EP0108003B1 - Double strip line resonators and filter using such resonators - Google Patents

Description

The present invention relates to a high frequency filter made from electromagnetic resonators which can be called "two-band resonators".

In the high frequency range called UHF (practically from 300 MHz to 3 Ghz), the resonators and filters produced from these elements are often made up of line sections. These can be air coaxial lines or coaxial lines loaded with dielectric as mentioned in the article: "Bandpass filter with dielectric materials used for broadcasting channel filter" by K. Wakino and Y. Konishi published in the review I.E.E.E. Transactions on Broadcasting, vol. BC-26, No. 1, March 1980. It is also known to manufacture resonators and filters from microstrip lines as indicated in the article: "750 MHz microstrip bandpass filter on barium tetratitanate substrate" by G. Ohm and G. Schmoller published in Electronics Letters, vol. 18, No. 15 of July 22, 1982, as also described in GL Matthaei's "Microwave filters impedance-matching networks and coupling structures", pages 422-425 and 770 of 1964 published by McGraw-Hill Book Company , New-York USA.

The coaxial line technique allows the manufacture of independent resonators whose natural frequencies can be adjusted before their assembly to form filters. This assembly can be carried out in the case of a bandpass filter by placing the various resonators end to end, the couplings between two sections of consecutive lines being determined by the distances which separate their faces placed opposite. However, to obtain interesting overvoltage coefficients (greater than 500) it is necessary to have sections having a fairly large section. Typically, a silver-plated 20 mm diameter resonator can have an overvoltage coefficient Q greater than 1000 for a frequency of 1 GHz. In addition, the coupling of quarter-wave resonators remains delicate and the very realization of the coaxial structure is quite complex because of the different operations of machining and metallization of elements with circular section.

Resonators can be designed by metallizing two opposite faces of a substrate as described in the article by J. Watkins "Radiation Loss from Open-Circuited Dielectric Resonators" published in IEEE Transactions on Microwave Theory and Techniques, October 1973, pages 636-639, or according to the technique of microstrip lines. They are generally produced from a relatively large dielectric substrate, one face of which is entirely metallized and the other of which receives a metallic conductor in the form of a thin strip. This technique has two drawbacks. On the one hand, the inherent overvoltage coefficients Q of the resonators are always low (less than 500) and consequently the performance of filters formed from these resonators is always modest (high insertion losses, greater than 3 dB towards 1 GHz). On the other hand, once the filter has been produced, by depositing ribbons on the same substrate, it is practically impossible to adjust the natural frequencies of the resonators as well as their mutual couplings. This prohibits the industrial production of filters comprising a high number of poles due to the inevitable dispersions of the characteristics: in particular, the dielectric constant of the substrate.

In order to overcome these drawbacks, the invention relates to filters comprising resonators produced from a parallelepiped made of dielectric material. A line is produced by metallizing two opposite faces of the parallelepiped and a resonator À / 4 or À / 2 depending on the length and termination of the line.

• - un boitier comportant une face inférieure recouverte d'un substrat isolant;
• - au moins un résonateur fixé audit substrat et comprenant un milieu diélectrique à six faces, caractérisé en ce que quatre au plus desdites faces étant recouvertes par une métallisation et deux autres faces non recouvertes étant opposées l'une à l'autre, lesdites faces métallisées étant perpendiculaires au plan du substrat.

The object of the invention is therefore a high frequency filter comprising:

• - a housing comprising a lower face covered with an insulating substrate;
• - at least one resonator fixed to said substrate and comprising a dielectric medium with six faces, characterized in that at most four of said faces being covered by a metallization and two other non-covered faces being opposite to each other, said metallized faces being perpendicular to the plane of the substrate.

• - la figure 1 représente une ligne microruban;
• - les figures 2 et 3 représentent des résonateurs demi-onde selon l'invention;
• - la figure 4 représente un résonateur quart d'onde selon l'invention;
• - les figures 5 et 6 représentent un filtre passe-bande selon l'invention;
• - la figure 7 est un diagramme donnant la résponse d'un filtre passe-bande;
• - la figure 8 représente un filtre coupe-bande selon l'invention.

The invention will be better understood and other advantages will appear during the description which follows and from the appended figures among which:

• - Figure 1 shows a microstrip line;
• - Figures 2 and 3 show half-wave resonators according to the invention;
• - Figure 4 shows a quarter wave resonator according to the invention;
• - Figures 5 and 6 show a bandpass filter according to the invention;
• - Figure 7 is a diagram giving the response of a bandpass filter;
• - Figure 8 shows a notch filter according to the invention.

représente la longueur d'onde dans le vide et Àg la longueur d'onde dans le guide bi-ruban. Cet indice dépend de la constante diélectrique du matériau et de la géométrie de la ligne. Ainsi, en utilisant un barreau de diélectrique en tétratitanate de baryum BaTi₄0₉ de constante diélectrique 37, et de dimensions a et b comprises entre 5 et 10 mm, on obtient, pour 1 GHz, n_e=4,7.Figure 1 shows a waveguide called microstrip line (microstrip line in English). This line is formed by a flat dielectric substrate 1 covered on its underside with a metallization 2. The opposite face of the substrate receives a conductive tape 3. This is a fairly well-known technique for producing a waveguide. One can envisage waveguides produced by the metallization of two faces of a dielectric solid with six faces and having a different appearance. This is the object of FIG. 2. This solid can be a parallelepiped. In this figure, there is a parallelepiped 4 of dielectric material having a rectangular section with sides a and b. Metallizations 5 and 6 cover two opposite faces of the dielectric. Unlike the microstrip line, the bi-ribbon line has two similar electrodes. Side a is the distance between the two electrodes 5 and 6. Such a line propagates electromagnetic waves with an effective index

represents the wavelength in vacuum and λg the wavelength in the bi-ribbon guide. This index depends on the dielectric constant of the material and the geometry of the line. Thus, using a bar of dielectric made of barium tetratitanate BaTi ₄ 0 ₉ of dielectric constant 37, and of dimensions a and b between 5 and 10 mm, we obtain, for 1 GHz, n _e = 4.7.

• - résonateur demi-onde:
  

• - résonateur quart d'onde:
  

Such a line can have natural resonant frequencies depending on whether the value of its length L is an even or odd multiple of λg / 4 and depending on the boundary conditions. In practice, we will focus on the following two cases:

• - half-wave resonator:
  

• - quarter wave resonator:
  

Ils peuvent aussi être du type représenté à la figure 3. On voit sur cette figure un guide d'onde bi-ruban formé par un barreau diélectrique 7 recouvert sur deux de ses faces de dépôts métalliques 8 et 9. Les conditions aux limites: métallisations des extrémités 10 et 11 en font un résonateur λ_g/₂ pour

Half-wave resonators can be present as shown in FIG. 2, that is to say in open circuit, with

They can also be of the type shown in Figure 3. We see in this figure a dual-ribbon waveguide formed by a dielectric bar 7 covered on two of its faces with metallic deposits 8 and 9. Boundary conditions: metallizations ends 10 and 11 make it a λ _g / ₂ resonator for

A λ _g / 4 resonator is shown in FIG. 4. It is formed by a bi-ribbon line defined by a dielectric bar 12, metallizations 13 and 14 and a short circuit 15 caused by the metallization of one of the ends. from the bar. Its length L is equal to λ _g / 4.

Using a material of dielectric constant 37 and metallizations carried out in screen-printed silver, the results summarized in Table 1 are obtained. The measurements were carried out on square section resonators (a = b) of type Àg / 4 and Àg / 2. Table 1 also gives the values of the resonant frequency fo, of the overvoltage coefficient Q at the resonance, of the volume V for each resonator. In fact, what is especially important for the cross-section of the bar is the distance a which separates the metallizations.

It can be seen, on reading Table 1, that the overvoltage Q is proportional to the edge a and that, at constant section, the overvoltage varies as the square root of the frequency. We can write Q = √f, K being a coefficient of proportionality. If we consider the overvoltage / volume ratio of the dielectric, we see that the dual-ribbon structure makes it possible to obtain resonators having excellent performance compared to their size. For a given overvoltage and volume, one can choose between the two types of resonators. For example, resonators 4 and 5 are equivalent from this point of view.

In order to have temperature stable resonators, it is advantageous to choose an appropriate dielectric. One can for example use a material such as those which were the subject of the patent of the Applicant No. 80.04 601 filed on February 29, 1980. These materials have relative molar proportions t Ti0 ₂ , x Sn0 ₂ , y Zr0 ₂ , a NiO, b La ₂ 0 ₃ and c Fe where the parameters t, x, y, a, b and c satisfy the following inequalities:

For x close to 0.35 the coefficient of thermal variation is zero. The high dielectric constant (around 37) of such materials allows a reduction in volume of the resonators for a given wavelength.

These resonators are typically intended for the production of band-pass and band-cut filters in the UHF range. They can also be used to stabilize oscillators. Examples of filters in the vicinity of 1 GHz are presented below. They can be easily transposed to other frequencies and can be made indifferently using Àg / 2 or Àg / 4 resonators.

For the production of filters, the type of resonators, their length and their section must be chosen according to the required performance.

FIG. 5 represents an embodiment of a bandpass filter with four resonators 20, 21, 22 and 23. These correspond, for example, to number 3 in Table 1, ie a = b = 7 mm and of type Àg / 4 . They are arranged in a housing 24 connected to ground. Figure 5 is a top view of the housing from which the cover has been removed. A cut was made at the holes 25 and 26 for input and output of the signal. The hole 25 allows the passage of a conductor 27 which forms a coupling loop 29, serving as excitation means, with the resonator 20. The end of the conductor 27 is then connected to the housing. The device allowing the output of the signal is similarly constituted by a conductor 28 which forms a loop 30, which serves as collecting means, at the level of the resonator 23 and the end of which is connected to ground. The bottom of the housing is covered with an insulating substrate 31 which has, for example, a very low dielectric constant. The resonators are fixed to the substrate 31, for example by gluing. The metallizations of the quarter wave resonators are respectively mutually parallel and perpendicular to the substrate as shown in FIG. 5. The coupling between resonators is made by mutual inductance. The natural frequencies of each resonator have been previously adjusted either by manufacturing or by running in. The development of the filter is then greatly facilitated. The resonators can also be separated by spacers made of dielectric material of low dielectric constant. The distances between each resonator can be of the order of edge a.

Figure 6 is a sectional view of the filter shown in Figure 5, the section being taken along AA. In these 2 figures, the same references represent the same objects. A metal cover 32 closes the housing and helps to remove the filter from external influences. It can be fixed to the housing by screws not shown. In order to make fine adjustments to the couplings between resonators, it is possible to place adjustment screws along axes 33, 34 and 35. These screws, located between the resonators, modify the electromagnetic field, depending on the state of their depression. reigns between the resonators.

• - des pertes d'insertion dans la gamme B_o inférieures ou égales à 2,5 dB,
• - une fréquence centrale f_o=1070 MHz,
• - une ondulation dans la bande B_o≤0,5 dB, .
• - la bande passante Bo=24 MHz,
• - les bandes passantes à 20 dB et 40 dB, B₂₀=50 MHz et B₄₀=90 MHz.

For example, the curve of the coefficient | S ₂₁ | of the diffusion matrix as a function of frequency f, that is to say the shape of the frequency response of the four-pole band pass filter described above. This is the object of FIG. 7. The frequency response of the filter is represented by curve 40. The ordinate axis is graduated in decibels. The curve has a maximum and two fairly steep sides which define a bandpass filter. The filter is characterized by a central frequency f _o , a pass band B _x at x dB, the ripple presented by the maximum which determines a pass band B _o , insertion losses. The natural frequencies of the resonators 20, 21, 22 and 23 are respectively 1060, 1080, 1080 and 1060 MHz. From the diagram in Figure 7, we note:

• - insertion losses in the B _o range less than or equal to 2.5 dB,
• - a central frequency f _o = 1070 MHz,
• - a ripple in the band B _o ≤0.5 dB,.
• - the bandwidth Bo = 24 MHz,
• - the bandwidths at 20 dB and 40 dB, B ₂₀ = 50 MHz and B ₄₀ = 90 MHz.

The measurements made on this filter also give | S ₁₁ | in B _o <0.1.

For comparison, other measurements were made on a bandpass filter comprising 3 resonators with configurations identical to the previous ones (a = b = 6 mm, h = 15 mm, ε _r = 37) and with natural frequencies 1060 MHz for the input resonator, 1080 MHz for the middle one and 1060 MHz for the output one. The characteristics of this 3-pole filter are then: f. = 1070 MBz, B _o = 20 MHz, B ₂₀ = 50 MHz and B ₄₀ = 110 MHz. The coefficient S ₁₁ of the diffusion matrix is less than 0.1.

The resonators according to the invention also lend themselves very well to the production of notch filters. Figure 8 shows such a filter. The housing was cut as in FIG. 5. The housing 50 is recognized on which a cover, not shown, is fixed. The bottom of the housing is covered with a substrate 51 of dielectric material of low dielectric constant. The filter includes 3 quarter-wave resonators 52, 53 and 54, holes 55 and 56 which allow the passage of a signal input conductor 57 and an output conductor 58, a line 59 which can be l soul of a coaxial line. The housing and its cover are connected to the ground. The distances separating the resonators from each other and between the line 59 are of the order of magnitude of the edge a. It is also possible to obtain with this kind of filters an adjustment of the couplings by the presence of screws modifying the electromagnetic field between the resonators.

The band-cut and band-pass filters produced using quarter-wave resonators exhibit a first spurious response at a frequency substantially triple their operating frequency.

Claims (3)

1. A high frequency filter comprising:

- casing (24), the bottom of which is covered by an insulating substrate (31),

- at least one resonator fixed to said substrate and comprising a dielectric medium having six surfaces, characterized in that at the most four of said surfaces are metal coated and that two non-metal coated other surfaces are opposed to each other, said metal coated surfaces being perpendicular to the plane of the substrate.

2. A high frequency filter according to claim 1, characterized in that said resonators (20, 21, 22 and 23) are arranged between stimulating means (29) and collector means (30) constituting the input and output terminals of the filter such that the incident electromagnetic energy is successively filtered by said resonators, the presence of said resonators causing the filter to become a band-pass filter.

3. A high frequency filter according to claim 1, characterized in that said resonators (52, 53 and 54) are arranged for withdrawing electromagnetic energy transferred by a propagation line (59) which connects the input and output terminals of the filter, the presence of said resonators causing the filter to become a bad-rejection filter.

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