Electrical filter structure

1. An electrical filter for forwarding an electrical signal from a first port to a second port in a frequency-selective manner, the filter comprising:

the first port comprising a first characteristic port impedance;

the second port comprising a second characteristic port impedance;

a filter core comprising a working impedance, wherein the working impedance is different from the first characteristic port impedance and from the second characteristic port impedance, wherein the filter core comprises a first filter core port, a second filter core port, a plurality of series impedance elements electrically coupled between the first filter core port and the second filter core port, and a plurality of shunt impedance elements, wherein the shunt impedance elements of the filter core are implemented using transmission lines electrically coupled to nodes which are electrically coupled between subsequent series impedance elements of the first filter core, wherein the shunt impedance elements of the first filter core are arranged in different conducting layers of a multi-layer structure, and wherein a transmission line used to implement a first shunt impedance element does not overlap a transmission line used to implement a second impedance element in a projection perpendicular to a main surface of the multi-layer structure;

a first matching circuit electrically coupled between the first port and the filter core, wherein the first matching circuit is configured to perform an impedance matching between the first characteristic port impedance and a characteristic impedance presented by the filter core to the first matching circuit; and

a second matching circuit electrically coupled between the second port and the filter core, wherein the second matching circuit is configured to perform an impedance matching between the second characteristic port impedance and a characteristic impedance presented by the filter core to the second matching circuit.

2. The filter according to claim 1 , wherein the working impedance of the filter core is smaller than the first characteristic port impedance; and

wherein the working impedance of the filter core is smaller than the second characteristic port impedance.

3. The filter according to claim 1 , wherein the filter core comprises a plurality of inductances implemented using transmission lines comprising transmission line impedances higher than the working impedance.

4. The filter according to claim 1 , wherein the filter core comprises a plurality of capacitances implemented using transmission lines comprising transmission line impedances smaller than the working impedance.

5. The filter according to claim 1 , wherein the first characteristic port impedance is equal to the second characteristic port impedance; and

wherein the characteristic impedance presented by the filter core to the first matching circuit is analogous to the characteristic impedance presented by the filter core to the second matching circuit.

6. The filter according to claim 1 , wherein the first matching circuit or the second matching circuit is a L-network comprising a shunt impedance element and a series impedance element.

7. The filter according to claim 6 , wherein the first matching circuit comprises a shunt capacitance and a series inductance, wherein the series inductance of the first matching circuit is merged with a front inductance of the filter core.

8. The filter according to claim 6 , wherein the first matching circuit comprises a shunt capacitance and a series inductance, wherein the second matching circuit comprises a shunt capacitance and a series inductance, wherein the series inductance of the second matching circuit is merged with a tail inductance of the filter core.

9. The filter according to claim 1 , wherein the first matching circuit or the second matching circuit is a II network comprising at least two shunt impedance elements and a series impedance element electrically coupled between the shunt impedance elements.

10. The filter according to claim 1 , wherein the first matching circuit or the second matching circuit is a T network comprising at least two series impedance elements and a shunt impedance element electrically coupled with a node which is electrically coupled between the two series impedance elements.

11. The filter according to claim 1 , wherein the first matching circuit or the second matching circuit comprises a plurality of cascaded L-networks.

12. The filter according to claim 1 , wherein the filter core is configured to form a low pass filter, and

wherein the filter core comprises a plurality of series inductances electrically coupled in series between a first end of the filter core, which is electrically coupled to the first matching circuit, and a second end of the filter core, which is electrically coupled to the second matching circuit,

wherein the series inductances of the filter core are implemented using transmission lines comprising a transmission line impedance which is higher than the working impedance.

13. The filter according to claim 12 , wherein the series inductances of the filter core are implemented using strip lines comprising a conductive strip arranged between two ground layers.

14. A double filter, comprising:

a first filter comprising a first filter core;

a second filter comprising a second filter core;

wherein the first filter core comprises a first filter core port, a second filter core port, a plurality of series impedance elements electrically coupled between the first filter core port and the second filter core port, and a plurality of shunt impedance elements, wherein the shunt impedance elements of the first filter core are implemented using transmission lines electrically coupled to nodes which are electrically coupled between subsequent series impedance elements of the first filter core, wherein the shunt impedance elements of the first filter core are arranged in different conducting layers of a multi-layer structure; and

wherein the second filter core comprises a third filter core port, a fourth filter core port, a plurality of series impedance elements electrically coupled between the third filter core port and the fourth filter core port, and a plurality of shunt impedance elements, wherein the shunt impedance elements of the second filter core are implemented using transmission lines electrically coupled to nodes which are electrically coupled between subsequent series impedance elements of the second filter core, wherein the shunt impedance elements of the second filter core are arranged in different conducting layers of the multi-layer structure, and wherein a transmission line used to implement a first shunt impedance element of the first filter core and a transmission line used to implement a second shunt impedance element of the second filter core are arranged in a common conducting layer of the multi-layer structure.

15. The double filter according to claim 14 , wherein a transmission line used to implement a second shunt impedance element of the first filter core and a transmission line used to implement a first shunt impedance element of the second filter core are arranged in a common conducting layer of the multi-layer structure.

16. The double filter according to claim 14 , wherein the transmission line used to implement the first shunt impedance element of the first filter core and the transmission line used to implement the first shunt impedance element of the second filter core are at least partly overlapping in a projection perpendicular to a main surface of the multi-layer structure, with a conductive shielding disposed in between.

17. The double filter according to claim 15 , wherein the transmission line used to implement the second shunt impedance element of the second filter core and the transmission line used to implement the second shunt impedance element of the first filter core are at least partly overlapping in a projection perpendicular to the main surface of the multi-layer structure, with a conductive shielding disposed in between.

18. The double filter according to claim 14 , wherein the shunt impedance elements of the first filter core are arranged in different conducting layers of the multi-layer structure, such that a via inductance is arranged in series with at least one of the shunt impedance elements of the first filter core; and

wherein the shunt impedance elements of the second filter core are arranged in different conducting layers of the multi-layer structure, such that a via inductance is arranged in series with at least one of the shunt impedance elements of the second filter core.