EP2517299B1 - Frequency-tunable microwave bandpass filter - Google Patents

Description

The invention particularly relates to a frequency tunable bandpass microwave filter produced by the waveguide technique.

Microwave transmissions require the use of transmit and receive filters to select the frequency band in which the signal is transmitted. At microwave frequencies, it is possible to use guide filters which make it possible to obtain low losses and great selectivity.

In some applications, it is interesting to be able to tune the filter within a frequency band in order to be able to configure the hardware or the device at any time and until operation according to the frequency of the signal to be transmitted.

There are several ways to make guide filters. Some use transverse partitions forming irises of inductive or capacitive type, others longitudinal partitions (septum). The narrowest filters may have a relative bandwidth of a fraction of a percent of the center frequency.

The patent US 5,808,528 [4] discloses a band pass filter which comprises a waveguide having a plurality of conductive walls and a movable wall defining the "large" dimension "a" of the waveguide. In this patent, the discontinuities created by a septum T with self-induced obstacles in the vicinity of the axis of symmetry of the guide are used to define the cavities and the couplings of the filter ( figure 1a ).

The equivalent diagram of a guide cavity is shown in figure 1b according to the prior art [2] page 697 in which L is an admittance line section Yo and JB an end-of-line admittance.

(où "a" et "d' " sont définis sur la figure 1c)
dépendent directement de la dimension du grand côté du guide « a » et varient considérablement quand « a » varie lorsque le petit côté « b » du guide est déplacé parallèlement à lui-même pour régler "a". La figure 1c représente un exemple d'iris selfique selon l'art antérieur.Devices according to the prior art comprise cavities with end inductive admittances (made by means of iris or septum), for which the values of the equivalent inductive admittances (jB) at the ends of the cavities: $$\mathrm{B}/\mathrm{Yo}~\mathrm{-}\left({\mathrm{\lambda }}_{\mathrm{boy\; Wut}}\right)/{\mathrm{a\; *\; <figure-callout\; id="2"\; label="cot"\; filenames="imgf0004.png"\; state="\{\{state\}\}">cot</figure-callout>}}^2\left(\mathrm{\pi d}\text{'}\mathrm{/}\left(2\mathrm{at}\right)\right)$$

(where "a" and "d" are defined on the figure 1c )
depend directly on the size of the long side of the guide "a" and vary considerably when "a" varies when the small side "b" of the guide is moved parallel to itself to set "a". The figure 1c represents an example of inductive iris according to the prior art.

The document Sichak entitled "Tunable Waveguide Filters", Proceedings of the IRE, IEEE, PISCATAWAY, NJ, US LNKD-DOI: 10.1109 / JRPROC.1951.273747.vol.39, No. 9, 1, September 1951, pages 1055-1059, XP011153423 , describes a device in which the capacitors are coupled by half-wave or quarter-wave transformers which do not have capacitive irises.

The document US 2,967,209 discloses a device having capacitive iris cavities, a coupling by means of half-wave or quarter-wave transformers, which uses, to vary the frequency, the displacement of a dielectric band across the long side of the guide.

The document GB 608 254 discloses a resonating resonator resonator which does not have resonant cavities.

The document US 5,808,528 discloses a septum inductive iris cavity device whose bandwidth varies with the center frequency.

• un guide d'onde de section rectangulaire comprenant une première partie conductrice fixe I et une deuxième partie conductrice mobile II,
• ladite première partie fixe I comprenant trois cloisons conductrices longitudinales formant trois côtés du guide d'onde G, la section du guide ayant un grand côté « a » défini par la position de la partie conductrice mobile II lorsqu'elle est insérée dans la partie I et un petit côté « b »,
• ladite première partie I comprenant plusieurs premières cloisons conductrices avec un ou plusieurs obstacles conducteurs associés aux ouvertures complémentaires dans la section du guide d'onde formant des iris de type capacitif, lesdites premières cloisons étant montées transversalement à la propagation de l'onde dans le guide et définissant plusieurs cavités Ki dans le sens longitudinal du guide et solidaires de la première partie I, et plusieurs secondes cloisons conductrices avec une ou plusieurs ouvertures i définissant des iris de type capacitif et qui forment, en association avec deux longueurs de guide adjacentes à une seconde cloison conductrice, des inverseurs d'immitance Ji, lesdites premières cloisons formant une succession de cavités Ki résonnantes couplées par des inverseurs d'immitance Ji,

The object of the present invention relates to a microwave frequency tunable band pass filter comprising in combination at least the following elements:

• a rectangular section waveguide comprising a first fixed conductive portion I and a second movable conductive portion II,
• said first stationary part I comprising three longitudinal conducting partitions forming three sides of the waveguide G, the section of the guide having a large side "a" defined by the position of the movable conductive part II when it is inserted in part I and a small side "b",
• said first portion I comprising a plurality of first conductive partitions with one or more conductive obstacles associated with the complementary openings in the waveguide section forming capacitive-type irises, said first partitions being mounted transversely to the propagation of the wave in the guide and defining several cavities Ki in the longitudinal direction of the guide and secured to the first part I, and several second conductive partitions with one or more openings i defining capacitive type irises and which form, in association with two lengths of guide adjacent to a second partition conductive, immitance reversers Ji, said first partitions forming a succession of resonant cavities Ki coupled by invertors of immitance Ji,

Said mobile conductive part II comprising a wall, parallel to the short side "b" of the guide, forming the fourth face of the waveguide G, said wall defining the dimension value "a" of the long side of the guide and thus the central frequency of the filter, the second part II comprising a plurality of slots receiving the partitions of part I which form the capacitive-type irises, the cavities Ki thus being formed when the part I and the part II are nested,
means for ensuring electrical contact between the first fixed conductive part I and the second movable conductive part II.

The capacitive type irises used to form the cavities Ki have, for example, an opening "d (x)" variable as a function of the abscissa x according to the side "a" which makes it possible to maintain the bandwidth of the filter constant when " a "varies. In a possible embodiment, the variable "d (x)" aperture as a function of the abscissa x along the long side "a" may be a linear function to give this aperture a trapezoidal shape.

In a possible embodiment, the filter comprises to ensure electrical continuity along the small side "b" movable guide a sliding metal contact spring copper alloy.

It is also possible to ensure the electrical continuity along the small moving "b" side of the guide by means of a sliding contact with charged elastomer conductive joints.

The filter may comprise to provide electrical continuity along the short side "b" movable guide a trap bringing a short circuit to sliding contact points "C" for a selected guided wavelength.

The filter may comprise means for moving the partitions of the capacitive irises of the cavities parallel to the short side "b" of the waveguide to vary the opening "d" identically to the ends of the each cavity and thus simultaneously change the value of the overvoltage coefficient Q for all the cavities Ki.

The filter may also comprise means adapted to vary the opening "d" of the capacitive irises of the cavities when the narrow adjustable side "b" of the guide moves with the mobile side II said means constituted by a separate motorized control or no, and common to all cavities.

The filter may also comprise means adapted to vary the opening "d" of the capacitive irises of the cavities when the narrow adjustable side "b" of the guide moves with the movable side II, said means being a compensated pusher device in the reverse direction adapted to push the irises of the cavities upwards parallel to the small side "b" of the guide to increase the value of "d" when decreasing the value "a" of the long side.

The movable partition associated with the movable conductor II side of the guide is moved mechanically parallel to itself by one or more rotary or linear motors or piezoelectric motors.

• La figure 1a un exemple de cavité utilisant un septum à obstacles inductifs selon l'art antérieur, la figure 1b le schéma équivalent d'une cavité en guide d'onde selon l'art antérieur, la figure 1c un exemple d'iris inductif utilisé selon l'art antérieur pour limiter une telle cavité,
• La figure 2, un filtre passe bande en guide d'ondes à section rectangulaire composé de deux parties conductrices I et II,
• La figure 3, la manière dont sont emboîtées les deux parties I et II de la figure 2 pour former le filtre en guide d'ondes,
• La figure 4, une vue en coupe d'une section transverse du guide d'onde,
• La figure 5, une vue en coupe d'une section longitudinale du filtre en guide composé d'une succession de cavités résonnantes et d'inverseurs d'immitance,
• La figure 6a plusieurs exemples de réalisation d'iris capacitifs,
• La figure 6b, un exemple d'iris ayant une ouverture en forme de trapèze permettant de maintenir la bande passante du filtre pratiquement constante quand la fréquence centrale du filtre varie avec "a",
• Les figures 7a, 7b et 7c, plusieurs modes de réalisation pour réaliser un contact assurant la continuité électrique entre les deux parties I et II, et
• La figure 8, un rappel de schéma utilisé pour le calcul d'inverseur d'immitance.

Other features and advantages of the device according to the invention will appear better on reading the description which follows of an example of embodiment given by way of illustration and in no way limiting attached to the figures which represent:

• The figure 1a an example of a cavity using an inductive obstacle septum according to the prior art, the figure 1b the equivalent diagram of a waveguide cavity according to the prior art, the figure 1c an example of inductive iris used according to the prior art for limiting such a cavity,
• The figure 2 a rectangular section waveguide bandpass filter composed of two conductive parts I and II,
• The figure 3 , the way in which the two parts I and II of the figure 2 to form the waveguide filter,
• The figure 4 , a sectional view of a transverse section of the waveguide,
• The figure 5 a sectional view of a longitudinal section of the guide filter composed of a succession of resonant cavities and immitance reversers,
• The figure 6a several examples of embodiments of capacitive irises,
• The figure 6b an example of an iris having a trapezoidal aperture to maintain the filter bandwidth substantially constant when the center frequency of the filter varies with "a",
• The Figures 7a, 7b and 7c , several embodiments for making a contact providing electrical continuity between the two parts I and II, and
• The figure 8 , a schema reminder used for calculating Immersion Inverter.

The description relates to a waveguide filter having stability in bandwidth when tuned to frequency. According to the last proposed embodiment, the bandwidth is practically insensitive to the change of frequency agreement.

• trois cloisons conductrices longitudinales 101, 102, 103 formant trois côtés d'une partie fixe I du guide d'onde G,
• des cloisons conductrices solidaires de la partie I : 105₁, 105₂ et 105₄, 105₅ avec un ou plusieurs obstacles conducteurs associés à des ouvertures complémentaires Oi dans la section du guide formant des iris de type capacitif (réf. [1] et [7]), cloisons montées transversalement à la propagation de l'onde dans le guide de E vers S. Les cloisons 105₁ et 105₂ définissent la cavité (K₁) 106₁ de longueur L1 et les cloisons 105₄, 105₅ la cavité (K₂) 106₂ de longueur L2. La cloison 105₃ avec une ouverture définissant un iris de type capacitif comporte une ouverture « i » (figure 5), et forme, avec les deux longueurs de guide adjacentes L3 et L4, un inverseur d'immitance J entre les deux cavités 106₁ et 106₂,
• une paroi latérale 104 formant la face de II qui constitue le quatrième côté du guide et qui se trouve face à la cloison 102. La partie II s'encastre ou s'insère dans la partie I sur le petit coté « b » du guide, permettant de définir la valeur de la dimension du grand côté du guide "a", laissant passer au moyen de fentes 108i ayant des dimensions choisies pour recevoir les cloisons de la partie I qui forment les iris de type capacitifs, et fermant le guide sur le quatrième côté 104 du guide de dimension intérieure "b". La référence 107 correspond à la cloison mobile extérieure du guide d'onde G lorsque la première partie fixe I et la deuxième partie mobile II sont emboîtées l'une dans l'autre.

The figure 2 describes a portion of a bandpass filter in a rectangular section waveguide which comprises for example the following elements:

• three longitudinal conducting partitions 101, 102, 103 forming three sides of a fixed part I of the waveguide G,
• conductive partitions integral with the part I: 105 ₁ , 105 ₂ and 105 ₄ , 105 ₅ with one or more conductive obstacles associated with complementary openings Oi in the section of the guide forming iris capacitive type (ref. [7]), partitions mounted transversely to the propagation of the wave in the guide from E to S. The partitions 105 ₁ and 105 ₂ define the cavity (K ₁ ) 106 ₁ of length L 1 and the partitions 105 ₄ , 105 ₅ the cavity (K ₂ ) 106 ₂ of length L2. The partition 105 ₃ with an opening defining a capacitive type iris has an opening "i" ( figure 5 ), and forms, with the two adjacent guide lengths L3 and L4, an immitance reverser J between the two cavities 106 ₁ and 106 ₂ ,
• a side wall 104 forming the face of II which constitutes the fourth side of the guide and which faces the partition 102. Part II is embedded or inserted in part I on the small side "b" of the guide, to define the value of the dimension of the long side of the guide "a", passing through slots 108i having dimensions selected to receive the partitions of Part I which form the capacitive-type irises, and closing the guide on the fourth side 104 of the inner dimension guide "b". The reference 107 corresponds to the outer movable partition of the waveguide G when the first fixed part I and the second movable part II are nested one inside the other.

The values of the parameters "a", "b" and "d" are chosen according to the frequency of the filter and the dimensions of the guide which are functions of this frequency. In practice we have, for example, "a" ~ "b" / 2 and "d" must allow the realization of the opening.

The two conductive parts I and II are nested as described on the figure 3 . The signal propagates between the accesses E and S of the waveguide G passing through the openings Oi of height "d" capacitive irises through the cavities Ki thus formed by the transverse partitions and the walls of the guide and to through the openings "i" of the walls whose particular function is to perform a function of inversion of immitance.

The shape of the capacitive openings and the dimension "d" or "i" of their opening under each transverse partition (iris) is determined to obtain the desired frequency response and selectivity for the filter (see figure 4 ).

The side of the movable portion II of the waveguide, which closes all the cavities, is manually or mechanically adjusted by means of a single adjustment to move the frequency filter in the desired band. The filter is therefore tuned throughout the band to be covered by this unique adjustment of the dimension of the "a" side. The moving conductive partition II of the guide can be moved mechanically parallel to itself by one or more rotary motors or linear or piezoelectric or other. Mechanical movement can be controlled by software. These displacement means are known to those skilled in the art and will not be represented for reasons of simplification.

Operation of the tunable waveguide filter according to the invention

où "a" est la dimension du grand côté du guide de section rectangulaire.In a rectangular guide of long side "a" and of small side "b", the guided wavelength "λ _g " of a signal at frequency f is equal to: $${\mathrm{\lambda }}_{\mathrm{boy\; Wut}}=1/{\left({\mathrm{<figure-callout\; id="2"\; label="f"\; filenames="imgf0004.png"\; state="\{\{state\}\}">f</figure-callout>}}^2/{\mathrm{vs}}^2-1/{\left(2\mathrm{at}\right)}^2\right)}^{\wedge }\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$$

where "a" is the dimension of the long side of the rectangular section guide.

By varying "a", it is possible to have the same guided wavelength λ _g in the filter for different frequencies f ₁ and f ₂ with sides "a ₁ " and "a ₂ " respectively: $${\mathrm{\lambda }}_{\mathrm{boy\; Wut}}=1/{\left({\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="imgf0004.png"\; state="\{\{state\}\}">f</figure-callout>}}_1{}^2/{\mathrm{vs}}^2-1/{\left(2{\mathrm{at}}_1\right)}^2\right)}^{\wedge }\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.=1/{\left({\mathrm{<figure-callout\; id="2"\; label="f"\; filenames="imgf0004.png"\; state="\{\{state\}\}">f</figure-callout>}}_2{}^2/{\mathrm{vs}}^2-1/{\left(2{\mathrm{at}}_2\right)}^2\right)}^{\wedge }\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$$

In cavity filters, for a rectangular guide, a cavity has a length L close to "λ _g " / 2, and its load overvoltage coefficient Q is a function of the openings of the end irises (couplings jB) (ref. 2]).

The overvoltage coefficients (Q) of each resonator being determined, the architecture or design of the filter is obtained by an association of resonators in series and parallel.

If two different frequencies have the same guided wavelength, "λ _g ", in the guide, as the cavities keep the same length L, and if the couplings between cavities remain equal, the response of the filter is similar to the two frequencies.

According to the approximate formulas given in ref. [1], we see that the couplings jB of capacitive type depend only on the height of the opening "d" and the dimension of the small side "b" which do not vary when "a" varies (see figure 4 ). For example, for an iris with a single rectangular opening of height "d" in a small side guide "b", we have: $$\mathrm{B}/\mathrm{Yo}~\mathrm{8\; b\; /}\left({\mathrm{\lambda }}_{\mathrm{boy\; Wut}}\right)*\mathrm{LN}\left(\csc \left(\mathrm{\pi \; d}/2b\right)\right)$$

With B admittance of the iris, admittance Yo reference to LN L N ogarithme épérien.

This independence property of the value of B with respect to "a" remains valid for all types of capacitive iris ([7] pages 218-221 or 248-255 or 404-406 depending on their shape and thickness).

• avoir sa fréquence centrale f qui varie,
• garder ses couplages et Q à peu près constants,

et de ce fait la bande passante du filtre va rester à peu près constante.Thus, a rectangular waveguide filter with capacitively coupled cavities whose large side "a" is varied by means of a movable partition on the short side "b" and for cavities with "λ _g " fixed, goes :

• have its central frequency f which varies,
• keep its couplings and Q roughly constant,

and therefore the bandwidth of the filter will remain approximately constant.

The dimensions and shape of the capacitive irises of cavities having dimensions (opening "d") achievable are determined for example in the manner described below. The "design" is obtained by fixing overvoltage values Q of the cavities Ki which make it possible to have reasonable iris openings and by coupling the cavities by means of immitance inverters. These immitance inverters of value J must also use capacitive-type irises so that their value is independent of "a" when the short side of the guide "b" is moved.

An example of an immitance inverter adapted to this application is known to those skilled in the art and in accordance with the scheme of the invention. figure 8 according to the example on page 63 of [5].

The design or architecture of the tunable filter according to the invention is obtained for example using methods known to those skilled in the art, as it is explained in [5] page 59 or in [6] page 559. For example, the structure of a filter of order 4 is obtained by placing the four cavities (Ki) between the immitance reversers Ji:
J1 K1 J2 K2 J3 K3 J4 K4 J5 (see part of this filter on the figure 5 : L ₁ and L ₂ represent the lengths of the cavity part K1 and K2, and J the inversion part of immitance)

The capacitive irises used may be thin or thick. The formulas that make it possible to calculate their respective equivalent schemes are known from the prior art, for example, [7] (pages 218-221 or 248-255 or 404-406 depending on their shape and their thickness).

Capacitive irises may include one or more transverse conductive obstacles associated with one or more corresponding, complementary openings in the section of the guide.

The figure 6a , non-limiting, represents several possible embodiments of this type of iris. For example, the conductive obstacle 51 associated with the two complementary openings O1 in the section of the waveguide forms such an iris. Similarly, the conductive obstacles 52 and 53 associated with the complementary central opening O2 in the section of the guide constitute an iris of this type. The two conductive obstacles 55 and 56 associated with the three complementary O3 openings also form a capacitive iris.

The transverse conductor obstacle 54 associated with its complementary opening O4 in the section of the guide is a capacitive iris similar to that shown in the guide of the figure 4 .

• la susceptance jB de l'iris (jB_o a la même valeur à la fréquence centrale f_o de la cavité quelle que soit sa valeur dans la bande à couvrir quand "a" varie),
• la longueur d'onde guidée " "λ_go", constante à la fréquence centrale f_o de la cavité quand "a" varie,
• la longueur de la cavité L, constante,
• la longueur d'onde dans l'air à la fréquence transmise f_o,

A more precise analysis of the overvoltage coefficient of a cavity shows that it is a function of:

• the susceptance jB of the iris (jB _o has the same value at the central frequency f _o of the cavity whatever its value in the band to cover when "a" varies),
• the guided wavelength "" λ _go ", constant at the center frequency f _o of the cavity when" a "varies,
• the length of the cavity L, constant,
• the wavelength in the air at the transmitted frequency f _o ,

avec k (B_o, λ_go, L) constant quand la fréquence centrale de la cavité varie.We have, as a first approximation: $$\mathrm{Q}={\mathrm{kf}}_{\mathrm{o}}{}^2$$

k (B _o, λ _go, L) constant when the center frequency of the cavity varies.

Like BW ', the bandwidth of the resonator is equal to f _o / Q, we obtain as a first approximation: $$\mathrm{BW}\text{'}=\mathrm{k}\text{'}/{\mathrm{f}}_{\mathrm{o}}$$

We see that for a frequency shift of +/- 5% (for example +/- 300 MHz at 6 GHz), the bandwidth of the resonator will vary by - / + 5% because fo varies (for example - / + 1 MHz for BW = 20MHz when f _o varies by +/- 5%).

According to another embodiment and in order to compensate for the variation of the bandwidth BW of the filter as a function of the central frequency f _o , one solution consists in varying B _o by changing the opening "d" of the capacitive irises of the cavities .

One way is to make this iris mobile parallel to the short side of the guide while maintaining electrical contact with the fixed and movable parts forming the cavity. One possible adjustment is to move the iris partition parallel to the small side "b" of the waveguide to vary the opening "d" identically to the ends of each cavity and thus simultaneously change the value Q to all the cavities. The change of "d" in practice is low, of the order of a few tenths of a millimeter. On the other hand, it is not necessary to change the values of the openings "i" of the capacitive irises used in immitation inverter J so that these inverters retain the same value J.

The frequency response of the filter thus adjusted, and regardless of the number of poles of the filter is then exactly the same throughout the covered band and requires only two settings "a" and "d" in all for the filter.

• par une commande séparée motorisée ou non, et commune à toutes les cavités,
• en poussant les iris des cavités vers le haut pour augmenter la valeur de « d » quand on diminue la valeur « a » du grand côté par un dispositif en poussoir compensé dans le sens inverse par exemple par un ressort.

The variation of the aperture "d" of the capacitive iris can be obtained when the narrow adjustable side "b" of the guide (partition 107) moves with the moving side II, for example, using one of two ways described below:

• by a separate motorized control or not, and common to all the cavities,
• by pushing the irises of the cavities upwards to increase the value of "d" when the value "a" of the long side is decreased by means of a compensated push-button device in the opposite direction, for example by a spring.

According to another exemplary embodiment, represented at figure 6b it is possible to increase the apparent value "d" of capacitive irises used for resonant cavities when "a" decreases.

To obtain this result, the opening "d" has a slightly variable value along the dimension x of the large side "a". When the small side of the guide associated with the movable conductive part II moves by increasing the value of "a", the apparent opening of the iris "d (x)" decreases, which makes it possible to slightly vary the coefficient of overvoltage Q to compensate for the variation of the bandwidth of the BW filter when f _o varies.

An approximate form of the opening is for example obtained from the calculation of "d (x)" at the two extreme points in frequency of the band to be covered by f _o . This form of the iris represented at figure 6b is then a rectangle trapezium whose large base 20 is on the wall "b" of the fixed part I of the guide, the smallest side 21 being on the side of the opening receiving the movable part II.

By increasing the number of calculation points in the frequency band covered by the filter when its center frequency f ₀ is varied by keeping its bandwidth BW constant or substantially constant, a more precise form is obtained for the opening "d (x)" as a function of the abscissa x along the long side "a".

Spurious responses

The parasitic responses of the filter, close to the cut-off frequency of the guide which is a function of "a", are suppressed, for example by putting in series with the tunable filter a suitable length of guide under the cut at these frequencies.

Sliding contacts

The tunable filter according to the invention can use at least three types of sliding contacts C to ensure electrical continuity along the small moving side of the guide.

The first possibility is to use a metal spring piece made of copper alloy fixed on the movable partition and providing a spring action to maintain the relative position of the movable partition and the conductive walls (see FIG. figure 7a ).

The second uses a sliding contact (see figure 7b ). The movable partition has one or more grooves along the movable partition in which conductive elastomeric seals 32 make it possible to maintain the ohmic contact.

The third solution is to ensure the contact according to the trap technique used to ensure good electrical continuity at the junction between the guides (see ref. [3]). It consists in bringing back by means of a trap (33) a short circuit at the sliding contact points ("C") for a selected guided wavelength (see Figure 7c ). The trap consists of the complete cut schematized by hatching. This solution seems interesting considering the fact that "λ _g " is constant in the guide when "a" varies, for any central frequency f _o of the filter.

• avoir sa fréquence centrale f qui varie
• garder ses couplages et Q à peu près constants, et
• donc la bande passante du filtre va rester à peu près constante.

Thus, a rectangular waveguide filter with capacitively coupled cavities whose large side "a" is varied by means of a movable partition on the small side "b" (and for cavities with "λ _g " fixed ), goes :

• have its central frequency f which varies
• keep its couplings and Q roughly constant, and
• therefore the bandwidth of the filter will remain approximately constant.

This is not the case for filters using inductive iris or septum for which the equivalent admittances at the end of cavities jB depend directly on the large width of the guide "a" and vary considerably when "a" varies.

References

1. [1] " Design of tunable resonant cavities with constant bandwidth "LDSmullin Technical Report No. 106 RLE / MIT 1949 .
2. [2] " Maximally flat filters in waveguide ", WWMumford, BSTJ October 1948, p 684-713 .
3. [3] " Circuits for ultrashort waves "E.Roubine ESE 1966 .
4. [4] US 5,808,528 .
5. [5] " Microstrip filters for RF / Microwave Applications "Jia-Sheng Hong and MJLancaster, John Wiley 2001 .
6. [6] " Handbook of Filter Synthesis "Anatol I Zverev, Wiley-Interscience .
7. [7] Waveguide Handbook "Marcuvitz N, Radiation Laboratory Series No. 10, McGraw-Hill, 1951 .

Claims (10)

1. Frequency-tunable microwave bandpass filter comprising in combination at least the following elements:

   a waveguide (G) of rectangular section comprising a first fixed conductive portion (I) and a second moving conductive portion (II),

   said first fixed portion (I) comprising three longitudinal conductive partitions (101), (102), (103) forming three sides of the waveguide G, the section of the waveguide having a large side (a) defined by the position of the moving portion (II) when it is inserted into the portion (I) and a small side (b),

   said first portion (I) comprising a number of first conductive partitions (105₁, 105₂ and 105₄, 105₅) constituted of one or more conductive obstacles which associated with complementary openings (O_i) in the section of the waveguide form the irises of capacitive type, said first partitions being mounted transversely to the propagation of the wave in the guide and defining a number of cavities (K_i) (106₁, 106₂) in the longitudinal direction of the guide and integral to the first portion (I) and a number of second conductive partitions (105₃) with one or more openings (i) defining the irises of capacitive type and which form, in association with two guide lengths (L3, L4) adjacent a second conductive partition (105₃) immitance inverters (Ji), said first partitions (105₁, 105₂ and 105₄, 105₅) forming a succession of resonant cavities (Ki) coupled together by the immitance inverters (Ji),

   said second moving conductive portion (II) comprising a wall (104), parallel to the small side (b) of the guide, forming the fourth face of the waveguide (G), said wall (104) defining the value of dimension (a) of the large side of the guide, and thus the centre frequency of the filter, the moving portion (II) comprising a number of slots (108i) receiving the partitions of the portion (I) which form the irises of capacitive type, the cavities Ki thus being formed when the portion (I) and the portion (II) are fitted together,

   means (30, 32, 33) for ensuring electrical contact between the first fixed conductive portion (I) and the second moving conductive portion (II).
2. Tunable filter according to claim 1 characterised in that the irises of capacitive type (105₁, 105₂ and 105₄, 105₅) used to form the cavities (Ki) have an opening of dimension "d(x)" according to the dimension of the small side (b) variable as a function of abscissa x according to the large side "a" which allows for keeping the bandwidth of the filter constant when "a" varies.
3. Tunable filter according to claim 2 characterised in that the opening of dimension "d(x)" according to the dimension of the small side (b) variable as a function of the abscissa x along the large side "a" is a linear function for giving this opening a trapezoidal form.
4. Tunable filter according to claim 1 characterised in that it comprises a metallic piece (30) fixed on the moving partition and providing a spring action for maintaining the relative position of the moving partition and the conductive walls.
5. Tunable filter according to claim 1 characterised in that said moving partition has one or more grooves along the moving partition in which conductive seals (32) of charged elastomer allow for maintaining ohmic contact.
6. Tunable filter according to claim 1 characterised in that it comprises for assuring electrical continuity between the fixed portion (I) and the moving portion (II) along the moving small side (b) of the guide a trap (33) bringing a short circuit to the sliding contact points (C) for a chosen guided wavelength.
7. Tunable filter according to claim 1 characterised in that it comprises means for displacing the partitions of the capacitive irises of the cavities parallel to the small side (b) of the waveguide for varying the opening (d) identically at the ends of each cavity and thus simultaneously changing the value of the overvoltage coefficient (Q) for all of the cavities (Ki).
8. Tunable filter according to claim 1 characterised in that it comprises means adapted to vary the opening (d) of the capacitive irises of the cavities when the narrow adjustable side (b) of the guide is displaced with the moving side (II), said means being a separate motorised or not control, and common to all of the cavities.
9. Tunable filter according to claim 1 characterised in that it comprises means adapted to vary the opening (d) of the capacitive irises of the cavities when the narrow adjustable side (b) of the guide is displaced with the moving side (II), said means being a thrust device compensated in the reverse direction adapted to push the irises of the cavities upward parallel to the small side (b) of the guide to increase the value of the opening (d) when the value (a) of the large side is reduced.
10. Tunable filter according to claim 1 characterised in that the moving partition (107) associated with the moving side (II) of the guide is displaced mechanically parallel to itself by one or more rotary or linear or piezoelectric motors.

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