EP2797161B1 - Microwave filter with dielectric element - Google Patents

Description

FIELD OF THE INVENTION

The present invention relates to the field of microwave wave filters, typically of frequency between 1 GHZ and a few tens of GHz.

The processing of a microwave wave, for example received by a satellite, requires the development of specific components, allowing the propagation, amplification, and filtering of this wave. Indeed, the microwave received by a satellite must be amplified before being sent back to the ground. This amplification is only possible by separating all the frequencies received into channels, each corresponding to a given frequency band. The amplification is then carried out channel by channel. Then the signal is recombined before being sent to the transmitting antenna.

Filters are thus used for producing an input multiplexer (referred to as IMUX) or an output multiplexer (referred to as OMUX). A filter can be excited only by a relatively narrow band of frequency around a resonant frequency.

The filter according to the invention comprises at least one cavity and a dielectric element disposed inside. More particularly, the filters according to the invention are suitable for producing OMUX-type multiplexers located after a power amplifier. Its role is to eliminate all the parasitic frequencies created by the power amplifier. The specifications of these filters are very strict in terms of quality factor and isolation (no stray modes in the band of interest) because of their situation between the power amplifier and the transmitting antenna.

STATE OF THE ART

Conventionally, the microwave wave filters comprise, in addition to one or more cavities coupled together in which dielectric resonators are arranged, means for coupling the energy microwave (RF) on the one hand to introduce RF energy to the input of the filter and, on the other hand, to extract RF energy at the output of the filter. In addition, they generally comprise tuning means for adjusting the frequency of the main modes of resonance of the filter.

Filters known from the prior art are described for example in the patent US5880650 . In this filter, a dielectric element consists of a flat plate having the shape of a parallelogram, and the maximum of the electric field is located in the dielectric element, which thus acts as a resonator.

An advantage of the filter described in the patent US5880650 is that the dielectric resonator is in mechanical and electrical contact with the walls of the metal cavity by the four corners of the plate. The vertices are truncated or rounded so as to match the shape of the side walls, flat or slightly curved depending on the shape of the cavity. The mechanical contact allows an exact and reproducible positioning of the resonant element in the cavity and the heat transfer between the resonator element and the walls is significantly improved.

A disadvantage of this filter is that due to the location of the electric field in the dielectric element, the dielectric losses are important. Conversely, an empty resonant cavity has significant metal losses. The quality factor Q depending on metal losses and dielectric losses, an empty cavity or a dielectric resonator cavity therefore each have the disadvantage of significant losses that is to say a non-optimal quality factor.

The document US6433652 describing a multimode filter having a dielectric resonator is considered to be the closest state of the art.

In addition the filter described in the patent US5880650 has been optimized for C-band operation (3 to 5 GHz). For it to operate at a higher frequency (for example in the Ku band from 10 to 13 GHZ), it is necessary to divide the dimensions by about three, which leads to a filter of small size, which is an advantage. However, the rise in frequency leads to a deterioration of the quality factor Q.

Another type of filter is described in the patent US8031036 . This filter includes a cylindrical metal cavity and inside an element also cylindrical dielectric comprising a collar, fixed to the walls of the cavity over its entire circumference by the collar through for example a ring or springs. In this filter the electric field is concentrated in the dielectric resonator with the aforementioned drawbacks. In addition the volume of the resonator cylinder is large, leading to a heavy filter, which is a significant disadvantage for components to be embedded on a satellite.

An object of the present invention is to overcome the aforementioned drawbacks.

DESCRIPTION OF THE INVENTION

• une première portion présentant une épaisseur selon ledit axe longitudinal et une section selon un plan perpendiculaire audit axe longitudinal dont les sommets sont répartis selon un polygone, et dont au moins deux sommets sont court-circuités entre eux par les parois conductrices de la cavité, via un contact électrique ou hyperfréquence entre les sommets et les parois,
• au moins une portion pyramidale comprenant un apex et une base coïncidant avec une section extrémale de la première portion.

The present invention relates to a microwave filter having at least one resonant mode comprising at least one cavity at least partially closed using conductive walls and having a cylindrical outer surface defined by a guide curve described by a generator and having a point of symmetry, an axis passing through a point of symmetry and parallel to said generator being called the longitudinal axis of the cavity and at least one dielectric element disposed in said cavity and comprising:

• a first portion having a thickness along said longitudinal axis and a section in a plane perpendicular to said longitudinal axis whose vertices are distributed in a polygon, and at least two vertices of which are short-circuited by the conductive walls of the cavity, via an electrical or microwave contact between the vertices and the walls,
• at least one pyramidal portion comprising an apex and a base coinciding with an extremal section of the first portion.

Advantageously, the guide curve is chosen from a square, a rectangle, a hexagon, a circle, an ellipse.

Advantageously, the base comprises vertices distributed along a regular polygon.

Advantageously, all the vertices of the section are short-circuited by the conductive walls of the cavity via an electrical or microwave contact between the vertices and the walls.

Advantageously, the filter according to the invention comprises an upper pyramidal portion and a lower pyramidal portion respectively comprising an upper base coinciding with an upper extremal section and a lower base coinciding with a lower extremal section of the first portion.

Advantageously, the upper pyramidal portion and the lower pyramidal portion are identical.

Advantageously, the apex is disposed on the longitudinal axis. Advantageously, the barycentre of said polygon is disposed on the longitudinal axis.

Advantageously, an angle between the base and a face of the pyramidal portion is less than or equal to 45.

Advantageously, the pyramidal portion is truncated along a plane perpendicular to the longitudinal axis.

Advantageously, the truncated pyramidal portion has a recess formed on an upper face of the truncated pyramidal portion. Advantageously, at least one recess is made at any point around the circumference of the dielectric element.

Advantageously, the filter according to the invention is dimensioned so that a resonance frequency of a resonant mode is between 3 GHz and 30 GHz.

Advantageously, an electromagnetic field corresponding to a resonant mode comprises an even number 2n of areas for which the electromagnetic field has a maximum, the zones being arranged in equal number n on either side of the first portion of the dielectric element, n being selected from 1, 2 or 3.

Advantageously, each of the zones is distributed partially inside and partially outside the pyramidal portion disposed on the same side as the zone.

Advantageously, the filter according to the invention comprises at least one input cavity and one output cavity and comprises input coupling means of a microwave wave coming from an external source with said input cavity, and output coupling means between said output cavity and an external waveguide, and comprises intermediate coupling means of the cavities therebetween.

• la figure 1 illustre schématiquement un filtre selon l'invention.
• la figure 2 décrit un exemple de structure pyramidale de l'élément diélectrique.
• la figure 3 illustre un mode de réalisation préféré de portion pyramidale.
• la figure 4 illustre une variante d'élément diélectrique comprenant une portion pyramidale tronquée
• la figure 5 illustre une variante d'élément diélectrique comprenant des évidements.
• la figure 6 illustre la distribution des lignes de champ pour un mode de réalisation du filtre selon l'invention.
• la figure 7 illustre la distribution des lignes de champ pour un autre mode de réalisation du filtre selon l'invention.
• la figure 8 illustre schématiquement une variante de réalisation de l'élément diélectrique du filtre selon l'invention.
• la figure 9 illustre la distribution des lignes de champ pour un filtre selon l'invention présentant un élément diélectrique tel que décrit figure 8.
• la figure 10 illustre un premier exemple de mode de réalisation d'un filtre selon l'invention.
• la figure 11 illustre un deuxième exemple de mode de réalisation d'un filtre selon l'invention.
• la figure 12 illustre un troisième exemple de mode de réalisation d'un filtre selon l'invention.
• la figure 13 illustre un exemple de réponse fréquentielle sur une large bande d'un filtre selon l'invention
• la figure 14 illustre un exemple de réponse fréquentielle au voisinage de la fréquence de résonance d'un filtre selon l'invention.

Other features, objects and advantages of the present invention will appear on reading the detailed description which follows and with reference to the appended drawings given by way of non-limiting examples and in which:

• the figure 1 schematically illustrates a filter according to the invention.
• the figure 2 describes an example of a pyramidal structure of the dielectric element.
• the figure 3 illustrates a preferred embodiment of a pyramidal portion.
• the figure 4 illustrates an alternative dielectric element comprising a truncated pyramidal portion
• the figure 5 illustrates an alternative dielectric element comprising recesses.
• the figure 6 illustrates the distribution of the field lines for an embodiment of the filter according to the invention.
• the figure 7 illustrates the distribution of the field lines for another embodiment of the filter according to the invention.
• the figure 8 schematically illustrates an alternative embodiment of the dielectric element of the filter according to the invention.
• the figure 9 illustrates the distribution of the field lines for a filter according to the invention having a dielectric element as described figure 8 .
• the figure 10 illustrates a first exemplary embodiment of a filter according to the invention.
• the figure 11 illustrates a second exemplary embodiment of a filter according to the invention.
• the figure 12 illustrates a third exemplary embodiment of a filter according to the invention.
• the figure 13 illustrates an example of frequency response over a wide band of a filter according to the invention
• the figure 14 illustrates an example of frequency response in the vicinity of the resonance frequency of a filter according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

An object of the invention is to provide a microwave filter having very good performance in both quality factor Q and insulation.

By isolation means the ability of the filter not to transmit unwanted modes other than the selected resonance modes of the filter. The frequency range around the resonant frequency for which no parasitic mode is transmitted is denominated in the English terminology "spurious free range". Of course, we want to obtain the widest possible range.

For example, for an OMUX application in the Ku band (10 to 13 GHz), a range of the order of 500 MHz is typically sought on either side of the resonant frequency, an empty quality factor at least equal to 18000 and power handling of at least 300 W per channel.

The figure 1a discloses a perspective view of a microwave filter (RF) 10 according to the invention. This filter has at least one resonance mode and comprises a cavity 11 at least partially closed using conductive walls 12, typically metal. The cavity 11 has a cylindrical outer surface defined by a directing curve C described by a straight line called generatrix of the cylinder. The guide curve of the filter cavity according to the invention has a point of symmetry Sy, which facilitates manufacture and simulation.

According to a preferred embodiment, for ease of manufacture, the guide curve C is a square, a rectangle, a hexagon, a circle or an ellipse. The longitudinal axis z of the hollow cylindrical cavity is defined as the axis parallel to a generating line and passing through the points of symmetry.

The filter 10 according to the invention also comprises at least one dielectric element 13 disposed in the cavity 11. The dielectric element 13 comprises a first portion 131 having a thickness e along the z axis and a section along a plane perpendicular to z of which the vertices S1, S2,... Sp are distributed according to a polygon P. For ease of understanding and in a nonlimiting manner, the polygon represented on the figure 1 is a square, but any polygon P is compatible with the invention.

According to a preferred variant, the polygon is regular (triangle, square, pentagon, hexagon ...) or rectangular, to allow a low-cost industrial production of the filter and easier optimization due to the presence of axes of symmetry.

According to a preferred embodiment, the polygon is a square so as to limit the contacts between the dielectric element 13 and the cavity 11, which makes it possible to favor certain modes and to guarantee the quality of the contacts.

Similarly on the figure 1 and in a nonlimiting manner the sides which join the vertices between them are straight segments, but any other form is compatible with the filter 10 according to the invention, variants of which are described below. The contact of the dielectric element 13 with the conducting wall is effected through the first portion 131, according to the same principle as that described in the patent US5880650 , that is to say that at least two vertices of the polygon are short-circuited between them by the walls 12, via an electrical or microwave contact between these vertices and the wall.

• Assemblage du filtre simplifié par un positionnement exact et absolu de l'élément diélectrique sans recourir à des éléments de maintien.
• Transfert thermique entre l'élément et les parois nettement amélioré ;

The method of fixing the dielectric element 13 to the walls 12 thus has the same advantages as those described in the patent US5880650 , such as :

• Simplified filter assembly by exact and absolute positioning of the dielectric element without the need for holding elements.
• Thermal transfer between the element and the walls significantly improved;

• Troncature ou arrondi des sommets, tel que décrit figure 1, pour épouser la forme des parois latérales, planes ou arrondies en fonction de la forme de la courbe directrice C de la cavité cylindrique,
• troncature des sommets selon des dimensions légèrement inférieures aux dimensions transversales de la cavité de manière à laisser un petit espace, qui peut être vide ou rempli d'un matériau diélectrique ou conducteur et/ou élastique,
• utilisation de piliers de maintien,
• troncature des sommets selon des dimensions légèrement supérieures aux dimensions transversales de la cavité et réalisation d'encoches,

The method of fixing the dielectric element to the walls is also compatible with the same variants, for example:

• Truncation or rounding of vertices, as described figure 1 to conform to the shape of the side walls, flat or rounded depending on the shape of the guide curve C of the cylindrical cavity,
• truncating the vertices in dimensions slightly smaller than the transverse dimensions of the cavity so as to leave a small gap, which may be empty or filled with a dielectric material or conductive and / or elastic,
• use of pillars,
• truncation of the vertices in dimensions slightly greater than the transverse dimensions of the cavity and making notches,

etc ....

It is not necessary for all the vertices of the polygon P to be short-circuited between them, it suffices that the vertices short-circuited by the walls 12 are in sufficient number to ensure correct positioning of the dielectric element in the cavity.

According to a preferred variant, for a better positioning accuracy, all the vertices S1 ... Sp of the polygon P are short-circuited by the conductive walls.

The dielectric element 13 also comprises at least one pyramidal portion 132, 133 as illustrated in FIGS. figures 1a (perspective view), 1b (side view) and 1c (top view). The pyramidal portion comprises an apex Asup, Ainf, top of the pyramid, and a base Bsup, Binf, which coincides with an extremal section 134, 135 of the first portion 131. The term extremal section is the upper section 134 and the lower section 135 of the first portion 131 of thickness e.

The particular shape of the dielectric element associated with an optimized dimensioning (cavity and dielectric element) makes it possible to obtain a filter with improved performances compared to those of the filters of the state of the art.

According to a variant, the dielectric element 13 comprises a single pyramidal portion, lower 132 or greater 133.

According to a preferred variant, the dielectric element 13 comprises two pyramidal portions on either side of the first portion 131, the upper base Bsup coinciding with the upper end section 134 and the lower base Binf coinciding with the lower end section 135 of the first portion 131.

In order to simplify the optimization calculations of the dielectric element in the cavity, according to a preferred embodiment the upper and lower pyramidal portions are identical. According to a preferred embodiment, the filter according to the invention comprises a plane of symmetry xy. The existence of a symmetry on the shape of the dielectric element makes it possible to obtain better insulation, because of the symmetry of modes that follows. A distortion of the modes makes the behavior of the filter not optimal.

Preferably, the filters according to the invention operate in a TE (transverse electric) mode.

The figure 2 schematically illustrates an example of a pyramidal structure whose base consists of vertices arranged on a polygon P of barycentre Ba, and Apex Asup. In this example, the centroid Ba and the Asup apex are not arranged on the longitudinal axis z of the cylindrical cavity.

In order to simplify the optimization calculations, according to a preferred variant illustrated Figures 1a, 1b and 1 C the apex Asup and Ainf of the pyramidal portions are arranged on the longitudinal axis z of the cavity 11.

In order to position and fix the dielectric element 13 more easily in the cavity 11, according to a preferred variant the centroid B of the polygon P serving as a base for the pyramidal portion is disposed on the longitudinal axis z of the cavity 11, as illustrated. figures 1 a, 1 b, 1 c.

Preferably, the dielectric element 13 is made of a single block, which has the advantage of facilitating the industrial production of the element 13, obtained by molding, machining or grinding or by additive manufacturing (stereolithography).

The figure 3 illustrates a preferred embodiment of pyramidal portion, whose base is a regular polygon (3a: square, 3b: pentagon) whose apex A has an orthogonal projection on the base, defining a height h, which passes through the centroid Ba of the polygon.

Examples of this particular case of pyramidal portion are the regular tetrahedron, the square pyramid ( figure 3a ), the pentagonal pyramid ( figure 3b ), the hexagonal pyramid ....

The angle α is the angle between the base of the pyramid and one side of the pyramid. According to a preferred embodiment, the angle α (or all the angles α when they are not equal) is less than or equal to 45 °.

The figure 4 represents an alternative dielectric element 13 inside a cylindrical cavity 11 of generating curve C in a circle (cylinder circular). The figure 4a illustrates a perspective view, the figure 4b a profile view.

According to this variant, the pyramidal portion is truncated, for example along a plane T perpendicular to the longitudinal axis z. The apex is then virtual.

Truncation is defined by a distance Dtr corresponding to the fraction k of the height for which the material has been deleted. $$\mathrm{Deuteronomists}=\mathrm{k}\times \mathrm{h}$$

Truncation has the advantage of limiting the sensitivity of the performance of the filter to the value of the angle α.

Preferably k is between 0.1 and 0.5. For lower k values, the advantage of truncation is not significant. For higher values of k, the quality factor Q decreases substantially.

The figure 5 represents another variant of dielectric element 13 inside a cylindrical cavity 11 of generating curve C in a circle. The figure 5a illustrates a perspective view, the figure 5b a view from above and the figure 5c a profile view. In the example of the figure 5 the polygon is a square: the four vertices S1, S2, S3 and S4 are distributed in a square.

In the example shown on the figure 5 the periphery 51 of the dielectric element is rounded (chamfer 55) near the vertices to match the shape of the cylindrical wall.

According to this illustrated variant figure 5 the periphery 51 of the dielectric element 13 does not coincide with the sides of the square in which the vertices are distributed. Thus, at least one recess 52 is made at any location around the periphery 51 of the dielectric element 13. Preferably, all the sides of the polygon have a recess 52 to ensure symmetry of the electromagnetic field.

This involves removing dielectric material in areas where the electric field is of low intensity. One advantage is to obtain a smaller dielectric volume. Another advantage is to obtain better insulation by controlling the frequency of the other modes (parasites) which depend more heavily on this portion of dielectric. Preferably the recess is made so as not to add edges at right angles.

The filter of the figure 5 has a dielectric element which reciprocally combines and truncates. These two variants are independent.

• des dimensions (épaisseur, et dimensions transversales de la première portion, hauteur de portion pyramidale) et de la forme (base carrée, pentagonale, hexagonale) de l'élément diélectrique,
• des dimensions et de la forme de la cavité résonante dans laquelle l'élément diélectrique est disposé,
• du matériau diélectrique utilisé pour la réalisation de ce dernier.

In the design of a microwave filter according to the invention, the resonance frequencies mainly depend on:

• dimensions (thickness, and transverse dimensions of the first portion, height of the pyramidal portion) and the shape (square, pentagonal, hexagonal base) of the dielectric element,
• dimensions and shape of the resonant cavity in which the dielectric element is disposed,
• dielectric material used for the realization of the latter.

The values of these variables therefore depend on the microwave band in which the filter operates. According to a preferred variant, the filter according to the invention is sized to operate in the C, X and Ku and Ka bands, that is to say comprising a resonance frequency in the range [3 GHz; 30 GHz].

• Cavité métallique :
  • cylindrique circulaire de diamètre compris entre 20 et 25 mm et de longueur comprise entre 20 et 25 mm.
• Elément diélectrique :
  • base carrée épousant la forme de la cavité pour les 4 sommets,
  • épaisseur e de la première portion comprise entre 2 et 4 mm
  • angle α de la pyramide : entre 8° et 11 °
  • permittivité diélectrique : 9.5 et 10.

An example of sizing for a 12 GHz resonance frequency is:

• Metal cavity
  • cylindrical circular diameter between 20 and 25 mm and length between 20 and 25 mm.
• Dielectric element:
  • square base matching the shape of the cavity for the 4 vertices,
  • thickness e of the first portion between 2 and 4 mm
  • angle α of the pyramid: between 8 ° and 11 °
  • dielectric permittivity: 9.5 and 10.

For these dimensions, a vacuum quality factor between 18000 and 19000 and a total range isolation between 1 GHz and 1.5 GHz around the resonance frequency have been calculated with a temperature compensated dielectric.

The presence of a recess improves the insulation range, the presence of truncation decreases the sensitivity of the resonant frequency to the value of the angle of the pyramid, thus relaxing the manufacturing constraints of the dielectric element.

From an electromagnetic point of view, two types of filters are conventionally distinguished according to the manner in which the dielectric element is used.

In a first type the dielectric element is used as a resonator, which means that the electric field is concentrated inside it. The "resonator modes" (also called dielectric modes) are thus modes whose electrical energy is mainly concentrated in the dielectric material (typically 90 to 95%). Their losses are essentially dielectric and depend on the characteristics of the material (loss tangent).

In contrast, in a second type called "cavity modes", the resonant cavity is said to be "charged" by the dielectric element which modifies the dielectric permittivity of the medium. The losses are essentially metallic.

An improved mode of operation of the filter according to the invention is called "hybrid", and consists in charging the cavity with a dielectric to partially concentrate the electrical energy, so as to reduce the metal losses while limiting the dielectric losses. The electromagnetic operation of the filter according to the invention thus combines the two types of conventional operation, which allows, partly thanks to the specific shape of the dielectric element, to minimize losses (high quality factor) while maintaining good insulation.

In "hybrid" operation, the resonant mode has an even number 2n of zones for which the electric field has a maximum, the zones being arranged in equal numbers on either side of the first portion 131 of the dielectric element 13.

In practice, only the values n = 1, n = 2, n = 3 and n = 4 are of practical interest. In fact, the more one ascends in order, the more maxima there are, and beyond 4 maxima on each side, the insulation becomes insufficient.

At constant dimensions, the larger the n, the greater the resonance frequency of the corresponding mode. It is therefore necessary to increase the dimensions to bring this resonant frequency to the frequency of the filter.

When a filter is made per channel, one option is to use for each channel a filter of identical structure and operating in the same mode, but with homothetic dimensions, to obtain proportional and determined resonant frequencies.

In an improved mode of the filter according to the invention illustrated figures 6 and 7 each of the areas for which the electric field has a maximum is distributed partially inside and partially outside the pyramidal portion disposed on the same side as the area in question. According to the state of the art "plate", the plates are positioned on the maximum fields to concentrate the electrical energy.

For a filter according to the invention, of hybrid nature, the first portion of the dielectric (common base of the pyramids) is positioned on a minimum of field (between the two field maxima). Since the dielectric always has a tendency to concentrate electrical energy, by adjusting the dimensions of the pyramid, this energy is partially concentrated, partly internally, partly outside the dielectric, optimally.

An advantage of using a "hybrid" mode where the maximum field is located partially outside the dielectric and partially inside is to obtain dielectric losses lower than those obtained for a conventional resonator and loss mode. lower than those obtained for a conventional cavity-type mode.

The figure 6 illustrates a filter 10 according to the invention operating in "hybrid" mode, whose dielectric element comprises two square pyramids, truncated and recessed in a circular cylindrical cavity 11 as illustrated figure 5 , the contact between element 13 and wall 12 is effected by the four corners of the square, as well as the distribution of the field lines of the resonant mode in the cavity. On the figure 6 is also illustrated the distribution of field lines for highlighting the position of field maxima, for example for a polarization. The figure 6a represents a profile view et al figure 6b a perspective view.

We see on the figure 6a that there are two zones that concentrate the electric field, each disposed on either side of the first portion of the dielectric element (case n = 1). Zones 61 and 62 correspond to the locations for which the electric field has a maximum. Each zone 61, 62 is partially distributed in the dielectric element and partially outside thereof. Concentrating electrical energy center of the cavity, partially in the dielectric, substantially reduces the metal losses, while limiting the dielectric losses.

The figure 7 illustrates the same filter as that of the figures 6 which has favored a resonant mode having eight zones, four in section (71 and 72, 73 and 74) which concentrate the electric field on either side of the first portion of the dielectric element (case n = 4). The figure 6a represents a profile view and the figure 6b a perspective view of the dielectric element 13 and the distribution of the field lines.

To obtain a resonant mode at eight maxima, it is appropriate for example to seek a resonance frequency in this mode, without modifying the dimensions of the cavity and the dielectric 13.

For example, the resonance frequency of the n = 4 mode is 14.5 GHz when the resonance frequency of the n = 1 mode is 12 GHz, all else being equal.

Another variant of the shape of the dielectric element is illustrated figure 8 . The truncated pyramid has a recess 80 made on an upper face of the truncated pyramidal portion.

The recess is of any shape, for example a through hole, or an inverted pyramid and is positioned in an area having a low electric field. This variant is interesting for the case n = 4 (see figure 7 ) in which the electric field at the center of the truncated portion is weak. The realization of this recess in optimal dimensions makes it possible to control the frequencies of the parasitic modes.

The figure 9 illustrates the distribution of the field lines of the resonant mode in the cavity, with a truncated pyramid, whose truncated flat portion is recessed, for a mode n = 4. The recess disturbs little the distribution of the maximum electric field, partially inside and partially outside the pyramidal portion.

A first exemplary embodiment of a filter 10 according to the invention is illustrated schematically figure 10 . The filter comprises at least one input cavity 101 and an output cavity 102, input coupling means 103 of a microwave wave from an external source with the input cavity 101 and output coupling means 104 between the output cavity 102 and an external waveguide and comprises intermediate coupling means 105 of the cavities between them. Metal transverse walls 106 and 107 at least partially close the inlet and outlet cavities.

The filter may also comprise one or more intermediate cavities coupled together, as described in the figure 1 of the document US5880650 . All these cavities are for example electrically defined inside a cylindrical waveguide section by means of a plurality of walls transverse to the longitudinal axis of the cylinder 106, 105, 107, which close the cavities at least partially to both of them. ends of each cavity. The construction materials of the waveguide and transverse walls are those commonly used by those skilled in the art for such an embodiment. The input and output coupling means are also those commonly used by those skilled in the art.

The intermediate coupling means are conventionally different forms of slots or irises, or capacitive probes, inductive iris or a combination of both.

The filter according to the invention may also comprise tuning means of the resonant frequency known to those skilled in the art.

On the figure 10 a dielectric element according to the invention is disposed inside a cavity, but the filter according to the invention may also comprise a plurality of pyramidal dielectric elements per cavity, possibly combined with plate-type dielectric elements as described in FIG. patent US5880650 .

For example with a single dielectric element per cavity, the element is preferably positioned in the middle of the cavity. With two dielectric elements per cavity, an element is positioned on either side of the middle of the cavity.

Another exemplary embodiment of a filter according to the invention is described figure 11 , for which the input and output coupling means 103, 104 are positioned on the transverse walls 106, 107, in a configuration called "in line".

The figure 12 illustrates a third exemplary embodiment of a filter according to the invention comprising an input cavity and an output cavity. According to this example, there is no single section of waveguide, but two cylindrical portions of parallel longitudinal axes, each cavity being at least partially closed by transverse walls 106, 107 for the inlet cavity and 108 , 109 for the exit cavity. The input and output coupling means are arranged on the cylindrical wall of the corresponding cavity.

With a filter according to the illustrated configurations figures 10 or 12 , it is possible to implement an external temperature compensation system, as described in the patent US2006097827 , on covers 106, 107, 108 and 109. It is thus possible to use dielectrics that are not temperature compensated, which makes it possible to substantially increase the quality factor when empty (typically 25,000).

The Figures 13 and 14 illustrate the frequency response of a filter 10 according to the invention as illustrated figure 10 and sized for a resonance frequency of 12 GHz. Parameter S is a parameter that accounts for filter performance in terms of reflection and transmission. The curve S11 corresponds to the reflection and S12, or S21 to the transmission.

The tuning of the filter makes it possible to obtain a transmission maximum (reflection minimum) for a given frequency band. The bandwidth of the filter is determined at S11 (or S22) equi-ripple, for example at 15dB or 20 dB of reflection reduction with respect to its out-of-band level.

The figure 13 illustrates the broadband response and shows good isolation from parasitic modes. The figure 14 is a zoom around the resonant frequency and illustrates the response inside the bandwidth. The filter has 4 poles and is centered around 11.950 GHz, and the bandwidth is 40 Mhz.

Claims (16)

1. A microwave filter (10) having at least one resonant mode and comprising:

   - at least one cavity (11) that is at least partially closed by conducting walls (12), said filter being characterised in that it has:

   a cylindrical outer surface defined by a directrix (C) described by a generatrix and having a point of symmetry (Sy), with an axis passing through a point of symmetry and parallel to said generatrix being called the longitudinal axis (z) of said cavity (11); and

   - at least one dielectric element (13) disposed inside said cavity and comprising:

   - a first portion (131) having a thickness (e) along said longitudinal axis (z) and a section along a plane perpendicular to said longitudinal axis (z), the vertices (s1, s2, s3, s4) of which portion are distributed as a polygon (P) and at least two vertices of which are short-circuited together by said conducting walls (12) of said cavity via an electric or microwave contact between said vertices and said walls;

   - at least one pyramidal portion (132, 133) comprising an apex (Asup, Ainf) and a base (Bsup, Binf) that coincide with an end section (134, 135) of said first portion (131).
2. The filter according to claim 1, wherein said directrix (C) is selected from among a square, a rectangle, a hexagon, a circle, an ellipse.
3. The filter according to any one of the preceding claims, wherein said base comprises vertices (s) that are distributed as a regular polygon (P).
4. The filter according to any one of the preceding claims, wherein all of said vertices of said section are short-circuited together by said conducting walls (12) of said cavity via an electric or microwave contact between said vertices and said walls.
5. The filter according to any one of the preceding claims, comprising an upper pyramidal portion (132) and a lower pyramidal portion (133), respectively comprising an upper base (Bsup) that coincides with an upper end section (134) and a lower base (Binf) that coincides with a lower end section (135) of said first portion (131).
6. The filter according to claim 5, wherein said upper pyramidal portion (134) and said lower pyramidal portion (135) are identical.
7. The filter according to any one of the preceding claims, wherein said apex (Ainf, Asup) is disposed on said longitudinal axis (z).
8. The filter according to any one of the preceding claims, wherein the barycentre (O) of said polygon (P) is disposed on said longitudinal axis (z).
9. The filter according to any one of the preceding claims, wherein an angle (α) between said base and a face of said pyramidal portion is less than or equal to 45°.
10. The filter according to any one of the preceding claims, wherein said pyramidal portion is truncated along a plane (T) perpendicular to said longitudinal axis (z).
11. The filter according to any one of the preceding claims, wherein said truncated pyramidal portion has a recess that is made on an upper face of said truncated pyramidal portion.
12. The filter according to any one of the preceding claims, wherein at least one recess (52) is made at any point on the perimeter (51) of said dielectric element (13).
13. The filter according to any one of the preceding claims, dimensioned so that a resonant frequency of a resonant mode is between 3 GHz and 30 GHz.
14. The filter according to any one of the preceding claims, wherein an electromagnetic field corresponding to a resonant mode comprises an even number 2n of zones (61, 62, 71, 72, 73, 74), for which said electromagnetic field has a maximum, said zones being disposed in equal numbers n on both sides of said first portion (131) of said dielectric element (13), with n being selected from among 1, 2, 3 or 4.
15. The filter according to claim 14, wherein each of said zones is distributed partially inside and partially outside said pyramidal portion that is disposed on the same side as said zone.
16. The filter according to any one of the preceding claims, comprising at least one input cavity (101) and one output cavity (102), and comprising means (103) for coupling the input of a microwave originating from an external source with said input cavity (101) and means (104) for coupling the output between said output cavity (102) and an external waveguide, and comprising intermediate means (105) for coupling said cavities together.

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