Tunable RF filter circuit

Abstract

A tunable RF filter circuit (AHF) is specified which enables good electrical properties, good tunability and simple driving despite low complexity. In this case, the filter circuit comprises a first and a second signal route (SW1, SW2) in a signal path (E, A). At least three resonant circuits (RK1, RK2, RK3) are arranged one after another in the second signal route and interconnect the second signal route with ground. The resonant circuits are electrically and/or magnetically coupled (K) and each comprise a tunable impedance element. The second signal route contains an impedance element (IMP).

Claims

1. A tunable RF filter circuit, comprising: an input, an output and therebetween a signal path having a first signal route and a second signal route in parallel with the first signal route; an impedance element interconnected in the first signal route, the impedance element having a first terminal directly connected to a first node and a second terminal directly connected to a second node; and a plurality of resonant circuits, each of the plurality of resonant circuits interconnecting the second signal route with electric ground, wherein: the plurality of resonant circuits comprise N resonant circuits, N being greater than 3; each of the plurality of resonant circuits is directly connected to a different node of the second signal route, the different nodes being between first and second terminals of the second signal route, the first terminal of the second signal route being directly connected to the first node and the second terminal of the second signal route being directly connected to the second node; each of the plurality of resonant circuits is at least one of electrically or magnetically coupled to another resonant circuit of the plurality of resonant circuits and each of the plurality of resonant circuits comprises a tunable impedance element; and the plurality of resonant circuits have a higher quality factor Q than coupling elements which couple the plurality of resonant circuits together.

2. The tunable RF filter circuit of claim 1, wherein each of the plurality of resonant circuits comprises an inductive element magnetically coupled to the inductive element of another one of the plurality of resonant circuits.

3. The tunable RF filter circuit according to claim 1, wherein the tunable impedance element of each of the plurality of resonant circuits comprises a tunable capacitive element.

4. The tunable RF filter circuit according to claim 3, wherein each of the tunable capacitive elements have a quality factor Q>100.

5. The tunable RF filter circuit according to claim 3, wherein a ratio of capacitance values of each of the tunable capacitive elements in the plurality of resonant circuits is constant.

6. The tunable RF filter circuit according to claim 1, wherein each of the plurality of resonant circuits comprises an oscillatory circuit section selected from: an LC resonant circuit, a ceramic resonator, an MEMS resonator, an acoustic resonator, a resonator with a wave guiding arrangement integrated in a substrate, and a cavity resonator.

7. The tunable RF filter circuit according to claim 1, wherein: four of the plurality of resonant circuits are arranged one after another in the second signal route; the impedance element in the first signal route is an inductive element; and the signal path comprises a respective capacitive element on an input side and on an output side of the signal path.

8. The tunable RF filter circuit according to claim 1, wherein the signal path comprises a respective tunable capacitive element on at least one of an input side or an output side of the signal path.

9. The tunable RF filter circuit according to claim 1, further comprising a control logic which is interconnected with the respective tunable impedance elements of the plurality of resonant circuits by means of control lines and is configured to control impedance values of the corresponding tunable impedance elements.

10. A method for driving the tunable RF filter according to claim 1, wherein: the tunable RF filter further comprises control logic that controls an impedance value of each of the tunable impedance elements; and the control logic maintains a constant ratio of the impedance values of the tunable impedance elements.

11. The tunable RF filter circuit according to claim 1, wherein a transfer curve of the tunable RF filter circuit includes poles.

12. At least one of a transmitting filter or a receiving filter of a communication device comprising the tunable RF filter circuit according to claim 1.

13. The tunable RF filter circuit according to claim 1, wherein: the impedance element in the first signal route has a quality factor Q<100; each of the plurality of resonant circuits arranged in the second signal route has a quality factor Q>100; and the coupling elements have a quality factor Q<200.

14. The tunable RF filter circuit according to claim 2, wherein each of the plurality of resonant circuits comprises a tunable capacitive element.

15. The tunable RF filter circuit according to claim 14, wherein a ratio of capacitance values of the tunable capacitive elements in the plurality of resonant circuits is constant.

16. A tunable RF filter circuit, comprising: an input, an output and therebetween a signal path having a first signal route and a second signal route in parallel with the first signal route; an impedance element interconnected in the first signal route, the impedance element having a first terminal directly connected to a first node and a second terminal directly connected to a second node; and a plurality of resonant circuits, each of the plurality of resonant circuits interconnecting the second signal route with electric ground, wherein: the plurality of resonant circuits comprise N resonant circuits, N being greater than three; each of the plurality of resonant circuits is directly connected to a different node of the second signal route, the different nodes being between first and second terminals of the second signal route, the first terminal of the second signal route being directly connected to the first node and the second terminal of the second signal route being directly connected to the second node; each of the plurality of resonant circuits is at least one of electrically or magnetically coupled to another resonant circuit of the plurality of resonant circuits and each of the plurality of resonant circuits comprises a tunable impedance element; the tunable impedance elements have a quality factor Q>100; and each resonant circuit of the plurality of resonant circuits has a higher quality factor Q than a coupling element which couples the resonant circuit to another resonant circuit.

17. The tunable RF filter circuit according to claim 16, wherein each of the plurality of resonant circuits comprises an oscillatory circuit section selected from: an LC resonant circuit, a ceramic resonator, an MEMS resonator, an acoustic resonator, a resonator with a wave guiding arrangement integrated in a substrate, and a cavity resonator.

18. A tunable RF filter circuit, comprising: an input, an output and therebetween a signal path having a first signal route and a second signal route in parallel with the first signal route; an impedance element interconnected in the first signal route, the impedance element having a first terminal directly connected to a first node and a second terminal directly connected to a second node; and a plurality of resonant circuits, each of the plurality of resonant circuits interconnecting the second signal route with electric ground, wherein: the plurality of resonant circuits comprise N resonant circuits, N being greater than 3; each of the plurality of resonant circuits is directly connected to a different node of the second signal route, the different nodes being between first and second terminals of the second signal route, the first terminal of the second signal route being directly connected to the first node and the second terminal of the second signal route being directly connected to the second node; each of the plurality of resonant circuits is at least one of electrically or magnetically coupled to another resonant circuit of the plurality of resonant circuits and each of the plurality of resonant circuits comprises a tunable impedance element; and the tunable impedance element of each of the plurality of resonant circuits comprises a tunable capacitive element, each of the tunable capacitive elements having a quality factor Q>100.

Description

(1) In the figures:

(2) FIG. 1: shows an equivalent circuit diagram of a tunable RF filter circuit,

(3) FIG. 2: shows the equivalent circuit diagram of a filter circuit comprising additional impedance elements,

(4) FIG. 3: shows the equivalent circuit diagram of a filter circuit comprising four resonant circuits,

(5) FIG. 4: shows the equivalent circuit diagram of a filter circuit comprising four resonant circuits and further capacitive elements,

(6) FIG. 5: shows a possible realization of the resonant circuits as LC resonant circuit,

(7) FIG. 6: shows inductively coupled resonant circuits,

(8) FIG. 7: shows resonant circuits having acoustic resonators,

(9) FIG. 8: shows one possible form of the circuit in which the input and/or output impedance is adjustable,

(10) FIG. 9: shows calculated frequency dependencies of the matrix elements S.sub.1,1 and S.sub.2,1,

(11) FIG. 10: shows a cross section through a package for and comprising a tunable filter.

(12) FIG. 1 shows an equivalent circuit diagram of the tunable RF filter circuit AHF, in which a signal path SP is arranged between an input E and an output A. In this case, the signal path SP comprises two parallel-connected partial sections, namely the first signal route SW1 and the second signal route SW2. An impedance element IMP is interconnected in the first signal route SW1. The impedance element IMP can be realized as a capacitive element or as an inductive element. The three resonant circuits RK1, RK2, RK3 are arranged one after another in the second signal route SW2. The resonant circuits are electrically or magnetically coupled and each comprise at least one tunable impedance element. Each of the three resonant circuits interconnects the second signal route with ground.

(13) In this case, the first resonant circuit RK1 is coupled to the input E. In this case, the third resonant circuit RK3 is coupled to the output A. Those resonant circuits which are coupled to the input E or to the output A directly rather than via another resonant circuit constitute the so-called outer resonant circuits. These two outer resonant circuits thus enclose the other resonant circuit(s), which thus constitute inner resonant circuits.

(14) In the equivalent circuit diagram in FIG. 1, therefore, the first resonant circuit RK1 and the third resonant circuit RK3 constitute the outer resonant circuits, while the second resonant circuit RK2 constitutes the (sole) inner resonant circuit.

(15) The electrical and/or magnetic coupling of the resonant circuits is symbolized by the coupling designated by K. In this case, the first resonant circuit RK1 is electrically and/or magnetically coupled to the second resonant circuit RK2. The second resonant circuit RK2 is also coupled to the third resonant circuit RK3 besides the first resonant circuit RK1.

(16) Via the coupling of the resonant circuits, an electrical signal can be forwarded from resonant circuit to resonant circuit, such that an RF signal can propagate in the second signal route SW2 as well.

(17) FIG. 2 shows an equivalent circuit diagram of the tunable RF filter circuit in which an impedance element IMP is arranged between the input E and the first resonant circuit RK1. In this case, the impedance element is realized as a capacitive element KE. An inductive element at this point is likewise possible, however.

(18) An impedance element IMP likewise realized as a capacitive element is arranged between the third (i.e. the second outer) resonant circuit RK3 and the output A. This capacitive element, too, can be replaced by an inductive element.

(19) FIG. 3 shows the equivalent circuit diagram of the RF filter circuit in which four resonant circuits are present in the second signal route SW2. A fourth resonant circuit RK4 is thus arranged between the third resonant circuit RK3 and the output A. Therefore, the resonant circuits RK1, RK4 form the outer resonant circuits, while the resonant circuits RK2, RK3 form the inner resonant circuits. The couplings between the resonant circuits have the strengths K1 and K2, e.g. as a result of a symmetrical construction.

(20) Furthermore, it is possible for the second signal route SW2 to comprise 5, 6, 7, 8, 9 or 10 resonant circuits that are correspondingly arranged in series between the input E and the output A.

(21) FIG. 4 shows the equivalent circuit diagram of the tunable RF filter circuit in which four resonant circuits RK4 are arranged in the second signal route and in which a capacitive element KE is arranged between the input E and the first resonant circuit RK1.

(22) Furthermore, a further capacitive element KE is arranged between the last outer resonant circuit RK4 and the output A.

(23) FIG. 5 shows an equivalent circuit diagram of the tunable RF filter circuit in which the resonant circuits are realized as LC circuits. Each resonant circuit, shown here on the basis of the example of the first resonant circuit RK1comprises a parallel connection of an inductive element IE and a tunable capacitive element AKE. The tunable capacitive element AKE in this case constitutes the tunable impedance element of the corresponding resonant circuit. Conversely, each resonant circuit could also comprise a tunable inductive element. The corresponding parallel-connected impedance element of the resonant circuit would then be a capacitive element.

(24) The tunable capacitive element AKE is interconnected with a control logic STL. The control logic STL comprises circuit elements that can be used to receive a control signal of an external circuit environment. The control signal of the external circuit environment is interpreted and control signals are output to the individual tunable capacitive elements AKE via corresponding signal lines SL.

(25) The electromagnetic coupling between the resonant circuits is realized by a capacitive coupling of capacitive elements KE as coupling elements KO. For this purpose, each resonant circuit essentially comprises an electrode of a capacitive element KE via which it is coupled to the adjacent resonant circuit or the adjacent resonant circuits. In this case, a coupling via capacitive elements KE essentially constitutes a capacitive electrical coupling. In this case, the quality factor Q of said capacitive elements is permitted to be lower than the quality factor Q of the elements used in the resonant circuits.

(26) FIG. 6 shows the equivalent circuit diagram of the tunable RF filter circuit in which the coupling between the resonant circuits RK is effected inductively. In this case, each resonant circuit has at least one inductive element IE via which a coupling to another inductive element of the corresponding resonant circuit is effected. Since the first resonant circuit RK1 is only inductively coupled to the second resonant circuit RK2, the first resonant circuit RK1 needs only one inductive element IE1 for coupling. The second resonant circuit RK2 is inductively coupled both to the first resonant circuit RK1 and to the third resonant circuit and therefore requires two inductive elements.

(27) Whether the resonant circuits are coupled inductively or capacitively is unimportant for the fact that RF signals can be transmitted, such that the series arrangement of resonant circuits constitutes the second signal route SW2.

(28) The capacitive elements for coupling between the resonant circuits in FIG. 5 and the inductive elements for coupling the resonant circuits in FIG. 6 are in this case arranged and configured such that the correct degree of coupling is obtained. In this case, the degree of coupling can be set by the distance between the electrodes or the electrode area or the coil shape, coil size and coil distance.

(29) In each case two inductively coupled inductive elements of adjacent resonant circuits here essentially form a transformer circuit.

(30) FIG. 7 shows an equivalent circuit diagram of the tunable RF filter circuit in which the resonant circuits comprise an acoustic or ceramic resonator AR besides a tunable capacitive element AKE. Acoustic or ceramic resonators are distinguished by high quality factors and at the same time by small dimensions. However, since they cause comparatively high production costs and require measures for decoupling and for protection against interfering ambient conditions on account of their mechanical mode of operation, the use of LC components may be preferred.

(31) FIG. 8 illustrates the possibility for impedance matching on the basis of the example of the input impedance. By varying the capacitances of the capacitive elements AKE of the first resonant circuit RK1 and of the capacitive element AKE interconnected at the input E, the filter circuit is able to adjust the input impedance of the circuit. On the output side, too, it is possible to use corresponding tunable impedance elements, e.g. capacitive elements, in series at the output A or with respect to ground in the last resonant circuit for adjusting the output impedance. The regulation can likewise be effected by means of the control logic STL. A capacitance of the capacitive element at the input E with a magnitude of 5 pF and a capacitance of the capacitive element in the first resonant circuit RK1 with a magnitude of 34.34 pF enable an input impedance of 5, for example, such that for instance an impedance matching to an amplifier circuit can be obtained. A capacitance of the capacitive element at the input E with a magnitude of 18 pF and a capacitance of the capacitive element in the first resonant circuit RK1 with a magnitude of 38.81 pF enable an input impedance of 50, for example, such that for instance standard impedance of this magnitude can be obtained.

(32) The profile of the transfer function remains substantially unchanged as a result of the adjustment of the input or output impedances.

(33) FIG. 9 shows calculated frequency dependencies of the insertion loss |S.sub.2,1| and the reflection |S.sub.1,1|. In the insertion loss, a passband with steep edges in the transition region is formed. The insertion loss is low within the passband. Outside the passband, the degree of reflection is so great that virtually no RF power can pass through the filter circuit.

(34) Two poles exist outside the passband.

(35) Four poles exist within the passband, and can be attributed to four resonant circuits.

(36) FIG. 10 shows a simple exemplary embodiment of a package in which the RF filter circuit with its components can be integrated. The package is constructed on a substrate S, which is a mono- or multilayer substrate and has at least one wiring plane VE1. The wiring plane VE1 can be formed on the surface of the substrate S or, as illustrated in FIG. 1 between two insulating layers of a multilayer substrate. On the substrate, at least one semiconductor component HLB is mounted and electrically connected to the first wiring plane VE1. Besides the semiconductor component HLB, further discrete, integrated or other components (not illustrated in the figure) can be arranged on the substrate S in the first component position KL1. The semiconductor component HLB comprises at least high-quality-factor tunable passive components.

(37) A control unit can be integrated in the semiconductor component HLB. The control unit can also be realized as a further separate semiconductor component and be arranged in the first component position KL1.

(38) The components of the first component position KL1 are covered with a dielectric layer DS or embedded into a dielectric layer DS, which terminates toward the top with an approximately planar surface. A second component position KL2 is provided above the dielectric layer DS with the first component position KL1 embedded or arranged underneath in a covered manner. Discrete passive components DP having a high quality factor are arranged in said second component position. The discrete passive components DPB having a high quality factor are electrically interconnected with the components of the first component position KL1. This can be effected directly via plated-through holes DK from the components of the second component position KL to the contacts of the semiconductor components HLB in the first component position KL1. However, it is also possible, as illustrated in the figure, to provide a second wiring plane VE2 between the first and second component positions KE1, KE2. The line sections of the second wiring plane VE2 are electrically connected to the corresponding contacts of the discrete passive components DPB and likewise to contacts of the semiconductor components by means of plated-through holes DK. The second wiring plane VE2 can be embedded between two layers of a dielectric.

(39) External contacts AK are provided at the underside of the substrate S, said external contacts being connected via plated-through holes DK either directly to the components of the first component position KL1 or, as illustrated in the figure, to the first wiring plane SE1. FIG. 1 does not illustrate further passivations or protective coverings which seal the components of the package P against environmental influences. Such a passivation may be for example at least one layer deposited or applied directly onto the surface of the discrete passive components, in particular a thin-film layer. The passivation can also comprise a layer sequence of deposited or applied layers. By way of example, it is possible to apply a first positively locking covering to the discrete passive components DPB, which covering terminates with the surface of the dielectric layer DS. This may be a thermoplastic film, for example. Said film can subsequently be provided with a metallization which, if appropriate, can also be reinforced electrolytically or in an electroless manner.

(40) It is also possible to enlarge the surface of the substrate S compared with the region provided with components and to cause the passivation to terminate with the then projecting substrate surface. Furthermore, it is possible to place a rigid and mechanically dimensionally stable cap onto the surface of the dielectric layer DS or onto projecting surface regions of the substrate S and to seal it with respect thereto. Both with a positively locking covering and with a rigid cap, afterward a potting of the entire package P can also be effected, wherein advantageously either a glob top compound is applied or the whole thing is encapsulated with a plastics compound by injection molding.

(41) The tunable RF filter circuit is not restricted to the circuit details shown. Filter circuits having further circuit elements such as e.g. capacitive elements, inductive elements or resonant circuits are likewise encompassed by the filter circuit.

LIST OF REFERENCE SIGNS

(42) |S1,1|: Reflection |S2,1|: Insertion loss A: Output AHF: Tunable radio-frequency (RF) filter circuit AK: External contact AKE: Tunable capacitive element AR: Acoustic resonator DK: Plated-through hole DPE: Discrete passive component DS: Dielectric layer E: Input HLB: Semiconductor component IE: Inductive element IMP: Impedance element K: Electrical and/or magnetic coupling KE: Capacitive element KL1, KL2: First, second component position KO: Coupling element P: Package RK,RK1-4: Resonant circuit S: Substrate SL: Control line SP: Signal path STL: Control logic SW1: First signal route SW2: Second signal route VE1, VE2: First, second wiring plane