EP0346806B1 - Band-pass-filter with dielectric resonators - Google Patents

Description

The invention relates to a bandpass filter with dielectric resonators. The invention relates more particularly to a microwave bandpass filter using dielectric resonators resonating in TE011 mode: These dielectric resonators are placed in a guide used at its cutoff frequency.

In radio-relay systems, connection filters are needed to select the different channels in a given band (for example from 7.1 to 7.7 GHz). The filters must therefore be tunable in each of the channels included in the band.

It is known to make filters with dielectric resonators tunable in a frequency band. It suffices to strongly push the adjustment screws above the resonators in order to raise the resonant frequency of the latter, but then the overvoltage of the filter is reduced. Another method consists in changing the dimensions of the dielectric resonators (height and diameter) in order to maintain a good overvoltage coefficient: the diameter to height ratio must be close to 1.8. In this case, to produce filters having the same characteristics according to the channels, it is necessary to modify the distance between the dielectric resonators to compensate for their different diameters, therefore to modify the mechanical dimensions. It is then necessary to make as many different filters as there are channels in the band.

• la demande de brevet EP-0 176 966 décrit un filtre passe-bande dans lequel plusieurs résonateurs diélectriques sont disposés en alignement, deux plots métalliques de longueur peu différente du quart de la longueur d'onde de la fréquence centrale du filtre étant positionnés aux deux extrémités du filtre ; l'un entre le premier résonateur et le connecteur d'entrée, l'autre entre le dernier résonateur et le connecteur de sortie.
• la demande de brevet EP-0 209 878 décrit un filtre hyperfréquence à résonateurs diélectriques dans lequel une plaque de base en métal supporte plusieurs résonateurs, un élément de couplage d'entrée et un élément de couplage de sortie, un connecteur d'entrée et un connecteur de sortie, cette plaque étant protégée par un couvercle muni d'une cavité pour former ainsi un guide d'onde.

Devices of the known art are described in European patent applications EP-0 176 966 and EP-0 209 878:

• patent application EP-0 176 966 describes a bandpass filter in which several dielectric resonators are arranged in alignment, two metal studs of length little different from a quarter of the wavelength of the central frequency of the filter being positioned at the two ends of the filter; one between the first resonator and the input connector, the other between the last resonator and the output connector.
• patent application EP-0 209 878 describes a microwave filter with dielectric resonators in which a metal base plate supports several resonators, an input coupling element and an output coupling element, an input connector and a output connector, this plate being protected by a cover provided with a cavity to thereby form a waveguide.

But in the filters thus described, the adjustment can only take place at a given frequency, these filters having numerous parasitic responses favored by the discontinuities created by the metallic adjustment screws allowing the tuning of the dielectric resonators.

The object of the invention is to eliminate these various drawbacks as much as possible.

As a state of the art, document WO-A-8,700,350 which also relates to a bandpass filter with dielectric resonators of the type currently considered, this filter comprising a microwave guide in which the dielectric resonators are placed, each dielectric resonator being integral with a support of dielectric material, this filter comprising at least one screw providing an adjustable "mode suppressor", which is placed parallel to the supports of the dielectric resonators.

Documentary may finally be cited in document EP-A-0.047.203 which describes a tunable filter with dielectric resonators for which the propagation mode is, unlike the filters concerned by the present application, a TM11 mode.

• . en ce que le support diélectrique de chaque résonateur diélectrique est déplacable axialement de façon à permettre de modifier la position du résonateur correspondant dans le guide et d'ajuster ainsi la fréquence de résonance de chaque résonateur en l'éloignant ou en le rapprochant d'une paroi du guide ;
• . et en ce que lesdits résonateurs ont un même diamètre, quel que soit le canal utilisé dans la bande de fréquences considérée, seule leur hauteur variant suivant le canal utilisé, de sorte que finalement ce filtre utilise une seule et même mécanique pour chacun de ces canaux en ne changeant donc que la hauteur des résonateurs diélectriques pour passer d'un canal à l'autre.

The invention provides a bandpass filter with dielectric resonators, more particularly with dielectric resonators resonating in TE011 mode, comprising a microwave guide in which the dielectric resonators are arranged, each dielectric resonator being integral with a support made of dielectric material, this filter comprising at least one screw producing an adjustable "mode suppressor", which is placed parallel to the supports of the dielectric resonators, characterized:

• . in that the dielectric support of each dielectric resonator is axially displaceable so as to allow the position of the corresponding resonator to be modified in the guide and thus to adjust the resonant frequency of each resonator by moving it away or bringing it closer to a wall of guide;
• . and in that said resonators have the same diameter, whatever the channel used in the frequency band considered, only their height varying according to the channel used, so that ultimately this filter uses one and the same mechanism for each of these channels thus only changing the height of the dielectric resonators to go from one channel to another.

In the filter of the invention, therefore, the same mechanics are used for each of the channels, only changing the height of the dielectric resonators: their diameter remains constant whatever the channels to be covered in the band, only their height varies according to the channel. used. The overvoltage of the resonators is no longer optimum but the differences observed in the losses of the filters, according to the channels, are negligible.

• d'un guide monobloc,
• d'un accès en guide,
• d'au moins deux résonateurs diélectriques,
• de supports de résonateurs, en matériau diélectrique fileté,
• de rondelles élastiques et d'écrous de blocage de la position de chaque support,
• de manchons métalliques filetés permettant le passage des résonateurs diélectriques dans le guide,
• d'écrous de blocage pour maintenir les manchons,
• d'un fond métallique fermant le guide, maintenu par vis,
• de vis avec écrou de blocage permettant le réglage du couplage entre résonateurs, placées perpendiculairement aux supports de résonateurs diélectriques,
• d'un accès coaxial formant une antenne afin d'exciter le résonateur adjacent,
• et d'une vis avec son écrou de blocage pour régler le couplage entre l'antenne et le résonateur adjacent.

In an advantageous embodiment, the filter according to the invention is composed:

• a one-piece guide,
• guide access,
• at least two dielectric resonators,
• resonator supports, made of threaded dielectric material,
• elastic washers and nuts for locking the position of each support,
• threaded metal sleeves allowing the passage of the dielectric resonators in the guide,
• locking nuts to hold the sleeves,
• a metal bottom closing the guide, held by screws,
• of screws with locking nut allowing adjustment of the coupling between resonators, placed perpendicular to the supports of dielectric resonators,
• a coaxial access forming an antenna in order to excite the adjacent resonator,
• and a screw with its locking nut to adjust the coupling between the antenna and the adjacent resonator.

• les figures 1 à 3 illustrent schématiquement deux vues en coupe longitudinale (respectivement selon le plan de coupe I-I représenté à la figure 2, et selon le plan de coupe II-II représenté à la figure 1) et une vue de face du filtre selon l'invention ;
• la figure 4 représente une courbe illustrant le couplage de deux résonateurs du filtre selon l'invention ;
• les figure 5 à 9 représentent différentes courbes illustrant le fonctionnement du filtre selon l'invention.

The characteristics and advantages of the invention will become apparent from the description which follows, by way of nonlimiting example, with reference to the appended figures in which:

• Figures 1 to 3 schematically illustrate two views in longitudinal section (respectively along the section plane II shown in Figure 2, and along the section plane II-II shown in Figure 1) and a front view of the filter according to l invention;
• FIG. 4 represents a curve illustrating the coupling of two resonators of the filter according to the invention;
• Figures 5 to 9 show different curves illustrating the operation of the filter according to the invention.

• d'un guide 10, par exemple monobloc, réalisé par exemple en extrudé d'aluminium recouvert d'une fine couche d'argent (environ 10 µm) pour diminuer les pertes électriques,
• d'un accès en guide 11, par exemple une bride standard vissée sur le guide 10 à l'aide de vis 9,
• d'au moins deux résonateurs diélectriques 13,
• de supports 14 de résonateurs, en matériau diélectrique comme par exemple du polypenco ou de la rexolite, filetés,
• de rondelles élastiques 15 et d'écrous de blocage 16 de la position de chaque support 14,
• de manchons métalliques 17 filetés permettant le passage des résonateurs diélectriques 13 dans le guide 10,
• d'écrous de blocage 18 pour maintenir les manchons 17,
• d'un fond métallique 19 fermant le guide, maintenu par des vis 20,
• d'au moins une vis 21 avec son écrou de blocage 22 réalisant un "suppresseur de mode", placée parallèlement aux supports 14 des résonateurs diélectriques,
• de vis 23 avec écrou de blocage 24 permettant le réglage du couplage entre résonateurs, placées perpendiculairement aux supports de résonateurs diélectriques 13,
• d'un accès coaxial 25 formant une antenne 26 afin d'exciter le résonateur adjacent,
• et d'une vis 27 avec son écrou de blocage 28 pour régler le couplage entre l'antenne 26 et ledit résonateur adjacent.

The filter according to the invention, as shown in Figures 1 to 3, is composed:

• a guide 10, for example in one piece, produced for example from extruded aluminum covered with a thin layer of silver (around 10 μm) to reduce electrical losses,
• access to guide 11, for example a standard flange screwed onto guide 10 using screws 9,
• at least two dielectric resonators 13,
• supports 14 of resonators, of dielectric material such as for example polypenco or rexolite, threaded,
• elastic washers 15 and locking nuts 16 for the position of each support 14,
• of threaded metal sleeves 17 allowing the passage of the dielectric resonators 13 in the guide 10,
• locking nuts 18 to hold the sleeves 17,
• a metal bottom 19 closing the guide, held by screws 20,
• at least one screw 21 with its locking nut 22 making a "mode suppressor", placed parallel to the supports 14 of the dielectric resonators,
• of screws 23 with locking nut 24 allowing adjustment of the coupling between resonators, placed perpendicular to the supports of dielectric resonators 13,
• a coaxial access 25 forming an antenna 26 in order to excite the adjacent resonator,
• and a screw 27 with its locking nut 28 for adjusting the coupling between the antenna 26 and said adjacent resonator.

• une première courbe (30) avec : D=7,3 mm h=2,70 mm fo=7700 MHz
• une seconde courbe (31) avec : D=7,3 mm h=3,78 mm fo=7100 MHz

The curves in FIG. 4 show that the coupling of two dielectric resonators varies very little when the dielectric resonators have the same diameter D and different heights h; Indeed we have:

• a first curve (30) with: D = 7.3 mm h = 2.70 mm fo = 7700 MHz
• a second curve (31) with: D = 7.3 mm h = 3.78 mm fo = 7100 MHz

By varying the distance d between two resonators, the frequency difference (Δ f) corresponding to the resonance of the two dielectric resonators is measured. The Δ f is directly linked to the value of the coupling coefficient: (see "MICROWAVE FILTERS IMPEDANCE-MATCHING, NETWORKS AND COUPLING STRUCTURES" by: G. MATTHAEI, L. YOUNG, E.N.T. JONES ARTECH HOUSE BOOKS pages 666-667).

: f₂-f₁ représente la bande passante du filtre (la fréquence centrale est : ${\text{f}}_{\text{o}}\text{= 0,5 (f₁+f₂)}$

g_i et g_i+1 représentent les éléments du passe-bas prototype.We deduce the following formula:

: f₂-f₁ represents the bandwidth of the filter (the central frequency is: ${\text{f}}_{\text{o}}\text{= 0.5 (f₁ + f₂)}$

g _i and g _{i + 1} represent the elements of the prototype low pass.

The small difference between these two curves allows the coupling to be taken up by the screws 23. It is thus easy to adjust the bandwidth of the filter, whatever the channel used, while keeping the same mechanics.

The dielectric resonators 13 are fixed on their support 14, for example by gluing. The latter is made of dielectric material, having a low relative permittivity ε _r as well as a good loss tangent δ; it can be for example: polypenco, rexolite ... This support 14 is threaded, which makes it possible to modify the position of the dielectric resonators 13 in the guide 10. It is thus possible to adjust the resonant frequency of each dielectric resonator by keeping him away or by bringing it closer to a wall of the guide. When the dielectric resonator is placed in the middle of the guide, its resonant frequency can no longer be reduced because then the slightest movement would tend to bring the dielectric resonator either to the bottom or to the top of the guide. To overcome this defect and in order to increase the range of adjustability, the support 14 is screwed into a metal sleeve 17 itself threaded and screwed into the wall of the guide 10.

By unscrewing this sleeve 17, the wall of the guide is artificially removed and, thus, the resonance frequency of the dielectric resonator is reduced. A locking nut 18 comes to hold the sleeve 17, once the filter has been adjusted. In this way, the filter tuning frequency can be varied from fo ± 20 MHz, without degrading the characteristics of the filter. Under no circumstances should the sleeve 17 enter the guide 10.

• les mauvais contacts entre le guide et son couvercle,
• les pertes supplémentaires,
• le surcoût de fabrication.

In addition, this sleeve 17 has a diameter greater than that of the dielectric resonators, making it possible to mount them in the guide 10. The guide 10 can therefore be in one piece and all the problems inherent in the two-part guide are eliminated, with regard in particular to :

• poor contact between the guide and its cover,
• additional losses,
• the additional manufacturing cost.

Another aspect of the invention relates to the suppression of parasitic responses (or higher modes). Indeed, although one does not introduce metal adjusting screws into the guide to adjust the resonant frequency of the dielectric resonators, the latter resonate in many modes beyond the TE011 mode. To overcome this defect, it is known to reduce the interior height of the guide in order to have the highest possible cutoff frequency. However, if one does not want to degrade the filter overvoltage, the interior height of the guide must be approximately twice the height of the dielectric resonators. Under these conditions, the parasitic modes are not sufficiently attenuated.

• Il est très difficile d'introduire ces "suppresseurs de modes" dans le guide lors d'une production en série,
• le diélectrique de maintien des plaques crée inévitablement des pertes supplémentaires.

It is known to use "mode suppressors"("Mode suppressor for dielectric resonator filter" of CL REN, IEEE MTTS DIGEST 1982 p. 389), which consist in inserting a thin metal plate between two dielectric resonators crossing the guide in his height. Such a plate is held by dielectric supports. This system has two drawbacks:

• It is very difficult to introduce these "mode suppressors" in the guide during mass production,
• the plate holding dielectric inevitably creates additional losses.

In the invention, this plate is replaced by at least one screw 21 placed between two dielectric resonators, in an axis parallel to their supports.

This assembly is simple, inexpensive and does not introduce a dielectric in the guide. In addition, by varying the position of the screw 21 relative to the top of the guide, the effectiveness of the attenuation of the higher modes can be increased by creating an end capacitance between the end of the screw 21 and the top of the guide 10.

The effect of this booster capacity is added with the "mode suppressor" effect described in the article cited above. Such screws 21 are preferably placed in the middle area of the filter.

In the embodiment shown in FIGS. 1 to 3, the filter 10 has a guide access 11 and a coaxial access 25. These two accesses can be either in guide or in coaxial. In the case of coaxial access, the height of the adjacent dielectric resonator must be increased so that its resonant frequency is approximately 20 MHz lower than the filter adjustment frequency. Indeed, the antenna 26 has the effect of increasing the resonant frequency of this dielectric resonator because it approaches the metal near the latter. In the case of the guide access 11, a screw 27 is introduced into the flange, in order to adjust the input coupling. This screw 27 is in the same plane as the other coupling screws 23.

The guide 10 is in one piece but can also be produced in several parts, in different metals.

The maximum number of 21 "mode suppressor screws is n-1 if n is the number of dielectric resonators making up the filter. But such a number is not necessary because the influence of the additional screws becomes negligible on the attenuation parasitic responses while the level of losses due to these screws tends to become significant.

The curves, represented in FIG. 5, show the results of a filter with six dielectric resonators according to the invention, with regard to the transmission losses Pt as well as the reflection losses Pr ("return loss"). It is a 6-pole TCHEBYTCHEFF filter, with a 0.02 dB ripple and an equi-ripple band B = 20.5 MHz. The dielectric resonators used are made of zirconium titanate having the following characteristics: D = 7.3 mm; h = 2.9 mm; ε _r = 37; Tf = 5.5 ppm per degree C (Tf: Temperature drift coefficient of dielectric resonators). The guide has the following internal dimensions, internal width l = 12.62 mm; interior height H = 9 mm.

The curves shown in Figure 6 show this same filter shifted 18 MHz lower in frequency, by unscrewing the "metal sleeves".

The curves represented in FIGS. 7, 8 and 9 give the transmission curves of the filter of the invention over a very wide frequency band.

The curve, shown in Figure 7, shows this screwless "mode suppressor" filter. There are many spurious responses: at f = 10 GHz, f = 12 GHz, f = 13.5 GHz and f = 15.5 GHz.

The curve shown in Figure 8 shows this filter with three "mode suppressor" screws, in contact with the top of the guide: there is no end capacity effect. We note that the responses at f = 10 GHz are attenuated by more than 50dB, at f = 11.7 GHz the attenuation is 20dB and the same for the responses around f = 15.5 GHz.

The curve, shown in Figure 9, shows this filter with three "mode suppressor" screws, without contact with the top of the guide: this time we adjust the "mode suppressor" screws to further reduce spurious responses, we use the end capacity of these screws. The responses between 10 GHz and 12 GHz are attenuated by 10dB compared to the case of FIG. 8, those around 13.2 GHz are attenuated by 40dB and those around 15.5 GHz are attenuated by 20dB. Note that the attenuation of the two harmony in the band 7.1-7.9 GHz is greater than 60dB (band 14 to 16 GHz).

It is understood that the present invention has only been described and shown as a preferred example and that its constituent elements can be replaced by equivalent elements without, however, departing from the scope of the invention.

Claims (12)

1. Band-pass filter with dielectric resonators, in particular dielectric resonators which resonate in TE011 mode, comprising a microwave guide (10) in which the dielectric resonators (13) are disposed, each dielectric resonator (13) being fastened to a dielectric material support (14), the filter comprising at least one screw (21) forming a variable "mode suppressor" disposed parallel to the supports of the dielectric resonators (13), characterised in that:

   . the dielectric support (14) of each dielectric resonator (13) is movable axially to modify the position of the respective resonator in the guide (10) and thus to adjust the resonant frequency of each resonator by moving it away from or towards a wall of the guide;

   . and said resonators (13) have the same diameter, irrespective of the channel used in the frequency band in question, only their height varying according to the channel used, so that the filter uses the same mechanical design for each channel, only the dielectric resonator height changing from one channel to another.
2. Filter according to claim 1 characterised in that each support (14) is screwed into a screwthreaded metal sleeve (17) which is screwed into the wall of the guide (10).
3. Filter according to claim 2 characterised in that each support is associated with a spring washer (15) and a locknut (16) for immobilising the support in the respective sleeve (17).
4. Filter according to claim 2 or claim 3 characterised in that a locknut (18) is associated with each sleeve (17) to secure the latter in the wall of the guide (10).
5. Filter according to any one of claims 1 to 4 characterised in that it comprises respective adjuster screws (23) between the resonators (13) and perpendicular to the supports (14) of the latter to enable the coupling between the latter to be adjusted.
6. Filter according to any one of claims 1 to 5 characterised in that the guide (10) is a monobloc guide on which the two ports (11, 25) of the filter are implemented and which is provided at one end with a flange (11) which defines a waveguide port and at its other end with a metal backing plate (19) closing the guide (10).
7. Filter according to claim 6 characterised in that the flange (11) is provided with a screw (27) for adjusting the input coupling.
8. Filter according to any one of the preceding claims characterised in that it comprises a coaxial port (25) forming an antenna (26) to excite the adjacent resonator.
9. Filter according to claim 8 characterised in that it comprises a screw (27) for adjusting the coupling between the antenna (26) and the adjacent resonator.
10. Filter according to any one of the preceding claims characterised in that the guide (10) is of extruded aluminium covered with a thin layer of silver to reduce electric losses.
11. Filter according to any one of the preceding claims characterised in that the supports (14) of the dielectric resonators are of polypenco or rexolite.
12. Filter according to any one of the preceding claims characterised in that in the case of a coaxial port the resonator adjacent the antenna is higher than the other resonators so that its resonant frequency is approximately 20 MHz lower.

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