EP0738022A1 - Passband cavity filter with comb structure and radio altimeter with an input filter of this kind - Google Patents

Abstract

The filter includes a casing (1) which houses a first space followed by a first filter access comprising a rod (Be) and screwed adjuster (Le) and a second space followed by the comb teeth (B1-B3) and adjusters (V1-V3). A space leads to a second access filter (Bs,Ls) before a fourth space meets the wall. The access filters and the teeth lie in one plane while between 0 and 5 rejection filters (R1e,R2e,R2s,R1s) are placed perpendicularly in the spaces. At least one rejection resonator has a resonance frequency above the central maximum of the passband filter. If two resonators are used, they both have a frequency above the central maximum and are placed in the second and third space.

Description

The present invention relates to band pass filters of the cavity type and with comb structure which are, in particular, used as input filters for radio frequency receivers; it also relates to radio receivers, and more particularly radio altimeters, equipped with such filters.

There are commercially available radio altimeters equipped with band pass filters, with cavities, with comb structure called "comb-line" structure in Anglo-Saxon literature. The filters used only cause a small insertion loss and have no parasitic bandwidths around the bandwidth. On the other hand, the flanks of the passband are those of a TCHEBYSCHEFF filter, that is to say that they are not very stiff, which is troublesome in certain uses, especially for the flank which limits the high frequencies of the bandwidth.

The object of the invention is to avoid or, at the very least, to reduce this drawback without, for this, adding a notch filter in series with the band pass filter considered.

This is obtained by adding to the filter, inside its housing, in one or more suitably chosen places, respectively one or more resonators whose tuning frequency is, itself, suitably chosen.

According to the invention there is provided a band-pass filter, with cavities, with comb structure, of central frequency Fo, comprising a parallelepipedic housing with a first and a second end, two first internal walls parallel to each other and two second internal walls mutually parallel and orthogonal to the first walls and a series assembly with successively the first end, a first interval, a first filter access, a second interval, comb teeth in series, a third interval, a second filter access, a fourth interval and the second end, the access points and the teeth each comprising a bar mounted perpendicular to the first walls, characterized in that n, with n integer and 0 <n <5, of the four intervals each include a resonator mounted perpendicular to the second walls .

• les figures 1 et 2 des vues schématiques d'un filtre selon l'art antérieure,
• les figures 3 et 4, des vues schématiques d'un filtre selon l'invention,
• la figure 5, des courbes relatives aux filtres selon les figures 1 à 4.

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

• FIGS. 1 and 2 schematic views of a filter according to the prior art,
• FIGS. 3 and 4, schematic views of a filter according to the invention,
• FIG. 5, curves relating to the filters according to FIGS. 1 to 4.

In FIGS. 1 to 4, the corresponding elements are designated by the same symbols.

Figures 1 and 2 are respectively a longitudinal sectional view and a bottom view, cover removed, of a bandpass filter, cavity, comb structure, three teeth, according to the prior art. In FIG. 2, the section according to FIG. 1 has been identified by an axis line corresponding to the projection of the cutting plane in the plane of FIG. 2, and by two arrows XX which indicate which of the two cut planes is drawn. in figure 1.

The filter of FIGS. 1 and 2 comprises a parallelepipedal box, of metal, with a hollow part, 1, and a cover, 2.

Two metal bars, Be, Bs, are integral with the bottom of the hollow part and are arranged perpendicular to this bottom; three adjustment screws, metallic V1, V2, V3, the heads of which are external to the housing, pass right through and perpendicular to the bottom of the hollow part in order to be able to sink more or less deeply inside the housing . The Be bar, the screws V1, V2, V3 and the bar Bs are aligned in this order.

Two metal access points Pe, Ps as well as three metal bars B1, B2, B3 are integral with the cover 2. The sockets are arranged in the extension of holes drilled in the cover and the three bars B1, B2, B3, which constitute the three teeth of the comb are mounted perpendicular to the cover. Coaxial cables Le, Ls of which only the ends have been shown, penetrate respectively into the access sockets Pe, Ps, with their internal conductor which is isolated from the cover by an insulator and which emerges from the cover on the sides opposite to the socket; the external conductor of these cables is in contact with the internal wall of the socket. In FIG. 2, the internal conductors of the cables Le, Ls, as well as the bars B1, B2, B3 have been drawn in broken lines, in the position they occupy, inside the housing, when the cover 1 is placed on the hollow part 2, as shown in FIG. 1. The cables Le, Ls constitute the input and output conductors of the filter according to figures 1 and 2.

The bars B1, B2, B3 constitute resonators and the screws V1, V2, V3, disposed respectively opposite the bars B1, B2, B3, constitute the elements for adjusting the resonance frequencies of these resonators. A filter like that of FIGS. 1 and 2 is considered to be a filter with three cavities where the cavities are the spaces in the vicinity of each of the bars B1, B2, B3.

In the embodiment which is described here, the housing 1 and the cover 2 as well as the bars Be, B1, B2, B3 are made of light alloy, chromated on all the walls internal to the waveguide which constitute the housing and lid; as for the adjustment screws V1, V2, V3, they are made of beryllium bronze.

• dimensions extérieures : longueur 60 mm, largeur des côtés parallèles au plan de la figure 1 17 mm, largeur des côtés parallèles au plan de la figure 2 18 mm
• dimensions intérieures : longueur 55 mm, largeur des parois parallèles au plan de la figure 1 10 mm, largeur des parois parallèles au plan de la figure 2 12 mm.

The filter housing according to Figures 1 and 2 has the following dimensions:

• external dimensions: length 60 mm, width of the sides parallel to the plane of figure 1 17 mm, width of the sides parallel to the plane of figure 2 18 mm
• internal dimensions: length 55 mm, width of the walls parallel to the plane of Figure 1 10 mm, width of the walls parallel to the plane of Figure 2 12 mm.

It is a bandpass filter whose amplitude response, A, with respect to the frequency, F, represented by the curve Ch according to FIG. 5, is substantially centered on 4.3 GHz. The curve Ch is a classic curve of a Chebyshev filter, generally spelled Chebyshev or Chebishev in Anglo-Saxon literature. This kind of filter has a low insertion loss and has no parasitic bandwidths in the cut band; on the other hand, the slope of the transition between the pass band and the cut band is fairly low, in particular compared to that of the Cauer filters; but it is not known to produce these latter filters in mechanical structure, that is to say in the form of cavity filters, in comb; however it is possible to make Cauer's filters in other technologies, for example in microstrips (microstrip in the literature Anglo-Saxon), the drawback is that, in such embodiments, parasitic bandwidths appear in the cut band.

Figures 3 and 4 show a bandpass filter obtained by adding to the filter according to Figures 1 and 2, four rejection resonators, R1e, R2e, R1s, R2s which, in the example described, are made of beryllium bronze. It is in fact a question, in these figures, of showing in which positions can be placed rejection resonators to improve the response of the filter; in the example described the improvement sought only required the implementation of the resonators R2e and R2s; in other applications it may prove that a single rejector resonator is sufficient or that it is necessary to use three of them, or even all four.

These four resonators are arranged in the vicinity of the input and the output of the filter: R1e between the input bar Be and the end of the waveguide near this bar, R2e between the input bar Be and the resonator B1-V1, R2s between the resonator B3-V3 and the output bar Bs and R1s between the output bar Bs and the end of the waveguide near the bar Bs.

The four rejector resonators are mounted perpendicular to those of the internal walls of the housing, the plane of which is parallel to that of FIG. 1, that is to say that they are mounted perpendicular to the filter resonators.

The advantage of this choice of the positions of the rejector resonators, in the vicinity of the access points of the filter, perpendicular to the resonators proper of the filter, is that thus the rejector resonators play their role of rejectors well, that is to say take from energy at their tuning frequency, while not greatly disturbing the pass band of the filter, as shown in FIG. 5.

In FIG. 5 have been drawn, in addition to the curve Ch representative of the amplitude / frequency response of the filter according to FIGS. 1 and 2, a curve Ci representative of the amplitude / frequency response of the filter used for the description, that is to say that is to say, as has been said above, of a filter corresponding to that of FIGS. 3, 4 but without the rejection resonators R1e, R1s.

The comparison of the curves Ci and Ch shows that the introduction of rejection resonators R2e, R2s in the filter according to FIGS. 1 and 2, improves filtering by around 30 decibels in the upper part of the filter bandwidth; the bandwidth according to the curve Ci is centered on Fo = 4.3 GHz and, compared to that according to the curve Ch, it is slightly reduced at the top of the band and very slightly more extended at the bottom of the band.

In the example described, the rejection resonators R2e, R2s are respectively tuned to resonance frequencies of approximately 4.5 and 4.6 GHz, which explains the dips in the curve Ci at these frequencies, dips which are due to the energy taken by these resonators at their tuning frequency. It should be noted that the two rejector resonators are set to tuning frequencies higher than the frequencies of the passband of the filter, that is to say that in fact they function correctly only under these conditions. For tuning frequencies lower than the passband of the filter, a large parasitic passband appears in the response curve, which is generally not desirable.

The present invention is not limited to the example described. Thus the comb structure may have a number of teeth other than three. The bars such as Be, Bs and the teeth of the comb, instead of being perpendicular to the widest of the longitudinal internal walls of the waveguide, can be perpendicular to the narrowest of these walls; in this case the rejector resonators, to remain orthogonal to them, will be mounted perpendicular to the widest of the longitudinal internal walls of the waveguide.

Application, in particular, in radio altimeters.

Claims (4)

Bandpass filter, with cavities, with comb structure, of central frequency Fo, comprising a parallelepiped housing (1, 2) with a first and a second end, two first internal walls parallel to each other and two second internal walls parallel to each other and orthogonal to the first walls and a series assembly with successively the first end, a first interval, a first filter access (Be, Le, Pe), a second interval, comb teeth in series (B1, B2, B3), a third interval, a second filter port (Bs, Ls, Ps), a fourth interval and the second end, the ports and the teeth each comprising a bar (Be, B1, B2, B3, Bs) mounted perpendicular to the first walls , characterized in that n, with n integer and 0 <n <5, of the four intervals each include a resonator (R1e, R2e, R2s, R1s) mounted perpendicular to the second walls. Bandpass filter according to claim 1, characterized in that at least one of the n resonators (R1e, R2e, R2s, R1s) has a tuning frequency greater than Fo. Bandpass filter according to claim 1, characterized in that n is equal to 2 and in that the two resonators (R2e, R2s) each have a tuning frequency greater than Fo and are located respectively in the second interval and in the third interval. Altimeter characterized in that it comprises at least one filter according to one of the preceding claims.

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