EP0060174A1 - Band-pass filter with dielectric resonators - Google Patents

Abstract

1. Pass band filter of dielectric resonator type, comprising a waveguide (1) operating below its cut-off frequency, an input antenna (Ae ) and an output antenna (As ) respectively disposed at the two ends of the guide, and n resonators (n positive integer) (R1 -R4 ) disposed in series inside the guide, characterized in that, in a manner to have the resonators oscillate in the TM011 mode and to excite a TM mode within the guide the cut-off frequency of which exceeds that of the mode TM011 of the resonators, the dimensions of the transverse section of the guide are of the order of 2.5 times the transverse dimensions of the resonators, the resonators being substantially disposed along the longitudinal axis (XX) of the guide and the antennas being disposed in the propagation sense in the guide.

Description

The present invention relates to a bandpass filter with dielectric resonators, comprising a waveguide operating below its cutoff frequency, an input antenna and an output antenna respectively disposed at the two ends of the guide and n resonators (n positive integer) arranged in series inside the guide.

Filters of this type are known which operate by direct coupling of the dielectric resonators with the input and output coupling elements. This direct coupling is done by the fields radiated by the dielectric resonators. The no-load overvoltage coefficients of these filters do not exceed 4000 due to the TE011 resonance mode used for the resonators, the latter not having a very high overvoltage coefficient (for example 6000 to 5 GHz), and due that, in order to obtain the important coupling necessary for the operation of the filter, the coupling element (antenna or loop) must be very close to the resonator and thus deteriorates the overvoltage coefficient.

In fact the TE 0 ₁₁ mode is used in these known filters because it is easily excitable by using antennas or loops; the height and width of the waveguide are taken on the order of three times greater than, respectively, the height and the diameter of the resonators.

There are also bandpass filters with dielectric resonators which operate in evanescent mode; in these filters the coupling between resonators is carried out by means of the fields of a waveguide operating below its cut-off frequency while the coupling of the extreme resonators with the input and output coupling elements, they are neighbors is done as in the previous case, that is to say that the input and output coupling elements are directly placed in the field lines of the resonators. But, again, the overvoltage coefficients do not exceed 4,000.

The object of the present invention is to produce filters with dielectric resonators, having a significantly higher overvoltage coefficient than that of known dielectric resonator filters.

This is achieved, in particular, by using the resonance mode TMOll of the resonators which causes an overvoltage greater than that of the TE ₀₁₁ mode.

It should be noted that it was demonstrated, during work carried out in the microwave electronics laboratory of the University of Limoges in a working group led by MM. GARAULT and GUILLON, that the TM ₀₁₁ mode of a cylindrical dielectric resonator has a higher overvoltage coefficient than that of the TE ₀₁₁ mode, which suggested that the use of the TM ₀₁₁ mode could be advantageous in the context of the operation of filters with dielectric resonators. It should also be noted that the realization of such a filter is not obvious given that, in the TM ₀₁₁ mode, the electromagnetic energy is more confined (confined mode in the Anglo-Saxon literature) in the vicinity of the resonator, that is to say that the energy gradient is greater and that, therefore, the coupling problems are more difficult to solve.

According to the invention a filter of the type described at the beginning of this text is mainly characterized in that, so as to make the resonators resonate according to the mode TM ₀₁₁ and to propagate in the guide a mode TM whose cut-off frequency is higher to that of the TM ₀₁₁ mode of the resonators, the dimensions of the cross section of the guide are of the order of 2.5 times the transverse dimensions of the resonators, the resonators are arranged substantially along the longitudinal axis of the guide and the antennas are arranged in the direction of propagation in the guide.

• - la figure 1, un filtre selon l'invention, couvercle enlevé,
• - la figure 2, une vue en coupe du filtre de la figure 1 avec son couvercle,
• - la figure 3 une vue partielle en coupe longitudinale d'un autre filtre selon l'invention,
• - la figure 4 une autre vue partielle du filtre selon la figure 3.

The present invention will be better understood and other characteristics will appear with the aid of the description below and of the figures relating thereto which represent:

• FIG. 1, a filter according to the invention, cover removed,
• FIG. 2, a sectional view of the filter of FIG. 1 with its cover,
• FIG. 3 is a partial view in longitudinal section of another filter according to the invention,
• - Figure 4 another partial view of the filter according to Figure 3.

To have a bandpass filter, the resonant frequency of the resonators must be lower than the cutoff frequency of the lowest propagation mode excited in the guide.

To resonate a resonator, placed in a waveguide, according to the mode TM ₀₁₁ it is not necessary to excite the fundamental mode (TEO 1) of the guide whose cut-off frequency is lower than the resonance frequency of the mode TM ₀₁₁ of the dielectric resonators; to avoid having this problem, it would suffice to take a waveguide of sufficiently small section compared to the working wavelength, but this would result in significant Joule effect losses in the walls of the guide.

The principle is to avoid exciting a TE mode of the guide in order to excite only a TM type mode, which entails the use, as coupling elements, of antennas oriented along the direction of propagation in the guide. .

Furthermore, in order to avoid losses by Joule effect in the walls, the width and the height of the section of the guide will be taken in the order of 2.5 times respectively the diameter and the height of the resonators, it being understood that is an average value and if, for example, the height of the guide is only twice greater than the height of the resonator, on the other hand its width will be taken of the order of three times greater than the diameter of the resonator.

With regard to the coupling mode between the input antenna and the first resonator, that is to say the resonator closest to this antenna, it should be noted that the configuration of the field lines of the guided mode TM ₁₁ makes it possible to excite the mode TM ₀₁₁ in the first resonator; to excite the TM ₁₁ mode in the guide, the input antenna must be arranged along the longitudinal axis of the guide because this is where the electric field is maximum in TM ₁₁ mode; on the other hand, the input antenna can be placed relatively far from the first resonator so as not to disturb it while being effectively coupled to it by the field lines.

With regard to the coupling between two successive resonators, it should be noted that a resonator resonating according to the TM ₀₁₁ mode is comparable to an electric dipole; or an electric dipole placed in a waveguide excites all the modes of the waveguide which have an electric field component parallel to the direction of the dipole therefore in particular the TM ₁₁ mode, which allows the transmission of energy from one resonator to the other via the fields of this mode.

As regards the coupling between the last resonator and the output antenna, what has been said for the coupling between the input antenna and the first resonator applies, in the direction of propagation near the waves.

These considerations have led to the embodiments according to FIGS. 1 to 4 which correspond to filters according to the invention.

Figure 1 shows the filter seen from above, cover removed and Figure 2 shows this filter, cover put back, seen in a section through a plane perpendicular to the plane of Figure 1 and passing through a line XX of Figure 1. These figures show that the filter is formed by an associated waveguide 1, to form a housing, with two plates, 2 and 3, integral with its ends. The cover which has been discussed above is a cover, 10, which appears in FIG. 2, it consists of one of the sides of the waveguide. This waveguide is a square section waveguide with a side of 30 mm, and a length of 206 mm.

FIGS. 1 and 2 show the longitudinal axis XX of the waveguide, i.e. the axis passing through the point of intersection of the diagonals of the sections of the guide. Two antennas A, A _s , respectively secured to plates 2 and 3 by means of connection sockets 20, 30 which isolate them from the plates, are arranged along the axis XX, that is to say according to the direction of propagation of the waves in the guide. Between the antennas A _e and A are successively arranged, substantially along the axis XX, four dielectric resonators R ₁ , R ₂ , R ₃ , R ₄ which rest respectively on four supports S ₁ , S ₂ , S ₃ , S ₄ . These four supports are crown supports made of a dielectric material with low permittivity; they rest on the face of the waveguide opposite to the cover 10. This arrangement of the antennas A _e A and of the resonators R ₁ , R ₂ , R ₃ , R ₄ , as indicated in the above, allows the coupling of the antenna A _e with the resonator R ₁ , of the successive resonators between them and of the resonator R ₄ with the output antenna A _s , is done by means of the field lines of the guided mode TM ₁₁ . In order to make it possible to readjust the filter setting, tuning screws V to V ₅ are provided; these tuning screws pass through the cover 10, substantially halfway between the input antenna A and the resonator R ₁ between the resonators R and R ₂ , R ₂ and R ₃ , R ₃ and R ₄ and between the resonator R ₄ and

the output antenna A _s .

• - bande passante : 20 MHz
• - réglage possible de la bande passante de + 10 %.

The filter which has just been described is a flat filter, with four poles, designed to operate at an average frequency of 4.9 GHz and having a Q ₀ overvoltage coefficient of 7000 to 4.9 GHz. The other features of the filter are:

• - bandwidth: 20 MHz
• - possible adjustment of the bandwidth of + 10%.

Figures 3 and 4 correspond to another embodiment of the filter according to the invention. In this embodiment the filter tuning device according to Figures 1 and 2, consisting of tuning screws (V to V5, Figure 2), is removed and the filter is magnetically tunable. But the waveguide still works below its cutoff frequency, the antennas and the resonators are arranged as in the case of the figure and the dimensions of the cross section of the guide are still around 2.5 times the transverse dimensions of the resonators; there too, the resonators resonate according to the TM ₀₁₁ mode and the propagation in the guide is done according to a TM mode whose cut-off frequency is higher than that of the TM ₀₁₁ mode, in this case it will mainly be the TM ₁₁ mode.

Figure 3 shows a rectangular guide section 1 made of brass; this guide section has an internal section of 30 mm for its long side and 20 mm for its short side. Two metal plates 2, 3, also made of brass, close the guide section respectively at its two ends; these metal plates are each pierced with a hole traversed by a connection socket 20, 30 the central part of which is constituted by a linear metallic conductor which extends inside the guide section over a length of approximately 6 mm to constitute an antenna of the electric dipole type, A, A _s .

• - un support S en forme d'anneau à section circulaire de 16 mm de diamètre extérieur et de 4 mm d'épaisseur ; ce support est réalisé en un matériau diélectrique à faible permittivité,
• - un résonateur diélectrique classique 11 formé par un disque en titanate de zirconium dont la permittivité est égale à 36 ; ce disque a un diamètre de 10 mm et une épaisseur de 5 mm ;
• - un élément intercalaire 12 réalisé en un matériau diélectrique à faible constante diélectrique et faibles pertes ; cet élément intercalaire se présente sous la forme d'un disque de 10 mm de diamètre et d'environ 2 mm d'épaisseur,
• - et un élément ferrimagnétique 13, constitué d'un grenat de fer Yttrium se présentant également sous la forme d'un disque de 10 mm de diamètre et de 2,5 mm d'épaisseur.

Inside the guide section 1 are arranged, on one of the long sides, three resonant members R ₁ -S ₁ , R ₂ -S ₂ , R ₃ -S ₃ ; these three resonant members are aligned in the direction of the largest dimension of the guide section and are regularly spaced. From the long side of the guide section 1, on which it is fixed by bonding, the resonant member R ₁ -S ₁ successively comprises:

• - a ring-shaped support S with a circular section of 16 mm outer diameter and 4 mm thick; this support is made of a dielectric material with low permittivity,
• - a conventional dielectric resonator 11 formed by a zirconium titanate disc whose permittivity is equal to 36; this disc has a diameter of 10 mm and a thickness of 5 mm;
• - An intermediate element 12 made of a dielectric material with low dielectric constant and low losses; this intermediate element is in the form of a disc 10 mm in diameter and approximately 2 mm thick,
• - And a ferrimagnetic element 13, consisting of a Yttrium iron garnet also in the form of a disc 10 mm in diameter and 2.5 mm thick.

The intermediate element 12 produces an offset, upwards in the frequency domain, of the operating band of the filter and this offset is all the greater the greater its thickness. This element therefore makes it possible, by a suitable choice of its thickness, to adjust, during manufacture, the position of the operating frequency band of the filter while using for this filter the same dielectric resonators and the same ferrimagnetic elements as for other filters with slightly different operating bands.

The parts S ₁ -11-12-13, constituting the resonant member R ₁ -S ₁ , are assembled by gluing.

The resonant members R ₂ -S ₂ and R ₃ -S ₃ are identical, from the point of view of form and constitution, to the resonant member R ₁ -S ₁ .

The resonant members R ₁ -S ₁ , R ₂ -S ₂ , R3 -S ₃ are respectively subjected to magnetic fields H1 , H3 , H3 produced by electromagnets outside the guide section; the electromagnet that produces the field H1 to which the resonant member R ₁ -S ₁ is subjected will appear in FIG. 4.

The frequency matching of the filter according to FIG. 3 is possible thanks to the ferrimagnetic elements, such as 13, of the resonant members. This agreement is obtained by modifying, separately for each resonant member, the value of the magnetic field to which each resonant member is subjected, that is to say by modifying the value of the excitation current of the electromagnet producing this field. magnetic. This possibility of ac stringing the filter is explained by the gyromagnetic character specific to ferrimagnetic materials: depending on the size of the field applied to the ferrimagnetic elements of the resonant organs R ₁ -S ₁ , R ₂ -S ₂ , R ₃ -S _{3 '} the distribution of the lines of the electromagnetic field inside the guide section 1 varies more or less, which results in a more or less significant variation in the resonant frequency of the filter.

This variation in the resonant frequency of the filter is mainly linked to the nature and geometry of the dielectric resonator and of the ferrimagnetic element of the resonant members R ₁ -S ₁ , R ₂ -S ₂ , R ₃ -S _3.

In the case of the example described where the magnetic field provided by the electromagnets can vary from 0 to 1500 gauss, the central frequency of the filter can vary from 4.8 GHz to 5.5 GHz, i.e. a relative variation of nearly 15%.

Figure 4 is another view of the filter already partially shown in Figure 3. Figure 4 shows the guide section 1 seen at the end; the plate 2 and the connection socket 20 have been removed so as to make it possible to see the resonant member R ₁ -S ₁ placed inside the guide section. In this figure also appears an electromagnet L1; this electromagnet LI is the one that produces the magnetic field H1 which was discussed during the description of FIG. 3. This electromagnet L comprises a U-shaped part, 5, made of extra soft iron (in the example described, this iron is of the type known as iron from Sweden); the ends of the U are respectively pierced with two threaded holes through which two adjustment screws 50 and 51 pass, also made of extra soft iron; these adjustment screws make it possible to modify the spacing between the two poles of the electromagnet. A coil 52 is wound around the U-shaped part and has its conductor which is extended at its two ends by, respectively, two connecting wires 53 and 54; the winding 52 is supplied by a variable current source 4. As has been said above, the variation in current from the source 4 makes it possible to vary the magnetic field supplied by the electromagnet from 0 to 1500 gauss.

The invention is not limited to the embodiments described, thus it is possible to use cylindrical guides with resonators operating according to the TM ₀₁₁ mode; in this case so that the mode of operation of the guide either, as with the rectangular guides, a TM mode whose cut-off frequency is higher than that of the TM ₀₁₁ mode of the resonators, it is necessary to take the TM _Ol mode which leads to the same considerations, as regards the dimensions and the arrangement of the elements, only with the rectangular guides.

It should be noted that the intermediate elements such as 12 (Figures 3 and 4) can be deleted. The ferrimagnetic elements are then directly in contact with the dielectric resonators and the adjustment of the position of the operating frequency band of the filter must then be done by modifying the dielectric resonator or the ferrimagnetic element, or both at the same time; in practice, such modifications are more troublesome than those which consist in changing the thickness of the disc made of dielectric material with low losses, constituting the intermediate element.

Claims (7)

1. Bandpass filter with dielectric resonators, comprising a waveguide (1) operating below its cut-off frequency, an input antenna (A) and an output antenna (A _s ) respectively disposed at both ends of the guide and n resonators (n positive integer) (R ₁ -R ₄ ) arranged in series inside the guide, characterized in that, so as to cause the resonators to resonate according to mode TM ₀₁₁ and to propagate in the guide a TM mode whose cut-off frequency is greater than that of the TM ₀₁₁ mode of the resonators, the dimensions of the cross section of the guide are of the order of 2.5 times the transverse dimensions of the resonators, the resonators are arranged substantially along the longitudinal axis (XX) of the guide and the antennas are arranged in the direction of propagation in the guide. 2. Bandpass filter according to claim 1, characterized in that the guide is a rectangular guide, in that the propagation mode in the guide is TM ₁₁ mode and in that the resonators are cylindrical resonators. 3. Bandpass filter according to claim 1, characterized in that it comprises at least one adjustment element constituted by a tuning screw (V ₁ - V ₅ ) which passes through the wall of the waveguide and the penetration depth in the waveguide is adjustable. 4. Bandpass filter according to claim l, characterized in that, so that the filter is magnetically tunable over a large bandwidth, on the one hand the n resonators are composite resonators (R ,, R ₂ , R ₃ ) each comprise a dielectric resonator (11) proper and a ferrimagnetic element (13) arranged one above the other, and on the other hand n electromagnets (LI) external to the guide are associated respectively with n composite resonators to create magnetic fields ( H1 , H2 , H3 ) adjustable by changing their excitation current. 5. Bandpass filter according to claim 4, characterized in that the n magnetic fields ( H1 , H2 , H3 ) are adjustable separately. 6. Bandpass filter according to claim 4, characterized in that the n electromagnets (L1) each comprise an adjustment means (50, 51) of the spacing of their respective poles. 7. Bandpass filter according to claim 4, characterized in that each composite resonator also comprises an intermediate element (12) disposed between the dielectric resonator proper (11) and the ferrimagnetic element (13).

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