EP0071509A1 - Band-pass filter with open-ended linear resonators - Google Patents

Abstract

The present invention relates to bandpass filters requiring the addition of bandstop filtering. To avoid having to use a notch filter in series with a bandpass filter with linear resonators open at their two ends (H1-H4), linear resonators (P1 and P4) tuned to a frequency to be rejected are connected to the bandpass filter resonators to produce a notch function. Application to microwave filtering.

Description

The present invention relates to bandpass filters produced by means of linear resonators open at their two ends, such as hair pin resonators (hair pin resonators in Anglo-Saxon literature) also called U resonators, and such as straight resonators, formed by a straight section of line. It is recalled on this subject that these resonators, also called resonators in 2, being open at their two ends, resonate on a frequency corresponding, in the filter, to a wavelength λ equal to twice the electrical length of the resonator; the bandwidth of the filter is centered on this resonant frequency.

In general for filtering problems and in particular for filters with linear resonators open at their two ends when, to a bandpass function, a bandstop function must be added, for example to eliminate an undesirable frequency, a notch filter is placed in series with the notch filter. This has the disadvantage of requiring two filters and therefore of taking up space and being expensive.

The object of the present invention is to avoid having to use two filters.

This is achieved by integrating a notch function in an existing band pass filter.

According to the invention, there is provided a bandpass filter with n linear resonators open at both ends, called λ 2 resonators (n: positive integer; λ: mean wavelength of the passband in the filter), is mainly characterized in that, to each of p (p integer at least equal to 1 and at most equal to n) resonators in 2 is connected, in the vicinity of its medium, that is to say where the electric field at its minimum value, an additional resonator whose resonant frequency is a frequency to be rejected thanks to this additional resonator.

• - les figures 1 à 4 des schémas de filtres selon l'invention,
• - la figure 5 une vue détaillée d'un filtre selon l'invention,
• - la figure 6 un graphique relatif, en partie, au filtre selon la figure 5.

The invention will be better understood and other characteristics will appear from the following description and the figures relating thereto which represent:

• FIGS. 1 to 4 of diagrams of filters according to the invention,
• FIG. 5 a detailed view of a filter according to the invention,
• - Figure 6 is a graph relating, in part, to the filter according to Figure 5.

In the various figures, the corresponding elements bear the same references.

FIG. 1 is a schematic view of a band-pass filter produced using U-shaped resonators. This filter comprises two access lines A ₁ A ₂ , parallel in them and, between these two lines, five resonators in U, H ₁ to H ₄ , whose vertical bars of the U are parallel to the lines A ₁ A ₂ . The length of the U-shaped resonators, H ₁ to H ₄ , is λ 2 (λ being the wavelength corresponding to the average frequency of the pass band of the band pass filter). The resonators H ₁ and H ₄ are associated respectively with two U-shaped resonators, P ₁ and P ₄ , the length of which is equal to λ 2 (λ 'being a wavelength corresponding to a frequency to be rejected in the filter).

In the filter according to FIG. 1, as moreover in the filters which will be described later, the ports such as A and A ₂ and the resonators such as H ₁ to H ₄ and P ₁ , P ₄ are constituted by metal deposits placed on one side of a dielectric substrate, the other side of which is covered with a metal sheet forming a ground plane. The dielectric substrate and the metal sheet do not appear in FIG. 1 for the sake of simplification.

The diagram in FIG. 1, without the resonators P ₁ and P ₄ , corresponds to a bandpass filter of conventional type. In such a filter, the coupling coefficient existing between two resonators is defined by the distance which separates them, by their line width and by the distance which separates the two branches of the same resonator. The resonators being open at their two ends, their resonant frequency is the frequency corresponding to the wavelength λ in the filter, where λ is as indicated above equal to twice the length of the resonators ^H ₁ to H ₄ .

A notch function was obtained in the filter according to FIG. 1 by the two U-shaped resonators, P ₁ and P ₄ , placed respectively on the back of the resonators H ₁ and H ₄ , that is to say placed so to have the horizontal bar of the U common with that of the resonators H ₁ and H ₄ . Thus arranged at the place where the electric field is at its minimum value in the resonators H ₁ and H ₄ , the resonators P ₁ and P4 bring practically no modification of the characteristics of the band-pass filter obtained thanks to the resonators H ₁ at H ₄ ; and these resonators P ₁ and P ₄ behave like a notch filter put in series with the band pass filter.

Another possibility of making the filter according to the invention is given by FIG. 2 which is a diagram differing from that according to FIG. 1 only by the fact that the resonators P ₁ and P ₄ are replaced by resonators of shorter length. , P ' ₁ , P' ₄ , but at the ends of which are connected respectively two variable capacitors C ₁ , C ₄ adjusted so as to give the sets P ' ₁ C ₁ and P' ₄ C ₄ an electrical length equal to half the length λ 'which was discussed in Figure 1 (At' wavelength in the filter corresponding to the frequency to be rejected by the filter).

où λ' est la longueur d'onde dans le filtre correspondant à la fréquence à rejeter. Un exemple concret de réalisation d'un tel filtre est donné à l'aide des figures 5 et 6.FIG. 3 is the diagram of a filter according to the invention which differs from the filter of FIG. 1 by the replacement of the resonators P _l and P ₄ of the U type by two simple resonators Q ₁ Q ₄ , that is to say ie by resonators each having only one of their ends isolated; at their other end, these two simple resonators are respectively connected substantially in the middle of the horizontal bar of the resonators H ₁ and H ₄ . These simple resonators Q ₁ and Q ₄ are formed by pieces of line of length

where λ 'is the wavelength in the filter corresponding to the frequency to be rejected. A concrete example of embodiment of such a filter is given with the aid of FIGS. 5 and 6.

The invention does not only apply to filters of the bandpass type with U-shaped resonators, it also applies as shown in FIG. 4 to bandpass filters with parallel lines. Figure 4 shows such a filter; this filter comprises an input line A and an output line A ₂ parallel to each other and between these lines four straight resonators, L to L ₄ , of length equal to λ 2 (λ wavelength corresponding to the average frequency of the filter bandwidth), open at both ends. Four simple resonators Q ₁ to Q ₄ of length equal to λ '4 (λ' wavelength in the filter corresponding to a frequency to be rejected by this filter) are respectively connected, at one of their ends, in the middle of the straight resonators L to L ₄ . Here again, as in the case of the preceding figures, the added resonators (Q ₁ to Q ₄ ) bring about a notch function at the frequency corresponding to the wavelength λ '. Also, as in the case of the previous figures, these added resonators are connected in the middle of the resonators giving the bandpass function, that is to say where the electric field has its minimum value, so as not to disturb the bandpass function of the filter.

Figure 5 is a detailed view of a filter according to the invention corresponding to the type shown in the diagram of Figure 3; a graduated scale from 0 to 1cm is placed next to the filter to show the enlargement ratio of the drawing.

• - deux lignes d'accès du filtre, A₁, A₂, parallèles entre elles et respectivement reliées au conducteur intérieur des prises 11 et 12,
• - six résonateurs en U, H₁ à H₆, disposés entre les lignes d'accès A₁ et A₂ et dont les barres verticales sont parallèles à ces mêmes lignes,
• - et deux résonateurs simples, Q₁ et Q₆, respectivement reliés à l'une de leurs extrémités au milieu des résonateurs en U, H₁ et H₆ ; il est à noter que, dans cette réalisation, à l'inverse de ce qui a été dessiné sur la figure 3, les résonateurs Q₁ et Q₆ ne sont pas droits comme les résonateurs Q₁ et Q₄ mais sont courbés de manière à ne pas nécessiter un substrat de dimensions plus importantes que le substrat nécessaire à la réalisation des lignes d'accès A et A₂ et des résonateurs en U, H₁ à H₆.

In Figure 5 is shown a housing 1 whose cover has been removed to show the interior. This housing 1 is associated with two connection sockets 11, 12 of the coaxial type. Inside the housing there is a dielectric plate 2 on which are arranged the lines constituting respectively:

• - two filter access lines, A ₁ , A ₂ , mutually parallel and respectively connected to the inner conductor of sockets 11 and 12,
• - six U-shaped resonators, H ₁ to H ₆ , arranged between the access lines A ₁ and A ₂ and the vertical bars of which are parallel to these same lines,
• - and two simple resonators, Q ₁ and Q ₆ , respectively connected at one of their ends in the middle of the U-shaped resonators, H ₁ and H ₆ ; it should be noted that, in this embodiment, contrary to what has been drawn in FIG. 3, the resonators Q ₁ and Q ₆ are not straight like the resonators Q ₁ and Q ₄ but are curved so as to do not require a substrate of larger dimensions than the substrate necessary for the production of the access lines A and A ₂ and of the U-shaped resonators, H ₁ to H ₆ .

In FIG. 5 also appear two short circuits, K ₁ and K ₂ , connected respectively between the access line A ₁ and the resonator H ₁ and between the access line A ₂ and the resonator H ₆ ; these short circuits have have been studied to adapt the impedance of the filter according to the circuit in which this filter is intended to be mounted.

In addition to what appears in FIG. 5, the filter comprises on the hidden face of the dielectric substrate 2 a metal sheet electrically connected to the housing 1 and serving as a ground plane; the external conductor of the plugs 11 and 12 is also electrically connected to the housing 1.

FIG. 6 is a graph showing the attenuation A brought, as a function of the frequency, by the filter according to FIG. 5 (curve G ₁ in solid lines) and showing the attenuation brought by the bandpass filter according to the art known corresponding to the filter of Figure 5, that is to say without the resonators Q ₁ and Q ₆ (curve G ₀ in dashed lines). The curve G ₀ shows that the conventional filter (without the resonators Q _I and Q ₆ ) has a bandwidth, centered on an average frequency of 825 MHz, which is the useful bandwidth of the filter, that is to say the bandwidth for which it was designed; this conventional filter also has a bandwidth whose lower frequency is at 1200 MHz and which constitutes a parasitic bandwidth which can be troublesome in certain applications. The curve G ₁ in FIG. 6 shows that the addition of the resonators Q _I and Q ₆ to the other elements of the filter in FIG. 5 makes it possible to eliminate this parasitic pass band by performing a band-cut function.

et sur d'autres de ces résonateurs en

λ 2 des résonateurs simples en 4 tels que Q_I et Q₄ (figure 3). Il est à noter également que, au lieu des condensateurs variables C₁ et C₄ de la figure 2, peuvent être montés des condensateurs fixes obtenus en même temps et de la même façon que les résonateurs, c'est-à-dire par dépôt métallique sur une plaquette ou à partir d'une plaquette métalisée dont une partie de la couche métallique a été retirée par attaque chimique ou mécanique ; ces condensateurs fixes sont alors constitués d'une rangée de languettes parallèles disposées entre les branches du U des résonateurs P' et P'₄ , perpendiculairement à ces branches, deux languettes successives étant respectivement solidaires des deux branches du U.Other embodiments of bandpass circuits can be envisaged without departing from the scope of the invention. Thus, for example, in the case of FIG. 1, resonators identical to the resonators P and P ₄ can be associated with the resonators H ₂ and H ₃ . Likewise to perform the strip cutter function it is possible to connect on some of the resonators in λ 2 (H ₁ H ₂ H ₃ H4) of figure 1 or of figure 3 resonators in U

and on others of these resonators in

λ 2 of the simple resonators in 4 such as Q _I and Q ₄ (Figure 3). It should also be noted that, instead of the variable capacitors C ₁ and C ₄ of FIG. 2, fixed capacitors can be mounted obtained at the same time and in the same way as the resonators, that is to say by deposition metallic on a wafer or from a metallic wafer from which part of the metallic layer has been removed by chemical or mechanical attack; these fixed capacitors then consist of a row of parallel tongues arranged between the branches of the U of the resonators P 'and P' ₄ , perpendicular to these branches, two successive tongues being respectively integral with the two branches of the U.

It should be noted that the central frequency of the notch function which is integrated in a band pass filter can be just as much a frequency higher than the pass band of the band pass filter, as a lower frequency, or even that a frequency included in this bandwidth; it suffices to determine the electrical length of the resonators which produce this notch function, as a function of the wavelength of the central frequency of the notch function to be obtained.

In general, the number of resonators intended to add a predetermined notch function in a bandpass filter with linear resonators, can be chosen between 1 and n, n being the number of resonators defining the bandpass function of the filter considered; the choice of the number and the position of the resonators intended to add the notch function is a means of acting on the shape of the response curve of the filter.

Claims (3)

1. Bandpass filter with n linear resonators (H ₁ -H ₄ ) open at both ends, called λ 2 resonators (n: positive integer; λ: mean wavelength of the passband in the filter), characterized in that, to each of p (p integer at least equal to 1 and at most equal to n) of the n resonators in λ 2 (H ₁ , H ₄ ) is connected, in the vicinity of its medium, i.e. say where the electric field has its minimum value, an additional resonator (P ₁ , P ₄ ) whose resonant frequency is a frequency to be rejected using this additional resonator. 2. Filter according to claim 1, characterized in that at least one of the p additional resonators (P ₁ , P ₄ ) is a U-shaped resonator and in that this additional resonator is connected to the resonator at 2 by the horizontal bar of his U. 3. Filter according to claim 1, characterized in that at least one of the p additional resonators is a resonator (Q ₁ , Q ₄ ) having an isolated end and an end connected to the resonator in corresponding λ 2.

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