EP0071508B1 - Small-dimensioned microwave filter with linear resonators - Google Patents

Description

The present invention relates to filters made by means of linear resonators open at their two ends, known under the name of linear resonators λ / 2 in which enters the category of hair pin resonators (hair pin resonators in Anglo-Saxon literature ) or U-shaped resonators.

Filters with linear resonators open at their two ends have been known in the art for a long time and are for example described in the review "IRE Transactions on Microwave Theory and Techniques •, vol. MTT-6-, n ° 2, April 1958, p. 223 to 231, and in the review “IEEE Transactions on Microwave Theory and Techniques •, vol. MTT-20, November 1972, p. 719 to 729.

Filters are also known, such as those called “comb-line filters for example, using λ / 4 resonators, one of the ends of which is grounded. These filters, which use a technology radically different from that of open resonators in X / 2, have in the past been sometimes associated with tuning capacities, as shown in US Pat. No. 3,534,301 and the article published in the journal "IEEE Transactions on Microwave Theory and Techniques", Vol. MTT-18, May 1970, p. 278 to 280.

These filters with linear resonators open at / λ2 have the main drawback of providing, in addition to the useful bandwidth centered on the useful frequency fo, a “parasitic bandwidth centered on the frequency 2fo.

The attached FIG. 1 shows a band pass filter with linear U-shaped resonators of the conventional type. In the case of the example described, this is a filter produced on a wafer, 1, twice two inches, that is 5.08 x 5.08 cm, having a thickness of 1.27 mm; a graduated scale from 0 to 1 cm is placed next to the plate to show the enlargement ratio of the drawing.

The filter according to FIG. 1 comprises, in addition to the plate 1 made of alumina, a ground plane constituted by a gold deposit 10 μm thick covering all of that of the two faces of the plate which is not visible on the drawing. On the face of the plate visible in the drawing, two lines L _i , L ₂ are made by depositing gold 10 μm thick, and, between these two lines, six U-shaped resonators, H ₁ to H _s . The lines L ₁ , L ₂ constitute the ports of the filter; they are mutually parallel and parallel to the vertical bars of the U of the resonators.

A filter of the type of that according to FIG. 1 is of course with fixed bandwidth since it does not have a tuning element whose characteristic would be easily adjustable.

The attached FIG. 2 is a graph which gives, as a function of the frequency F expressed in megahertz, the value of the attenuation A, expressed in decibels, which the filter entails according to FIG. 1. This graph shows that the filter has a bandwidth centered on 825 MHz and with a width of 55 MHz at 3 dB from the value at 825 MHz; it is for this bandwidth that the filter of figure 1 has been calculated, the U-shaped resonators of which have been chosen with a length equal to 70 mm, ie λ / 2, where λ is the wavelength in alumina corresponding to the frequency of 825 MHz. It should be noted that, next to this bandwidth centered on 825 MHz, the filter according to FIG. 1 has the drawback of having, as it appears from FIG. 2, a bandwidth which can be called “parasitic bandwidth • , and which is due to the harmonic 2 of the U-shaped resonators; this parasitic bandwidth is situated substantially between 1000 and 1400 MHz and, in this bandwidth, the attenuation varies in wave form between -3 and -20 decibels.

Furthermore, the size of these filters is linked to the length of the resonators, but in these known filters this length which, being substantially equal to half the average working wavelength in the filter, is not negligible. In addition, these known filters are not tunable: it should indeed be possible to easily modify the length of the resonators which is of course not the case.

The object of the present invention as claimed is to provide filters with linear resonators open at their two ends, such as U-shaped resonators, of dimensions significantly smaller than those of conventional filters of the same type, which can be easily tunable, and whose the parasitic bandwidth is further from the central frequency of the filter than in the case of filters of this type previously known. It therefore relates to a microwave filter with n linear hairpin resonators (n positive integer) open at their two ends of the so-called half-wavelength type, said filter being characterized in that each of the resonators is length less than the half wavelength which corresponds to the central frequency of the band to be filtered, and in that each of said resonators further comprises a capacitor connected between its two open ends, the value of the capacitance of said capacitor being that which gives, for the resonant circuit which it constitutes with the associated resonator, a resonant frequency equal to said central frequency of the band to be filtered.

• la figure 1, un filtre selon l'art connu,
• la figure 2, un graphique relatif à la figure 1,
• la figure 3, un filtre selon l'invention
• la figure 4, un graphique relatif à la figure 3,
• la figure 5, une partie d'un filtre selon l'invention,
• la figure 6, un schéma d'un 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 filter according to the known art,
• FIG. 2, a graph relating to FIG. 1,
• Figure 3, a filter according to the invention
• FIG. 4, a graph relating to FIG. 3,
• FIG. 5, part of a filter according to the invention,
• Figure 6, a diagram of a filter according to the invention.

Figures 1 and 2, relating to the prior art, have have already been described previously.

Figure 3 is the representation of a filter according to the invention; this filter, like the filter according to FIG. 1, was calculated to present a passband of 55 MHz centered on 825 MHz. This filter is produced on an alumina wafer, 1, twice an inch, ie 2.54 x 2.54 cm having a thickness of 1.27 mm; a graduated scale of 0 to 1 cm, placed next to the plate, gives the enlargement ratio of the drawing.

Like the filter according to FIG. 1, the filter according to FIG. 3 comprises a ground plane here consisting of a gold deposit 10 μm thick and which completely covers all of that of the two faces of the plate which is not not visible in the drawing; on the face of the wafer 1 visible in the drawing, two lines L ′ ₁ , L ′ _z parallel to each other and, between these lines, five U-shaped resonators are produced by a deposit of gold 10 μ thick, H ' ₁ to H' _s whose vertical bars are parallel to the lines L ' _i , L' _z . Between two points located, in the case of FIG. 3, in the vicinity of the ends of the U-shaped resonators, H ' ₁ to H' ₅ , are connected five variable capacitors, Ci to C ₅ , associated respectively with the resonators H ' ₁ to H '₅; these variable capacitors, symbolically represented by two parallel bars crossed by an arrow, are miniature commercial capacitors whose capacity can be adjusted between 0.3 and 1.2 picofarads.

The length of the U-shaped resonators of FIG. 3 is 40 mm, which, for a half-wavelength having this value in alumina, corresponds to a frequency of 1450 MHz; but, as shown in FIG. 4, with the filter according to FIG. 3, the passband is no longer centered on a frequency whose corresponding half-wavelength is equal to the length of the U-shaped resonators.

FIG. 4 is a graph which represents, with the same scales on the abscissa and on the ordinate as in FIG. 2, the attenuation as a function of the frequency produced by the filter according to FIG. 3. This graph shows that the filter of FIG. 3 has a bandwidth centered on 825 MHz, almost identical to that of the filter according to FIG. 1. On the other hand, the “parasitic bandwidth of the filter of FIG. 1 (between 1,000 and 1,400 MHz) does not exist with the filter of Figure 3; in fact the capacitors Ci to C ₅ have been adjusted to, in the example described, lower the average frequency of the pass band of the filter by about one octave, which results in a doubling of the electrical length of the resonators while, the physical length of the resonators remaining unchanged, and known, as can be observed experimentally, only this physical length determining the frequency of the parasitic harmonics, the first annoying harmonic in this case becomes the harmonic four (3300 MHz) of the real frequency Okay.

Thus, in general, there is a reduction in the size of the filter compared to filters of the kind of that according to FIG. 1 and the distance from the parasitic bandwidth; the capacitor completing the electrical length of the resonator with which it is associated, it is possible to give this resonator a physical length of the order of λ / 4 as in the case of the filter corresponding to FIG. 3 and to adjust the capacitor to give the curve of Figure 4. By choosing the capacitors appropriately. variables it is even possible to give resonators in U a length less than λ / 4.

By continuously varying the value of the capacitors of the filter according to FIG. 3, a continuous variation of the frequency matching of this filter is obtained while retaining a very distant parasitic passband.

Another possibility of producing the tuning capacitors of the filter according to the invention is the realization of the etched circuit type. In this embodiment, the capacitors are obtained at the same time and in the same way as the resonators: metallic deposition on a wafer or metallized wafer from which part of the metallic layer has been removed by chemical or mechanical attack. FIG. 5 shows a resonator, H, with the associated capacitor, C, obtained in the manner indicated above. The capacitor C consists of a row of parallel tongues, arranged between the branches of the U of the resonator H. perpendicular to its branches; two successive tongues are respectively integral with the two branches of the U. It should be noted that filters produced in this way are not adjustable but have, compared to filters with variable capacitors of the kind of that according to FIG. 3, the advantage of '' reduced thickness and faster production.

Different variants of the filters described with the aid of Figures 3 to 5 can be proposed without departing from the scope of the invention. This is how band-cut filters can be made in the same way; Figure 6 is a schematic view of such a filter.

FIG. 6 represents a notch filter which comprises a single access line, L, the two ends of which constitute the inlet and the outlet of the filter respectively. Three U-shaped resonators, H ₀₁ , H ₀₂ , H ₀₃ , are arranged in the same plane as line L, with their branches parallel to line L and are placed on either side of this line. Variable capacitors C ′ _i , C ′ ₂ , C ′ ₃ make it possible to obtain the same advantages as in the case of FIG. 3 but in a transposition to the strip cut function.

In other variants, the coupling between the access line or lines and the resonators can be done by an electrical connection between one of the resonators and the line considered. Similarly, an access line, or both, may be perpendicular to the bars of the U-shaped resonators and terminate in an electrical connection to the resonator; this is how, for example, in FIG. 3, the line L ′ can be replaced by a connection starting from the left edge of the plate and ending at a point on the left bar of the resonator H ' _{1 ′} this dependent point by the neck input range to be achieved.

Furthermore, instead of making filters of which the lines, the resonators and possibly the capacitors consist of metallic deposits on a substrate, it is possible, in a metal plate deposited on a substrate, to produce the lines, the resonators and possibly capacitors, by removing metal; we then have a filter with slotted lines and resonators ("slot lines and" resonant slots in Anglo-Saxon literature).

Claims (3)

1. A microwave filter comprising n linear hairpin resonators (H'₁-H'₅), n being a positive integer, characterized in that the length of each resonator is smaller than λ/2, \ being the wavelength corresponding to the center frequency of the filter band, and that a capacitor (C₁-C₂) is branched between the open ends of each resonator, the capacitance of said capacitor being such that a resonance frequency equal to said center frequency of the filter band is obtained in the resonant circuit constituted by said capacitor and the associated resonator.

2. A microwave filter according to claim 1, characterized in that the capacitors are variable capacitors.

3. A microwave filter according to claim 1, characterized in that, the resonators being obtained by a specified technique of depositing a metal on a substrate (1), the capacitors (C) are fixed value capacitors which are deposited by the specified technique on the substrate.

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