Impedance adjustment circuit for adjusting planar antennas

1. An impedance matching circuit for matching planar antennas, the circuit comprising:

a signal path with a signal path input and a signal path output;

a first capacitive element with variable capacitance connected between the signal path input and signal path output;

a second capacitive element with variable capacitance connected between the signal path and ground;

an additional capacitive element connected in parallel with one of the first and second capacitive elements, wherein a parallel combination of the first, second and additional capacitive elements has a higher Q factor than the one of the first and second capacitive elements without the additional capacitive element;

a first inductive element connected between the signal path input and ground, a second inductive element connected between the signal path output and ground; and an antenna line with an impedance between 30 and 60 ohm connected to the signal path output.

2. The impedance matching circuit as in claim 1 , where the second capacitive element is connected between the signal path output and ground.

3. The impedance matching circuit as in claim 1 , where the second capacitive element is connected between the signal path input and ground.

4. The impedance matching circuit as in claim 1 , further comprising a resistive element with voltage-dependent resistance connected between the signal path output and ground.

5. The impedance matching circuit as in claim 1 ,

wherein the first and second inductive elements have Q factors greater than or equal to 15 and inductances between 1 and 30 nH,

wherein the first capacitive element has a Q factor greater than 10 and a capacitance that is adjustable in an interval that lies between 0.5 and 13 pF, and

wherein the second capacitive element has a Q factor greater than or equal to 10 and a capacitance that is adjustable in an interval that lies between 0.5 and 8.0 pF.

6. The impedance matching circuit as in claim 1 for use in a mobile communications device with a sending path, a receiving path and a planar antenna of a PIFA type,

wherein the signal path input is connected to the sending path and the receiving path and

wherein the antenna line is connected between the signal path output and the planar antenna.

7. The impedance matching circuit as in claim 6 , which matches the sending path in a sending frequency band and the receiving path in a receiving frequency band to the planar antenna with a standing wave ratio in the sending path that is less than 3:1 and with a standing wave ratio in the receiving path that is less than 4:1.

8. The impedance matching circuit as in claim 1 for use in W-CDMA frequency bands between 500 and 4500 MHz.

9. The impedance matching circuit as in claim 1 , wherein one of the first and second variable capacitance capacitive elements has a tuning range of at least 2.5:1.

10. The impedance matching circuit as in claim 1 , wherein one of the first and second capacitive elements with variable capacitance: comprises an element selected from the group consisting of: an adjustable capacitor with a barium strontium titanate dielectric layer, a capacitive element made in CMOS technology, a circuit of MEMS capacitors, a semiconductor varactor diode, a capacitive element made in NMOS technology, a 5-bit array, and a gallium arsenide capacitive element.

11. The impedance matching circuit as in claim 1 , wherein both the first and second capacitive elements with variable capacitance comprise an element selected from the group consisting of: an adjustable capacitor with a barium strontium titanate dielectric layer, a capacitive element made in CMOS technology, a circuit of MEMS capacitors, a semiconductor varactor diode, a capacitive element made in NMOS technology, a 5-bit array, and a gallium arsenide capacitive element.

12. The impedance matching circuit as in claim 1 , wherein the first and second inductive elements are designed as metallizations in a multilayer substrate, chosen from the substrate comprising HTCC, LTCC, FR4, or laminate.

13. The impedance matching circuit as in claim 1 , wherein the first and second capacitive elements with variable capacitance are designed in the same way and are arranged in a same housing.

14. The impedance matching circuit as in claim 1 , further comprising control lines to vary a capacitance that are jointly used by the two capacitive elements with variable capacitance.

15. The impedance matching circuit as in claim 1 , wherein the impedance matching circuit operates in a frequency range selected from the group consisting of 0.5 GHz.ltoreq.f.ltoreq.1.7 GHz, 1.7 GHz.ltoreq.f.ltoreq.2.5 GHz, and f.gtoreq.2.5 GHz.

16. The impedance matching circuit as in claim 1 , further comprising with a directional coupler connected in the signal path.

17. A multiband matching circuit that comprises two impedance matching circuits, each according to claim 1 , and a dual band directional coupler, wherein

the two impedance matching circuits are configured for use in different frequency bands, and

the dual band directional coupler is connected to signal paths of both impedance matching circuits.

18. A multiband matching circuit that comprises two or more impedance matching circuits, each according to claim 1 , and a directional coupler assigned to each impedance matching circuit, wherein

the two or more impedance matching circuits are configured for use in different frequency bands, and

the assigned directional couplers are each connected to the signal path of the impedance matching circuit.

19. A multiband matching circuit that comprises a first, a second, and a third impedance matching circuit, each according to claim 1 , and a dual band directional coupler, wherein

signal paths of the second and third impedance matching circuits are connected to the same dual band directional coupler.