EP0193162A1 - Microwave bandpass filter - Google Patents

Abstract

Bandpass filter made with ribbon waveguides and easily reproducible. The filter comprises, between its two ports (A, B), n half-wave resonators (1-7) connected in series. Capacitors (C0-C7) ensure a strong coupling between the successive elements of this series connection. Quarter-wave resonators (10, 11, 70, 71), connected by one of their ends to the series circuit, provide, at the limit of the passband of the filter thus obtained, an amplitude / frequency response with a steep edge. Application to band pass filters, wide band, for microwave frequencies.

Description

The present invention relates to bandpass filters for microwave frequencies, and, in particular, broadband filters and filters produced with ribbon waveguides.

It is known to produce microwave bandpass filters, for example in the technology of ribbon waveguides (stripline or microstrip in Anglo-Saxon literature), by combining in series a lowpass filter and a pass filter -high ; the low-pass filter consists of a succession of sections of thin lines and sections of wide lines which respectively constitute the inductive and capacitive elements of the filter; the high-pass filter comprises sections of thin lines connected to ground, which constitute set elements and which are connected to each other by open circuit lines or capacitors- Such filters require a large number of poles in the pass filter -bas and in the high-pass filter, that is to say a large number of line sections; they are therefore bulky and expensive.

Other known band-pass filters are produced by means of successive resonators, placed between the inlet and the outlet of the filter and very strongly coupled together; the resonators are very close to each other so as to obtain the strong coupling. Such filters are difficult to manufacture, even in ribbon waveguide technology, when the desired coupling requires a space between two successive resonators of less than 100 microns; indeed, this coupling must be perfectly constant from one filter to another to maintain the same characteristics for all filters of the same production.

The object of the present invention is to avoid, or at least to reduce the aforementioned drawbacks.

This is obtained mainly thanks to a resonator filter in which on the one hand the coupling between the successive resonators is reinforced by means of suitably arranged capacitors, and in which on the other hand a band-stop function has been introduced.

l'un de l'autre (avec k entier supérieur à -1).According to the invention, a bandpass filter for microwave, comprising, in series from the electrical point of view, n + 2 elements (n: positive integer) constituted by an input line, n linear resonators, open to both ends and all substantially of given length λ b / 2 and an output line, the resonators being ordered from the first to the n ^me respectively between the input line and the output line, is characterized by the combination of n + capacitive couplings for coupling respectively the input line at the first end of the first resonator, the second ends of the i the resonators at the first ends of the (i + 1) ^my resonators (with i integer ranging from 1 inclusive to n-1 inclusive) and the second end from the n ^me resonator to the output line, and at least one pair of linear resonators of length λ s / 4 - (λ s wavelength to reject, less than ab), the resonators of a pair each having one end connected to one of the n + 2 elements e t being at an electrical distance -

from each other (with k integer greater than -1).

• -la figure 1, un premier exemple de filtre selon l'invention,
• -la figure 2, des courbes de réponse en fréquence, relatives au filtre selon la figure 1,
• -les figures 3 à 5, des vues relatives à un second exemple de filtre selon l'invention.

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 first example of a filter according to the invention,
• FIG. 2, frequency response curves, relating to the filter according to FIG. 1,
• FIGS. 3 to 5, views relating to a second example of a filter according to the invention.

In the various figures, the corresponding elements are designated by the same references.

FIG. 1 represents a filter according to the invention: this filter, produced in waveguides with ribbons, comprises a support, P, made of polytetrafluoroethylene glass (better known under the registered trademark "teflon glass") constituted by a rectangular plate 45 x 65 mm and 1.6 mm thick. The hidden face of the support P is entirely covered by a copper deposit which constitutes a ground plane; on the visible side of the copper deposits A, 1 to 7, 10, 11, 70, 71, B respectively constitute an input line, seven linear resonators open at their two ends, four linear, auxiliary, short-circuit resonators at one of their ends and an exit line. It should be noted that, in this description and in the claims, it is a question of filters having an input line, such as A in FIG. 1 and an output line such as B in this same figure, but of course the role of these lines can be reversed, line A then becoming the exit access and line B the entry access. The filter according to FIG. 1 is a filter of the type with parallel lines, in fact the resonators 1 to 7 are constituted by sections of lines in parallel to reduce the size of the filter; these sections are arranged between the input and output lines, A and B. The resonators 1 to 7 are resonators of the half-wave line type, all substantially of length λ b / 2 or λ b is the wavelength corresponding to the center frequency of the filter bandwidth. To obtain a significant coupling between the successive sections of this filter and to make this coupling easily reproducible from one filter to another of the same production, variable capacitors CO to C7 respectively connect line A to the first end of the resonator 1, the second end of the resonator 1 at the first end of the resonator 2, ..., the second end of the resonator 6 at the first end of the resonator 7 and the second end of the resonator 7 at line B; the line sections 1 to 7, with the capacitors C1 to C6, thus form a zigzag assembly.

The four auxiliary resonators 10, 11, 70, 71, of quarter-wave line type, are mounted in short circuit by connection of one of their ends to the resonator 1 for 10 and 11 and to the resonator 7 for 70 and 71. These short-circuit resonators are intended to provide a notch function in the filter so that, as will appear in FIG. 2, the amplitude / frequency response curve of the filter has a bandwidth with a steeper edge in high frequency limit; for this the length of these auxiliary resonators is chosen to be equal to X s / 4 where X s is a wavelength to be rejected, less than λ b and corresponding substantially to the central frequency of the frequency band to be eliminated by the cut function -bandaged. These auxiliary resonators are associated in pairs, 10-11 and 70-71 and the resonators of the same pair are arranged at a distance equal to (2 k + ₁ ) λ b / 4 from each other with k positive integer taken, in the example described, equal to 1; the choice of this distance between the auxiliary resonators allows mutual compensation, in the bandwidth of the filter, of the inductive and capacitive disturbances brought by each of the resonators of the same pair. It should also be noted that the auxiliary resonators of this cut-off function strip can practically be arranged in any place of the electrical path which connects the two accesses of the filter, as long as, between them, the distance of (2k + 1) λ b / 4 is respected. The filter which has just been described has a bandwidth at 3 decibels which goes from 950 to 1700 MHz with, on both sides, a rapid attenuation up to 30 decibels.

There figure 2 is a graphic representation of amplitude / frequency responses relating to the circuit of figure 1. Three curves, G1, G2, G3, have been represented.

The curve G1 relates to the circuit of FIG. 1 with coupling capacitors CO to C7 of high value - (CO and C7 = 20 pF and C ₁ to C6 = 5 pF) and without the auxiliary resonators 10, 11, 70, 71; this curve is substantially that of a high-pass filter whose attenuation, greater than 30 decibels below 200 MHz, goes from 30 to 1 decibel between 200 and 500 MHz, is of the order of 1 to 2 decibels between 500 and 1600 MHz (flat response) then varies from approximately ₁ to ₁₁ decibels in the rest of the frequencies where the measurement was made, that is to say between 1600 and 3750 MHz. This frequency response is far from corresponding to the bandwidth of the filter in Figure 1: 950-1700 MHz.

The curve G2 in FIG. 2 relates to the amplitude / frequency response of the circuit of FIG. 1 without the auxiliary resonators 10, 11, 70, 71 but with the capacitors CO to C7 as they are set in the filter (CO and C7 = 15 pF, C1 and C6 = 3 pF, C2 to C5 = 1.5 pF). The answer is that sought for the low frequencies but, on the side of the high frequencies, the attenuation is not fast enough and strong enough.

The curve G3 relates to the amplitude / frequency response of the circuit described using FIG. 1; the comparison of this curve with the curve G2 shows that the addition of the notch filter, presenting a cut band centered on approximately 2300 MHz and obtained thanks to the quarter wave lines whose resonance frequencies are chosen in the band 1850 -2500 MHz, has the effect of causing a sudden loss variation near the high frequencies of the bandpass filter: loss of less than 3 decibels below 1750 MHz and loss of the order of 20 to 30 decibels for frequencies from 1800 MHz to more than 2500 MHz; this attenuation becomes less important again for frequencies of the order of 2700 MHz and more, but these frequencies are sufficiently distant from the frequencies of the passband (950-1700 MHz) so that it can result in a drawback in the majority of cases use of the filter.

Another embodiment of a filter according to the invention is described with the aid of Figures 3 to 5; it is in fact a filter having the same characteristics as the filter according to FIG. 1 but produced in two layers plus a ground plane on flexible supports and in which the capacitors corresponding to the capacitors C1 to C6 of FIG. 1, are obtained by overlapping the ends of lines separated by the thickness of a flexible support.

Figure 3 shows a flexible support in polyamide, S1, on which five copper deposits have been made: A, 1 + ₁ 0 and 11, 3, 5, 7 + 70 and 71, B. Figure 4 shows another flexible polyamide support, S2, on which three copper deposits have been made: 2, 4, 6. The supports S1 and S2 are two rectangular plates of 35 x 144 mm which are then glued to each other, to give the assembly shown in Figure 5; under the plates S1 and S2 is also bonded a ground plane constituted by a polyamide support covered, on a surface, by a copper deposit; this ground plane is not visible in FIG. 5.

To the assembly constituted by the plates S1 and S2 with their deposits and the ground plane, it suffices to add two fixed miniature capacitors, of 15 picofarads each, CO, C7, to constitute a filter comparable to the filter of FIG. 1. As in the filter of FIG. 1, the input and output lines are designated respectively by the letters A and B while the resonators in half-wave lines have the marks 1 to 7 and that the resonators, in quarter lines d wave, of the notch function, bear the marks 10, 11, 70, 71. The capacitive couplings between the line A and the resonator 1 and between the resonator 7 and the line B are respectively provided by the capacitors CO and C7 . On the other hand, the couplings between the half-wave resonators are obtained by placing the ends to be coupled face to face; the facing surfaces, separated by the dielectric from the polyamide support, thus form the two plates of the coupling capacitors; these capacitors bear the references C1 to C6 in FIG. 5.

Various other embodiments of a bandpass filter are possible without departing from the scope of the invention. Thus, in particular, the filter can be designed in a tri-plate structure, that is to say with the resonators arranged in the space separating two parallel ground planes. Similarly, from the embodiment according to FIG. ₁ , the capacitors C1 to C7 can be obtained by means of metal tongues deposited on a dielectric substrate; these tabs are arranged so that, for example to replace the capacitor C1 of FIG. 1, the two ends of the tab are respectively opposite the ends of the resonators 1 and 2 to which the capacitor C1 was connected; the facing surfaces determine the coupling between the successive resonators. Capacitors such as CO and C7 (Figures 1 and 5) can also be obtained by this technique of facing copper surfaces or by that presented in Figure 5; or equivalent; this is possible by giving the ends opposite sufficient surfaces taking into account the thickness and the permittivity of the dielectric which separates them and the capacity to obtain.

It is also possible to produce a filter comprising only a single half-wave resonator and a single pair of quarter-wave resonators.

Claims (2)

1. Bandpass filter for microwave, comprising, in series from the electrical point of view, n + 2 elements (n: positive integer) constituted by an input line (A), n linear resonators (1 -7), open at their two ends and all substantially of given length λ b / 2 and an output line (B), the resonators being ordered from the first to the n ^me between the input line and the output line respectively, characterized by the combination of n + 1 capacitive couplings (CO-C7) to respectively couple the input line (A) to the first end of the first resonator (1), the second ends of the i resonators to the first ends of the (i + ₁ ) ^my resonators (with i being an integer from 1 inclusive to n-1 inclusive) and the second end of ^me n resonator (7) to the output line (B), and at least a pair of linear resonators (10-11, 70 -71) of length λ s / 4 (Xs wavelength to reject, less than Xb), the resonators of a pair each having one end connected to one of the n + 2 elements (A, 1-7, B) and being at an electrical distance

from each other (with k integer greater than -1). 2. Filter according to claim 1, in which the n + 2 elements are produced according to the technique of ribbon waveguides, characterized in that the n + elements (A, ₁ -7, B) are distributed on both sides and d '' of the same dielectric support (S1) and in that at least one (C1-C6) of the n + capacitive couplings between two elements (1-2, 2-3, 3-4, 4-5, 5- 6, 6-7) is obtained by placing one end of one of the two elements considered face to face with one end of the other of the two elements considered, the two elements considered being, for this purpose, arranged on both sides 'other support and the surface of their opposite ends being determined according to the ability to obtain (Figure 5).

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