EP0921587B1 - Microwave filter with a steep upper edge - Google Patents

Microwave filter with a steep upper edge Download PDF

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Publication number
EP0921587B1
EP0921587B1 EP19980402978 EP98402978A EP0921587B1 EP 0921587 B1 EP0921587 B1 EP 0921587B1 EP 19980402978 EP19980402978 EP 19980402978 EP 98402978 A EP98402978 A EP 98402978A EP 0921587 B1 EP0921587 B1 EP 0921587B1
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EP
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Prior art keywords
guides
filter
waveguide
filter according
cutoff frequency
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EP19980402978
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German (de)
French (fr)
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EP0921587A1 (en
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Olivier Maas
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Thales SA
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Thales SA
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01P—WAVEGUIDES; RESONATORS, LINES, OR OTHER DEVICES OF THE WAVEGUIDE TYPE
    • H01P1/00—Auxiliary devices
    • H01P1/20—Frequency-selective devices, e.g. filters
    • H01P1/207—Hollow waveguide filters
    • H01P1/209—Hollow waveguide filters comprising one or more branching arms or cavities wholly outside the main waveguide

Definitions

  • the present invention relates to a microwave filter with steep upper flank.
  • the first and the most widespread, consists in producing a sufficiently wide band-pass filter, that is to say the lower flank of which is situated below the frequency band of the useful signal, the upper flank of this filter. band pass corresponding to the side of the low pass filter that we want to obtain.
  • band pass corresponding to the side of the low pass filter that we want to obtain.
  • the number of resonant cells of this bandpass filter depends on the ratio ⁇ '/ ⁇ ' 1 ( ⁇ ' 1 and ⁇ ' being respectively the maximum frequency of the bandwidth for which the attenuation of the filter is considered negligible, and the frequency, greater than ⁇ ' 1 , for which the attenuation specified on the upper side of the filter is obtained).
  • the second known possibility starts from the technique known as “leaky wave ”, generally used to make harmonic filters. She consists of connecting to a propagation line (waveguide or cable coaxial) a series of waveguides (connected guides) whose dimensions are such that their cut-off frequency is beyond the band of the useful signal and below the band of harmonic signals (including frequency is an integer multiple of the fundamental signal). In the band passing of the useful signal, the connected guides do not provide disturbance, and the wanted signal travels along the main line without couple in the connected guides. The whole then presents no attenuation in this bandwidth. Beyond the cutoff frequency of the connected guides, the useful signal couples in these connected guides, inside of which it is absorbed by a charge placed at the end of these guides connected.
  • each connected guide takes a small fraction of the useful signal.
  • At the end of the main line there is an attenuation proportional to the number of guides connected and the degree of coupling of each of them with the main line.
  • This filtering technique presents the advantage of introducing very low losses in the bandwidth, because the connected guides are not crossed by the incident wave.
  • we obtains an attenuation by absorption of the frequencies located beyond the cut-off frequency of the connected guides which is advantageous in particular for harmonic filters arranged at the output of transmitters with tubes.
  • the response curve of these filters does not allow them to be use as low pass filters in applications where search for extremely low losses up to the cut-off frequency Fc. Indeed, as shown in Figure 1, well before this frequency of cutoff, (which is determined by the width of the connected guides), we observes a phenomenon of increased filter losses: this is zone Z in figure 1.
  • the subject of the present invention is a low-pass filter for a transmission line, in particular for a microwave waveguide, exhibiting extremely low loss cutoff frequency low (less than about 0.3 dB) and high attenuation (at least 20 dB) from a frequency higher than the cut-off frequency and which as close as possible to this cutoff frequency.
  • the low-pass filter according to the invention comprises a section of propagation line inserted in the transmission line on which are connected several waveguides whose cutoff frequency is located above the required cut-off frequency of the filter, a load being placed at the end of each of the connected guides and these guides being arranged substantially regularly along the line section of propagation, the filter being characterized in that the length of the guides plugged in is greater than about eight times the wavelength in air corresponding to the filter cutoff frequency.
  • the pitch of the guides along of the section of line to which they are connected is approximately 1 / 6th of the wavelength in air corresponding to the cut-off frequency of the filter.
  • the lengths of the successive connected guides are uneven, and advantageously distributed randomly along the line.
  • the invention is described above with reference to a low-pass filter inserted into a microwave transmission line which is a guide waves, but it is understood that the invention can also be applied to very high frequencies (less than about 1 Ghz) and the lines of coaxial transmission.
  • the filter described below has a cutoff frequency of 9.1 Ghz, but it is understood that it can be different.
  • this waveguide 1 (FIG. 3) is of the classic type with rectangular section with double central rib longitudinal, that is to say that it has on the internal face of its walls large 2, 3 a central longitudinal “rib” 4, 5 respectively.
  • the filter 6 has a main waveguide portion 7 inserted between the two parts of the waveguide 1.
  • the inner section of the waveguide 7 has dimensions close to those of waveguide 1, but it has only one edge 8.
  • This edge 8 is formed on the face inside of the wide wall 9, corresponding to the wall 3 of the waveguide 1.
  • the edge 8 is aligned with the edges 5 of the two parts of the guide 1 and has substantially the same dimensions as the latter.
  • the wide wall of the waveguide section 7 opposite the wall 9 is formed by the wall "Lower” 10 of the set of connected guides 11, that is to say the wall in which the coupling irises of the connected guides are practiced.
  • the assembly 11 consists of two metal half-shells (12, 13), symmetrical with respect to the plane of symmetry 14 of the waveguide 7.
  • the filter body 6 consists of three metal parts 12, 13 and 15 assembled together for example by mechanical tightening.
  • Exhibit 15 in "U” shape constitutes with the two "lower” half-walls of the parts 12 and 13 (together forming the wall 10) the waveguide 7 to which are coupled the connected guides formed in parts 12 and 13.
  • the assembly of parts 12, 13 and 15 constitutes, with the charges absorbent described below, an easy to assemble component for both parts of the waveguide 1.
  • This component is called here "low pass filter ", Although strictly speaking the main waveguide 7 does not part of the filter (at least in theory, the main waveguide 1, forming the transmission line, could be uninterrupted and the guides connected, constituting the filter itself, could be coupled to such a guide main).
  • the connected guides are produced by precision milling in metal plates intended to form the half-shells 12 and 13.
  • long parallel grooves 16 are formed in these plates, leaving a peripheral partition around all of these grooves.
  • the different sides of this peripheral partition are referenced 10, 17 and 18 (these two last sides being parallel to grooves 16) and 19 (side opposite to the side 10).
  • These fillers 20 have, for example a wedge shape triangular.
  • the useful length of the connected guides formed by the grooves is greater than or equal to about eight times the length wave (corresponding to the filter cutoff frequency) in the air.
  • the corresponding wavelength is approximately 33 mm, which means that the useful length of connected guides (between wall 10 and load 20 corresponding) is greater than about 260 mm.
  • the lengths of the different successive branched guides are unequal, so to avoid any periodicity of their effects.
  • the width of all these grooves (measured parallel to the longitudinal axis of guide 7) is approximately 4 mm (this width determines their cutoff frequency, which must be the same for all connected guides) and their pitch of about 5 mm.
  • the number of connected guides is at least 30 approximately for an X-band cutoff frequency, and attenuation of at least minus 30 dB above the cutoff frequency.
  • the coupling between the connected guides and the main guide 7 is made by irises 21, that is to say openings made in the partition 10.
  • the width (measured parallel to the longitudinal axis of the guide 7) of these irises is equal to or slightly less than that of the guides connected.
  • Their length (measured perpendicular to the longitudinal axis of the guide 7, in a plane parallel to the wall 10) is variable in the way following ( Figure 5, on which the embodiment example at 30 is illustrated trendy guides): the ten central irises (referenced IC as a whole) have the same length L1, which is about 15 mm.
  • the two extreme irises denoted IE
  • IE have a length L2 of approximately 8 mm each
  • the eight irises intermediate (on each side of the central irises, not including the IE irises), noted I.I. have a length Ln which increases substantially linearly between the values L1 and L2, which allows to compensate by a coupling transition crescent (iris of increasing length) the passage between the parts of the guide main 1 without guides connected and guide 7 with guides connected.
  • a coupling transition crescent iris of increasing length
  • the length of the connected guides is such that the loads (20) placed at their ends do not absorb evanescent waves (which in art filters previous, were absorbed, causing losses in the band of these filters).
  • the stiffness of the flank results from the sudden passage of the guides connected in the on state, this which causes at a frequency very close to the cutoff frequency Fc (and greater than the latter) a strong attenuation of the waves passing through the transmission line. This attenuation is also a function of the number of guides connected.
  • These connected guides must all have the same cutoff frequency so that the stiffness of said anterior flank is the best possible. However, in the filter bandwidth, the losses are very low (0.2 dB or better). This is achieved in particular thanks to the three-part embodiment of the filter, which can be machined with high precision. Integration of coupling irises in the same room mechanical that their connected guides avoid electrical contact in an area of maximum electric field, where any resistance of contact generates losses.
  • the filter is passed between 4.75 Ghz and 9.1 Ghz, and its insertion losses are less than 0.3 dB, and from a frequency equal to Fc + 200 MHz, it has attenuation greater than 30 dB.
  • the ratio ⁇ '/ ⁇ ' 1 of the filter of the invention is less than 1.025 (whereas for a reactive filter of the prior art with 10 cells, this ratio is 1.18, its losses being approximately 1 dB in bandwidth).

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Description

La présente invention se rapporte à un filtre hyperfréquences à flanc supérieur raide.The present invention relates to a microwave filter with steep upper flank.

Pour réaliser une fonction de filtrage de type passe-bas dans un circuit à guides d'ondes, on connaít deux techniques différentes. La première, et la plus répandue, consiste à réaliser un filtre passe-bande suffisamment large, c'est-à-dire dont le flanc inférieur est situé en-dessous de la bande de fréquences du signal utile, le flanc supérieur de ce filtre passe-bande correspondant au flanc du filtre passe-bas que l'on veut obtenir. Toutefois, une telle technique présente les limitations suivantes. Le nombre de cellules résonantes de ce filtre passe-bande dépend du rapport ω'/ω'1 (ω'1 et ω' étant respectivement la fréquence maximale de la bande passante pour laquelle l'atténuation du filtre est considérée comme négligeable, et la fréquence, supérieure à ω'1, pour laquelle on obtient l'atténuation spécifiée sur le flanc supérieur du filtre). Plus le rapport ω'/ω'1 est faible, plus le nombre de cellules doit être grand. En outre, la complexité et les pertes d'insertion d'un tel filtre augmentent avec le nombre de cellules. En pratique, on est obligé de limiter ce nombre à une valeur d'environ 10, ce qui limite le rapport ω'/ω'1. A titre d'exemple, si on souhaite obtenir une atténuation de 30 dB avec un filtre passe-bas correctement adapté (avec une ondulation de 0,2 dB dans la bande passante) à dix cellules, avec un rapport ω'/ω'1 = 1,18, et ω'1 = 10 Ghz, on aura ω' = 11,8 Ghz. Ceci signifie qu'avec une telle technique, on ne peut réaliser un filtre qui soit passant à 10 Ghz et présente une atténuation de 30 dB à une fréquence plus basse que 11,8 Ghz.To perform a low-pass type filtering function in a waveguide circuit, two different techniques are known. The first, and the most widespread, consists in producing a sufficiently wide band-pass filter, that is to say the lower flank of which is situated below the frequency band of the useful signal, the upper flank of this filter. band pass corresponding to the side of the low pass filter that we want to obtain. However, such a technique has the following limitations. The number of resonant cells of this bandpass filter depends on the ratio ω '/ ω' 1 (ω ' 1 and ω' being respectively the maximum frequency of the bandwidth for which the attenuation of the filter is considered negligible, and the frequency, greater than ω ' 1 , for which the attenuation specified on the upper side of the filter is obtained). The lower the ratio ω '/ ω' 1 , the greater the number of cells. In addition, the complexity and the insertion losses of such a filter increase with the number of cells. In practice, we are obliged to limit this number to a value of around 10, which limits the ratio ω '/ ω' 1 . For example, if we want to obtain an attenuation of 30 dB with a properly adapted low-pass filter (with a ripple of 0.2 dB in the bandwidth) to ten cells, with a ratio ω '/ ω' 1 = 1.18, and ω ' 1 = 10 Ghz, we will have ω' = 11.8 Ghz. This means that with such a technique, it is not possible to produce a filter which is passing at 10 GHz and has an attenuation of 30 dB at a frequency lower than 11.8 GHz.

La deuxième possibilité connue part de la technique dite « leaky wave », généralement employée pour réaliser des filtres d'harmoniques. Elle consiste à brancher sur une ligne de propagation (guide d'ondes ou câble coaxial) une série de guides d'ondes (guides branchés) dont les dimensions sont telles que leur fréquence de coupure se situe au-delà de la bande du signal utile et en deçà de la bande des signaux harmoniques (dont la fréquence est un multiple entier du signal fondamental). Dans la bande passante du signal utile, les guides branchés n'apportent pas de perturbation, et le signal utile transite le long de la ligne principale sans se coupler dans les guides branchés. L'ensemble ne présente alors aucune atténuation dans cette bande passante. Au-delà de la fréquence de coupure des guides branchés, le signal utile se couple dans ces guides branchés, à l'intérieur desquels il est absorbé par une charge placée à l'extrémité de ces guides branchés. Ainsi, chaque guide branché prélève une petite fraction du signal utile. A l'extrémité de la ligne principale, on observe une atténuation proportionnelle au nombre de guides branchés et au degré de couplage de chacun d'eux avec la ligne principale. Cette technique de filtrage présente l'avantage d'introduire de très faibles pertes dans la bande passante, car les guides branchés ne sont pas traversés par l'onde incidente. En outre, on obtient une atténuation par absorption des fréquences situées au-delà de la fréquence de coupure des guides branchés, ce qui est avantageux en particulier pour les filtres d'harmoniques disposés à la sortie des émetteurs à tubes. Cependant, la courbe de réponse de ces filtres ne permet pas de les employer en tant que filtres passe-bas dans des applications où l'on recherche des pertes extrêmement faibles jusqu'à la fréquence de coupure Fc. En effet, comme représenté en figure 1, bien avant cette fréquence de coupure, (qui est déterminée par la largeur des guides branchés), on observe un phénomène d'augmentation des pertes du filtre : c'est la zone Z sur la figure 1.The second known possibility starts from the technique known as “leaky wave ”, generally used to make harmonic filters. She consists of connecting to a propagation line (waveguide or cable coaxial) a series of waveguides (connected guides) whose dimensions are such that their cut-off frequency is beyond the band of the useful signal and below the band of harmonic signals (including frequency is an integer multiple of the fundamental signal). In the band passing of the useful signal, the connected guides do not provide disturbance, and the wanted signal travels along the main line without couple in the connected guides. The whole then presents no attenuation in this bandwidth. Beyond the cutoff frequency of the connected guides, the useful signal couples in these connected guides, inside of which it is absorbed by a charge placed at the end of these guides connected. Thus, each connected guide takes a small fraction of the useful signal. At the end of the main line, there is an attenuation proportional to the number of guides connected and the degree of coupling of each of them with the main line. This filtering technique presents the advantage of introducing very low losses in the bandwidth, because the connected guides are not crossed by the incident wave. In addition, we obtains an attenuation by absorption of the frequencies located beyond the cut-off frequency of the connected guides, which is advantageous in particular for harmonic filters arranged at the output of transmitters with tubes. However, the response curve of these filters does not allow them to be use as low pass filters in applications where search for extremely low losses up to the cut-off frequency Fc. Indeed, as shown in Figure 1, well before this frequency of cutoff, (which is determined by the width of the connected guides), we observes a phenomenon of increased filter losses: this is zone Z in figure 1.

Des exemples de réalisation de filtres comportant une série de guides d'ondes branchés sur une ligne principale et absorbant les signaux dont la fréquence est supérieure à la bande passante souhaitée sont notamment présentés dans le texte du brevet français FR-A-1.252.509.Examples of embodiment of filters comprising a series of guides of waves connected to a main line and absorbing the signals whose frequency is higher than the desired bandwidth are notably presented in the text of the French patent FR-A-1,252,509.

La présente invention a pour objet un filtre passe-bas pour une ligne de transmission, en particulier pour un guide d'ondes hyperfréquences, présentant en-dessous de la fréquence de coupure des pertes extrêmement faibles (inférieures à environ 0,3 dB) et une atténuation élevée (d'au moins 20 dB) à partir d'une fréquence supérieure à la fréquence de coupure et qui soit la plus proche possible de cette fréquence de coupure.The subject of the present invention is a low-pass filter for a transmission line, in particular for a microwave waveguide, exhibiting extremely low loss cutoff frequency low (less than about 0.3 dB) and high attenuation (at least 20 dB) from a frequency higher than the cut-off frequency and which as close as possible to this cutoff frequency.

Le filtre passe-bas conforme à l'invention comporte un tronçon de ligne de propagation inséré dans la ligne de transmission sur lequel sont branchés plusieurs guides d'ondes dont la fréquence de coupure est située au-delà de la fréquence de coupure requise du filtre, une charge étant placée à l'extrémité de chacun des guides branchés et ces guides étant disposés sensiblement régulièrement le long du tronçon de ligne de propagation, le filtre étant caractérisé en ce que la longueur des guides branchés est supérieure à environ huit fois la longueur d'onde dans l'air correspondant à la fréquence de coupure du filtre. The low-pass filter according to the invention comprises a section of propagation line inserted in the transmission line on which are connected several waveguides whose cutoff frequency is located above the required cut-off frequency of the filter, a load being placed at the end of each of the connected guides and these guides being arranged substantially regularly along the line section of propagation, the filter being characterized in that the length of the guides plugged in is greater than about eight times the wavelength in air corresponding to the filter cutoff frequency.

Selon une caractéristique de l'invention, le pas des guides le long du tronçon de ligne sur lequel ils sont branchés est d'environ 1/6ème de la longueur d'onde dans l'air correspondant à la fréquence de coupure du filtre.According to a characteristic of the invention, the pitch of the guides along of the section of line to which they are connected is approximately 1 / 6th of the wavelength in air corresponding to the cut-off frequency of the filter.

Selon une autre caractéristique de l'invention, les longueurs des guides branchés successifs sont inégales, et avantageusement réparties aléatoirement le long de la ligne.According to another characteristic of the invention, the lengths of the successive connected guides are uneven, and advantageously distributed randomly along the line.

La présente invention sera mieux comprise à la lecture de la description détaillée d'un mode de réalisation, pris à titre d'exemple non limitatif et illustré par le dessin annexé, sur lequel :

  • la figure 1, déjà mentionnée ci-dessus, est un diagramme de l'atténuation en fonction de la fréquence pour un filtre passe-bas connu,
  • la figure 2 est une vue en coupe, d'un mode de réalisation du filtre de l'invention,
  • la figure 3 est une vue en coupe d'un guide d'ondes sur lequel est branché le filtre de la figure 2,
  • la figure 4 est une vue en perspective d'un demi-ensemble de guides branchés du filtre de la figure 2, et
  • la figure 5 est une vue de dessous de l'ensemble de guides branchés de la figure 2, montrant les iris de couplage des guides branchés.
The present invention will be better understood on reading the detailed description of an embodiment, taken by way of nonlimiting example and illustrated by the appended drawing, in which:
  • FIG. 1, already mentioned above, is a diagram of the attenuation as a function of the frequency for a known low-pass filter,
  • FIG. 2 is a sectional view of an embodiment of the filter of the invention,
  • FIG. 3 is a sectional view of a waveguide to which the filter of FIG. 2 is connected,
  • FIG. 4 is a perspective view of a half-set of connected guides of the filter of FIG. 2, and
  • Figure 5 is a bottom view of the set of connected guides of Figure 2, showing the coupling irises of the connected guides.

L'invention est décrite ci-dessus en référence à un filtre passe-bas inséré dans une ligne de transmission hyperfréquences qui est un guide d'ondes, mais il est bien entendu que l'invention peut aussi s'appliquer aux très hautes fréquences (inférieures à environ 1 Ghz) et aux lignes de transmission coaxiales. D'autre part, le filtre décrit ci-dessous a une fréquence de coupure de 9,1 Ghz, mais il est bien entendu qu'elle peut être différente.The invention is described above with reference to a low-pass filter inserted into a microwave transmission line which is a guide waves, but it is understood that the invention can also be applied to very high frequencies (less than about 1 Ghz) and the lines of coaxial transmission. On the other hand, the filter described below has a cutoff frequency of 9.1 Ghz, but it is understood that it can be different.

Le filtre décrit ici est inséré entre deux parties d'un guide d'ondes principal 1. Dans l'exemple considéré, ce guide d'ondes 1 (figure 3) est du type classique à section rectangulaire à double nervure centrale longitudinale, c'est-à-dire qu'il comporte sur la face interne de ses parois larges 2, 3 une « nervure » (« Ridge » en anglais) longitudinale centrale 4, 5 respectivement. Le filtre 6 comporte une portion de guide d'ondes principal 7 inséré entre les deux parties du guide d'ondes 1. La section intérieure du guide d'ondes 7 a des dimensions voisines de celles du guide d'ondes 1, mais elle ne comporte qu'une arête 8. Cette arête 8 est formée sur la face interne de la paroi large 9, correspondant à la paroi 3 du guide d'ondes 1. L'arête 8 est alignée avec les arêtes 5 des deux parties du guide 1 et a sensiblement les mêmes dimensions que ces dernières. La paroi large du tronçon de guide d'ondes 7 opposée à la paroi 9 est constituée par la paroi « inférieure » 10 de l'ensemble de guides branchés 11, c'est-à-dire la paroi dans laquelle sont pratiqués les iris de couplage des guides branchés. L'ensemble 11 se compose de deux demi-coquilles métalliques (12, 13), symétriques par rapport au plan de symétrie 14 du guide d'ondes 7. Ainsi, le corps du filtre 6 se compose de trois pièces métalliques 12, 13 et 15 assemblées entre elles par exemple par serrage mécanique. La pièce 15 en forme de « U » constitue avec les deux demi-parois « inférieures » des pièces 12 et 13 (formant ensemble la paroi 10) le guide d'ondes 7 auquel sont couplés les guides branchés formés dans les pièces 12 et 13. L'assemblage des pièces 12, 13 et 15 constitue, avec les charges absorbantes décrites ci-dessous, un composant facile à assembler aux deux parties du guide d'ondes 1. Ce composant est dénommé ici « filtre passe-bas », bien qu'en toute rigueur le guide d'ondes principal 7 ne fasse pas partie du filtre (au moins en théorie, le guide d'ondes principal 1, formant la ligne de transmission, pourrait être ininterrompu et les guides branchés, constituant le filtre proprement dit, pourraient être couplés à un tel guide principal).The filter described here is inserted between two parts of a waveguide principal 1. In the example considered, this waveguide 1 (FIG. 3) is of the classic type with rectangular section with double central rib longitudinal, that is to say that it has on the internal face of its walls large 2, 3 a central longitudinal “rib” 4, 5 respectively. The filter 6 has a main waveguide portion 7 inserted between the two parts of the waveguide 1. The inner section of the waveguide 7 has dimensions close to those of waveguide 1, but it has only one edge 8. This edge 8 is formed on the face inside of the wide wall 9, corresponding to the wall 3 of the waveguide 1. The edge 8 is aligned with the edges 5 of the two parts of the guide 1 and has substantially the same dimensions as the latter. The wide wall of the waveguide section 7 opposite the wall 9 is formed by the wall "Lower" 10 of the set of connected guides 11, that is to say the wall in which the coupling irises of the connected guides are practiced. The assembly 11 consists of two metal half-shells (12, 13), symmetrical with respect to the plane of symmetry 14 of the waveguide 7. Thus, the filter body 6 consists of three metal parts 12, 13 and 15 assembled together for example by mechanical tightening. Exhibit 15 in "U" shape constitutes with the two "lower" half-walls of the parts 12 and 13 (together forming the wall 10) the waveguide 7 to which are coupled the connected guides formed in parts 12 and 13. The assembly of parts 12, 13 and 15 constitutes, with the charges absorbent described below, an easy to assemble component for both parts of the waveguide 1. This component is called here "low pass filter ", Although strictly speaking the main waveguide 7 does not part of the filter (at least in theory, the main waveguide 1, forming the transmission line, could be uninterrupted and the guides connected, constituting the filter itself, could be coupled to such a guide main).

Les guides branchés sont réalisés par fraisage de précision dans des plaques métalliques destinées à former les demi-coquilles 12 et 13. Par ce fraisage, on forme de longues rainures parallèles 16 dans ces plaques, en laissant subsister une cloison périphérique autour de l'ensemble de ces rainures. Pour la demi-coquille 12, représentée en figure 4, les différents côtés de cette cloison périphérique sont référencées 10, 17 et 18 (ces deux derniers côtés étant parallèles aux rainures 16) et 19 (côté opposé au côté 10). On dispose à l'extrémité de chaque rainure 16 (près du côté 19) une charge absorbante 20. Ces charges 20 ont, par exemple une forme en coin triangulaire.The connected guides are produced by precision milling in metal plates intended to form the half-shells 12 and 13. By this milling, long parallel grooves 16 are formed in these plates, leaving a peripheral partition around all of these grooves. For the half-shell 12, shown in FIG. 4, the different sides of this peripheral partition are referenced 10, 17 and 18 (these two last sides being parallel to grooves 16) and 19 (side opposite to the side 10). There is at the end of each groove 16 (near the side 19) a absorbent filler 20. These fillers 20 have, for example a wedge shape triangular.

Selon l'invention, la longueur utile des guides branchés formés par les rainures est supérieure ou égale à environ huit fois la longueur d'onde (correspondant à la fréquence de coupure du filtre) dans l'air. Dans un exemple de réalisation, pour une fréquence de coupure de 9,1 Ghz, la longueur d'onde correspondante est d'environ 33 mm, ce qui fait que la longueur utile des guides branchés (entre la paroi 10 et la charge 20 correspondante) est supérieure à environ 260 mm. De façon avantageuse, les longueurs des différents guides branchés successifs sont inégales, afin d'éviter toute périodicité de leurs effets. La largeur de toutes ces rainures (mesurée parallèlement à l'axe longitudinal du guide 7) est d'environ 4 mm (cette largeur détermine leur fréquence de coupure, qui doit être la même pour tous les guides branchés) et leur pas d'environ 5 mm. En effet, on a constaté que lorsque ce pas augmente, l'atténuation au-delà de la fréquence de coupure augmente, mais le ROS (rapport d'ondes stationnaires) se dégrade. Par conséquent, ce pas doit être optimisé pour obtenir le meilleur compromis entre une atténuation suffisante et un ROS acceptable. Dans un exemple de réalisation, le nombre de guides branchés est au moins de 30 environ pour une fréquence de coupure en bande X, et une atténuation d'au moins 30 dB au-delà de la fréquence de coupure.According to the invention, the useful length of the connected guides formed by the grooves is greater than or equal to about eight times the length wave (corresponding to the filter cutoff frequency) in the air. In an exemplary embodiment, for a cutoff frequency of 9.1 GHz, the corresponding wavelength is approximately 33 mm, which means that the useful length of connected guides (between wall 10 and load 20 corresponding) is greater than about 260 mm. Advantageously, the lengths of the different successive branched guides are unequal, so to avoid any periodicity of their effects. The width of all these grooves (measured parallel to the longitudinal axis of guide 7) is approximately 4 mm (this width determines their cutoff frequency, which must be the same for all connected guides) and their pitch of about 5 mm. Indeed, we have found that when this step increases, the attenuation beyond the frequency cutoff increases, but the ROS (standing wave ratio) is degraded. Therefore, this step must be optimized to obtain the best compromise between sufficient attenuation and acceptable ROS. In one example of embodiment, the number of connected guides is at least 30 approximately for an X-band cutoff frequency, and attenuation of at least minus 30 dB above the cutoff frequency.

Le couplage entre les guides branchés et le guide principal 7 se fait grâce à des iris 21, c'est-à-dire des ouvertures pratiquées dans la cloison 10. La largeur (mesurée parallèlement à l'axe longitudinal du guide 7) de ces iris est égale ou légèrement inférieure à celle des guides branchés. Leur longueur (mesurée perpendiculairement à l'axe longitudinal du guide 7, dans un plan parallèle à la paroi 10) est variable de la façon suivante (figure 5, sur laquelle est illustré l'exemple de réalisation à 30 guides branchés) : les dix iris centraux (référencés IC dans leur ensemble) ont la même longueur L1, qui est d'environ 15 mm. Les deux iris extrêmes, notés IE, ont une longueur L2 d'environ 8 mm chacun, et les huit iris intermédiaires (de chaque côté des iris centraux, non compris les iris IE), notés I.I., ont une longueur Ln croissant sensiblement linéairement entre les valeurs L1 et L2, ce qui permet de compenser par une transition à couplage croissant (iris à longueur croissante) le passage entre les parties du guide principal 1 sans guides branchés et le guide 7 à guides branchés. Bien entendu, le passage entre les parties du guide 1 à deux nervures (4, 5) et le guide 7 à une seule nervure se fait de façon progressive.The coupling between the connected guides and the main guide 7 is made by irises 21, that is to say openings made in the partition 10. The width (measured parallel to the longitudinal axis of the guide 7) of these irises is equal to or slightly less than that of the guides connected. Their length (measured perpendicular to the longitudinal axis of the guide 7, in a plane parallel to the wall 10) is variable in the way following (Figure 5, on which the embodiment example at 30 is illustrated trendy guides): the ten central irises (referenced IC as a whole) have the same length L1, which is about 15 mm. The two extreme irises, denoted IE, have a length L2 of approximately 8 mm each, and the eight irises intermediate (on each side of the central irises, not including the IE irises), noted I.I., have a length Ln which increases substantially linearly between the values L1 and L2, which allows to compensate by a coupling transition crescent (iris of increasing length) the passage between the parts of the guide main 1 without guides connected and guide 7 with guides connected. Well heard, the passage between the parts of the guide 1 with two ribs (4, 5) and the guide 7 with a single rib is done gradually.

Ainsi, dans le filtre passe-bas de l'invention, la longueur des guides branchés est telle que les charges (20) placées à leur extrémité n'absorbent pas les ondes évanescentes (qui, dans les filtres de l'art antérieur, étaient absorbées, ce qui causait des pertes dans la bande passante de ces filtres). La raideur du flanc (au-dessus de la fréquence de coupure) résulte du passage brutal des guides branchés à l'état passant, ce qui provoque à une fréquence très proche de la fréquence de coupure Fc (et supérieure à cette dernière) une forte atténuation des ondes passant dans la ligne de transmission. Cette atténuation est aussi fonction du nombre de guides branchés. Ces guides branchés doivent tous avoir la même fréquence de coupure pour que la raideur dudit flanc antérieur soit la meilleure possible. Par contre, dans la bande passante du filtre, les pertes sont très faibles (0,2 dB ou mieux). Ceci est obtenu en particulier grâce au mode de réalisation en trois pièces du filtre, qui peuvent être usinées avec une haute précision. L'intégration des iris de couplage dans la même pièce mécanique que leurs guides branchés permet d'éviter un contact électrique dans une zone de maximum de champ électrique, là où toute résistance de contact engendre des pertes.Thus, in the low-pass filter of the invention, the length of the connected guides is such that the loads (20) placed at their ends do not absorb evanescent waves (which in art filters previous, were absorbed, causing losses in the band of these filters). The stiffness of the flank (above the frequency of cut) results from the sudden passage of the guides connected in the on state, this which causes at a frequency very close to the cutoff frequency Fc (and greater than the latter) a strong attenuation of the waves passing through the transmission line. This attenuation is also a function of the number of guides connected. These connected guides must all have the same cutoff frequency so that the stiffness of said anterior flank is the best possible. However, in the filter bandwidth, the losses are very low (0.2 dB or better). This is achieved in particular thanks to the three-part embodiment of the filter, which can be machined with high precision. Integration of coupling irises in the same room mechanical that their connected guides avoid electrical contact in an area of maximum electric field, where any resistance of contact generates losses.

Dans l'exemple de réalisation cité ci-dessus, le filtre est passant entre 4,75 Ghz et 9,1 Ghz, et ses pertes d'insertion sont inférieures à 0,3 dB, et à partir d'une fréquence égale à Fc + 200 MHz, il présente une atténuation supérieure à 30 dB. Ainsi, le rapport ω'/ω'1 du filtre de l'invention est inférieur à 1,025 (alors que pour un filtre réactif de l'art antérieur à 10 cellules, ce rapport est de 1,18, ses pertes étant d'environ 1 dB dans la bande passante).In the embodiment cited above, the filter is passed between 4.75 Ghz and 9.1 Ghz, and its insertion losses are less than 0.3 dB, and from a frequency equal to Fc + 200 MHz, it has attenuation greater than 30 dB. Thus, the ratio ω '/ ω' 1 of the filter of the invention is less than 1.025 (whereas for a reactive filter of the prior art with 10 cells, this ratio is 1.18, its losses being approximately 1 dB in bandwidth).

Claims (7)

  1. Bandpass filter for a transmission line, comprising a propagation line piece (7), inserted into the transmission line (1) onto which are branched several waveguides (11) whose cutoff frequency lies beyond the required cutout frequency of the filter, a load (20) being placed at the end of each of the branch guides, and these guides being arranged substantially regularly along the propagation line piece, characterized in that the length of the branch guides is greater than around eight times the wavelength in air corresponding to the cutoff frequency of the filter.
  2. Filter according to Claim 1, characterized in that the spacing of the guides along the line piece to which they are branched is around 1/6th of the wavelength in air corresponding to the cutoff frequency of the filter.
  3. Filter according to Claim 1 or 2, characterized in that the lengths of the successive branch guides are unequal.
  4. Filter according to Claim 3, characterized in that the lengths of the successive branch guides are randomly distributed.
  5. Filter according to one of the preceding claims, characterized in that its body is made in three parts: two half-shells (12, 13) which are symmetric with respect to the plane of symmetry (14) of the propagation line piece and a "U" shaped member (15) forming a waveguide with one of the walls (10) of the assembly of the two half-shells.
  6. Filter according to one of the preceding claims, in which the coupling between the propagation line piece and the branch guides is achieved through coupling irises (21), this piece being a waveguide with a single longitudinal central ridge (8) and being hooked up to a waveguide with two longitudinal central ridges (3, 4), characterized in that the coupling irises have the same length (L1) in the middle of the piece, a smaller length (L2) at the ends, and a length (Ln) which increases substantially linearly between the values (L2) at the ends and the value in the middle (L1).
  7. Filter according to one of the preceding claims, having a cutoff frequency in the X band, characterized in that it comprises at least 30 branch guides approximately.
EP19980402978 1997-12-03 1998-11-27 Microwave filter with a steep upper edge Expired - Lifetime EP0921587B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR9715228 1997-12-03
FR9715228A FR2771857B1 (en) 1997-12-03 1997-12-03 HIGH STRAIGHT HYPERFREQUENCY FILTER

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EP0921587A1 EP0921587A1 (en) 1999-06-09
EP0921587B1 true EP0921587B1 (en) 2003-09-03

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DE (1) DE69817744T2 (en)
FR (1) FR2771857B1 (en)

Family Cites Families (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB845957A (en) * 1958-12-15 1960-08-24 Standard Telephones Cables Ltd Improvements in or relating to electric wave filters
FR1252509A (en) * 1960-03-31 1961-01-27 Thomson Houston Comp Francaise Microwave filter
US4060778A (en) * 1976-07-12 1977-11-29 Microwave Research Corporation Microwave harmonic absorption filter
FR2565416B1 (en) * 1984-05-30 1987-06-26 Alcatel Thomson Faisceaux MAIN WAVEGUIDE FILTER LOADED BY SIDE WAVEGUIDES

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DE69817744D1 (en) 2003-10-09
DE69817744T2 (en) 2004-07-15
FR2771857A1 (en) 1999-06-04
FR2771857B1 (en) 2000-02-18
EP0921587A1 (en) 1999-06-09

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