EP0423667A1 - Microwave band filter of the comb-line type - Google Patents

Abstract

Microwave band filter of the comb-line type, including an element for broadening its pass band. This element consists of a low-impedance line (9) which is interposed, at the earth stems of the fingers (1 to 5), between these earth stems and earth. A pass band broadening which depends on the length (l) of this low-impedance line is thereby obtained. <IMAGE>

Description

The present invention relates to a band filter for microwave frequencies, this band filter being of the comb filter or "COMB-LINE" type.

Among the band filters for microwave frequencies, the comb filter, or "Comb-Line" is the distributed constant filter which offers the best compromise, for a reduced bulk, between insertion losses and parasitic responses.

• La figure 1 jointe montre schématiquement un filtre Comb-Line de type connu. Ce filtre, qui dans cet exemple est à cinq pôles, est constitué de cinq portions de ligne parallèles 1 à 5, de longueur égale au huitième de la longueur d'onde centrale de fonctionnement dans la bande. Ces lignes, ou "doigts", ont chacune une de leurs deux extrémités court-circuitée à la masse, et leur autre extrémité est libre et est reliée à la masse par une capacité d'accord 6 qui a pour but de régler la fréquence de résonance de chacun des résonateurs tel que 1, 6. Des capacités de couplage 7, 8 constituent respectivement les organes d'entrée et de sortie de ce filtre.

Its design, in tri-plate or ribbon (micro-strip) technique, makes it directly transposable to so-called "MMIC" integrated circuit techniques, and provides an answer to certain filtering problems.

• The attached FIG. 1 schematically shows a Comb-Line filter of known type. This filter, which in this example has five poles, consists of five parallel line portions 1 to 5, of length equal to one-eighth of the central wavelength of operation in the band. These lines, or "fingers", each have one of their two ends short-circuited to ground, and their other end is free and is connected to ground by a tuning capacitor 6 which aims to adjust the frequency of resonance of each of the resonators such as 1, 6. Coupling capacitors 7, 8 respectively constitute the input and output members of this filter.

This type of filter has the disadvantage of having a band frozen once and for all according to its particular physical realization. However, in particular in satellite telecommunications systems, it is generally necessary to have two bandwidths (in general 36 and 72 MHz), which means that two types of comb filters of radically dimensions must be provided. different. However, the use of comb filters is strongly desired, because of the reduced dimensions of this kind of filter, necessary for the integration of the frequency transpositions used in C and KU bands.

The invention aims to remedy this drawback.

It relates for this purpose to a band filter for microwave of the comb filter type comprising several portions of parallel line of equal lengths, each having a first end short-circuited to ground along a first line perpendicular to the direction of said portions of line, characterized in that these line portions each have their second end, free, connected to ground by a tuning capacity, coupling capacities constituting respectively the input and output members of this filter, and in this that said filter comprises a low impedance common line which connects the first ends of these line portions along a second line parallel to the first such that the edge farthest from said common line with respect to the first line is at a determined distance from this one.

• - Figures 2 à 7 sont des schémas de principe de six exemples d'exécution de ce filtre ;
• - Figure 8 est une vue perspective éclatée partielle d'une réalisation tri-plaque de ce filtre et ;
• - Figure 9 est une vue en coupe du filtre de la figure 8.

The invention will be clearly understood, and its advantages and other characteristics will emerge from the following description of some nonlimiting exemplary embodiments, with reference to the appended schematic drawing in which:

• - Figures 2 to 7 are block diagrams of six examples of execution of this filter;
• - Figure 8 is a partial exploded perspective view of a tri-plate embodiment of this filter and;
• - Figure 9 is a sectional view of the filter of Figure 8.

The filter represented in FIG. 2 is derived from the conventional comb filter of FIG. 1 by interposing, at the ground feet of the five fingers 1 to 5 of this filter, a common line 9 at low impedance between these fingers 1 to 5 and the mass. In this example, the line 9 has a width L substantially equal to that of the filter, and a length l of the same order of magnitude as the width of each of the fingers 1 to 5. This line 9 is, in this embodiment, reported by welding on the ground feet of the five fingers 1 to 5. It is then observed that the pass band of the comb filter, which was for example (in the case of FIG. 1) of the order of 15 to 17 MHz, passes (in the case of Figure 2) at about 36 Megahertz. It is observed that with this new filter (FIG. 2), the performance of adaptation to the accesses is preserved, a readjustment of the frequency tuning capacities 6 being however necessary due to the decrease in the characteristic impedance of the fingers 1 to 5.

FIG. 3 shows a filter produced in the same way as that of FIG. 2, but this time interposing a low-impedance line 10 of the same width L but of length l ′ double that of, 9, of this FIG. 2 We then observe that the bandwidth of this other new filter is even wider, reaching around 72 Megahertz.

By increasing this length l, the bandwidth further increases, and it is thus possible to vary the latter in a ratio of 1 to 10.

A very convenient embodiment of a filter with the same performance as that of FIG. 2 is shown in FIG. 4: it is simply produced by welding together, at a distance l from the mass, on the side of the "feet of mass "of the fingers 1 to 5, a tinned wire 11 parallel to the ground plane, the wire 11 replacing, from the electrical point of view, the metal strip 9 of FIG. 2.

Likewise, FIG. 5 shows that it is possible to simply produce a filter of the same performance as that of FIG. 3 by relating this same wire 11 to the distance l ′ with respect to the mass.

Finally, FIGS. 6 and 7 respectively show an embodiment of the filters according to FIGS. 2 and 3, by total integration of the low impedance lines 9 and 10 into the structure of the filter, without discontinuities, the assemblies (1 to 5 and 9 ) and (1 to 5 and 10) each being made of a single block.

This type of filter lends itself as well to production by ribbon lines, by tri-plate technology, as by integrated technology (possible application in MMIC). The attached tapes 9, 10 can be welded, screwed, or thermocompressed.

As an example of an embodiment, FIGS. 8 and 9 show a practical embodiment in tri-plate technology.

We distinguish in these figures the fingers 12 and the low impedance line 13, which are made in one piece, tapped holes 14 being provided to be able to attach, by screwing, another transverse plate to widen, if necessary, the strip bandwidth of this filter. A box 15 is conventionally provided both for this comb filter and for the remaining part of the circuit (such as for example a Group Propagation Time Corrector).

The comb filter itself is closed by a screwed cover 16 (screws 17 and 18), while a cover 20 closes, thanks to the screws 18 and 19, the housing itself 15.

The filter which has just been described allows:
- the choice of the location of a specific band;
- easy modification of bandwidth during the life of the product;
- to achieve the best compromise, for very wide bandwidths, between the sensitivity of the couplings, linked to the positioning accuracy of two fingers, and the physical production of the line at low impedance (for a maximum capacity value).

The production techniques by ribbon lines or by tri-plate technology allow similar realizations from 100 MHz to 10 GHz.

Other propagation media can be used, but the MMIC application is especially interesting for the realization of integrated filters (from 500 MHz to 3 GHz in bandwidth), for future integrations between 3 and 20 GHz.

It goes without saying that the invention is not limited to the exemplary embodiments which have just been described, but is on the contrary capable of being produced in other equivalent forms.

Claims (5)

1 / Band filter for microwave of the comb filter type comprising several portions of parallel line (1 to 5) of equal length, each having a first end short-circuited to ground along a first line perpendicular to the direction of said portions of line , characterized in that these line portions each have their second end, free, connected to the ground by a tuning capacity (6), coupling capacities (7, 8) constituting respectively the input and output members of this filter, and in that said filter comprises a common low impedance line (9, 10) which connects the first ends of these line portions along a second line parallel to the first such that the edge furthest from said common line relative to the first line or at a determined distance (1) from it. 2 / Band filter according to claim 1, characterized in that the width (L) of this common line with low impedance (9, 10) is substantially equal to the width of the filter. 3 / Band filter according to claim 1 or claim 2, characterized in that the low impedance line (9, 10) is attached to the mass feet of the fingers (1 to 5) of the filter. 4 / Band filter according to one of claims 1 to 3, characterized in that the low impedance line is produced by welding, at a distance (l, l ′) determined relative to the mass, on the side of feet of mass of the fingers (1 to 5) of the filter and parallel to the ground plane, a tinned wire (11). 5 / Band filter according to claim 1 or claim 2, characterized in that the low impedance line (9 to 10) is integrated without discontinuity in the structure of the filter, the fingers of the filter (1 to 5) of this line (9, 10) being made in one piece.

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