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Title:
SLOT LINE RESONATOR
Document Type and Number:
WIPO Patent Application WO/2015/114004
Kind Code:
A1
Abstract:
Slot line resonator resonator comprising: a first conductive layer and a second parallel conductive layer, a slot line (10; 20; 31) provided in the first conductive layer, and being folded in a first spiral pattern with a shape factor such that the slot line has parallel or concentric sections, a feed structure (11,12,13; 21,22,23; 32,33) provided on the second conductive layer and configured to feed the slot line by coupling, wherein the feed structure comprises a non-resonant patch (11; 21; 32) located in correspondence with the spiral pattern, said patch being extended by a feed line (12; 22; 33).

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Inventors:
LOUZIR, Ali (Technicolor R&D France, 975 avenue des Champs BlancsZAC des Champs Blanc, CS 17616 Cesson-Sévigné, F-35576, FR)
JOSHI, Chetan (C-5, CEERI Campus Pilani,India, Rajasthan PIN 1, 33303, IN)
ROBERT, Jean-Luc (2 rue Paul Gauguin, Betton, F-35830, FR)
Application Number:
EP2015/051703
Publication Date:
August 06, 2015
Filing Date:
January 28, 2015
Export Citation:
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Assignee:
THOMSON LICENSING (1-5 rue Jeanne d'Arc, Issy-les-Moulineaux, F-92130, FR)
International Classes:
H01P1/201; H01P5/10; H01P7/08; H01Q13/16; H01P5/02
Domestic Patent References:
WO2003094293A12003-11-13
Foreign References:
KR20110031614A2011-03-29
US20060017527A12006-01-26
FR1450223A2014-01-13
Other References:
HYO RIM BAE ET AL: "A Crooked U-Slot Dual-Band Antenna With Radial Stub Feeding", IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, IEEE, PISCATAWAY, NJ, US, vol. 8, 1 January 2009 (2009-01-01), pages 1345 - 1348, XP011331185, ISSN: 1536-1225, DOI: 10.1109/LAWP.2009.2038937
JIUN-PENG CHEN ET AL: "A spirally complementary split-ring resonators antenna for circular polarization and RFID reader application", ANTENNAS AND PROPAGATION SOCIETY INTERNATIONAL SYMPOSIUM (APSURSI), 2012 IEEE, IEEE, 8 July 2012 (2012-07-08), pages 1 - 2, XP032472219, DOI: 10.1109/APS.2012.6349199
S.C. LIN; C.H. WANG; C.H. CHEN: "Novel patch-via-spiral resonators for the development of miniaturized bandpass filters with transmission zeros", IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, vol. 55, no. 11, January 2007 (2007-01-01), pages 137 - 146
Attorney, Agent or Firm:
BARR, Angela (Technicolor, 1-5 rue Jeanne d'Arc, Issy-les-Moulineaux, F-92443, FR)
Download PDF:
Claims:
CLAIMS

1 -Slot line resonator for a filter, the resonator comprising: a first conductive layer and a second parallel layer, a slot line (10; 20; 31 ) provided in the first conductive layer, and being folded in a first spiral pattern with a shape factor such that the slot line has parallel or concentric sections, a feed structure (1 1 ,12,13; 21 ,22,23; 32,33) provided in the second conductive layer and configured to feed the slot line by coupling, wherein the feed structure comprises a non-resonant patch (1 1 ; 21 ; 32) located in correspondence with the spiral pattern, said patch being extended by a feed line (12; 22; 33).

2. A resonator according to claim 1 wherin the coupling between the slot line and the non-resonant patch is a non-electromagnetic coupling.

3. A resonator according to claim 1 or 2 in which the size of the non-resonant patch is determined based on the resonant frequency of the slot line.

4. A resonator according to any one of the preceding claims wherein the non- resonant patch is arranged to entirely cover the slot line. 5. A resonator according to any one of the preceding claims wherein the non- resonant patch and the slot line are interconnected by a first metal-plated via (14; 24; 36).

6. A resonator according to claim 5, wherein the via is positioned either at the centre of the patch or on one of the edges. 7. A resonator according to any one of the preceding claims wherein an impedance transformer (13; 23) is inserted between the patch and the feed line.

8. A resonator according to any preceding claim wherein the first conductive layer is provided on a first surface of a dielectric substrate and the second layer is provided on a second parallel surface of the dielectric substrate.

9. A resonator according to any preceding claim comprising, a microstrip transmission line (25; 34; 37) folded in a second spiral pattern between the first conductive layer and the second conductive layer.

10. A resonator accordring to claim 9 comprising a a multi-layer substrate wherein the microstrip transmission line is provided on a layer between the first conductive layer and the second conductive layer.

1 1 . A resonator according to claim 9 or 10 wherein the microstrip transmission line is connected to the patch by a second metal-plated via.

12. A resonator according to claim any one of claims 9 to 1 1 , wherein the first and second spiral patterns are folded in the same direction.

13. A resonator according to claim 1 1 or 12, wherein the first and second vias are collinear.

14. A resonator according to any one of claims 1 1 to 13, wherein the first via is positioned on an edge of the patch and the second via is positioned at the centre of the patch.

15. A resonator according to any one of the preceding claims, wherein the shape of the patch and of the first and second spiral patterns is polygonal such as square, rectangular, triangular, or circular.

16. A resonator according to any one of the preceding claims wherein the slot line has an electrical length L less than or equal to k λ/2 where λ is the guided wavelength in the slot at the harmonic frequency of order (k-1 ) and k is an integer greater than or equal to 1 (the fundamental mode being the zero harmonic)

17. A resonator according to any one of the preceding claims wherein the electric field of two parallel sections of the slot line is in phase opposition,

18. Single or multiple passband filter, comprising at least one slot line resonator (40; 41 ) according to any one of the preceding claims.

Description:
SLOT LINE RESONATOR

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a slot line resonator for a filters, for example a compact resonator for a selective filter on cmulti-layer substrates. The present invention also relates to band-pass filters including such resonators, such filters being adapted notably but not exclusively to wireless or mobile communication devices.

TECHNOLOGICAL BACKGROUND

With the growing demand for new services, devices used for mobile communications and in home networks must be able to operate at different frequencies and according to several standards. In this case, it is necessary, in order to maintain the integrity of the signals corresponding to these different standards, to use very narrow-band filters constituted of high quality factor resonators. In general, the implementation of such filters requires a compromise between on one hand the electrical performance of the filter and on the other hand its cost and size. The performance of a filter depends on the quality factor Q of the resonator used. The higher the quality factor, the better the performance. However, a high quality factor Q involves the use of technologies whose cost is high and the filters used in these technologies such as SMD (surface mounted device) filters are most often bulky, which is hardly compatible with the necessities of mobile devices.

To overcome these disadvantages, French patent application number 14 50223 filed on 13 January 2014 in the name of Thomson Licensing proposed a new type of printed resonator which is small and has a high quality factor Q and a reduced sensitivity to the parameters of the substrate. In this case, the resonator is made using a slot line folded in a spiral pattern and fed by electromagnetic coupling according to the Knorr principle.

The use of a slot line folded in a spiral to make resonators for band-pass filters was also proposed in the article IEEE Transactions on Microwave Theory and Techniques, Vol. 55, issue 1 1 , pp.137-146 dated January 2007 in the name of S.C. LIN, C.H. WANG and C.H. CHEN entitled "Novel patch-via-spiral resonators for the development of miniaturized bandpass filters with transmission zeros".

Figures 1A and 1 B diagrammatically show a spiral slot line resonator as described in said article. Part A diagrammatically shows a plan view of said resonator while part B shows a diagrammatic perspective view of it. As shown in figure 1 B, a dielectric substrate 1 is equipped on one of its faces with a conductive layer 2 wherein a slot line 3 has been etched in a spiral pattern. This slot line has a width Ws and a length which is a function of the operating frequency. On the face of the substrate opposite the conductive layer 2, a patch 4 made of a conductive layer has been implemented. This patch 4 has a width Wp and a length Dp covering the spiral pattern as shown in figure 1A. Moreover, the slot line 3 is folded in a spiral pattern such that the spacing between two parallel slots is equal to Gs. As shown in figure 1A, the spiral pattern 3 is interconnected with the patch 4 represented by dashes, forming a microstrip line/coplanar waveguide transition using a metal-plated via 7. Moreover, the resonant element 3 is connected to a feed line 5 implemented on the same face as the patch 4 via the intermediary of a conductive via 6 mounted between the line 5 and the conductive layer 2. As described in the article, different resonators of this type can be coupled together to make band-pass filters; however, the performance of the resonator depends largely on the different sizes used and is very sensitive to the parameters of the substrate and to the production tolerances.

The purpose of the present invention is therefore to propose a resonator printed on a single-layer or multi-layer substrate which is particularly compact and not very sensitive to the parameters of the substrate and to the production tolerances. This resonator overcomes the disadvantages of the slot line resonator described in the article above as will be explained in more detail hereafter. The present invention has been devised with the foregoing in mind.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a slot line resonator for a filter, the resonator comprising:a first conductive layer and a second layer parallel to the first layer,a slot line (10; 20; 31 ) provided in the first layer, and being folded in a first spiral pattern with a shape factor such that the slot line has parallel or concentric sections, ,a feed structure (1 1 ,12,13; 21 ,22,23; 32,33) provided on the second layer and configured to feed the slot line by coupling, wherein the feed structure comprises a non-resonant patch (1 1 ; 21 ; 32) located in spatial correspondence with the spiral pattern, said patch being extended by a feed line (12; 22; 33).

In an embodiment the first layer is provided on one surface of a substrate and the second layer is provided on an opposite surface of the substrate. In another embodiment the first conductive layer and the second layer are provided in a multilayer substrate.

In an embodiment the coupling between the slot line and the non-resonant patch is a non-electromagnetic coupling. In an embodiment the size of the non-resonant patch is determined based on the resonant frequency of the slot line.

In an embodiment the non-resonant patch is arranged to entirely cover the slot line.

In an embodiment the non-resonant patch and the slot line are interconnected by a first metal-plated via.

In an embodiment the via is positioned either at the centre of the patch or on one of the edges.

In an embodiment an impedance transformer is inserted between the patch and the feed line. In an embodiment, a microstrip transmission line folded in a second spiral pattern is provided between the conductive layer and the second layer..

In an embodiment the microstrip transmission line is connected to the patch by a second metal-plated via.

In an embodiment the first and second spiral patterns are folded in the same direction, that is to say either in the clockwise direction or in the anti-clockwise direction.

In an embodiment the first and second vias are collinear.

In an embodiment the first via is positioned on an edge of the patch and the second via is positioned at the centre of the patch. In an embodiment the shape of the patch and of the first and second spiral patterns is polygonal such as square, rectangular, triangular, or circular.

In an embodiment the slot line has an electrical length L less than or equal to k λ/2 where λ is the guided wavelength in the slot at the harmonic frequency of order (k-1 ) and k is an integer greater than or equal to 1 (the fundamental mode being the zero harmonic)

In an embodiment the electric field of two parallel sections is in phase opposition, A further aspect of the invention provides a single or multiple passband filter, comprising at least one slot line resonator (40; 41 ) according to any embodiment of the first aspect of the invention.

A further aspect of the invention provides a slot line resonator for single or multiple passband filters comprising:

- a dielectric substrate with at least one first face equipped with a conductive layer and a second parallel face,

a slot line etched in the conductive layer, the slot line having an electrical length L less than or equal to kA/2 where λ is the guided wavelength in the slot at the harmonic frequency of order (k-1 ) and k is an integer greater than or equal to 1 (the fundamental mode being the zero harmonic) and being folded in a first spiral pattern with a shape factor such that the slot line has parallel or concentric sections, the electric field of two parallel sections being in phase opposition,

a feed structure implemented on the second face of the substrate and feeding the slot line by coupling,

wherein the feed structure comprises a non-resonant patch located under the spiral pattern, said patch being extended by a feed line.

Preferably, the non-resonant patch and the slot line are interconnected by a first metal-plated via. According to different embodiments, the via is positioned at the centre of the patch or on one of the edges of the patch, the positioning of the via making it possible to modify the resonant frequency. Moreover, an impedance transformer may be inserted between the patch and the feed line.

In an embodiment, the substrate is a multi-layer substrate, a microstrip transmission line folded in a second spiral pattern is implemented on at least one layer between the two outer layers of the substrate, said microstrip line being connected to the patch by a second metal-plated via. This embodiment makes further reduction of the surface area of the resonator possible by taking advantage of the multi-layer structure of the substrate. It also makes it possible to produce a second resonance. According to an embodiment, the first and the second spiral pattern are folded in the same direction, that is to say either in the clockwise direction or in the anti-clockwise direction. Moreover, the first and second vias can be collinear or one of the vias can be placed at the centre of the patch while the other via is positioned on an edge of the patch.

According to another characteristic of the present invention, the shape of the patch and the shape of the first and second spiral patterns are similar and chosen from among polygonal shapes, namely square, rectangular, triangular or other shapes, or the circular shape.

Embodiments of the invention provide a resonator printed on a single-layer or multi-layer substrate which is particularly compact and not very sensitive to the parameters of the substrate and to the production tolerances.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the present invention will appear upon reading the description of different embodiments, this description being made with reference to the enclosed drawings, wherein:

Figures 1A and 1 B already described show in A a very diagrammatic plan view of a slot line resonator according to a prior art and in B a perspective view of the same resonator.

Figure 2 is a diagrammatic plan view of an embodiment of a slot line resonator in accordance with the present invention.

Figure 3 is a curve giving the resonance as a function of the frequency for the embodiment of figure 2. - Figures 4A and 4B show in A different curves giving the resonance as a function of the frequency for different dimensions of the patch in the case of the embodiment of the prior art shown in figures 1A and 1 B and in B different curves giving the resonance as a function of the frequency for the same patch dimensions but in the case of the embodiment of figure 2 in accordance with the present invention. Figure 5 diagrammatically shows different positions for the via interconnecting the slot line with the patch in the case of the embodiment of figure 2.

Figure 6 shows the curves of resonance as a function of the frequency for the different positions of the via given in figure 5.

Figure 7 is a perspective view of a second embodiment of a slot line resonator in accordance with the present invention.

Figures 8A, 8B, 8C respectively show in A a diagrammatic plan view of the resonator of figure 7, in B a cross-section at the level of the patch shown in part A and in C a curve giving the resonance as a function of the frequency for the embodiment shown in A and B.

Figure 9 diagrammatically shows the dimensions obtained for the patch and the spiral pattern of the slot line, respectively for the embodiment of figure 2 and for the embodiment of figure 7 at a same operating frequency.

Figures 10A and 10 B show in A a perspective view of a third embodiment of a slot line resonator in accordance with the present invention and in B a diagrammatic cross-sectional view at the level of the patch of the resonator shown in A.

Figures 1 1 A and 1 1 B diagrammatically show in A different shapes for the patch and in B different shapes for the spiral pattern associated with the patch.

Figure 12 is a diagrammatic perspective view of a fourth embodiment of a slot line resonator in accordance with the present invention.

Figure 13 is a diagrammatic perspective view of a filter using two resonators in accordance with the present invention.

Figure 14 is a diagrammatic cross-sectional view of the filter of figure 13, and Figure 15 shows a curve giving the resonance as a function of the frequency for the embodiment of figures 13 or 14.

DESCRIPTION OF DIFFERENT EMBODIMENTS

The different embodiments described hereafter are given as examples but in no way limit the scope of the attached claims.

A description will first be given, with reference to figures 2 to 6, of a first embodiment of a non-radiating slot line resonator in accordance with the present invention. As shown in figure 2, a slot line 10 has been made by etching in the ground plane of a substrate (not shown). This slot line 10 has an electrical length L less than or equal to kA/2 where λ is the guided wavelength in the slot at the harmonic frequency of order (k-1 ) and k is an integer greater than or equal to 1 , the fundamental mode being the zero harmonic. The slot line 10 is folded in a spiral pattern with, in the case of figure 2, a square shape factor. The slot line may for example be as described in French patent application number 14 50223 of 13 January 2014 in the name of Thomson Licensing. A metal patch 1 1 is etched on the face of the substrate opposite the face receiving the slot line. This patch 1 1 is non- resonant and is configured to feed the slot line 100 as explained hereafter.

While in the illustrated embodiment the slot line 10 is provided on one surface of a substrate and the metal patch is provided on an opposite surface of the substrate, in another embodiments the slot line and the patch may be provided in multi-layer structure the slot line being provided in a layer parallel to a layer on which the patch is provided.

As shown in figure 2, the patch 1 1 is connected to a feed line 12 which is in general a line of 50 ohms impedance. In the embodiment shown, the patch 1 1 is connected to the 50 ohms line 12 via the intermediary of an impedance transformer 13 so that the impedance provided by the patch corresponds to the impedance of the feed line 12. In a practical embodiment, to obtain a resonance at 5GHz, a slot line resonator of length 27.9mm has been implemented; this length makes it possible to obtain a resonance at 5GHz when a standard substrate known as FR4 with a dielectric constant er= 4.6 is used as the substrate. Table 1 below gives the values used for the lengths and widths of the different elements to obtain a resonance at 5GHz. Table 1

tan5=0.02

As shown by reference numeral 14, a metal-plated via is used to connect the spiral slot line 10 to the metal patch 1 1 . The via is positioned, in the embodiment of figure 2, at the centre of the patch 1 1 and of the spiral pattern 10. As shown in figure 3 which relates to the curve of resonance as a function of the frequency, it can be seen that this type of resonator for single passband filters resonates very precisely at a frequency which is in the range of 0-10GHz and that this resonance is spectrally very pure. If two resonators of this type are combined such that their energy can be sufficiently coupled at the resonant frequency, then it is possible to obtain a very effective filtering device.

To demonstrate certain advantages of the slot line resonator in accordance with the present invention with respect to the resonator of the prior art shown in figure 1A and 1 B, simulations varying the size of the patch have been carried out. The effects of varying the size of the patch in the resonator of figures 1 are shown in the curve in figure 4A. In this case, when the dimensions of the patch are varied, starting from the original configuration wherein the patch is a square having a width of 5.08mm x 5.08mm then using the following dimensions namely a rectangle of 5mm x 6mm, of 6mm x 5mm, of 6mm x 4.5mm, of 7mm x 5mm or of 4.5mm x 6mm or a square of 4.5mm x 4.5mm or 6mm x 6mm, resonance curves are obtained which are offset respectively on each side of the resonance curve of the original structure represented by M1. Identical tests were carried out on the resonator in accordance with the present invention shown in figure 2. The results of the tests give the resonance curves of figure 4B. In this case, it is noted that the resonator is much less sensitive to the variations in dimensions of the patch. In fact, in the present invention the patch is not resonant and is used to feed the slot line resonator and not as a resonant element.

In the embodiment of figure 2, the patch 1 1 and the slot line 10 are interconnected by a via 14 positioned at the centre of the patch. However, according to the present invention, it is possible to position the via in different positions as shown in figure 5. Thus the via can be positioned at the centre 14a of one of the edges of the patch called edge 1 centre or at the centre 14b of another edge called edge 2 centre, in 14c at the end of the spiral, in 14d at a corner 1 , or in 14e at another corner referred to as corner 2.

The resonance measurements performed for the different positions of the via of figure 5 show, as shown on the curves of figure 6, that modifying the position of the via 14a to 14e makes it possible to lower the resonant frequency of the slot line resonator for single or multiple passband filters and thus to miniaturise said resonator. Thus, when the via is positioned in the middle of the edge opposite the edge where the patch is fed, this via generates a resonance at low frequency. The position of the via determines the resonant frequency by determining the resonant mode propagated in the resonator.

A description will now be given, with reference to figures 7 and 8A, 8B, 8C of a second embodiment of a slot line resonator for single or multiple passband filters in accordance with the present invention. In this case, the resonator is implemented on a multi-layer substrate. Thus, as shown diagrammatically in figure 7, a slot line in a spiral pattern with a square shape factor as described with reference to figure 2 has been implemented in the ground plane M of a substrate. As for figure 2, a patch 21 whose dimensions correspond to the dimensions of the spiral pattern 20 has been implemented on the outer face opposite the face receiving the ground plane M. The patch 21 is connected to an input port 22 at 50 ohms via the intermediary of an impedance transformer 23. As shown more specifically with reference to figures 8A and 8B, a microstrip line 25 has been etched on an intermediate layer between the layer receiving the slot line 20 and the metal patch 21. This line has been etched in a spiral pattern as shown in figures 7 and 8a. Moreover, a metal-plated via 24 respectively connects the microstrip line 25 to the metal patch 21 and to the slot line 20. This via is positioned in the middle of the patch as shown in figure 8B.

A slot line resonator as described above has been tested for an initial resonant frequency of 5GHz. The curve of resonance dB (S (1 ,1 )) as a function of the frequency shown in figure 8C shows the appearance of a new resonant frequency at 2.4GHz as represented by ml . The resonant frequencies due to this specific structure can be tuned individually or collectively by choosing the position of the vias and by modifying the length of the spiral patterns of the slot line and/or of the microstrip line. This very compact resonator can be used to make coupled multi- band filters.

Moreover, when the properties of the slot line resonator of figure 2 are compared with the slot line resonator of figure 8A, it is apparent that, as shown in the left part of figure 9, the resonator is constituted of a square patch 1 1 and of a spiral pattern 10 having a dimension of 5.08mm for a resonant frequency of 5GHz while the resonator of the right part of figure 9 corresponding to the resonator of figure 8A with a patch 21 , a slot line 20 having a square spiral pattern and a microstrip line 25 on an intermediate layer between the patch 21 and the slot line 20 with a spiral pattern has a dimension of 2.6mm for a resonant frequency of 5GHz. A smaller resonator than the resonator without microstrip line is therefore obtained.

A variant embodiment of the resonator shown in figure 7 will now be described with reference to figures 10A and 10B. In this case as shown in figure 10A, the substrate is a multi-layer substrate comprising a metal layer 30 wherein a slot line 31 has been etched in a spiral pattern. A patch 32 facing the spiral pattern 31 has been etched on the outer face of the substrate. This patch 32 is connected via the intermediary of an impedance transformer 33 to a feed line not shown. A microstrip line 34 has been etched in a spiral pattern on an intermediate layer between the metal layer 30 and the face receiving the patch 32. This micrtostrip line is positioned between the spiral pattern of the slot line 31 and the patch 32. As shown more clearly in figure 10B, the metal patch 32 is interconnected with the ground plane comprising the slot line via the intermediary of a metal-plated via 36 positioned, in this embodiment, on the edge of the patch opposite the edge of connection to the feed. The microstrip line 34 is connected via the intermediary of a via 35 to the middle of the ground plane 30. As mentioned above, interconnecting the patch 32 to the ground plane of the slot line 30 at an edge of the patch makes it possible to lower the resonant frequency.

The spiral pattern used for the embodiments of figures 2, 7 and 10 is a spiral pattern having a square shape factor. However, as shown in figure 1 1 , other shape factors can be used to make both the patch and the spiral pattern for the slot line. Thus, a patch and a spiral pattern having a circular, rectangular, square, triangular or other polygonal shape factor as shown in figure 1 1 can be used, one of the constraints being that the patch covers the spiral pattern.

Generally, in order to ensure an optimum coupling between the patch providing the excitation and the resonant spiral pattern(s), the shape factor of the patch is such that it covers the spiral pattern(s).

Figure 12 shows an embodiment originating from the embodiment of figure 10 with, in the case of a multi-layer substrate, the implementation of spiral microstrip lines on two intermediate layers of the substrate. More specifically, reference numeral 30 represents the conductive layer of the substrate wherein the slot line 31 is implemented in a spiral pattern. Reference numeral 32 represents the patch implemented opposite the spiral pattern 31 , on the face of the substrate opposite the conductive layer 31. The patch 32 is connected by an impedance transformer 33 to a feed line not shown. A first microstrip line 34 in a spiral pattern with a square shape factor has been etched on a first intermediate layer between the patch 32 and the spiral pattern 31 , this microstrip line 34 being connected to the ground plane 31 by a via 35. Moreover, the patch 32 is interconnected to the ground plane 31 by a via 36 provided in the middle of the edge opposite the edge of the patch connected to the impedance transformer 33. According to this embodiment, a spiral pattern implemented by a second microstrip line 37 has been etched on a second intermediate layer between the spiral pattern of the first microstrip line 34 and the spiral pattern of the slot line 31. Preferably, the winding direction of the spiral patterns 34 and 37 is identical and corresponds to that of the spiral pattern of the slot line 31 implemented in the ground plane of the lower layer. This structure makes further reduction of the trace (i.e. the surface area occupied by the resonator) possible by taking advantage of the additional layer of metallisation to include the spiral microstrip line 37 here, the effect of which is to increase the total length of the spiral microstrip line and therefore lower the resonant frequency.

A description will now be given, with reference to figures 13 and 14, of the use of a slot line resonator in accordance with an embodiment of the invention to provide a single or multiple passband filter. As shown in figure 13, a first resonator 40 is associated with a second resonator 41 in order to provide a filtering device. The feed of the resonator 40 forms the input of the filter while the feed of the resonator 41 forms the output port of the filter.

As shown clearly in figure 14, the two resonators 40 and 41 are stacked one on top of the other with the slot lines in a spiral pattern facing each other. The faces 40b and 41 b comprising the spiral patterns are separated from each other by a distance of approximately 0.1 mm as shown in figure 14. Moreover, the patches 40a and 41 a form the outer surfaces of the assembly. The filter obtained is a filter with two coupled resonators which is the object of the present invention. These resonators are coupled to the input and output ports by (in this case square) patch patterns 40a and 41 a. The coupling between the two resonators is adjusted by the separation between the two metal layers containing the spiral pattern slot lines (in this case 0.1 mm). Figure 15 shows a curve giving the resonance S (1 ,1 ) and the transmission S (2,1 ) as a function of the frequency of the filter as described in figures 13 and 14. A good band-pass filter characteristic is therefore observed in the region of 2.45GHz. The insertion losses of the compact resonators-based filter which is the object of the invention are less than 0.3 dB in the band with a matching better than 25 dB. The slot line resonator for single or multiple passband filters as described above has numerous advantages. It is possible to obtain a very compact resonator at a low cost. The resonator can act as a single-pole filter with a good band-pass characteristic. Moreover, the resonator is not very sensitive to the parameters of the substrate and to the production tolerances. Furthermore, the resonant frequency of the resonator can be controlled by changing the length of the spiral slot line. Furthermore, the use of multiple metallised layers makes it possible to obtain a more compact and non-planar design.