EP0649571B1 - Pass-band filter with coupled resonators - Google Patents

Abstract

According to the invention, each elementary cell comprises two resonators (R1, R2). Each resonator (R1, R2) comprises a microband (MR1, MR2) of which one branch (L1, L2) is used for coupling and has one end (e1, e2) connected to frame (22) and a capacitor (C1, C2) which is also connected to frame (22). The cells are connected directly to each other. Application to the fabrication of pass-band filters, particularly in the FM band.

Description

Technical area

The present invention relates to a filter bandpass with coupled resonators. She finds a application in electronics, especially in realization of band-pass filters whose frequency of work is located in the FM band, or substantially from 70 to 120 MHz.

State of the art

The filter of the invention is of the resonator type coupled. The French request FR-A-2-626-716 (or the request corresponding European EP-A-0 326 498) describes a filter with coupled resonators which is illustrated on the figure 1. As shown, this filter includes five resonators C1 to C5 deposited on the same substrate 10. Each resonator includes a conductive microstrip line 14 (in copper for example) forming a loop with an opening 16. Connected through this opening there is a capacitor 18 adjustable, or adjusted once for all. The whole line and capacitor forms an LC resonant circuit. The length of the microstrip is of the order of λ / 8 if λ is the length wave associated with the resonant frequency of the circuit.

The substrate 10 is made of dielectric material (by example in epoxy glass, Teflon, ...). On the face bottom of this substrate is a conductive layer not shown (in copper for example) forming ground plan.

The different resonators are coupled together to the others by parallel and adjacent sides.

The filter is completed by an input microstrip E and an output microstrip S.

Such filters work in the frequency band ranging appreciably from 950 to 1750 MHz, in particular in signal receiving stations television broadcast by satellite.

Although satisfying in some respects, these filters have drawbacks. First, their dimensions increase rapidly when the frequency of work decreases (since these dimensions are of the order of a fraction of the wavelength). They would become prohibitive in the FM band, around 100 MHZ.

Then, they present a significant attenuation at the center of the bandwidth, on the order of 6dB, attenuation which increases further if the width of band decreases.

Finally, they are difficult to simulate and calculate, due to the multiple couplings between cells, always difficult to quantify exactly.

The document entitled "Kammleitungsfilter aus gekoppelten Mikrostreifenleitungen "by Von Roland Briechle, published in the journal FREQUENZ, vol. 30, n ° 8, August 1976 describes a filter comprising a large number resonators each consisting of a microstrip playing the role of inductance and a capacitor, both united to the mass. Each resonator thus forms a "finger" and the complete filter consists of a large number of such fingers, coupled to each other the along the microstrips.

In addition, this filter requires circuits input and output adaptation.

Such a filter causes losses due to coupling between resonators and it's very difficult to simulate, because no software is capable of simulating so many and, for some, also distant. As soon as we change the number of resonators, for example to vary the band busy, the couplings change and you have to start again all the simulation.

Even if we reduced the number of resonators to its theoretical minimum which is 3, we would obtain another complex filter with an input finger, a output finger and an intermediate finger, the assembly being difficult to simulate.

These filters therefore do not resolve either the problem of simulation and computation.

Statement of the invention

The purpose of the present invention is to remedy these drawbacks. To this end, it proposes a filter with reduced dimensions (practically by a factor of 10 compared to the filter of the document FR-A-2 626 716 already cited) and fall to approximately λ / 100. The filter can therefore work at frequencies falling below 500 MHz. In addition, the filter of the invention has very low losses, of the order of 2dB. Its bandwidth can be adjusted between a narrow band (2%) and a wide band (40%).

All these results are obtained thanks to a structure particular which exclusively includes two coupled resonators constituting a filtering. According to the invention, the resonators of a cell are each formed by a line at microstrip playing, at working frequency, essentially the role of an inductor and by a tuning capacitor. It is therefore still a LC type resonator. But, according to a first characteristic of the resonator, the line is not in shape loop with opening. The tuning capacitor is therefore not inserted into an opening but connected at one end of the line and it has a armature to electrical ground. According to another characteristic, the line includes a part (or branch) straight line used to couple the two resonators of the same cell and, to do this, the two rectilinear branches specific to the two resonators are juxtaposed. The length of these branches, as well that their width makes it easy to adjust the coupling to the appropriate value.

Such a cell with exclusively two resonators is not contemplated in the prior art, even in the FREQUENZ document cited above, where the number of fingers is always very important and in any state of cause, cannot be less than 3.

The invention therefore already covers a cell elementary of the type described. But it also covers the if the filter includes several cells elementary, connected directly to each other others consecutively, access specific to the second resonator of a cell being connected to the clean access to the first resonator in the next cell. The losses due to cascading are thus reduced to their minimum compared to the filter of the FREQUENZ document already mentioned where the cascading of resonators was done by coupling. A total number of equal resonators, the filter of the invention has lower losses and its simulation is found greatly simplified.

• un ligne à microruban conducteur jouant, à la fréquence de travail, le rôle d'une inductance, cette ligne comprenant une première partie rectiligne représentant une partie du microruban, cette première partie ayant une extrémité reliée à une masse électrique, les deux premières parties propres à deux résonateurs d'une même cellule étant juxtaposées et assurant le couplage entre les résonateurs, la ligne comprenant encore une seconde partie cette seconde partie présentant une extrémité,
• un condensateur d'accord ayant une armature reliée à l'extrémité de la seconde partie et une autre armature reliée à la masse électrique,
• un accès situé en un point entre l'extrémité de la seconde partie et le condensateur d'accord la seconde partie de chaque microruban faisant un certain angle avec la première partie, les deux secondes parties des microrubans des deux résonateurs faisant entre elles un angle double.

Specifically, the present invention therefore relates to a bandpass filter with coupled resonators, characterized in that it comprises at least one elementary filtering cell, each elementary cell being formed of resonators exclusive of two, ci -after designated first and second resonator, each resonator of a cell comprising:

• a conductive microstrip line playing, at the working frequency, the role of an inductor, this line comprising a first rectilinear part representing a part of the microstrip, this first part having one end connected to an electrical ground, the first two clean parts with two resonators of the same cell being juxtaposed and ensuring coupling between the resonators, the line further comprising a second part, this second part having one end,
• a tuning capacitor having an armature connected to the end of the second part and another armature connected to the electrical ground,
• an access located at a point between the end of the second part and the tuning capacitor, the second part of each microstrip making an angle with the first part, the two second parts of the microstrips of the two resonators making a double angle between them .

The filter of the invention can comprise several cells like this, in which case two cells are connected directly to one the other, the proper access to the second resonator of a cell being connected to its own access to the first resonator of the next cell.

If the first part or branch, used for coupling of the two resonators of the same cell, is necessarily straight, the rest of the line at microstrip, that is to say the second branch, can have any shape and layout: inclined, at right angles, etc.

The microstrip line can therefore have various shapes in L, Γ, etc ...

As for the widths of the microstrip branches, they are not necessarily identical. They may be different from each other. They can even vary gradually, or by jumps, along of the same branch.

All known or future techniques allowing to make microstrips are applicable to the invention: use of a dielectric substrate, triple plate technology, suspension in a box, existence of a ground plane under a substrate or use metal walls of a housing to constitute ground, etc. The use of a dielectric at high permittivity makes it possible to reduce the dimensions. But, if these become too weak, the use of air as a dielectric allows find reasonable dimensions.

Brief description of the drawings

• Figure 1, already described, shows a filter according to the prior art with coupled cells;
• Figure 2 shows schematically a filter according to the invention with a single cell;
• Figure 3 illustrates an embodiment with fully rectilinear microstrip;
• FIG. 4 illustrates an embodiment with microstrip with inclined branches;
• FIG. 5 illustrates an embodiment with microstrip with width coupling branch variable;
• FIG. 6 illustrates an embodiment microstrip with second width branch variable;
• Figure 7 shows a mask for the realization a two-cell filter;
• Figure 8 is an electrical diagram of a two cell filter;
• Figure 9 shows the bandwidth obtained with the filter of figure 8, in a range from 78 to 118 MHz;
• Figure 10 shows the filter attenuation as a function of frequency, within a range ranging from 1 to 200 MHz;
• Figure 11 shows the attenuation towards the high frequencies up to 2000 MHz;
• Figure 12 shows the standing wave rate in a frequency range from from 1 to 200 MHz.

Detailed description of embodiments

We see, in Figure 2, a dielectric substrate 20 on the underside of which a metal layer 22 has been filed to constitute a ground plan. Sure the upper side, there are two resonators R1, R2 each consisting of a microstrip MR1, MR2, and a capacitor C1, C2. Each microstrip includes a first part (or branch) rectilinear L1 (respectively L2) and a second part (or branch) L'1 (L'2) which, in the illustrated variant, forms, with the part L1 (L2), a Γ. The end e1 (e2) of the branch L1 (L2) is connected to the ground plane 22 by a 24/1 conductor pad and passage (24/2). The end e'1 (e'2) of the branch L'1 (L'2) is connected to one of the plates of a capacitor C1 (C2), the other capacitor armature being connected to the ground plane 22 by a stud and a conductive passage 26/1 (26/2).

Optionally, a single conductive pad and a single conductive passage can be used to bring together the ends e1, e2 to the ground plane. The lines are therefore well short-circuited at one of their ends.

The cell entry E takes place between C1 and L'1 and the exit S between C2 and L'2. Naturally, the device is symmetrical and we can enter S and exit at E.

All of these means form a C cell.

Figures 3, 4, 5 and 6 illustrate some modes of the various branches of the microstrip.

In FIG. 3, first of all, the branches L1 (L2) and L'1 (L'2) are an extension of the other and the microstrip is straight. There is more to strictly speaking of the second part, the first can be considered as forming the entire ribbon.

In FIG. 4, the branches L'1 (L'2) not serving not at coupling are tilted by a certain angle () on the branches L1 (L2) used for coupling. Branches L'1, L'2 thus form, between them, an angle double (2). We can take for example  = 45 °, at which the branches L'1, L'2 are at right angles.

But, we could also take  = 90 °, in which case the branches L'1, L'2 would be an extension from each other and would form an L with the branch of coupling L1, L2.

In FIG. 5, the coupling branch L1 (L2) sees its width increase from one end (in this case the one that is grounded) to the other, the reverse also being possible.

In FIG. 6, it is the branch L'1 which sees its width increase.

Figures 7 and 8 illustrate an embodiment particular of a filter according to the invention in the case where this filter comprises two cells.

Figure 7 first shows the mask used to constitute the printed circuit on the face top of the substrate. This mask is shown at scale 3 which allows to appreciate the dimensions of the inventive filter. This mask includes two parts symmetrical about a point O. Each part includes an ME entry access strip and MS outlet, and two juxtaposed T-bands forming a set M1,2 (M3,4) which will correspond to the two cells.

Figure 8 shows the corresponding electrical diagram in FIG. 7, once the capacitors C1, C2, C3, C4 have been reported. We recognize the resonators coupled R1 and R2 for the first cell C1-2 and the coupled resonators R3, R4 for the second C3-4. The coupled branches are respectively L1, L2 for the first cell and L3, L4 for the second.

In the diagram of Figure 8, we see that each cell is directly connected to the next. The connection ribbon is referenced 30. There is therefore no longer coupling, as in the prior art, but a simple serialization.

In addition, the two cells are arranged so that they are as far apart as possible each other to avoid any coupling between they. Thus, the second C3-4 cell is not arranged following on from the first C1-2, but placed symmetrically to the element 30.

If the filter included more than two cells, it would always be so, with alternating cells sometimes oriented in one direction sometimes in the other to form a cascade of staggered cells.

Figures 9 to 12 illustrate the performance of the filter in Figures 7 and 8.

Figure 9, first, shows the attenuation of the filter in the band from 78 to 118 MHz. We see that the attenuation at the center of the bandwidth is very low (around 2 dB).

Figure 10 gives the same attenuation but on a wider frequency range from 1 MHz to 200 MHz.

Figure 11 shows the high attenuation frequencies, up to 2000 MHz. We see a peak appear parasitic but with considerable attenuation of around 70 dB. This peak therefore does not matter in the practice.

Finally, Figure 12 shows the standing wave rate (TOS) as a function of frequency. In the bandwidth, this rate drops to around -22 dB.

Claims (9)

1. Band-pass filter with coupled resonators, characterized in that it comprises at least one elementary filtering cell, each elementary cell being formed by a number of resonators exclusively equal to two, hereafter referred to as the first and second resonator, each resonator of a cell comprising:

   a conducting microstrip line (MR1, MR2) which, at the working frequency, fulfils the function of an inductor, this line comprising a straight first part (L1, L2) representing a part of the microstrip, this first part (L1, L2) having an end (e1, e2) connected to an electrical reference earth (22), the two first parts (L1, L2) belonging to the first and second resonators (R1, R2) in the same cell (C) being juxtaposed and providing coupling between the first and second resonators (R1, R2), the line furthermore comprising a second part (L'1, L'2), this second part having an end (e'1, e'2),

   a tuning capacitor (C1, C2) having one plate connected to the end (e'1, e'2) of the second part and another plate connected to the electrical reference earth (22),

   an access (E, S) at a point lying between the end (e'1, e'2) of the second part and the tuning capacitor (C1, C2), the second part (L'1, L'2) of each microstrip making a certain angle () with the first part (L1, L2), the two second parts (L'1, L'2) of the microstrips of the two resonators making a double angle (2) between them.
2. Band-pass filter according to Claim 1, characterized in that it comprises a plurality of elementary filtering cells connected directly to one another consecutively, the access belonging to the second resonator in a cell being connected to the access belonging to the first resonator in the following cell.
3. Band-pass filter according to Claim 1, characterized in that the angle () which the second part (L'1, L'2) makes with respect to the first (L1, L2) is equal to 90°, each microstrip having an L-shape.
4. Band-pass filter according to Claim 1, characterized in that the microstrip has a Γ-shape.
5. Band-pass filter according to Claim 1, characterized in that the first part (L1, L2) of the microstrip has a variable width.
6. Band-pass filter according to Claim 1, characterized in that the second part (L'1, L'2) of the microstrip has a variable width.
7. Band-pass filter according to any one of Claims 1 to 5, characterized in that the ends (e1, e2) of the first parts (L1, L2) of the two microstrips belonging to two coupled resonators (R1, R2) are connected to one and the same electrical reference earth (24/1, 24/2).
8. Band-pass filter according to Claim 1, characterized in that the successive cells (C1-2, C3-4) are mounted in a zigzag.
9. Band-pass filter according to any one of Claims 1 to 8, characterized in that it operates in a frequency band centred at close to 100 MHz.

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