HF-FILTER

Description

[0001] The invention relates to high-frequency filters (HF filters) that can be used, for example, in portable communication devices.

[0002] Portable communication devices, such as mobile phones, can now enable communication across a wide variety of frequency bands and transmission systems. To achieve this, they typically include a multitude of RF filters, each designed for a specific frequency and transmission system. While modern RF filters can now be manufactured in small dimensions, the sheer number of filters and the complexity of their interconnections mean that the front-end modules housing them are still relatively large, and their production is both costly and expensive.

[0003] Tunable RF filters could provide a solution. Such filters have an adjustable center frequency, which is why one tunable filter can, in principle, replace two or more conventional filters. Tunable RF filters are known, for example, from US 2012 / 0313731 A1 and EP 2 530 838 A1. In these filters, the electroacoustic properties of resonators operating with acoustic waves are modified by tunable impedance elements.

[0004] Reconfigurable filters using switches are known from the article "Reconfigurable Multiband SAW Filters for LTE Applications", IEEE SiRF 2013, pp. 153-155, by Lu et al.

[0005] WO 2006 / 045176 A1 describes bandstop filters with a first and a second acoustic resonator and a phase shifter that connects the first and the second acoustic resonator.

[0006] US 5,617,065 A describes components for frequency selection using bulk acoustic waves.

[0007] WO 2010 / 027310 A1 describes a reconfigurable filter apparatus which has at least first and second passive filter sections connected in parallel to produce a common filter response.

[0008] EP 0 949 756 A1 describes monolithic filters that use devices with thin-film-based acoustic volume waves and passive components to control the shape and width of a bandpass response.

[0009] WO 99 / 23757 A1 describes an acoustic wave filter in conductor structure with lossy inductors connected in parallel with resonators.

[0010] EP 1 035 648 A2 describes a band-switching filter comprising a resonator circuit with a surface acoustic wave resonator, a switching element, and an independence element. This filter selectively passes or attenuates a signal through only one of two different passbands. The resonant frequency of the resonator circuit can be shifted to any other frequency by switching the connection state of the surface acoustic wave resonator and the impedance element by opening or shorting the switching element. This allows for a reduction in the number of filter stages. Both the band-switching filter and an antenna duplexer using the band-switching filter can be miniaturized by the use of a permanent magnet.

[0011] EP 2 416 496 A1 describes a filter for elastic waves with satisfactory filtering properties. This elastic wave filter comprises: multiple series-arm resonators, an inductor, and multiple parallel-arm resonators. The multiple series-arm resonators are connected in series along a single series arm, which connects an input terminal to an output terminal. The inductor is connected to the series arm in such a way that it is parallel to two or more of the multiple series-arm resonators. The multiple parallel-arm resonators are arranged along parallel arms. The connecting nodes of the inductor and the series arm are connected to ground potential. A parallel arm connects a connecting node of adjacent series-arm resonators to ground potential.

[0012] A particular problem with known tunable RF filters is that the tuning process itself alters important filter characteristics. For example, the insertion loss, input impedance, and / or output impedance change during tuning.

[0013] It is therefore a task to specify RF filters that allow tuning without changing other important parameters and that provide the person skilled in the art with additional degrees of freedom in the design of filter modules.

[0014] These problems are solved by an RF filter according to the claimed invention as defined in claim 1.

[0015] Further "aspects", "implementations", "examples", etc., which are described below and do not fall within the scope of the invention as defined above, are to be understood as technical background information intended to promote understanding of the claimed invention.

[0016] Basic elements in RF filters are known, for example, from ladder-type structures, where a basic element comprises a series resonator and a parallel resonator. Several such basic elements connected in series essentially produce the filtering effect, provided that the resonant frequencies and the antiresonant frequencies of the series and parallel resonators are suitably matched.

[0017] The basic elements presented here can therefore be understood as roughly halved basic elements of a ladder-type circuit.

[0018] Impedance converters can be either impedance inverters or admittance inverters. While an impedance converter transforms an arbitrary load impedance into an input impedance, the effect of impedance inverters and admittance inverters is much more specific. Impedance inverters and admittance inverters can be described using the same principles as two-port networks.

[0019] The chain matrix with matrix elements A, B, C, D describes the effect of a two-port device whose output port is connected to a load by specifying how a voltage applied to a load UL and a current flowing through a load IL into a voltage present at the entrance gate U IN and a current flowing into the entrance gate I IN be transformed: U IN I IN = A B C D U L I L

[0020] The impedance Z is defined as the ratio between voltage and current: Z = U I

[0021] Thus, a load impedance is determined. ZL into an input impedance Z IN transformed: Z IN = AZ L + B CZ L + D

[0022] The load impedance ZL From the outside, it looks like the input impedance Z IN out of.

[0023] An impedance inverter is now characterized by the following chain matrix: A B C D K = 0 iK i / K 0

[0024] It follows Z IN = K 2 Z L

[0025] The impedance is inverted. The proportionality factor is K 2< .

[0026] An admittance inverter is characterized by the following chain matrix: A B C D J = 0 i / J iJ 0

[0027] This implies the following for the admittance Y: Y IN = J 2 Y L

[0028] The admittance is inverted. The proportionality factor is J 2< .

[0029] It was found that the simultaneous presence of parallel and series resonators significantly affects the variability of important parameters when tuning the RF filter. It was further found that tuning has less influence on these parameters when only one type of resonator is present. Thus, when only series or only parallel resonators are present, the RF filter behaves more stably with respect to insertion loss, input impedance, and / or output impedance during tuning. It was also found that the aforementioned impedance converters are suitable for making series resonators appear as parallel resonators and vice versa. In particular, a series connection of two impedance converters with a series resonator in between appears to its circuit environment as a parallel resonator.A series connection of two admittance inverters with a parallel resonator in between looks like a series resonator to its circuit environment.

[0030] These series connections therefore make it possible to create RF filter circuits that are easier to tune.

[0031] It is therefore possible to design the RF filter in such a way that the impedance converters are impedance inverters and the resonators are series resonators.

[0032] Such filters do not require parallel resonators. If the filters are designed as bandpass or bandstop filters, they typically exhibit a steep right-hand slope. The filter can be used in a duplexer. Due to the steep right-hand slope, it is preferably used as a transmit filter, specifically when the transmit band is below the receive band. If the relative arrangement of the transmit and receive bands is reversed, the filter with series resonators is preferably used as a receive filter.

[0033] It is also possible to design the RF filter in such a way that the impedance converters are admittance inverters and the resonators are parallel resonators.

[0034] Such filters do not require series resonators. If the filters are designed as bandpass or bandstop filters, they typically exhibit a steep left-hand slope. The filter can also be used in a duplexer. Due to the steep left-hand slope, it is preferably used as a receive filter, specifically when the receive band is above the transmit band. If the relative arrangement of the transmit and receive bands is reversed, the filter with series resonators is preferably used as the transmit filter.

[0035] It is possible for impedance converters to include both capacitive and inductive elements as impedance components. However, it is also possible for impedance converters to include only capacitive elements or only inductive elements. In the latter case, the impedance converters consist solely of passive circuit elements. In particular, if the impedance converters include few or no inductive elements, they can easily be implemented as structured metallizations within metal layers of a multilayer substrate.

[0036] It is possible that the impedance converters include phase-shifting conductors in addition to inductive or capacitive elements. It is also possible that the impedance converters consist entirely of phase-shifting conductors. Phase-shifting conductors can also be integrated into a multilayer substrate in a simple and compact design.

[0037] It is possible that the filter is described by a symmetric description matrix B.

[0038] There are filter circuits that are completely described by a description matrix B. The matrix B contains matrix elements that characterize the individual circuit components of the filter.

[0039] A filter circuit comprising three resonators R1, R2, R3 connected in series and connected on the input side to a source impedance ZS and on the output side to a load impedance ZL would have the following form: B = Z S 0 0 0 0 0 R 1 0 0 0 0 0 R 2 0 0 0 0 0 R 3 0 0 0 0 0 Z L

[0040] The circuit would not function as a bandpass filter.

[0041] If the two outer series resonators are masked by impedance inverters so that they each appear as parallel resonators, a structure is obtained that behaves like a ladder-type structure and is described by the following description matrix. B = Z S K S 1 0 0 0 K S 1 R 1 K 12 0 0 0 K 12 R 2 K 23 0 0 0 K 23 R 3 K 3 L 0 0 0 K 3 L Z L

[0042] This is K S 1 for the impedance inverter between the source impedance ZSand the first resonator. K 12 represents the impedance inverter between the first and second resonators. IA denotes the indices of the inverter sizes, the resonators between which the corresponding inverters are arranged. Bij = Bji, meaning the matrix is ​​symmetrical with respect to its diagonal. The filter circuit corresponding to equation (9) is shown in Figure 1 The resonators are described by quantities on the diagonal of the matrix. The impedance converters are described by quantities on the off-diagonals directly above and below the diagonal.

[0043] It is possible that the filter includes a second impedance converter connected in parallel to a segment of the filter. This segment comprises a series circuit with a basic element and two impedance converters.

[0044] The description matrix then contains entries that are located above the upper or below the lower subdiagonal.

[0045] It is possible that at least one of the resonators of the basic elements is tunable.

[0046] In principle, and especially if one of the resonators is tunable, BAW resonators (BAW = Bulk Acoustic Wave), SAW resonators (SAW = Surface Acoustic Wave), GBAW resonators (GBAW = Guided Bulk Acoustic Wave), and / or LC resonators are suitable. Resonator elements operating with acoustic waves essentially have an equivalent circuit with a parallel connection of a capacitive element C0 on the one hand and a series connection of an inductive element L1 and a capacitive element C1 on the other. Such a resonator element has its resonant frequency at ω 0 = 1 L 1 C 1 and its anti-resonance frequency at ω p = ω 0 1 + C 1 C 0 = 1 L 1 C 1 1 + C 1 C 0

[0047] If the resonator includes tunable elements such as tunable inductive or capacitive elements connected in series and / or parallel to the resonator element, a resonator with a variable frequency response is formed. The resonant frequency depends on L1 and C1, but not on C0. The antiresonance additionally depends on C0. By varying the impedance of the tunable impedance elements, C0 and L1 of the equivalent circuit can be varied independently. This allows the resonant frequency and the antiresonance frequency to be set independently.

[0048] As an alternative to, or in addition to, resonators with resonator elements whose characteristic frequencies can be varied by means of tunable impedance elements, a tunable resonator can comprise an array of resonator elements, each of which can be coupled to or disconnected from the resonator by means of a switch. This is then an array of m resonator elements per tunable resonator.

[0049] This allows the construction of RF filters that can realize m different filter transfer curves, depending on the currently active resonator element. Each of the m resonators can be assigned to exactly one filter transfer curve. However, it is also possible for several simultaneously active resonator elements to be assigned to one filter transfer curve. Thus, m resonator elements enable up to m! (factorial of m) different filter transfer curves. m can be 2, 3, 4, 5, 6, 7, 8, 9, 10, or even more. If the resonator elements are connected in parallel, 2m different filter transfer curves are possible.

[0050] The switches can be semiconductor-based switches such as CMOS switches (CMOS = Complementary Metal-Oxides Semiconductor), GaAs (Gallium Arsenide)-based switches, or JFET switches (JFET = Junction FET [FET = Field-Effect Transistor]). MEMS switches (MEMS = Microelectromechanical System) are also possible and offer excellent linear characteristics.

[0051] It is therefore possible that all resonators can be tuned to different frequency bands.

[0052] In particular, it is possible that the tunability of the resonators allows for compensation of temperature fluctuations, adjustment of the filter with respect to impedance matching, adjustment of the filter with respect to insertion loss, or adjustment of the filter with respect to isolation.

[0053] It is also possible that each resonator comprises the same number of resonator elements, which can be controlled via switches that are addressable via a MIPI interface (MIPI = Mobile Industry Processor Interface).

[0054] It is possible for one or more impedance converters to include or consist of passive impedance elements. The impedance converter can therefore comprise two parallel capacitive elements and one parallel inductive element. This refers to cross-branches, e.g., to ground, which contain a corresponding capacitive or inductive element.

[0055] It is also possible for an impedance converter to comprise three parallel capacitive elements.

[0056] It is also possible for an impedance converter to comprise three parallel inductive elements.

[0057] It is also possible for an impedance converter to comprise two parallel inductive elements and one parallel capacitive element.

[0058] It is possible, mathematically, for individual impedance elements to exhibit negative impedance values, such as negative inductances or negative capacitances. However, negative impedance values ​​are unproblematic if the corresponding impedance elements are connected to other impedance elements of the RF filter, so that the combined impedance of the connected elements is positive. In this case, the connection of the originally intended elements would be replaced by the element with the positive impedance value.

[0059] The following section explains important principles and provides a non-exhaustive list of exemplary and schematic circuits to illustrate key aspects of the RF filter.

[0060] The figures and combinations of features expressly designated as "examples" below are not part of the invention and serve only for illustration. The invention is defined in claim 1, and only the features specified therein are part of the invention.

[0061] They show: Fig. 1: An RF filter F with three resonators and four impedance converters, Fig. 2: A filter with three resonators and two impedance converters, Fig. 3: A duplexer D with a transmit filter TX and a receive filter RX, which are connected to an antenna via an impedance matching circuit, Fig. 4: An RF filter F in which a series resonator S is connected centrally and a series resonator is connected peripherally to two impedance converters each, Fig. 5: An RF filter F that exclusively uses parallel resonators, Fig. 6: An RF filter F in which an impedance converter directly connects a first resonator to a third resonator, Fig. 7: An RF filter F in which an admittance inverter directly connects a first resonator to a third resonator, Fig. 8: An RF filter with tunable Resonators, Fig. 9A to Fig. 9K: various designs of tunable resonators, Fig.Fig. 10A: a tunable resonator with series resonator elements activatable by switch, Fig. 10B: a tunable resonator with parallel resonators activatable by switch, Fig. 11A to Fig. 11F: various examples of an impedance inverter, Fig. 12A to Fig. 12F: various examples of an admittance inverter, Fig. 13A to Fig. 13C: various levels of abstraction in the design of an RF filter, Fig. 14A to Fig. 14H: various concrete examples of an RF filter with two tunable series resonators and three impedance converters, Fig. 15A to Fig. 15H: embodiments of an RF filter with two tunable resonators, three impedance converters and one bridging capacitive element each, Fig. 16: the insertion loss of a Resonator (A) and a corresponding bandpass filter (B), Fig. 17: the passband curves of the RF filter from . Fig. 16, wherein tunable impedance elements are changed in their impedance to obtain a new position of the passband B, Fig. 18: the admittance (A) of a resonator and the insertion loss (B) of a corresponding bandpass filter with admittance inverters, Fig. 19: the RF filter for Fig. 18, where impedance values ​​of tunable impedance elements were varied to obtain a different position of the passband, Fig. 20: Insertion losses (B, B') of an RF filter in which different frequency positions of the passband are obtained by tuning resonators, Fig. 21: Different passband curves (B, B') of an RF filter with parallel resonators and admittance inverters in which different impedance values ​​result in different positions of the passband, Fig. 22: Insertion losses of a tunable duplexer: The curves B1 and B3 denote a tunable transmit frequency band. Curves B2 and B4 represent the insertion losses of an adjustable receiving frequency band, Fig. 23: a stressed filter circuit, Fig. 24: a possible form of integration of circuit components in one component, Fig. 25: transfer functions of a tunable, stressed filter according to Fig. 23 .

[0062] Figure 1Figure 1 shows an example of an RF filter circuit F with three resonators and four impedance converters IW. The middle resonator represents a fundamental element GG. The middle resonator can be a parallel resonator P or a series resonator S. The two impedance converters IW surrounding the first resonator cause it to appear externally as either a series or a parallel resonator. If the middle resonator is a parallel resonator, then the first resonator can also be a parallel resonator, appearing externally as a series resonator. Similarly, the third resonator would then also be a parallel resonator, appearing externally as a series resonator. Conversely, the middle resonator can be a series resonator S. In that case, the two outer resonators would also be series resonators, appearing externally as parallel resonators.Thus, using the impedance converter IW, a ladder-type similar filter structure can be obtained, even if only series resonators or only parallel resonators are used.

[0063] Figure 2 shows a filter circuit in which the middle resonator is masked by the surrounding impedance converters IW in such a way that the filter appears to the outside as an alternating sequence of parallel and series resonators, although only one type of resonator is used.

[0064] Figure 3Figure 1 shows a duplexer D in which both the transmit filter TX and the receive filter RX comprise series connections of impedance converters and resonators, interconnected in such a way that only one type of resonator is required per filter. Since series resonators are suitable for forming a steep right-hand filter slope of a passband, and since transmit frequency bands are generally lower in frequency than receive frequency bands, it is advantageous to use series resonators in the transmit filter TX. Similarly, parallel resonators would be used in the receive filter RX. If the transmit frequency band is higher than the receive frequency band, then series resonators in the receive filter and parallel resonators in the transmit filter would be advantageous.

[0065] The TX and RX filters are connected to an antenna ANT via an impedance matching circuit (IAS). From the perspective of the IAS, each of the two filters (TX and RX) looks like a conventional ladder-type filter circuit, so practically no additional effort is required in the design of the other circuit components such as the antenna and impedance matching circuit.

[0066] Figure 4 An example is shown where the middle resonator is implemented as a series resonator S. Due to the effect of the impedance converters IW, a series resonator element can also be used in each of the two outer resonators, even though the combination of impedance converters and series resonator appears externally as a parallel resonator P. To make series resonators appear externally as parallel resonators, impedance inverters K are preferably used.

[0067] In contrast, it shows Figure 5An example of an RF filter F, in which only parallel resonators are used. Using admittance inverters J as embodiments of the impedance converters IW, the two outer parallel resonators appear as series resonators S. Together with the central, middle resonator, a parallel resonator P, the RF filter F forms a quasi-ladder-type structure.

[0068] Figure 6 Figure 1 shows an example where the two outer resonators are directly connected via another impedance converter, e.g., an impedance inverter. This direct connection of the outer resonators via an additional impedance converter represents a new degree of freedom through which an RF filter can be further optimized.

[0069] Figure 7Figure 1 shows an example of an RF filter F, which uses parallel resonators and admittance inverters J. The two outer resonators are also directly interconnected via another admittance inverter J.

[0070] Figure 8 shows an example of an RF filter where the resonators are tunable.

[0071] Figure 9Figure 1 shows an example of a tunable resonator R. The resonator R comprises a resonator element RE. The resonator element RE can be a resonator element that operates with acoustic waves. A capacitive element CE is connected in parallel to the resonator element RE. Another capacitive element CE is connected in series with the parallel connection. The two capacitive elements CE are tunable, meaning their capacitance can be adjusted. Depending on the capacitive elements used, the capacitance can be adjusted continuously or in discrete values. For example, if the capacitive elements comprise varactors, the capacitance can be adjusted continuously by applying a bias voltage. If a capacitive element CE comprises a bank of individual capacitive elements that can be individually controlled by one or more switches, the capacitance of the corresponding capacitive element CE can be adjusted in discrete steps.

[0072] Figure 9B shows an alternative possibility of a resonator R in which the series connection of a tunable capacitive element CE with a resonator element RE is connected in series with a tunable inductive element IE.

[0073] Figure 9C Figure 1 shows an example of a tunable resonator R, in which a resonator element RE is connected in parallel to a tunable inductive element IE. This parallel connection is connected in series with a tunable capacitive element CE.

[0074] Figure 9D This shows an example of a tunable resonator R. In this case, compared to... Figure 9C - the parallel circuit with a tunable inductive element IE connected in series.

[0075] Figure 9E shows an example of a tunable resonator in which a resonator element RE is simply connected in parallel with a tunable capacitive element CE.

[0076] Figure 9F Figure 1 shows an example of a tunable resonator R. A resonator element RE is connected in parallel to a tunable inductive element IE.

[0077] Figures 9E and 9F They show relatively simple examples of a tunable resonator R. Figures 9A to 9D Figure 1 shows embodiments of a tunable resonator R which, through an additional tunable element, allow further degrees of freedom in tuning. In this respect, the embodiments shown can be connected in series or parallel with further capacitive and inductive elements with fixed or variable impedance to obtain additional degrees of freedom, e.g., for a wider tuning range.

[0078] Figure 9G shows an example of a tunable resonator R, in which the resonator element RE is connected in parallel to a series connection comprising an inductive element IE and a tunable capacitive element CE.

[0079] Figure 9H shows an example of a tunable resonator R, in which the resonator element RE is connected in parallel to a parallel circuit comprising an inductive element IE and a tunable capacitive element CE.

[0080] Figure 9I shows an example of a tunable resonator R, in which the resonator element RE is connected in series to a series connection comprising an inductive element IE and a tunable capacitive element CE.

[0081] Figure 9J Figure 1 shows an example of a tunable resonator R, in which the resonator element RE is connected on the one hand in series to a series connection comprising an inductive element IE and a tunable capacitive element CE, and on the other hand in parallel to a parallel connection comprising an inductive element IE and a tunable capacitive element CE.

[0082] Figure 9KFigure 1 shows an example of a tunable resonator R, in which the resonator element RE is connected on the one hand in series to a series connection comprising a tunable inductive element IE and a tunable capacitive element CE, and on the other hand in parallel to a parallel connection comprising a tunable inductive element IE and a tunable capacitive element CE.

[0083] Furthermore, in addition to continuously tunable elements such as varactors and switchable elements of constant impedance, switchable tunable elements are also possible, e.g. varactors that can be added by means of a switch.

[0084] More generally, in a resonator, the resonator element can be connected in series with a series network and in parallel with a parallel network. The series network and the parallel network can each comprise impedance elements of fixed or variable impedance.

[0085] Figure 10 shows an example of a tunable resonator R, which includes a multitude of resonator elements RE and a multitude of switches SW. Figure 10A This shows resonator elements RE connected in series in the signal path SP. This represents a tunable series resonator. By individually opening and closing the individual switches SW, specific resonator elements RE can be coupled into the signal path SP. The tunable resonator R comprises... Figure 10A With m resonator elements RE, 2< different switching states can be obtained.

[0086] Figure 10B Figure 1 shows an example of a tunable resonator R, in which resonator elements connect the signal path SP to ground. Since the order in which the individual resonator elements RE are connected to the signal path SP is fundamentally relevant, m! (m factorial) different resonator states can be obtained.

[0087] The Figures 11A to 11F They provide various examples of an impedance inverter.

[0088] Figure 11A This diagram shows a form of impedance converter, which represents an impedance inverter. Two capacitive elements are connected in series in the signal path. One capacitive element connects the common node of the two capacitive elements in the signal path to ground. The capacitive elements in the signal path are thus assigned a negative capacitance -C. The capacitive element in the parallel path to ground is assigned a positive capacitance C.

[0089] As described above, the capacity values ​​are derived solely from the calculation rules for two gates. The in Figure 11AThe T-circuit shown does not necessarily have to be implemented in this way in a circuit environment. Rather, the capacitive elements with negative capacitance in the series path can be combined with further capacitive elements with positive capacitance that are additionally connected in the series path, so that one or more capacitive elements with positive capacitance are obtained in total.

[0090] The same applies to the examples of Figures 11B, 11C and 11D as well as for the embodiments of the admittance inverters in the Figures 12A, 12B, 12C and 12D .

[0091] Figure 11B shows a T-circuit made of inductive elements, where the two inductive elements connected in series in the signal path formally have a negative inductance.

[0092] Figure 11Cshows a form of an impedance inverter, which is a Pi circuit with one capacitive element of negative capacitance in the series path and two capacitive elements of positive capacitance in separate parallel paths.

[0093] Figure 11D This shows an example of a Pi-shaped impedance inverter where the inductance of the inductive element in the signal path is negative. The inductances of the inductive elements in the corresponding two parallel paths are positive.

[0094] Figure 11E Figure 1 shows an example of an impedance inverter with a phase-shifting circuit and an inductive element with inductance L. The phase-shifting circuit preferably has the characteristic impedance of the signal line Z0. The phase shift Θ due to the phase-shifting circuit is suitably adjusted.

[0095] For example, in the case of an impedance inverter, Θ can be given by the equation Θ = − tan − 1 2 X Z 0 be determined. This is X = K 1 − k Z 0 2 and K through Z in = K 2 Z l determined. In the case of an admittance inverter, the following may hold: Θ = − tan − 1 2 B Y 0 . This is B = J 1 − J Y 0 2 and J through Y in = J 2 Z l certainly.

[0096] Similarly to Figure 11E shows Figure 11F An example where the inductive element is replaced by a capacitive element of capacitance C.

[0097] The Figures 12A to 12F show examples of an admittance inverter.

[0098] Figure 12A This shows an example of an admittance inverter in a T-configuration, where the two capacitive elements in the series path have positive capacitances. The capacitive element in the parallel path nominally has a negative capacitance.

[0099] Figure 12BFigure 1 shows an example of an admittance inverter in a T-configuration, where two inductive elements with inductance L are connected in series in the signal path. In a parallel path, which connects two electrodes of the inductive elements to ground, one inductive element with negative inductance -L is connected.

[0100] Figure 12C This shows an example of an admittance inverter in a Pi configuration, where the two capacitive elements in the two parallel paths have a negative capacitance. The capacitive element in the signal path has a positive capacitance.

[0101] Figure 12D This shows an example of an admittance inverter in a Pi configuration with three inductive elements. The inductive element in the series path has a positive inductance. The two inductive elements in the two parallel paths each have a negative inductance.

[0102] Figure 12EFigure 1 shows an example of an admittance inverter where an inductive element with positive inductance L is connected between two segments of a phase-shifting circuit. Each segment of the phase-shifting circuit has a characteristic impedance Z0 and shifts the phase accordingly.

[0103] According to the Figure 12E shows Figure 12F An example of an admittance inverter, which is also based on phase-shifting circuits. A capacitive element with positive capacitance C is connected between two segments of a phase-shifting circuit.

[0104] Figure 13 This shows the use of tunable resonators R together with impedance converters IW. The resonator can be a series resonator. By using impedance inverters K as impedance converters IW, a combination of two impedance converters IW and a series resonator connected between them results in a parallel resonator.

[0105] If you replace the impedance converters IW of the Figure 13A through impedance inverters, such as those found in the Figures 11A to 11F , e.g. 11A, are known, then the circuit structure of the Figure 13B The capacitive elements with negative capacitance appear problematic. However, considering that the resonators R themselves have properties of capacitive elements with positive capacitance, the need for capacitive elements with negative capacitance directly connected to the resonator elements is eliminated. This is in Figure 13C shown.

[0106] Furthermore, if one takes into account capacitive elements that are connected in the circuit environment of the RF filter, the need for the peripheral capacitive elements of negative capacitance is also eliminated. Figure 13COverall, a circuit structure as shown in 14A is obtained. Even though an external circuit environment of the RF filter offers no possibility to compensate for the negative capacitances -C in Figure 13 If [the necessary capacity] is available, the negative capacity can be compensated by the positive capacity of the capacitive element in the parallel path.

[0107] Figure 14A This shows a simple RF filter circuit with two tunable resonators and three impedance elements, the impedance of which is chosen such that one of the two resonators acts as a parallel resonator. Figure 14A It therefore essentially shows a basic element of a ladder-type filter circuit, although only series resonators are used.

[0108] Figure 14B shows an alternative to the RF filter of the Figure 14A, because the inductive element L between the resonators is replaced by a capacitive element C and the capacitive element in the load-side parallel path is replaced by an inductive element.

[0109] Figure 14C shows an example of an RF filter with two resonators, where three inductive elements are connected in a parallel path.

[0110] Figure 14D shows an example of an RF filter where the two left impedance elements are formed by inductive elements and the right impedance element by a capacitive element.

[0111] Figure 14E shows an example where the two outer impedance elements are formed by inductive elements and the central impedance element by a capacitive element.

[0112] Figure 14F shows an example where the two right impedance elements are formed by capacitive elements and the left impedance element by an inductive element.

[0113] Figure 14G shows an example where the two right impedance elements are formed by inductive elements and the left impedance element by a capacitive element.

[0114] Figure 14H shows an example where all three impedance elements are formed by capacitive elements.

[0115] The Figures 15A to 15H show further alternatives of the RF filters of the Figures 14A to 14H , wherein a further impedance element directly connects the signal input and the signal output. Alternatively to the bridging capacitive element, a bridging inductive element or other impedance converter designs can be used.

[0116] Figure 16 This shows the admittance of a resonator (curve A) and the transfer function of an RF filter with such a resonator (curve B). Series capacitive elements have a value of 2.4 pF. Parallel capacitive elements have a value of 0.19 pF.

[0117] Figure 17 The graph shows the corresponding curves, with series tunable capacitors set to a capacitance value of 30 pF and parallel tunable capacitors to a capacitance value of 3.7 pF. The impedance converters of the Figures 16 and 17 The associated filters are impedance inverters. The resonators are series resonators.

[0118] In comparison, the Figures 18 and 19 corresponding curves of RF filters with admittance inverters and parallel resonators. Figure 18 This shows the characteristic curves of a filter where series tunable capacitors have a value of 2.4 pF and parallel tunable capacitive elements have a value of 0.19 pF.

[0119] Figure 19 shows the corresponding curves of the RF filter, where the series tunable capacitances have a value of 30 pF and the parallel tunable capacitances have a value of 3.7 pF.

[0120] Figure 20 This shows the insertion losses of bandpass filters with admittance inverters and parallel resonators. The filter features tunable resonators, which are tuned to receive band 17 or band 5 by means of adjustable capacitive elements. The resonators include resonator elements that can be coupled via switches, as shown in Fig. 10B shown.

[0121] Figure 21 This shows passband curves of an RF filter with impedance inverters and series resonators, where the tunable values ​​are set to the transmit frequencies of band 17 and to the transmit frequencies of band 5. The resonators include resonator elements that can be coupled via switches, as shown in Fig. 10A shown. Figure 22 shows the insertion losses of the receive and transmit filters of a tunable duplexer, once tuned to band 17 and once to band 15.

[0122] Fig. 23Figure 1 shows a possible embodiment of the RF filter defined in claim 1. In the signal path SP, four capacitive elements are connected in series. In six cross-branches to ground, a switchable resonator is connected in each. Each of the switchable resonators comprises a resonator element and a switch connected in series with it. An inductive element is connected in parallel to two of the four capacitive elements.

[0123] Fig. 24This shows how circuit components of the filter circuit can be advantageously integrated into a multilayer module. The capacitive elements CE can be implemented as MIM capacitors (MIM = Metal Insulator Metal) together with sections of the signal path in one layer. Switches SW can be implemented at the bottom of this layer. In a layer below, vias can be provided, representing the interface between (semiconductor) switches and the resonator elements. Below the interface layer, the resonator elements, e.g., as SAW, BAW, GBAW, etc., can then be arranged.

[0124] Fig. 25 shows calculated passband curves for bands 34 and 39, between which switching is possible via a switch.

[0125] RF filters or duplexers with RF filters may further include additional resonators or impedance elements, in particular tunable impedance elements. Reference symbol list:

[0126] A: Admittance of a resonator ANT: Antenna B: Insertion loss of an RF filter B', B1, B2, B3, B4: Insertion losses of RF filters CE: Capacitive element D: Duplexer F: RF filter GG: Fundamental element IAS: Impedance matching network IE: Inductive element IW: Impedance converter J: Admittance inverter K: Impedance inverter P: Parallel resonator R: Resonator RE: Resonator element RX: Receive filter S: Series resonator SP: Signal path SW: Switch TX: Transmit filter Z0: Characteristic line impedance Φ: Phase shift

Claims

1. A high frequency filter comprising: - a signal path (SP) with four capacitive elements connected in series, - six switchable resonators, wherein each of the switchable resonators comprises a resonator element and a switch connected in series therewith, and wherein each of the switchable resonators is connected in a cross branch to ground, and - an inductive element, wherein two of the capacitive elements are each connected between two switchable resonators, and the inductive element is connected in parallel with these two capacitive elements.