EP1067617B1 - Bandpass filter - Google Patents

Description

The present invention relates to a bandpass filter. It applies in particular to microwave filters, for example narrow band, arranged at the output of a radar transmission chain. More generally, it applies to bandpass filters used in transmitters but also in receivers of electromagnetic systems.

Radars are partly characterized by their operating frequency band. The latter is chosen in particular according to the applications planned for the radar. A signal emitted by a radar is produced from a low power microwave signal, generally created from a reference oscillator. This signal is then amplified through a transmission chain so as to obtain at the output of the radar a signal sufficiently powerful to reach a target and allow the latter to return an echo sufficiently powerful so that it can be detected by the chain of radar reception. To obtain a pure microwave signal which is in the desired band, it is necessary to place in the transmission chain a suitable bandpass filter. Like any microwave circuit located in a microwave transmission or reception chain, this bandpass filter must have a good adaptation characterized by a standing wave rate, this adaptation being associated with insertion losses. In addition, the bandpass filter has characteristics specific to its function. An important characteristic is notably the stiffness of its blanks, that is to say the frequency interval which separates the filter from a state passing to a state where it almost completely attenuates the waves. The smaller this interval, the greater the stiffness of the filter. The greater this stiffness, the more effective the filter. Another important characteristic of this circuit is the power level accepted by the filter.

It is known to produce bandpass filters for radar transmission or reception chains which have good performance. These performances relate in particular to insertion losses, out-of-band rejection in particular at harmonic frequencies, the standing wave rate but also the stiffness of the filter as well as its power handling. The document CH-A-245 847 describes a bandpass filter comprising low-pass and high-pass cells placed in series, constituted by inductive and capacitive elements.
The known structures do not allow good performance to be obtained in a small space. An object of the invention is to allow the production of a bandpass filter which has both good microwave performance and which occupies a reduced volume. To this end, the subject of the invention is a bandpass filter, characterized in that it comprises one or more elementary cells of the bandpass type placed in series, an elementary cell grouping together a network carrying out a low pass filtering an inductive section and a capacitive section in coaxial technology and a quarter-wave resonator in series exerting a high-pass filtering, placed in the middle of the capacitive section.

The main advantages of the invention are that it makes it possible to obtain a very good level of frequency rejection outside the passband, that it adapts to different frequency bands and that it is simple to implement. .

• la figure 1, une illustration du gabarit d'un filtre passe-bande ;
• la figure 2, un exemple de réalisation possible d'un filtre selon l'invention ;
• la figure 3, une cellule élémentaire composant le filtre de la figure 2 ;
• la figure 4a, un schéma électrique équivalent d'un tronçon élémentaire passe-bas ;
• la figure 4b, un schéma électrique équivalent d'une cellule élémentaire passe-bande ;
• la figure 5, un exemple de fonction de filtrage obtenue pour un filtre élémentaire passe-bande ;
• les figures 6 et 7, un autre exemple de réalisation possible d'un filtre selon l'invention.

Other characteristics and advantages of the invention will become apparent from the following description given with reference to the appended drawings which represent:

• Figure 1, an illustration of the template of a bandpass filter;
• Figure 2, a possible embodiment of a filter according to the invention;
• FIG. 3, an elementary cell making up the filter of FIG. 2;
• FIG. 4a, an equivalent electrical diagram of an elementary low-pass section;
• FIG. 4b, an equivalent electrical diagram of an elementary bandpass cell;
• FIG. 5, an example of a filtering function obtained for an elementary bandpass filter;
• Figures 6 and 7, another possible embodiment of a filter according to the invention.

FIG. 1 illustrates by way of indication the function of a band-pass filter in a system of axes whose ordinate axis represents the level of attenuation A, for example in dB, and whose abscissa axis represents the frequencies f. The stiffness of the filter is characterized by the frequency interval Δf ₁ which characterizes the transition from the maximum attenuation state 1 to the passing state corresponding to the passing state 2. The latter corresponds to a substantially zero attenuation, corresponding to the residual insertion losses. The stiffness is also characterized by the frequency interval Δf ₂ corresponding to the passage from the on state 2 to the maximum attenuation state 1. The two intervals Δf ₁ , Δf ₂ are for example substantially equal. The effectiveness of the filter is linked to its stiffness. A bandpass filter is all the more effective the greater its stiffness, that is to say that the intervals Δf ₁ , Δf ₂ are low.

FIG. 2 shows a possible embodiment of a filter according to the invention. This filter comprises a propagation structure of coaxial type 1 of length L. FIG. 2 shows the whole of this structure according to a longitudinal section passing through the center. The filter according to the invention comprises a succession of identical elementary cells 10 arranged in series and fitting into each other.

FIG. 3 illustrates an elementary cell 10, by the same sectional view as that of FIG. 2. This cell comprises an elementary filtering network of the low-pass type which has symmetry of revolution about an axis of symmetry 31. This low-pass network is composed of two sections 32, 33 of respective radii R ₁ , R ₂ representing respectively the inductance L ₀ and the filtering capacity C ₀ . The elementary capacitance C ₀ of the cell 10 is obtained mechanically by the spacing between the section of large radius R ₂ and the metal sheath 34 of the coaxial structure. The elementary inductance L ₀ is obtained by the length of the section 32. This gives an equivalent diagram as shown in FIG. 4. The elementary capacitance C ₀ is connected between the metal sheath 34 and the section 33 of large radius R ₂ , the elementary inductance L ₀ is a function of the length I _e of the section of small radius R ₁ .

The elementary cell 10 L ₀ , C ₀ performs a filtering of the high frequencies, the cut-off frequency f _H being given by the following relation, in the absence of any other element, in particular of a resonator which will be described later : $${\text{f}}_{\text{H}}\text{=}\frac{\text{1}}{\text{2π}\sqrt{{\text{The}}_{\text{0}}{\text{VS}}_{\text{0}}}}$$

FIG. 5 illustrates by a first curve 51, in solid line, the filtering function relating to this cell, in the same system of axes as that of FIG. 1. Waves having a frequency greater than a frequency close to the frequency of previous cut f _H are attenuated by cell L ₀ , C ₀ .

An elementary cell 10 also includes a quarter-wave resonator 35, placed in the middle of the capacitive section 33. From a mechanical point of view, this microwave trap is produced using a ring 35 made of insulating material, capable of be for example the material called by the Teflon trademark, and whose cross-sectional shape recalls a double L. This shape is linked to the long length of the trap which it is necessary to fold back inside the structure so that the open circuit can be correctly transformed into a short circuit at the capacitive section. This electrical continuity is ensured if the length Lp of the trap is substantially equal to the quarter wave at the central frequency of the useful frequency band, that is: $${\text{Lp = λ}}_{\text{Gc}}\text{/ 4}$$

λ _Gc being the guided wavelength associated with the central frequency of the useful band of the final filter. This resonator turns into a short circuit for microwave waves. Outside the band, the impedance brought back by the resonator is no longer a short circuit, which creates a mismatch which is characterized by filtering of low frequencies. The role of this section is in particular to create the bottom flank of the filter. Waves having a frequency lower than the frequency f _B are attenuated. On the contrary, waves having a frequency higher than f _B encounter a short circuit.

In order to ensure mechanical maintenance of the overall structure, the rings 35 for example slightly protrude from capacitive sections and are then held by a sheath 36, made of insulating dielectric material of small thickness.

By the effect of the widening of the central conductor, whose filtering function is illustrated by the first curve 51 and by the effect of quarter wave resonator whose response is illustrated by the second curve 52, an elementary bandpass filter is thus obtained. The filter is passed between the low cutoff frequency f _B and the high cutoff frequency f _H. Figure 4b illustrates the equivalent electrical diagram of an elementary cell, taking into account the effect of the resonator. A first filter comprises an inductance L ₀ connected to a capacitor C _{0/2 which} itself is also connected to the potential of the external conductor 34. In series with this filter are arranged an inductance L _r and a capacitance C _r equal to C ₀ / 2, both in series, connected to a capacitance C _{r also} connected to the potential of the external conductor 34. L ₀ , C ₀ are the inductance and the capacitance of the low-pass filter illustrated by FIG. 4a, and L _r , C _r are the inductance and the capacity corresponding to the resonator 35.

The resonator 35 need not extend from the central conductor 33 to the sheath 36 or the external conductor 34. However, to ensure good mechanical strength of the assembly, it is preferable that the transformer impedance occupies the entire interior space. An electrical insulator, for example Teflon, occupies the space between the central conductor 12 and the sheath 36 or the outer conductor 34.

A filtering function as illustrated in FIG. 5 and corresponding to an elementary filter 10 is not perfect, in particular insofar as its stiffness is low. To obtain a filter which comes as close as possible to an ideal filter, that is to say having the greatest possible stiffness, a filter according to the invention comprises several elementary filters 10 in series as illustrated in particular Figure 2.

The generic dimensions of the inductance and capacitance sections, as well as those of the resonator 35, which acts as an impedance transformer, are adjusted so that the pass band of the filter (f _H -f _B ) produced by a elementary cell 10 is for example centered on the central frequency of the useful band of the radar.

Figure 6 shows, in a perspective view, another possible embodiment of a device according to the invention. This embodiment completes, for example, the device as presented above. In the embodiment of Figure 6, the previous coaxial structure 1 is arranged inside a set 61 of rectangular waveguides 62. This set consists for example of two parts, only one of which is represented in FIG. 6 for ease of presentation. The rectangular waveguides 62 are substantially perpendicular to the axis 31 of the coaxial waveguide 1. The rectangular waveguides 62 are for example machined from a block of metal. A part thus machined comprising a set of rectangular half-wave guides as illustrated in FIG. 6. The complete guides 62 are then obtained by the superposition of the two parts. In each part, a hollow in the shape of a half-cylinder is machined perpendicular to the rectangular waveguides 62 to place the coaxial structure 1. The filter as illustrated in FIG. 6 shows that the rectangular waveguides 62 are arranged perpendicularly on either side of the coaxial waveguide 1 along a plane passing through the axis of symmetry 31 of the latter. Furthermore in FIG. 6, the rectangular guides 62 are open at their ends.

Figure 7 shows, in a sectional view in the plane of symmetry of the filter, that the ends of the rectangular waveguides 62 opposite the coaxial structure 1 are closed by microwave loads 71. All of the waveguides rectangular 62 closed by the microwave charges 71 make it possible in particular to optimize the performance of the filter outside of its pass band, that is to say of improving the level of attenuation outside this band. This set makes it possible in particular to trap the harmonics of high orders or other parasitic frequencies, outside of the pass band, which the filter attenuates insufficiently, compared with a very severe level of performance requested. As illustrated in FIG. 7, a rectangular waveguide 62 is coupled to each elementary filter cell 10, more precisely, a rectangular waveguide 62 is placed on each side of an elementary filter cell 10. The harmonics not filtered by the coaxial filter structure as described in relation to FIGS. 2 and 3 are trapped in the rectangular waveguides 62. The microwave load 71 in fact plays the role of a microwave absorbent. The frequencies thus absorbed are eliminated, and therefore filtered. The passage of the harmonics, not sufficiently filtered by an elementary filtering cell 10, to its associated rectangular waveguides is done by microwave coupling. In particular, the dimensions of the rectangular guides 62 are provided as a function of the frequencies to be attenuated and of the aforementioned coupling. This coupling takes place via the external conductor 34 of the coaxial structure 1 which is in contact with the walls 63 of the rectangular guides 62. Preferably, a microwave load 71 has a triangular shape, the point of which is oriented towards the inside of the guide, the triangle also being symmetrical with respect to the central axis 64 of the guide. This makes it possible in particular to optimize the absorption of the waves picked up by the guide.

A filter according to the invention has very good electrical performance while having a reduced bulk and a compact structure. In particular, due to its propagation structure in coaxial technology, its insertion losses are very low. In addition, by its additional structure in rectangular waveguides, it has excellent properties of rejection of harmonic frequencies outside its bandwidth.

The invention advantageously applies to different types of frequency bands, it suffices to adapt it to a given band to dimension the coaxial structure on the one hand and possibly the rectangular guides on the other hand. The invention is moreover simple to implement insofar as it does not require particularly complex components or structures.

Claims (9)

1. Band-pass filter comprising one or more band-pass type elementary cells (10) placed in series, characterized in that each elementary cell includes:

   - a network providing low-pass filtering made up of an inductive section (32) and a capacitive section (33) in a coaxial structure;

   - and a quarter-wave resonator (35) in series providing high-pass filtering, placed in the middle of the capacitive section (33).
2. Filter according to Claim 1, characterized in that a low-pass network is made up of an inductive section (32) of diameter R₁ and a capacitive section (33) of diameter R₂, where R₂ is greater than R₁.
3. Filter according to either of the preceding claims, characterized in that the resonator (35) has the shape of a ring.
4. Filter according to Claim 3, characterized in that the resonator (35) has the shape of a double L in a view in section passing through the axis of symmetry (31) of the coaxial structure.
5. Filter according to any one of the preceding claims, characterized in that a sheath (36), made of an electrically insulating material, is placed between the quarter-wave resonator (35) and the outer conductor (34) of the coaxial structure.
6. Filter according to any one of the preceding claims, characterized in that the set of elementary cells (10) is arranged inside a set (61) of rectangular waveguides (62) closed by a microwave load (71), the rectangular waveguides (62) being arranged perpendicularly on either side of the elementary cells (10) according to a plane passing through their axis of symmetry (31), the rectangular waveguides picking up waves of frequencies present outside the passband of the filter.
7. Filter according to Claim 6, characterized in that the set (61) of rectangular guides is made up of two parts, between which the elementary cells (10) are sandwiched, the rectangular waveguides (62) being machined in a block of metal.
8. Filter according to either of Claims 6 and 7, characterized in that a rectangular waveguide (62) is coupled to each filtering elementary cell (10), in such a way that a rectangular waveguide (62) is placed on each side of an elementary cell (10).
9. Filter according to any one of Claims 6 to 8, characterized in that a microwave load (71) has a triangular shape the tip of which is oriented towards the inside of the rectangular guide, the triangle being symmetrical with respect to the central axis (64) of the guide.