The present invention relates to a radio frequency filter and to a communication device comprising such a radio frequency filter.

Background

As radios become more compact and integrated there is renewed demand to produce low- loss, high-power filters that are low volume or have a small form-factor. Primarily, this is to enable components to be tightly packed and used in conjunction with large antenna arrays for Ml MO systems.

The rejection specifications of such filters are stringent however, in order to meet both strict regulatory emission requirements and provide the necessary protection of sensitive receiver electronics, diplexed with high-power transmitter amplifiers.

In order to provide suitable filtering, high order filters are required to achieve acceptable pass- band roll-off and out-of-band rejection. However, as filter order increases, so does the total volume of the filters. In order to address this problem, multi-mode solid dielectric waveguide filters are being developed that combine multiple resonances - or poles of a filter function - within single physical sections. This solution is significantly more space efficient, owing to the concentrating effect on the electric field of high dielectric permittivity of certain bulk materials used. However, often the filter specifications are so stringent that it is not feasible to realise the filtering function with a basic band-pass filter of the theoretically necessary order. Instead, additional transmission paths, known as cross-couplings, are introduced in a filter which provide a means to realise transmission zeroes prescribed by a filtering transfer function. These transmission zeroes can be selectively placed around the pass-band in order to provide significant additional rejection, where required by the design.

In solid multi-mode dielectric filters however, it is not trivial to realise the additional coupling paths required owing to the limitations of the physical structure. In dual-mode waveguide filters in particular, the primary modes used are orthogonal to each other, with coupling only easily achieved between two modes of similar alignment. Often, complex external and supplementary components or structures must be used to achieve the required cross-couplings, which greatly increase production costs through the requirement of extra parts themselves, as well as the added complexity of configuring them during manufacture.

Alternatively, the designer may intentionally limit the number of transmission zeroes utilised to what is naturally and easily obtainable from a given physical structure. Whilst simpler and cost- effective, this results in a realisation that is not as space-efficient as is theoretically possible.

Summary

An objective of embodiments of the present invention is to provide a radio frequency filter which diminishes the problems with conventional solutions.

Another objective of embodiments of the present invention is to provide a radio frequency filter which provide additional coupling paths in relation to traditional radio frequency filters.

A further objective of embodiments of the present invention is to provide a radio frequency filter with a desired arbitrary filter topology using a first dual-mode resonator and a second dual- mode resonator.

An additional objective of embodiments of the present invention is to provide a radio frequency filter with a desired arbitrary filter topology using solid dielectric, dual-mode waveguide technology.

The above objectives are fulfilled by the subject matter of the independent claims. Further advantageous implementation forms of the present invention can be found in the dependent claims.

According to a first aspect of the present invention a radio frequency filter is provided. The radio frequency filter comprises a first dual-mode resonator having a first mode, a second mode, and a first spurious mode. The first dual-mode resonator comprises a first intra-coupling element coupling the first mode to the first spurious mode, a second dual-mode resonator having a third mode and a fourth mode, and an inter-coupling element arranged between the first dual-mode resonator and the second dual-mode resonator, the inter-coupling element coupling the first mode to the fourth mode, the second mode to the third mode, and the first spurious mode to the third mode. With a radio frequency filter according to the first aspect a more narrow filter function may be provided. This is due to the possibility of coupling the first mode to the third mode via the first spurious mode. In a first possible implementation form of a radio frequency filter according to the first aspect, the first dual-mode resonator and the second dual-mode resonator are arranged with their respective centres along a common centre axis. The inter-coupling element comprises a first elongated iris centred on the common centre axis and a second elongated iris arranged perpendicular to the first elongated iris and offset from the common centre axis.

With a radio frequency filter according to the first possible implementation form the magnetic field lines of the first spurious mode are parallel to the magnetic field lines of the third mode at the inter-coupling element, thus providing a coupling between the first spurious mode and the third mode.

In a second possible implementation form of a radio frequency filter according to the first possible implementation form, the magnetic field strength of the first spurious mode is at its minimum and the electric field strength of the first spurious mode is at its maximum along the common centre axis. This is a favourable orientation of the first spurious mode to provide the desired coupling between the first spurious mode and the third mode.

In a third possible implementation form of a radio frequency filter according to the first or second possible implementation form, the second elongated iris is offset from the common centre axis in a direction of a length component of the first elongated iris. The first elongated iris and the second elongated iris are connected to each other or a gap is arranged between the first elongated iris and the second elongated iris. The first elongated iris has a longest dimension along a length component. As is stated above the second elongated iris is arranged perpendicular to the first elongated iris. In the case that the first elongated iris and the second elongated iris are connected to each other the irises together form a T-shape.

A further advantage of the displaced second elongated iris is that, because the magnetic field vectors of the second mode and the third mode are of constant magnitude and direction for all iris positions, the coupling strength between the second mode and the third mode 3b is invariant to its position. This means that the coupling between the second mode and the third mode can be controlled by varying the iris length, and the coupling from the spurious mode, can be controlled by the position of the second elongated iris, thereby allowing independent control of both couplings, from a single feature.

In a fourth possible implementation form of a radio frequency filter according to anyone of the first to the third possible implementation forms, the length component of the first elongated iris is arranged in parallel to the electric field vector of the second mode. The length component of the second elongated iris is arranged in parallel to the electric field vector of the first mode.

In a fifth possible implementation form of a radio frequency filter according to anyone of the first to fourth possible implementation forms or to the first aspect as such, the first intra-coupling element is arranged at a border area between a first side and a second side of the first dual mode resonator where magnetic field lines of the first spurious mode and the first mode are at least partially parallel. This is a favourable position of the intra-coupling element in order to get a high coupling efficiency between the first mode and the first spurious mode.

In a sixth possible implementation form of a radio frequency filter according to the fifth possible implementation form, the first intra-coupling element is one of: a notch, at least a partial chamfer and at least a partial indentation. These forms of intra-coupling elements are favourable in order to get a high coupling efficiency between the first mode and the first spurious mode and a desirable low coupling efficiency between other modes. The notch may be most preferred in order to get a low coupling efficiency between other modes, while other forms may be more desirable to achieve the highest possible coupling efficiency between the first mode and the first spurious mode. The first dual-mode resonator may comprise a block of dielectric material in which the intra- coupling element is configured.

In a seventh possible implementation form of a radio frequency filter according to the fifth or sixth possible implementation forms, the second dual-mode resonator has a second spurious mode and comprises a second intra-coupling element coupling the second spurious mode to the third mode.

The second intra-coupling element is arranged at a border area between a first side and a second side of the second dual mode resonator where magnetic field lines of the second spurious mode and the third mode are at least partially parallel. This is a favourable position of the intra-coupling element in order to get a high coupling efficiency between the second spurious mode and the third mode.

The second intra-coupling element is one of: a notch, at least a partial chamfer and at least a partial indentation. These forms of intra-coupling elements are favourable in order to get a high coupling efficiency between the second spurious mode and the third mode and a desirable low coupling efficiency between other modes. The notch may be most preferred in order to get a low coupling efficiency between other modes, while the other forms mentioned may be more desirable to achieve the highest possible coupling efficiency between the second spurious mode and the third mode. The second dual-mode resonator may comprise a block of dielectric material in which the intra- coupling element is configured.

In an eighth possible implementation form of a radio frequency filter according to the seventh possible implementation form, the second intra-coupling element is one of: a notch, at least a partial chamfer and at least a partial indentation. These forms of intra-coupling elements are favourable in order to get a high coupling efficiency between the first mode and the first spurious mode and a desirable low coupling efficiency between other modes. The notch may be most preferred in order to get a low coupling efficiency between other modes, while other forms may be more desirable to achieve the highest possible coupling efficiency between the first mode and the first spurious mode.

The first dual-mode resonator may comprise a block of dielectric material in which the intra- coupling element is configured. In a ninth possible implementation form of a radio frequency filter according to the seventh or eighth possible implementation forms, the inter-coupling element is further coupling the first spurious mode to the second spurious mode. This is an alternative to the above described possible implementation forms. In a tenth possible implementation form of a radio frequency filter according to the ninth possible implementation form, the inter-coupling element comprises a third elongated iris arranged parallel to the second elongated iris and displaced from the centre axis on the opposite side of the centre axis compared to the second elongated iris. The function of the third elongated iris is to avoid coupling directly from the first spurious mode to the third mode. The magnetic field of the first spurious mode is rotational around the common centre axis while the magnetic field of the third mode is rotational around an axis being perpendicular to the common centre axis and parallel to the second elongated iris and the third elongated iris. Thus, any positive coupling from the first spurious mode to the third mode through the second elongated iris is largely cancelled out by a similar negative coupling from the first spurious mode to the third mode through the third elongated iris. It may be preferable to arrange the second elongated iris and the third elongated iris symmetrically around the common centre axis in order to optimize the cancellation of the coupling from the first spurious mode to the third mode. The optimization is due to a radial dependency of the magnetic field strength.

In an eleventh possible implementation form of a radio frequency filter according to anyone of the seventh to tenth possible implementation forms, the first mode, the second mode, the third mode and the fourth mode together form a filter pass band, and wherein the frequency of the first spurious mode and the frequency of the second spurious mode are outside the filter pass band.

In a twelfth possible implementation form of a radio frequency filter according to anyone of the seventh to eleventh possible implementation forms, at least one of the electric field vector of the first spurious mode and the electric field vector of the second spurious mode are parallel to the common centre axis.

In a thirteenth possible implementation form of a radio frequency filter according to anyone of the first to the twelfth possible implementation forms or to the first aspect as such, the first dual-mode resonator comprises a third intra-coupling element coupling the first mode to the second mode.

It is advantageous to have a strong coupling between the first mode and the second mode.

The third intra-coupling element is arranged at a border area between sides of the first dual mode resonator where magnetic field lines of the first mode and the second mode are at least partially parallel. This is a favourable position of the third intra-coupling element in order to get a high coupling efficiency between the first mode and the second mode.

The third intra-coupling element is one of: a notch, at least a partial chamfer and at least a partial indentation. These forms of intra-coupling elements are favourable in order to get a high coupling efficiency between the first mode and the first spurious mode and a desirable low coupling efficiency between other modes.

In a fourteenth possible implementation form of a radio frequency filter according to anyone of the first to the thirteenth possible implementation forms or to the first aspect as such, the second dual-mode resonator comprises a fourth intra-coupling element coupling the third mode to the fourth mode. It is advantageous to have a strong coupling between the third mode and the fourth mode.

The fourth intra-coupling element is arranged at a border area between sides of the second dual mode resonator where magnetic field lines of the third mode and the fourth mode are at least partially parallel. This is a favourable position of the fourth intra-coupling element in order to get a high coupling efficiency between the third mode and the fourth mode.

The fourth intra-coupling element is one of: a notch, at least a partial chamfer and at least a partial indentation. These forms of intra-coupling elements are favourable in order to get a high coupling efficiency between the third mode and the fourth mode and a desirable low coupling efficiency between other modes.

The second dual-mode resonator may comprise a block of dielectric material in which the intra- coupling element is configured.

In a fifteenth possible implementation form of a radio frequency filter according to anyone of the first to the fourteenth possible implementation forms or to the first aspect as such, the first dual-mode resonator comprises a first monoblock of solid dielectric material and a first conductive layer covering the first monoblock, and the second dual-mode resonator comprises a second monoblock of solid dielectric material and a second electrically conductive layer covering the second monoblock.

By having a first monoblock of dielectric material in the first dual-mode resonator and a second dielectric material in the second dual-mode resonator the size of the first dual-mode resonator and the second first dual-mode resonator. The size depends on the dielectric constant in the monoblock.

In a sixteenth possible implementation form of a radio frequency filter according to the fourteenth or fifteenth possible implementation forms, the first monoblock, the second monoblock, and the coupling element together form a unitary unit. By the first monoblock, the second monoblock, and the coupling element togetherforming a unitary unit the manufacturing of the radio frequency filter may be facilitated. According to a second aspect a communication device for a wireless communication system is provided, the communication device comprising a radio frequency filter according to any of the first to sixteenth possible implementation forms of the first aspect or to the first aspect as such.

Short description of the drawings

Fig. 1 a shows a dual-mode resonator having a first mode and a second mode. Fig. 1 b shows an alternative dual-mode resonator having a first mode and a second mode. Fig. 2a shows the dual-mode resonator of Fig. 1 a in which a first spurious mode is shown. Fig. 2b shows the dual-mode resonator of Fig. 1 b in which a first spurious mode is shown. Fig. 3 is a view of the first dual-mode resonator along the first spurious electric field lines.

Fig. 4 is a view of the first dual-mode resonator according to an alternative embodiment along the first spurious electric field lines.

Fig. 5 shows schematically a radio frequency filter according to an embodiment of the invention.

Fig. 6 visualizes the concept of an inter-coupling element arranged between a first dual-mode resonator and a second dual-mode resonator. Fig. 7 is a view of the first dual-mode resonator of Fig. 6 along the common centre axis.

Fig. 8 shows a radio frequency filter according to an embodiment of the invention.

Fig. 9 shows the coupling characteristics through the inter-coupling element in the radio frequency filter in Fig. 8.

Fig. 10 shows schematically a radio frequency filter according to an embodiment of the invention. Fig. 1 1 shows schematically a radio frequency filter according to an embodiment of the invention. Fig. 12 shows schematically a radio frequency filter according to an embodiment of the invention.

Fig. 13 shows schematically the first resonator with a different form on the third intra-coupling element.

Fig. 14-18 shows different examples of filter functions realised with radio frequency filters according to different embodiments of the invention. Fig. 19 shows schematically a radio frequency filter 100 according to an embodiment of the invention.

Fig. 20 shows a radio frequency filter according to the embodiment in Fig. 19. Fig. 21 shows schematically a communication device in a wireless communication system. Detailed description

In the following description of embodiments of the invention the same reference numerals will be used for the same or equivalent features in the different drawings.

In the following description of embodiments electromagnetic modes in dielectric monoblocks will be described to explain the embodiments.

Fig. 1 a shows a dual-mode resonator 102 having a first mode 1 a and a second mode 2a. Fig. 1 b shows an alternative dual-mode resonator 102 having a first mode 1 a and a second mode 2a. The first mode 1 a has a first electric field vector E1 and a first magnetic field vector H1. The second mode 2a has a second electric field vector E2 and a second magnetic field vector H2. The first electric field vector E1 is perpendicular to the second electric field vector E2. The frequency of the first mode 1 a is similar to the frequency of the second mode 2a and depends on the size of the dual-mode resonator 102 and the dielectric constant of the dual-mode resonator 102. The size of the dual-mode resonator 102 will thus be larger if the dual-mode resonator 102 consists of air enclosed by reflecting sides compared to if the dual-mode resonator 102 consists of a material with higher dielectric constant. Fig. 2a shows the dual-mode resonator 102 of Fig. 1 a in which a first spurious mode Sa is shown. Fig. 2b shows the dual-mode resonator 102 of Fig. 1 b in which a first spurious mode Sa is shown. The first spurious mode Sa comprises a first spurious electric field vector Es and first spurious magnetic field vectors Hs. The first spurious electric field vector Es is perpendicular to both the first electric field vector E1 and the second electric field vector E2. In the embodiment shown in Fig. 1 a and Fig. 2a the frequency of the first spurious mode Sa is lower than the frequency of the first mode 1 a and lower than the frequency of the second mode 2a due to the fact that the dimension of the first dual-mode resonator 102 is smaller along the first spurious electric field lines Es than the dimension of the first dual-mode resonator 102 along the first electric field lines E1 and the second electric field line E2. In the embodiment shown in Fig. 1 b and Fig. 2b the frequency of the first spurious mode Sa is higher than the frequency of the first mode 1 a and higher than the frequency of the second mode 2a due to the fact that the dimension of the first dual-mode resonator 102 is larger along the first spurious electric field lines Es than the dimension of the first dual-mode resonator 102 along the first electric field lines E1 and the second electric field line E2.

Fig. 3 is a view of the first dual-mode resonator 102 along the first spurious electric field lines Es as well as the common centre axis 126. The first spurious magnetic field vectors Hs are shown in Fig. 3. Also shown in Fig. 3 are a first elongated iris 128 centred on a common centre axis 126 and a second elongated iris 130 arranged perpendicular to the first elongated iris 128 and offset from the common centre axis 126. The first dual-mode resonator 102 has a first electrically conductive layer 1 10 covering the dual mode resonator 102. The first elongated iris 128 and the second elongated iris 130 are openings in the electrically conductive layer 1 10. As can be seen in Fig. 3 the first elongated iris 128 (in its length extension) is at least partly perpendicular to the first spurious magnetic field vectors Hs. The second elongated iris 130 (in its length extension) is at least partly parallel to the first spurious magnetic field vectors Hs. This is due to the offset of the second elongated iris 130 from the common centre axis 126.

The coupling of, the first spurious mode and the second spurious mode by the first elongated iris 128 is minimal when the width of the first elongated iris 128 is small, as is typical. This is due to there being no component of magnetic field along the greater length (length extension) of the first elongated iris 128, with only a small component across the short length (width extension), which is below the cut-off frequency of the first elongated iris 128.

Fig. 4 shows an alternative embodiment of the first dual-mode resonator 102. The difference between the embodiment in Fig. 3 and the embodiment in Fig. 4 is that the second elongated iris 130 is located on the opposite side of the first elongated iris 128.

In the two embodiments shown in Fig. 3 and Fig. 4 the first elongated iris 128 and the second elongated iris 130 are not connected. A gap d is arranged between the first elongated iris 128 and the second elongated iris 130. In further embodiments, the first elongated iris 128 and the second elongated iris 130 can be connected forming a T-shape. In a further embodiment the concept of Fig. 3 and Fig. 4 can be combined. In such an embodiment the first dual-mode resonator 102 has two second elongated irises 130, one arranged as shown in Fig. 3, the other as shown in Fig. 4. Also in this embodiment the first elongated iris 128 and the second elongated iris 130 can be isolated from each other or connected with each other (forming an H-shape). In all embodiment though the elongated irises 128, 130 arranged between two dual mode resonators form an inter-coupling element for coupling (wanted and spurious) modes from the first dual mode resonator to the second dual mode resonator.

Fig. 5 shows schematically a radio frequency filter 100 according to an embodiment of the invention. The radio frequency filter 100 comprises a first dual-mode resonator 102 having a first mode 1 a, a second mode 2a, and a first spurious mode Sa, wherein the first dual-mode resonator 102 comprises a first intra-coupling element 1 12 coupling the first mode 1 a to the first spurious mode Sa, a second dual-mode resonator 104 having a third mode 3b, and a fourth mode 4b, and an inter-coupling element 106 arranged between the first dual-mode resonator 102 and the second dual-mode resonator 104, the inter-coupling element 106 coupling the first mode 1 a to the fourth mode 4b, the second mode 2a to the third mode 3b, and the first spurious mode Sa to the third mode 3b. The frequency of the first spurious mode Sa is lower than the frequency of the first mode 1 a and lower than the frequency of the second mode 2a due to the fact that the dimension of the first dual-mode resonator 102 is smaller along the first spurious electric field lines Es as is shown in Fig. 1 a and Fig. 2a. It is also possible to have the dimension of the first dual-mode resonator 102 larger along the spurious electric field lines Es as is shown in Fig. 1 b and Fig. 2b. In this case the frequency of the first spurious mode Sa is higher than the frequency of the first mode 1 a and higher than the frequency of the second mode 2a. Coupling between the first spurious modes is not immediately beneficial, owing to them being at significantly different frequencies to the main dual-mode resonances used for forming the filter function. Indeed, these spurious frequencies are often problematic and undesirable in their occurrence. However, if energy from a spurious mode can be transmitted to the adjacent cavity (e.g. from the first dual-mode resonator 102 to the second dual-mode resonator 104), there becomes the possibility of using this spurious resonance as a by-pass resonator, in order to provide a diagonal cross-coupling. This concept is shown schematically in Fig. 5. The coupling between the first mode 1 a and the fourth mode 4b is provided for by a first elongated iris 128 between the first dual mode resonator 102 and the second dual mode resonator 104. The coupling between the second mode 2a and the third mode 3b, and the coupling between the first spurious mode Sa and the third mode 3b, is provided for by a second elongated iris 130 128 between the first dual mode resonator 102 and the second dual mode resonator 104. The first elongated iris 128 and the second elongated iris 130 will be explained in further detail below. A third intra-coupling element 154 couples the first mode 1 a to the second mode 2a. A fourth intra-coupling element 156 couples the third mode 3b to the fourth mode 4b. The third intra-coupling element 154 and the fourth intra-coupling element 156 will be explained in further detail below.

Fig 6 visualizes the concept of an inter-coupling element 106 (formed by above mentioned elongated irises 128, 130) arranged between the first dual-mode resonator 102 and the second dual-mode resonator 104 of a radio frequency filter 100. The radio frequency filter 100 comprises a first dual-mode resonator 102 and a second dual-mode resonator 104. The first dual-mode resonator 102 and the second dual-mode resonator 104 are arranged with their respective centres along a common centre axis 126. The inter-coupling element 106 comprises the first elongated iris 128 centred on the common centre axis 126 and the second elongated iris 130 arranged perpendicular to the first elongated iris 128 and offset from the common centre axis 126. The first spurious electric field vector Es is parallel to the common centre axis. The first elongated iris 128 is at a distance d from the second elongated iris 130. The first spurious magnetic field vector Hs are oriented around the first spurious electric field vector Es. Thus the first spurious electric field vector Es is essentially parallel to the second elongated iris 130. The magnetic field strength of the first spurious mode Sa is at its minimum and the electric field strength of the first spurious mode Sa is at its maximum along the common centre axis 126.

The second elongated iris 130 is offset from the common centre axis 126 in a direction of a length component of the first elongated iris 128, wherein a gap is arranged between the first elongated iris 128 and the second elongated iris 130.

The length component of the first elongated iris 128 is arranged in parallel to the electric field vector E2 of the second mode 2a. The length component of the second elongated iris 130 is arranged in parallel to the electric field vector E1 of the first mode 1 a. The length component of the first elongated iris 128 is also arranged in parallel to the electric field vector E3 of the third mode 3b. The length component of the second elongated iris 130 is arranged in parallel to the electric field vector of the fourth mode 4b. This allows coupling of the first mode 1 a to the fourth mode 4b via the first elongated iris 128 and coupling of the second mode 2a to the third mode 3b via the second elongated iris 130. Also, the first spurious mode Sa is coupled to the third mode via the second elongated iris 130. By offsetting from the centre, i.e., the common centre axis 126, the second elongated iris 130, providing the coupling between mode 2a and 3b, the vectors of magnetic flux from the first spurious mode Sa become substantially aligned with both the second elongated iris 130 and the vectors of magnetic flux of mode 3b.

In Fig. 6 no intra-coupling elements are shown. These will be shown and described in relation to Fig. 8 and Figs. 10-13. Intra-coupling elements couple the modes within a dual mode resonator 102. Fig. 7 is a view of the first dual-mode resonator 102 along the common centre axis 126. In Fig. 7 the first spurious magnetic field vectors Hs and the third magnetic field vectors H3 are shown to be essentially parallel to each other at the second elongated iris 130.

Fig. 8 shows an embodiment of a radio frequency filter 100. Only the features not described with reference to Fig. 6 will be described. In the embodiment in Fig. 8 the first elongated iris 128 and the second elongated iris 130 are connected to each other and together forming a T. As described before, the two elongated irises 128 and 130 together form an inter-coupling element 106. Also shown in Fig. 8 is a first intra-coupling element 1 12 arranged at a border area between a first side 134 and a second side 136 of the first dual mode resonator 102 where magnetic field lines of the first spurious mode Sa and the first mode 1 a are at least partially parallel. The first intra-coupling element 1 12 is a notch in the embodiment shown in Fig. 8. Because the first spurious mode Sa is significantly separated in frequency to the primary modes, a large chamfer/cut/notch is required in order to couple sufficient energy between modes. As such, a notch is preferred to a cut or chamfer along the full length of the first dual mode resonator 102 as it results in significantly less spurious coupling or interaction between other modes within the resonant block, thereby enabling the accurate design of a filter using these features.

The first dual-mode resonator 102 comprises a third intra-coupling element 154 coupling the first mode 1 a to the second mode 2a. The third intra-coupling element 154 is arranged at a border area between the second side 136 and a third side 168 of the first dual mode resonator 102 where magnetic field lines of the first mode 1 a (Fig. 5) and the second mode 2a (Fig. 5) are at least partially parallel. This is a favourable position of the third intra-coupling element in order to get a high coupling efficiency between the first mode 1 a and the second mode 2a. In the embodiment shown in Fig. 8 the third intra-coupling element 154 is in the form of an indentation. The second dual-mode resonator 104 comprises a fourth intra-coupling element 156 coupling the third mode 3b to the fourth mode 4b. The fourth intra-coupling element 156 is arranged at a border area between a first side 158 and a second side 160 of the second dual mode resonator 104. In this border area the magnetic field lines of the third mode 3b and the fourth mode 4b are at least partially parallel. This is a favourable position of the fourth intra-coupling element 156 in order to get a high coupling efficiency between the third mode 3b and the fourth mode 4b. In the embodiment shown in Fig. 8 the fourth intra-coupling element 156 is in the form of an indentation. Fig. 9 shows the coupling characteristics through the inter-coupling element 106 in the radio frequency filter in Fig. 8 in dependence of the position of the second elongated iris 130 from the common centre axis 126. The coupling characteristics is shown on the vertical axis in MHz. The solid lines show the coupling efficiency between the first spurious mode Sa and the second spurious mode Sb for different lengths L2 (Fig. 3, 4) of the second elongated iris 130. The dashed lines describe the coupling between the second mode 2a and the third mode 3b for different lengths L2 (Fig. 3, 4) of the second elongated iris 130. The coupling efficiency for the first spurious mode Sa to the second spurious mode Sb is shown for convenience as both resonances are at the same frequency. The coupling of the first spurious mode Sa to the third mode 3b is proportional to the coupling of the first spurious mode Sa to the second spurious mode Sb and has a similar dependency on iris position, albeit being at different frequencies. It is apparent that, although the magnetic flux density of mode 3b is largely uniform in the iris cross-section and of the same magnitude for all iris bridge positions, the component of magnetic flux of the spurious mode, Sa, is not. As such, as the second elongated iris 130 is displaced further from the common centre axis 126, the magnitude of the coupling from Sa to 3b will increase, accordingly. This is clearly apparent and illustrated in Fig. 9. Furthermore, as the spurious mode, Sa, has both a vertical and horizontal component of flux on either side of the common centre axis 126, displacing the inter-coupling element 106 in one direction will result in an opposite sign coupling with respect to that obtained from displacing it in the other direction.

A further advantage of the displaced second elongated iris 130, because the magnetic field vectors of the second modes 2a and the third mode 3b are of constant magnitude and direction for all iris positions, is that the coupling strength between the second mode 2a and the third mode 3b is invariant to its position. This means that the primary coupling from the second mode 2a to the third mode 3b can be controlled by varying the length L2 (Fig. 3, 4) of the second elongated iris 130, and the coupling of the first spurious mode and the second spurious mode can be controlled by the position of the second elongated iris, thereby allowing independent control of both couplings, from a single feature.

Fig. 10 shows schematically a radio frequency filter 100 according to an alternative embodiment of the invention. Only the differences between the embodiment in Fig. 8 and the embodiment in Fig. 10 will be described. In Fig. 10 the third intra-coupling element 154 has been arranged on the opposite side of the second side 136 where the magnetic field lines of the first mode 1 a and the second mode 2a are at least partially parallel. Fig. 1 1 shows schematically a radio frequency filter according 100 to an alternative embodiment of the invention. Only the differences between the embodiment in Fig. 10 and the embodiment in Fig. 1 1 will be described. In Fig. 1 1 the first intra-coupling element 1 12 has been arranged on the opposite side of the first resonator 102, where the magnetic field lines of the first mode 1 a and the first spurious mode Sa are at least partially parallel.

Fig. 12 shows schematically a radio frequency filter 100 according to an alternative embodiment of the invention. Only the differences between the embodiment in Fig. 1 1 and the embodiment in Fig. 12 will be described. In Fig. 12 the second elongated iris 130 is placed on the opposite side of the first elongated iris 128, when compared to the previous embodiment in Fig. 1 1.

The operation of the radio frequency filters 100 according to the embodiments in Fig. 8 and 10-12 is as follows. The first dual-mode resonator 102 is fed with an electromagnetic wave to be filtered into the first mode 1a. The first intra-coupling element 1 12 will provide coupling from the first mode 1 a to the first spurious mode Sa and the third intra-coupling element 154 will provide coupling from the first mode 1 a to the second mode 2a. The inter-coupling element 106 will then provide coupling from the first mode 1 a to the fourth mode 4b, from the first spurious mode Sa to the third mode 3b and from the second mode 2a to the third mode 3b. More specifically, the first elongated iris 128 provides coupling from the first mode 1 a to the fourth mode 4b, and the second elongated iris 130 provides coupling between the second mode 2a and the third mode 3b, and the coupling between the first spurious mode Sa and the third mode 3b. Finally, the fourth intra-coupling element 156 will provide coupling from the third mode 3b to the fourth mode 4b. The first mode 1 a, the second mode 2a, the third mode 3b and the fourth mode 4b together form a filter pass band. The frequency of the first spurious mode Sa and the frequency of the second spurious mode Sb are outside the filter pass band. Fig. 13 shows schematically the first dual-mode resonator 102 with a different form on the third intra-coupling element 154. In contrast to the third intra-coupling elements 154 as described above the third intra-coupling element 154 in Fig. 13 is in the form of a full length chamfer. Fig. 14-18 shows different examples of filter functions realised with radio frequency filters 100 according to different embodiments of the invention. The different filter functions show the transmission in dB through the different radio frequency filters 100 as a function of the frequency in GHz. The filter functions show the transmission S 21 in dB as a function of the frequency in GHz. The single or pairs of transmission zeros shown in Figs. 14-18 can be formed with many different combinations of circuit topologies and slot offsets - there is no specific position for the slot offset or notch location that would be required for a given transmission zero location - it depends on the complete circuit. In Figs. 14-18 the solid lines and dotted lines show the typical variation that is obtainable as the notch and offset parameters are varied, i.e. that the transmission zero position is controllable by design.

Fig. 14 shows an example of strong and weak inductive triplet realised with an embodiment of the invention. As can be seen in Fig. 14 the filter function has a transmission dip above the transmission peak. Fig. 15 shows an example of strong and weak capacitive triplet realised with an embodiment of the invention. As can be seen in Fig. 15 the filter function has a transmission dip below the transmission peak.

Fig. 16 shows an example of strong and weak inductive quadruplet realised with an embodiment of the invention. As can be seen in Fig. 16 the filter function has two transmission dips above the transmission peak.

Fig. 17 shows an example of strong and weak inductive triplet realised with an embodiment of the invention. As can be seen in Fig. 17 the filter function has two transmission dips below the transmission peak.

Fig. 18 shows an example of strong and weak capacitive quadruplet realised with an embodiment of the invention. As can be seen in Fig. 18 the filter function has one transmission dip below the transmission peak and one above the transmission peak. Fig. 18 shows an example on significant high-and low-side skew. Fig. 19 shows schematically a radio frequency filter 100 according to an alternative embodiment. The radio frequency filter 100 comprises a first dual-mode resonator 102 having a first mode 1 a, a second mode 2a, and a first spurious mode Sa, wherein the first dual-mode resonator 102 comprises a first intra-coupling element 1 12 coupling the first mode 1 a to the first spurious mode Sa, a second dual-mode resonator 104 having a third mode 3b, a fourth mode 4b, and a second spurious mode Sb, and an inter-coupling element 106 arranged between the first dual-mode resonator 102 and the second dual-mode resonator 104, the inter- coupling element 106 coupling the first mode 1 a to the fourth mode 4b, the second mode 2a to the third mode 3b, and the first spurious mode Sa to the second spurious mode Sb. The second dual-mode resonator 104 has a second spurious mode Sb and comprises a second intra-coupling element 132 coupling the second spurious mode Sb to the third mode 3b. The first spurious electric field vector Es is parallel to the common centre axis 126 (shown in Fig. 20). The first elongated iris 128 is at a distance d from the second elongated iris 130. The first spurious magnetic field vector Hs are oriented around the first spurious electric field vector Es. Thus the first spurious electric field vector is essentially perpendicular to the second elongated iris 130. The magnetic field strength of the first spurious mode Sa is at its minimum and the electric field strength of the first spurious mode Sa is at its maximum along the common centre axis 126. The coupling between the first mode 1a and the fourth mode 4b is provided by a first elongated iris 128. The coupling between the second mode 2a and the third mode 3b, and the coupling between the first spurious mode Sa and the second spurious mode Sb, is provided by a second elongated iris 130 and a third elongated iris 138. The first elongated iris 128, the second elongated iris 130 and the third elongated iris 138, forming the inter-coupling element 106 will be explained in further detail with reference to Fig. 20. A third intra-coupling element 154 couples the first mode 1 a to the second mode 2a. A fourth intra-coupling element 156 couples the third mode 3b to the fourth mode 4b. The third intra-coupling element 154 and the fourth intra-coupling element 156 will be explained in further detail with reference to Fig. 20.

Fig. 20 shows a radio frequency filter 100 according to the embodiment in Fig. 19. The first dual-mode resonator 102 comprises a first monoblock 108 of solid dielectric material and a first electrically conductive layer 1 10 covering the first monoblock 108, and the second dual- mode resonator 104 comprises a second monoblock 1 14 of solid dielectric material and a second electrically conductive layer 1 16 covering the second monoblock 1 14. The first monoblock 108, the second monoblock 1 14, and the coupling element 106 together form a unitary unit. This facilitates the production of the radio frequency resonator.

The first dual-mode resonator 102 and the second dual-mode resonator 104 are arranged with their respective centres along a common centre axis 126. The inter-coupling element 106 comprises a first elongated iris 128 centred on the common centre axis 126, a second elongated iris 130 arranged perpendicular to the first elongated iris 128 and offset from the common centre axis 126, and a third elongated iris 138 arranged parallel to the second elongated iris 130 and displaced from the centre axis 126 on the opposite side of the centre axis 126 compared to the second elongated iris 130. The second elongated iris 130 couples the first spurious mode Sa to the second spurious mode Sb. The third elongated iris 138 couples the first spurious mode Sa to the second spurious mode Sb. Thus both the second elongated iris 130 and the third elongated iris 138 couple the first spurious mode Sa to the third mode 3b. The magnetic field vector of the first spurious mode Sa rotates around the common centre axis 126 while the electric field vector of the third mode 3b is orthogonal to the common centre axis 126 (but in the same plane). This, means that the second elongated iris 130 will contribute with a positive coupling while the third elongated iris 138 will contribute with a negative coupling. If the second elongated iris 130 and the third elongated iris 138 are positioned at the same distance from the common centre axis 126 the contribution from the second elongated iris 130 will cancel contribution from the third elongated iris 130.

The magnetic field vectors of the first spurious mode Sa and the magnetic field vectors of the second spurious mode Sb are circumferential to the common centre axis 126. The first intra-coupling element 1 12 is arranged at a border area between a fifth side 170 and a sixth side 172 of the first dual mode resonator 102 where magnetic field lines of the first spurious mode Sa and the first mode 1 a are at least partially parallel. The first intra-coupling element 1 12 is a notch in the embodiment shown in Fig. 20. Because the spurious mode is significantly separated in frequency to the primary modes, a large chamfer/cut/notch is required in order to couple sufficient energy between modes. As such, a notch is preferred to a cut or chamfer along the full length of the first dual mode resonator 102 as it results in significantly less spurious coupling or interaction between other modes within the resonant block, thereby enabling the accurate design of a filter using these features. The second intra-coupling element 132 is arranged at a border area between a first side 158 and a third side 162 of the second dual mode resonator 104 where magnetic field lines of the second spurious mode Sa and the third mode 3b are at least partially parallel. The second intra-coupling element 132 is a notch in the embodiment shown in Fig. 20 for the same reasons as for the first intra-coupling element.

The first dual-mode resonator 102 comprises a third intra-coupling element 154 coupling the first mode 1 a to the second mode 2a. The third intra-coupling element 154 is arranged at a border area between a second side 136 and a fourth side 140 of the first dual mode resonator 102 where magnetic field lines of the first mode and the second mode are at least partially parallel. This is a favourable position of the third intra-coupling element in order to get a high coupling efficiency between the first mode and the second mode. In the embodiment shown in Fig. 20 third intra-coupling element is in the form of an indentation.

The second dual-mode resonator 104 comprises a fourth intra-coupling element 156 coupling the third mode 3b to the fourth mode 4b. The fourth intra-coupling element 156 is arranged at a border area between the first side 158 and a second side 160 of the second dual mode resonator 104 where magnetic field lines of the third mode 3b and the fourth mode 4b are at least partially parallel. This is a favourable position of the fourth intra-coupling element 156 in order to get a high coupling efficiency between the third mode 3b and the fourth mode 4b. In the embodiment shown in Fig. 20 the fourth intra-coupling element 156 is in the form of an indentation.

In operation the first dual-mode resonator 102 is fed with an electromagnetic wave to be filtered into the first mode 1 a. The first intra-coupling element 1 12 will provide coupling from the first mode 1 a to the first spurious mode and the third intra-coupling element will provide coupling from the first mode 1a to the second mode 2a. The inter-coupling element 106 will then provide coupling from the first mode 1 a to the fourth mode 4b, from the first spurious mode Sa to the second spurious mode Sb and from the second mode 2a to the third mode 3b. More specifically, the first elongated iris 128 provides coupling between the first mode 1 a and the fourth mode 4b, and the second elongated iris 130 and the third elongated iris 138 provides coupling between the second mode 2a and the third mode 3b, and the coupling between the first spurious mode Sa and the second spurious mode Sb. Finally, the second intra-coupling element 132 will provide coupling from the second spurious mode Sb to the third mode 3b and the fourth intra-coupling element 156 will provide coupling from the third mode 3b to the fourth mode 4b. The first mode 1 a, the second mode 2a, the third mode 3b and the fourth mode 4b togetherform a filter pass band. The frequency of the first spurious mode Sa and the frequency of the second spurious mode Sb are outside the filter pass band.

Fig. 21 shows schematically a communication device 300 in a wireless communication system 400. The communication device 300 comprises a radio frequency filter 100 according to an embodiment of the invention. The wireless communication system 400 also comprises a base station 500 which may also comprise a radio frequency filter 100 according to any one of the embodiments described above. The dotted arrow A1 represents transmissions from the transmitter device 300 to the base station 500, which are usually called up-link transmissions. The full arrow A2 represents transmissions from the base station 500 to the transmitter device 300, which are usually called down-link transmissions.

The present communication device 300 may be any of a User Equipment (UE) in Long Term Evolution (LTE), mobile station (MS), wireless terminal or mobile terminal which is enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The UE may further be referred to as mobile telephones, cellular telephones, computer tablets or laptops with wireless capability. The UEs in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice or data, via the radio access network, with another entity, such as another receiver or a server. The UE can be a Station (STA), which is any device that contains an IEEE 802.1 1 -conformant Media Access Control (MAC) and Physical Layer (PHY) interface to the Wireless Medium (WM). The present communication device 300 may also be a base station a (radio) network node or an access node or an access point or a base station, e.g., a Radio Base Station (RBS), which in some networks may be referred to as transmitter, "eNB", "eNodeB", "NodeB" or "B node", depending on the technology and terminology used. The radio network nodes may be of different classes such as, e.g., macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. The radio network node can be a Station (STA), which is any device that contains an IEEE 802.1 1 -conformant Media Access Control (MAC) and Physical Layer (PHY) interface to the Wireless Medium (WM).