FR2613538A1 - Microwave filter - Google Patents

Abstract

The invention relates to microwave filters. The main subject of the invention is a microwave filter including U-shaped resonators, each resonator 1 being connected to at least one variable capacitance 5. The variable-capacitance capacitors 5 enable the filter to be adjusted. In a particularly advantageous variant of the invention, the variable-capacitance capacitors 5 are connected to the U-shaped resonators 1 substantially at the level of the axis of symmetry 15 of the resonators 1. The invention applies mainly to the construction of microwave filters and to the devices using such filters.

Description

MICROWAVE FILTER
The present invention relates to filters, in particular microwave filters.

 In electronics, and in particular in microwave, it is essential to be able to filter a signal.

 I1 is known to repackage microwave filters comprising discrete elements, called localized constant filters. These filters have a large footprint, particularly for the lowest frequencies. This space is most often characterized by a large thickness due to the size and number of self-intuctucts used. Localized constant filters are costly very high manufacturing, reproducibility being difficult.

 On the other hand, it is known to make rilters with a constant constant. Such a filter includes resonataurs. The signal propagates by coupling between the consecutive resonators of the filter. The distributed constant filters are made in stripline technology, the resonators being deposited by metallization on one side of a dielectric with a low band, the metallization of the second side constituting the ground plane.

 Finally, it is known to produce filters called combline comprising straight resonators, the e> hoppers of which are connected, on the one hand directly, and on the other hand through a variable capacitor, to the ground plane. Combline type filters present difficulties in achieving and obtaining the desired filtering. In addition, the proximity of the variable capacitors poses problems of space for the construction of the filter as well as problems of <plages In addition, before carrying out the inveution it was believed that the fact of adding elements to c '> nstants located on a filter with distributed constant increased the overall dimensions.

 The filters according to the present invention include U-shaped resonators, also called hairpins (hairpine in English terminology) connected to at least one variable capacitor. The presence of the variable capacitor allows the frequency response of the constructed filter to be adjusted over a very wide range.

 The filters according to the present invention have excellent reproducibility because the adjustment range is very wide. Thus, it is possible to compensate for the manufacturing variation originating in particular from metallized holes inFluencing the frequency response of the filter, variations in tolerance on the dielectric permissiveness C R or the effects of line wholes.

 In addition, the setting allows a single filter for a wide frequency band.

 In addition, it is possible to separate the capacitors and thus limit the coupling between the capacitors and the resonators while making the best use of the surface of the filter for the mechanical implantation of said capacitors.

 In addition, the equivalent scheme of the filters according to the present invention is extremely simple which allows the translation, by means of the equivalent schemes of the filters according to the present invention. From these diagrams the geometry of the filters is likely to be generated by computer-aided design (CAD) software.

 The main object of the invention is a microwave filter comprising a plurality of U-shaped resonators, two successive resonators having an electromagnetic coupling zone, the first and last U-shaped resonators being connected to filter connection means, characterized by the fact that each U-shaped resonator is electrically connected to at least one variable capacitor.

 The invention will be better understood by means of the description below and the figures given as nonlimiting examples among which - FIG. 1 is a diagram of a first example of embodiment of a filter of known type - FIG. 2 is a second embodiment of a filter of known type - FIG. 3 is a first embodiment of a filter according to the present invention - FIG. 4 is a second embodiment of a filter according to the present invention - FIG. 5 is a third example of embodiment of the filter according to the present invention - FIG. 6 is a fourth example of embodiment of the filter according to the present invention - FIG. 7 is a section along line AA ′ in FIG. 6 - FIG. 8 is a first example of a diagram < electrical equivalent of a coupling between two resonators of the filter of figure 6 - figure 9 is a second example of equivalent electrical diagram of a coupling between two resonators of the e> example of realization n of FIG. 6 of the filter according to the present invention.

- Figure 10 is a curve showing the perform.nces of the device of Figure 7.

 In FIGS. 1 to 10, the same references have been used to designate the same elements.

 In FIG. 1, an example of embodiment of the filters with distributed constant of known type can be seen. The filter of FIG. 1 comprises a plurality of resonators 1 in the shape of the letter U. The resonators are also called hairpin resonators (hairpin in English terminology).

Each resonator has two branches of length L arranged symmetrically and orthogonally with respect to a base, the resonators 1 are staggered and d. so that the branches of the two successive resonators 1 have an electromagnetic coupling.

 In the example illustrated, the filters comprising six 1-U resonators. The first and last 1-U resonator are coupled with connection means 2.

 In the example illustrated in the figure, the connection means 2 comprise a branch of length L parallel to the branches of first and last resonators 1 as well as a metallized orthogonal strip terminated by a metallized hole 3.

 The electrical connection is made at the metallized hole 3.

 The filters illustrated in FIG. 1 have the disadvantage of being difficult to achieve and therefore, given a given manufacturing tolerance of poor performance. Indeed the edge of the lines as well as the mel? Llized holes 3 can influence the frequency response of the filter of known type.

 In addition, these filters are very large compared to the preferred embodiment of the device according to the present invention. The filter in Figure 1 is specialized in stripline technology. The patterns illustrated in the figure are metallizations deposited on the surface of a low loss dielectric, such as for example PTFE. The other face of the dielectric support with low composite loss metallization constituting the ground plane.

 In Figure 2, we can see a type filter (known as combline. The filters illustrated in Figure 2 comp.> Rte a plurality of straight resonators 10. The straight resonators 10 are arranged parallel to each other.

Each straight resonator 10 is connected, by a first of its ends to the ground 4, and by a second end to a first armature of a variable capacitor 5. The second armature of the variable capacitor 5 is connected to the mas 8 4.

 The filter illustrated in FIG. 2 can have parasitic couplings between the capacitors of variable capacitor 5 and the resonator 10 and between the capacitors themselves; because of their proximity. In addition, the size of the variable cond.nsators 5 poses problems at the level of the geometric construction of the filter due to their proximity.

 In Figure 3, we can see a first embodiment of the device according to the present invention. The filter of FIG. 3 comprises I-shaped resonators whose ends of the two branches are connected to the mas.'e 4 via capacitors with variable capacity 5. The presence of capacitors with variable capacity 5 allows fine adjustment of the filter which is particularly necessary with the U-shaped resonators. However, the adjustment is delicate because of the need to take into account the adjustment of a variable capacitor 5 when adjusting the second capacitor 5 connected to the same resonator 1. Dsns l 'example illustrated in Figure 3 the filter comprises five resonators 1, it is understood that a different number does not depart from the scope of the present invention.

 In Figure 4, we can see a second embodiment of the device according to the present invention. The filters according to the present invention comprise a greater number of U-shaped resonators. In the example illustrated in FIG. 4, the filter comprises seven resonators, it is clearly understood that a different number of resonators does not leave the frame. of the present invention.

 The end of a first branch of each resistor 1 in U is connected to a first armature of a variable capacitor 5. The second armature of each variable capacitor 5 is connected to ground 4 .

 On the other hand, the end of the second branch of each U-shaped resonator is connected to ground 4.

The use of a single capacitor per resonator 1 in
U makes it easier to adjust the filter.

 In Figure 5, we can see a third embodiment of the filter according to the present invention. The filters illustrated in FIG. 5 comprise five U-shaped resonators 1. It is understood that the use of a different number of U-shaped resonators does not depart from the scope of the present invention.

 The ends of the two branches of each rescnator 1 in U are connected by means of a variable capacitance capacitor 5. The variation of the capacity of the variable capacitors 5 allows the filter to be adjusted.

 In Figure 6, we can see a particularly advantageous alternative embodiment. of the filter according to the present invention. In the nonlimiting example illustrated in FIG. 6, the filters include six 1-U resonators. The base of each 1-U resonator is connected to a first armature of a variable capacitor 5. The second armature of the capacitors variables 5 is connected to ground 4.

 Advantageously, the connection of the base of the U-shaped resonator 1 to the first armature of the variable capacity capacitor 5 is carried out at an axis of symmetry 15 of said U-shaped resonator 1. In FIG. 6, the capacitors 5 are shown at the outside of the U formed by the resonator 1. It is understood that the variable capacitors 5 connected inside the U formed by the resonators 1 are not within the scope of the present invention.

The capacity of a variable capacitor 5 is four times greater than that required for other embodiments of the device according to the present invention. Thus, it is easier to adjust the filter of FIG. 6 and above all the value of the capacity. remains achievable phy '. ically if one goes up in frequency (band L,
For the various embodiments of the filter according to the present invention, the condensers sold under the Thintrim reference: 9402.0, 9402.1 or 9402.2 sold by the company Tekelec Airtronic were used, for example.

The length L of the branches of the resonators 1 <n U is less than X / 8 in the case of the device in FIG. 6,
g for # g / 4 g / 4 necessary for the realization of the same filter using U-shaped resonators of the prior art. Thus the filters produced according to the present invention have a smaller footprint. This reduction in size is particularly important for the production of filters for on-board equipment, for example on board an aircraft or satellite.

In FIG. 6, an example of rialization has been illustrated in which a direct coupling 20 has been used as the connection means. The direct coupling 20 is a melallization directly connected to the first and last resonator 1 in
U. Direct coupling eliminates the problem posed in the case of wide bandwidths. Indeed, in this case, the distance from an electromagnetic coupling is difficult to achieve ((100 Cul). The location of the U-shaped resonator branch to which direct connection 20 is made is determined by calculation using the specific software developed for the calculation of the filter elements. The connection at the end of metallization 20 constituting the direct coupling is carried out for example by means of a metallized hole 3. It is understood that the direct connection is not limited in the exemplary embodiment of FIG. 6 but can be used in the exemplary embodiments of the filter according to the present invention illustrated by FIGS. 3, 4 and 5.

 In addition, the behavior of the filter illustrated in Figure 6 varies little with temperature. Indeed the influence of the temperature to opposite effects on the resonates <'rs 1 in the shape of U and on the capacitors 5, these opposite effects largely compensating for each other.

 Advantageously, the filters according to the present invention are produced in triplate technology. An exemplary embodiment of a filter in triplate technology is illustrated in FIG. 7. FIG. 7 corresponds to a (lethal embodiment of the filter of FIG. 6 seen in section along the axis AA ′. In triplate technology, the U-shaped resonators 1 are placed substantially in an included plane in a low loss dielectric 7. At least two faces of the dielectric are covered by a metallization constituting the ground plane 4. Advantageously, the low-loss dielectric 7 forms a rectangular parallelepiped, the six shafts of which are covered by metallizations forming the ground plane 4 of said filter. The vertical connections carry the references 13. They allow, on the one hand to connect the ends branches of the resonator 1 in U to the ground plane 4, and on the other hand to connect the capacitor with variable capacity 5 to the base of the resonators 1 in U.

 In the example illustrated in FIG. 7, the metalliF ation of the ground plane 4 comprises savings 9 avoiding short-circuiting the bases of the U-shaped resonators with the ground.

 The variable capacitors 5 are shown diagrammatically in FIG. 7. In a real example, the variable capacitors 5 will be installed, for example, on the surface of the filter according to the present invention. In the case where the filter according to the present invention is enclosed in a hermetic casing, it is possible to allow the adjustment screws of the variable capacity capacitors 5 to protrude.

 The realization in triplate technology is not limited to the exemplary embodiment of the filter according to the invention of FIG. 6. The triplate technology can also be applied to the filters according to the present invention illustrated by FIGS. 3, 4 and 5.

 In FIG. 8, one can see a symbolic representation of the coupling between two resonators 1. The coupling is made in FIG. 8 between two lines 30 and 31 of impedan ZO and whose length is equal to the electrical angle 0. The line 30 has an input at point A and a connection to ground 4. Line 31 has an output at a point B ol) placed at point A and a connection to ground.

In Figure 9, we can see an equivalent diagram of a portion of the filter according to the present invention established from the book by Matthaei 1980 edition, Microwave Filters, llnpedance
Matching Networks and Coupling Structures. A portion corresponding to two coupled branches of the two resolvers 1 (the variable capacitor is not in the equivalent diagram) corresponds to a serial line 21 and two psrplele lines 22 (stubs in English terminology with electric angle e).

Line 21 of electrical angle e corresponds to the coupling between two resonators. The line 22 of electric angle e corresponds to the branches of the resonators 1 in form of U.

Advantageously, we translate the filter we want to get into its equivalent scheme using the criteria used in Matthaei's work. So it is possible to use a computer-aided design software for their realization, we can for example use the software 'CAD
ESOPE, SUPERCOMPACT or TOUCHSTONE.

 Advantageously, the translation is carried out by a computer to which the filter which one wishes to obtain is indicated.

 In FIG. 10, we can see the response curve of two identical filters, one made in microstrip technology, the other made in triplate technology. Curve 24 corresponds to the triple plate technology. Curve 23 corresponds to microstrip technology. The noise obtained is lower in triplate technology. The gain being of the order of 1 dB. Drag reduction is particularly important in applications requiring good rejection of spurious signals.

 An important advantage of the filter according to the present invention lies in the greatly improved manufacturing yield, the variations in characteristics of the filter being able to be compensated for by the adjustment of the capacitors with variable capacity 5.

 Thus, it is possible to integrate on a same microwave printed circuit a filter and an active circuit such as for example an amplifier. The risk that the entire circuit does not have the desired characteristics due to a faulty filter is reduced in very broad terms by using the filter according to the present invention.

In an exemplary embodiment, a filter according to the present invention corresponding to FIG. 6 has been optimized to operate at 728 MHz. This filter works perfectly, without deteriorating the behavior at 800 MHz, 958 MHz and 1 GHz frequencies. The technology according to the present invention can be used from high radio frequencies. Its efficiency is notably very important in the VHF bands,
UHF and L-band.

 The invention applies mainly to the production of microwave filters and to devices using such filters.

Claims (10)

 1. Microwave filter comprising a plurality of U-shaped resonators (1), two successive resonators (1) having an electromagnetic coupling zone, the first and last U-shaped resonators (1) being connected to connection means (2 , 20) of the filter, characterized in that each U-shaped resonator (1) is electrically connected to Inoins a variable capacitor (5).  2. Microwave filter according to claim 1, characterized in that each end of each resonator (1) in U is electrically connected to a first frame of a variable capacitor (5) the second frames of said variable capacitors (5) being connected to ground (4).  3. Microwave filter according to the resale; tion 1, characterized in that a first end of each U-shaped resonator (1) is electrically connected to a first frame of a variable capacitor (5), the second frames of said variable capacitors (5) as well as the second ends of each U-shaped resonator (1) being connected to ground (4).  4. Microwave filter as claimed; tion 1, characterized in that the ends of the U-shaped resonators (1) are connected by means of a variable capacitor (5).  5. Microwave filter according to claim 1, characterized in that each resonator (1) in U is connected to a variable capacitor (5) substantially at the level of the axis of symmetry (15) of said resonators (1).  6. Filter according to claim 5, characterized in that the length L of each branch of the resonators (1) in U is less, # g / 8, #g being the wavelength of the  g g center frequency of the waves propagated by the filter.  7. Filter according to one. any of the preceding claims, characterized in that the means of (connection (2, 20) of the filter comprises metal strips (20) electrically connected to the first and to the last U-shaped resonator (1).  8. Filter according to any one of the preceding revelsdications, characterized in that the U-shaped resonators (1) are placed substantially on a plane included in a dielectric (7) with low losses, at least two faces of the dielectric ( 7) being covered by a melallization constituting the ground plane (4) of the filter.  9. Filter according to claim 8, characterized in that the low loss dielectric (7) forms a rectangular parallelepiped whose six faces are re. opened by metallizations forming the mass (4) of said filter.  10. Filter according to any one of the preceding revei'dlcations, characterized in that the ends of the resonators (1) in U are electrically connected to ground (4).