance frequency and the lowest resonance frequency of the used resonator elements. BAW lattice circuits have the advantage that there is a deep stopband rejection far away from the passband.

According to preventing power losses, which causes attenuation in the passband, impedance matching between circuits within the signal path is crucial. In case generator and load impedance are equal, impedance matching can be achieved by scaling BAW resonator areas. However, when generator, for instance the antenna of a mobile communication unit, and load impedance, for instance, the impedance of a following low noise amplifier (LNA), are different, impedance matching requires also impedance transformation, e.g. from 50Ω in to a 150 to 200Ω differential-out. Moreover, balanced output is preferred as well; because usually low noise amplifiers (LNA) incorporated after receive filters often require a balanced input signal. Hence, unbalanced-in to balanced-out circuits are generally preferred in the receiving path of communication systems.

Therefore, it is an object of the present invention to provide a circuit arrangement, which provides impedance transformation between impedance levels at an input and output port of the circuit arrangement. It is a further objective to have a circuit arrangement, which is frequency selective by providing a predetermined passband. Moreover, it is a further objective to have the input and output impedances of the filter circuit structure substantially matched with the respective loads. Accordingly, a circuit arrangement for providing impedance transformation between two ports being an input port and an output port has different the impedance levels at the two ports. The port with the high impedance level is a high impedance port and the port with the low impedance level is a low impedance port.

Further the circuit arrangement according to the present invention comprises at least one resonator circuit section constructed of at least two types of resonator elements which are a first resonator element having a first resonance frequency f[ and a first anti- resonance frequency fj_A and a second resonator elements having a second resonance frequency f_2R and a second anti-resonance frequency f_2A, wherein the resonator circuit section is constructed in a lattice circuit configuration and the first resonator elements are arranged as series arm elements and the second resonator elements are arranged as lattice arm elements. Furthermore, a highest resonance frequency f _H of the both first resonance frequency fiR and second resonance frequency f_2R is different from a lowest anti-resonance frequency f_Ai of the both first anti-resonance frequency f_1A and the second anti-resonance frequency f_2A.

As to the resonator circuit section, having the first resonator elements arranged as series arms and having the second resonator elements arranged as lattice arms. It should be noted that series resonator elements and lattice resonator elements of the resonator lattice circuit section may be exchanged, provided that series resonator elements are of one type and lattice resonator elements are of the other type. Advantageously, at least at one side of the resonator lattice circuit balanced signal guidance is provided. Furthermore, there must be reactance elements connected at least to one of the input port and the output port. Such reactance elements according to the invention being connected to the resonator lattice circuit may comprise discrete circuit elements; preferably the elements are discrete circuit elements made with passive integration technologies. As afore-mentioned, at least at one of the input port and output port signal guidance is balanced. It should be emphasized that it goes without saying series reactance elements connected to a port providing balanced signal guidance must be symmetrically constructed with respect to balanced signal guidance. Moreover, in case the circuit arrangement has a port with unbalanced signal guidance, e.g. the input port, there is a connection to a fixed reference potential possible, e.g. ground potential of the circuit, if needed. Balanced output is most preferred, because as already mentioned such circuit arrangement is advantageously connected to, for instance, the balanced input of a low noise amplifier (LNA). In a first embodiment of the present invention the highest resonance frequency f_RH is made higher than the lowest anti-resonance frequency f_AL. Moreover, the reactance elements are inductive elements being a series inductance Ls connected in series to the low impedance port and a parallel inductance Lp connected in parallel to the high impedance port. Advantageously, this circuit arrangement provides impedance transformation between the impedance levels at the input port and the output port of the circuit arrangement. As an additional advantage, there is also a broader passband with respect to the frequency selective characteristics of the circuit arrangement than in a usual configuration of a, for instance, lattice resonator filter circuit where the highest resonance frequency f_RH is made substantially equal to the lowest anti-resonance frequency f_AL. In a second embodiment of the present invention the highest resonance frequency f_RH is made lower than the lowest anti-resonance frequency f_AL. Additionally, the reactance elements are capacitive elements being a parallel capacitance Cp connected in parallel to the high impedance port and a series capacitance Cs connected in series to the low impedance port. Advantageously, this circuit arrangement provides impedance transformation between the impedance levels at the input port and the output port of the circuit arrangement. Also, as an additional advantage, there is also a narrower passband with respect to the frequency selective characteristics of the circuit arrangement than in a usual configuration of a, for instance, lattice resonator filter circuit where the highest resonance frequency f _H is made substantially equal to the lowest anti-resonance frequency f_A . The resonator elements are preferably acoustic resonance elements, more preferably these elements are bulk acoustic wave (BAW) resonators. For example, when BAW resonator elements are used in the invention, a BAW resonator comprises a stack on a substrate with at least one or more acoustic reflective layer, a bottom electrode, a bulk, a top electrode, and an optional massload on top of the top electrode. Thereby, the bulk of the BAW resonator elements comprise a piezoelectric layer having a predetermined thickness and being made of an piezoelectric material such as aluminum nitride (A1N) or zinc oxide (ZnO) and having an optional additional dielectric layer, for instance, silicon oxide (SiO2). The combination of silicon oxide (SiO2) and aluminum nitride (A1N) in the BAW resonators reduces the coupling coefficient of the BAW resonator elements, as required in some applications with respect, for instance, to bandwidth or temperature stability. According to the fabrication process of such BAW resonator elements, advantageously, the thickness of the component layers of the bulk, and/or the massload, and or the electrode layers for each BAW resonator element can be used to arrange the BAW resonator elements to have a predetermined resonance frequency and a predetermined anti-resonance frequency. For so-called thickness modes the frequency of acoustic vibration is approximately inversely proportional to the thickness of the piezoelectric layer. The piezoelectric thickness is therefore of the order of 1 micron, so typically a thin-film semiconductor process is used. In one embodiment, the solidly-mounted bulk acoustic wave resonator (sometimes called SBAR) one or more acoustic layers are employed between the piezoelectric layer and the substrate. An alternative embodiment of thin-film BAW resonator elements (sometimes called FBAR) employs a membrane approach with the metal-piezo- metal sandwich suspended in air. BAW resonators are often employed in bandpass filters having various topologies. One advantage of BAW resonators is the intrinsically better power handling compared to the interdigitated structures used in surface acoustic wave (SAW) resonators, especially at frequencies of modern wireless systems where the pitch of the interdigital structures must be sub-micron.

The above and other objectives, features and advantages of the present invention will become more clear from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings. It is noted that through the drawings same or equivalent parts remain the same reference number. All drawings are intended to illustrate some aspects and embodiments of the present invention. Moreover, it should be noted that in case of different embodiments only the differences are be described in detail. Circuits are depicted in a simplified way for reason of clarity. It goes without saying that not all alternatives and options are shown and therefore, the present invention is not limited to the content of the accompanying drawings. In the following, the present invention will be described in greater detail by way of example with reference to the accompanying drawings, in which

Fig. 1 shows a circuit diagram of a circuit arrangement wherein two BAW lattice circuit sections are connected with a series inductance at the input port and a parallel inductance at the output port;

Fig. 2 is an equivalent element circuit of a resonator element; Fig. 3 depicts the impedance characteristic of the used resonator elements according to the first embodiment illustrated in Fig. 1 ; and

Fig. 4 shows the impedance characteristics of two BAW resonator elements drawn over signal frequency, wherein resonance frequencies, anti-resonance frequencies and center frequency are arranged as usual.

Fig. 1 shows a circuit arrangement 10 according to the present invention, which comprises a first port 1, e.g. being an input port, and a second port 2, e.g. being an output port. The input port 1 has a connection to a fixed reference potential, e.g. ground potential of the circuit, therefore there is unbalanced signal guidance at the input port. There is connected a first load 3 to the first port 1 and a second load 4 towards the second port 2. It is noted that both loads in principal could be equal, however, with respect to the issue of impedance transformation it is assumed that the first load 3 is smaller than the second load 4. Hence, the input port 1 is a low impedance port and the output port 2 is high impedance port. For example, the first load may represent an internal resistance of a generator that is driving a radio frequency signal as input for the circuit arrangement 10; in an application the generator, for instance, may be a receiving antenna of a communication unit. Further, the second load 4 represents the input resistance of a following stage like, for instance, a low noise amplifier (LNA).

As to the better understanding of providing impedance transformation, in the following it will be assumed that the first load causes a 50Ω impedance level and the second load causes a 200Ω differential impedance level. In such case, it is clear for the man skilled in the art that for optimal power transition impedance transformation is a must. In other words: the input impedances of the circuit arrangement has to be matched according to the respective loads 3 and 4, at least within a frequency band that corresponds to the used signal frequencies within such circuit. In the preferred embodiment of the invention as shown in Fig. 1, a main part of the circuit arrangement 10 for providing impedance transformation are two resonator lattice circuits 20a and 20b which are connected to each other in cascade. Since both resonator lattice circuits 20a and 20b are equal constructed, in the following both will be described together in detail. Therefore, for better understanding in the following similar elements of the resonator lattice circuits 20a and 20b are noted at the same time.

Accordingly, the resonator lattice circuit 20a and 20b, respectively, comprises two types of resonator elements wherein a first type comprises resonator elements 22-1, 22-2 and 26-1, 26-2, respectively, and a second type comprises resonator elements 24-1, 24-2 and 28-1, 28-2, respectively. The structure of each resonator lattice circuit 20a and 20b, respectively, is constructed with the respective four resonator elements 22-1, 22-2, 24-1, 24-2 and 26-1, 26-2, 28-1, 28-2, respectively, in the known principle of bridge circuits. Thus, respective two of the respective four resonator elements, i.e. 22-1, 24-2 and 26-1, 26-2, respectively, are connected in series building a first series path, and 24-1, 22-2 and 28-1, 28- 2, respectively, are connected in series building a second series path. The connection nodes between two resonator elements of the respective first and second series path represent respective one output node of the resonator lattice circuit 20a and 20b, respectively. Further, respective first and second series path of the bridge are connected in parallel to the input nodes of the resonator lattice circuit 20a and 20b, respectively. Due to the illustration of the lattice circuits 20a and 20b, respectively, resonator elements 22-1, 22-2 and 26-1, 26-2, respectively, are also called horizontal elements or series elements of the lattice circuit 20a and 20b, respectively, and resonator elements 24-1, 24-2 and 28-1, 28-2, respectively, are also called diagonal elements or lattice elements of the lattice circuit 20a and 20b, respectively. Moreover, according to this naming convention each branch of the lattice circuits 20a and 20b, respectively, is called an arm of the lattice circuit 20a and 20b, respectively, wherein horizontal element builds an horizontal or series arm, respectively, and diagonal element builds a diagonal or lattice arm, respectively. According to this preferred embodiment it will be assumed that resonator elements 22-1, 22-2, 24-1, 24-2 and 26-1, 26-2, 28-1, 28-2, respectively, are BAW resonator elements. It should be noted that even the resonator elements 22-1, 22-2, 26-1, 26-2 have same resonance frequency and resonator elements 24-1, 24-2, 28-1, 28-2 have same anti-resonance frequency, these resonator elements can differ in area on the substrate of the device.

Accordingly, in the illustrated embodiment of the present invention according to Fig. 1, the used two types of resonator elements, i.e. resonator elements 22-1, 22-2 and 26- 1, 26-2, respectively, are of the first type and resonator elements 24-1, 24-2 and 28-1, 28-2, respectively, are of the second type, are adjusted such that the highest resonance frequency fRH is made higher than the lowest anti-resonance frequency fAL- This is also shown in Fig. 3, where the impedance characteristic of the tow types of used resonator elements is drawn over the signal frequency f. In this embodiment, the first type of resonator elements 22-1, 22-2 and 26-1, 26-2, respectively, has a resonance frequency f_R] and an anti-resonance frequency f_Aj. Thus, the second type of resonator elements 24-1, 24-2 and 28-1, 28-2, respectively, has a resonance frequency f_R2 and an anti-resonance frequency f_A2. It now can easily be seen from Fig. 3 that f_A1 is made smaller than f_R2 that also causes the frequency band where the resonator lattice circuit 20a and 20b, respectively, will have a passband to be broader. This passband Δ f approximately corresponds to the difference of the highest anti-resonance frequency, which is here f_A2, and the lowest resonance frequency, which is here fki.

At the input port 1 of the circuit arrangement 10, there are reactance sections 30a and 30b, which are arranged for providing impedance transformation and matching simultaneously together with the resonator lattice circuits 20a and 20b at the input port 1 and the output port 2 of the circuit arrangement 10. In this example, a parallel inductance 32 is connected in parallel to the high impedance port with the high impedance level being the output port 2 of the circuit arrangement 10 and a series inductance 31 is connected in series to the low impedance port with the low impedance level being the input port 1. Advantageously, the circuit arrangement 10 provides impedance transformation between the impedance levels at the input port 1 and the output port 2 of the circuit arrangement 10. As an additional advantage, there is also a broader passband with respect to the frequency selective characteristics of the circuit arrangement 10 than in a usual configuration of a, for instance, lattice resonator filter circuit where the highest resonance frequency f_RH is made substantially equal to the lowest anti-resonance frequency f_AL.

The above presented invention has introduced a circuit arrangement (10) which is applicable for communication devices for instance handheld GPS or personal communication units. Accordingly, such a circuit arrangement (10) comprising a certain combination of at least one resonator lattice circuit (20a, 20b) with resonator elements (22-1, 22-2, 24-1, 24-2, 26-1, 26-2, 28-1, 28-2) that are preferably BAW resonator elements. This at least one resonator lattice circuit (20a, 20b) is combined with at least one reactance circuit element (31, 32) which can be an inductance and capacitance element. The circuit arrangement (10) provides impedance transformation between different impedance levels at its input port (1) and its output port (2). Moreover, the circuit arrangement (10) according to the invention provides a frequency selective behavior which can be made according to a first embodiment being a broad band or according to a second embodiment being a narrow band with respect to the application needs. Thus, the invention is applicable in modern communication units since communication standards like UMTS can be achieved with a small amount of circuit elements. Moreover, according to the signal guiding, implementation in unbalanced-in to balanced-out applications is possible.

It should be noted that the present invention is not restricted to the embodiments of the present invention; in particular the invention is not restricted to a circuit which has been used in this specification for reason of example. Moreover, the principle of the present invention can be applied to any application that needs in a high frequency environment a circuit that provides impedance transformation and frequency selective characteristics.

Claims

CLAIMS:

1. A circuit arrangement (10) for providing impedance transformation between two ports being an input port (1) and an output port (2), comprising:

- at least one resonator circuit section (20a, 20b) constructed of at least two types of resonator elements which are a first resonator element (22-1, 22-2, 26-1, 26-2) having a first resonance frequency fi and a first anti-resonance frequency f]_A and a second resonator element (24-1, 24-3, 28-1, 28-2) having a second resonance frequency f_2R and a second anti-resonance frequency f_2A, wherein said resonator circuit section (20a, 20b) is constructed in a lattice circuit configuration and said first resonator elements (22-1, 22-2, 26-

1. 26-2) are arranged as series arm elements and said second resonator elements (24-1, 24-3, 28-1, 28-2) are arranged as lattice arm elements; and

- reactance means (30a, 30b) connected at least to one of said input port (1) and said output port (2);

- wherein a highest resonance frequency f_RH of said both first resonance frequency fi_R and second resonance frequency f_2R is different from a lowest anti-resonance frequency f_Aι of said both first anti-resonance frequency f_1A and said second anti-resonance frequency f_2A.

2. The circuit arrangement according to claim 1, wherein the impedance level at the input port (1) is different to the impedance level at the output port (2) and a port with the higher impedance level is a high impedance port and a port with the lower impedance level is a low impedance port.

3. The circuit arrangement according to claim 1 or 2, wherein said highest resonance frequency f_RH is higher than said lowest anti-resonance frequency f_AL-

4. The circuit arrangement according to claim 3, wherein said reactance means

(30a, 30b) are inductive elements being a series inductance Ls (31) connected in series to said low impedance port and a parallel inductance L (32) connected in parallel to said high impedance port.

5. The circuit arrangement according to claim 1 or 2, wherein said highest resonance frequency f_RH is lower than said lowest anti-resonance frequency f_AL-

6. The circuit arrangement according to claim 5, wherein said reactance means are capacitive elements being a parallel capacitance Cp connected in parallel to said high impedance port and a series capacitance Cs connected in series to said low impedance port.

7. The circuit arrangement according to one of the preceding claims, wherein said resonator elements (22-1, 22-2, 24-1, 24-2, 26-1, 26-2, 28-1, 28-2) are acoustic resonance elements, preferably bulk acoustic wave (BAW) resonators.

8. The circuit arrangement according to claim 7, wherein said BAW resonator elements comprise a stack on a substrate with at least one or more acoustic reflective layer, a bottom electrode, a bulk, a top electrode, and an optional massload at one or both of said bottom and top electrodes.

9. The circuit arrangement according to claim 7 or 8, wherein said bulk of said BAW resonator elements comprise a piezoelectric layer having a predetermined thickness and being made of an piezoelectric material such as aluminum nitride (AIN) or zinc oxide (ZnO) and having an optional additional layer of any non-piezoelectric dielectric material, preferably silicon oxide (SiO2).

10. The circuit arrangement according to claim 8 or 9, wherein said thickness of said bulk and/or said massload at said top electrode, and/or said bottom electrode and/or said top electrode of each type of said BAW resonator elements is arranged for providing a predetermined resonance frequency and a predetermined anti-resonance frequency.

11. The circuit arrangement according to one of the preceding claims, wherein at least one of said input port (1) and output port (2) signal guidance is balanced.

12. The circuit arrangement according claim 11, wherein series reactance means connected to a port providing balanced signal guidance are symmetrically constructed with respect to balanced signal guidance.

13. The circuit arrangement according to one of the preceding claims, wherein a port with unbalanced signal guidance has a connection to a fixed reference potential.