RF filter circuit, RF filter with improved attenuation and duplexer with improved isolation

1. An RF filter circuit, comprising:

an input port with an input terminal;

an output port with an output terminal;

a signal path between the input port and the output port, the signal path having a first series resonator;

a ground terminal;

a first coupling conductor segment that is a part of the signal path, the first coupling conductor segment being directly connected to the input terminal; and

a second coupling conductor segment directly connected to the output terminal or to the ground terminal, wherein the first coupling conductor segment and the second coupling conductor segment are arranged next to each other and are electromagnetically coupled such that the coupling enhances attenuation of the RF filter circuit in a rejection frequency band,

wherein the first coupling conductor segment and the second coupling conductor segment are aligned in parallel,

wherein the first coupling conductor segment and the second coupling conductor segment are dedicated to conduct RF signals in opposite directions,

wherein the first coupling conductor segment and the second coupling conductor segment establish a segment of a coupled differential signal line, and

wherein the first coupling conductor segment and the second coupling conductor segment are dedicated to carry currents that have opposite directions and are out of phase.

2. The RF filter circuit of claim 1 , wherein the second coupling conductor segment is directly connected to the output terminal.

3. The RF filter circuit of claim 1 , wherein the second coupling conductor segment is directly connected to the ground terminal.

4. The RF filter circuit of claim 1 , wherein the RF filter circuit comprises a ladder type structure and comprises:

a second series resonator electrically connected in the signal path in series to the first series resonator;

a first parallel branch electrically connecting the signal path to ground and comprising a first parallel resonator; and

second parallel branch electrically connecting the signal path to ground and comprising a second parallel resonator.

5. The RF filter circuit of claim 1 , wherein the attenuation is enhanced by cancellation of unwanted RF signals in the first and second coupling conductor segments.

6. The RF filter circuit of claim 1 , wherein the first series resonator is dedicated to work with acoustic waves.

7. The RF filter circuit of claim 1 , wherein the first series resonator comprises resonator components arranged on a chip.

8. The RF filter circuit of claim 7 , wherein the chip is arranged on a carrier substrate.

9. The RF filter circuit of claim 8 , wherein the first coupling conductor segment and the second coupling conductor segment are arranged on the chip.

10. The RF filter circuit of claim 8 , wherein the first coupling conductor segment and the second coupling conductor segment are arranged on the carrier substrate.

11. The RF filter circuit of claim 1 , wherein the electromagnetic coupling comprises a magnetic coupling as a main constituent.

12. An RF filter comprising the RF filter circuit of claim 1 .

13. A duplexer comprising the RF filter of claim 12 , wherein the rejection frequency band is a reception band and where the enhanced attenuation of the RF filter circuit in the reception band enhances isolation of the duplexer.

14. The duplexer of claim 13 , further comprising:

a transmission filter;

an impedance matching circuit; and

a reception filter,

wherein the first series resonator is an element of the transmission filter, and

wherein the impedance matching circuit is connected between the transmission filter and the reception filter.

15. The duplexer of claim 14 , wherein the reception filter comprises a double mode surface acoustic wave (DMS) structure.