EP0649571A1

EP0649571A1 - Pass-band filter with coupled resonators

Info

Publication number: EP0649571A1
Application number: EP94915581A
Authority: EP
Inventors: Henri Havot; Yvon Dutertre
Original assignee: Telediffusion de France ets Public de Diffusion; France Telecom SA
Current assignee: Telediffusion de France ets Public de Diffusion; Orange SA
Priority date: 1993-05-04
Filing date: 1994-05-03
Publication date: 1995-04-26
Anticipated expiration: 2014-05-03
Also published as: DE69420924D1; FI950033A0; FI950033A; FR2704983B1; FI115332B; WO1994025996A1; FR2704983A1; DE69420924T2; EP0649571B1

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

BANDPASS FILTER WITH COUPLED RESONATORS

DESCRIPTION

Technical area

The present invention relates to a bandpass filter with coupled resonators. It finds an application in electronics, in particular in the production of bandpass filters whose working frequency is located in the FM band, that is to say substantially from 70 to 120 MHz.

State of the art

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

The substrate 10 is made of dielectric material (for example epoxy glass, Teflon, etc.). On the underside of this substrate is a conductive layer (not shown in copper for example) forming a ground plane. The different resonators are coupled to each other 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 stations for receiving television signals. broadcast by satellites.

Although satisfactory in certain respects, these filters have drawbacks. First, their dimensions increase rapidly when the working frequency 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 have a non-negligible attenuation at the center of the bandwidth, of the order of 6 dB, attenuation which increases further if the bandwidth decreases.

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

The document entitled "Kam leitungsfilter aus gekoppelten Mikrostreifenleitungen" by Von Roland Briechle, published in the review FREQUENZ, vol. 30, no.8. August 1976, described a filter comprising a large number of resonators each consisting of a microstrip playing the role of inductance and a capacitor, both joined to ground. Each resonator thus forms a "finger" and the complete filter consists of a large number of such fingers, coupled to one another along the microstrips.

In addition, this filter requires input and output matching circuits. Such a filter causes losses due to the coupling between resonators and it is very difficult to simulate, because no software is capable of simulating such numerous couplings and, for some, so distant. As soon as the number of resonators is changed, for example to vary the bandwidth, the couplings change and the entire simulation must be repeated.

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

These filters do not therefore solve the problem of simulation and calculation either.

Statement of the invention

The object of the present invention is precisely to remedy these drawbacks. To this end, it offers a filter whose dimensions are reduced

(practically by a factor of 10 compared to the filter of 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. Furthermore, the filter of the invention has very low losses, of the order of 2 dB. Its bandwidth can be adjusted between a narrow band (2%) and a wide band (40%).

All these results are obtained thanks to a particular structure which exclusively comprises two coupled resonators constituting a filtering cell. According to the invention, the resonators of a cell are each constituted by a microstrip line playing, at the working frequency, essentially the role of an inductor and by a tuning capacitor. It is therefore still an LC type resonator. But, according to a first characteristic of the resonator, the line is not in the form of a loop with opening. The tuning capacitor is therefore not inserted into an opening but connected to one of the ends of the line and it has an armature to the electrical ground. According to another characteristic, the line comprises a straight portion (or bran¬ che) used to couple together the two resonators of the same cell and, to do this, the two straight branches specific to the two resonators are juxtaposed. The length of these branches, as well as their width, makes it easy to adjust the coupling to the appropriate value. Such a cell with exclusively two resonators is not envisaged in the prior art, even in the FREQUENZ document cited above, where the number of fingers is always very large and, in any event, cannot be less than 3 The invention therefore already covers an elementary cell of the type described. But it also covers the case where the filter comprises several elementary cells, connected directly to each other consecutively, the access specific to the second resonator of a cell being connected to the access specific to the first resonator of the cell which follows . The losses due to the cascading are thus reduced to their minimum compared to the filter of the FREQUENZ document already cited where the cascading of the resonators was carried out by coupling. With an equal total number of resonators, the filter of the invention has lower losses and its simulation is greatly simplified. 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 line with a conductive microstrip playing, at the working frequency, the role of an inductor, this line comprising a first rectilinear part representing at least one part of the microstrip, this first part having one end connected to an electrical ground, both first parts specific to two resonators of the same cell being juxtaposed and ensuring coupling between the resonators, the line further comprising a second part if the first does not constitute the whole of the line, 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 filter of the invention can comprise several cells of this kind, in which case two consecutive cells are connected directly to each other, the access specific to the second resonator of a cell being connected to the access specific to the first resonator of the next cell. If the first part or branch, used to couple the two resonators of the same cell, is necessarily rectilinear, the rest of the microstrip line (if the first part does not constitute the entire line), i.e. -to say the second branch, can have any shape and arrangement: inclined, at right angles, in continuation of the first, etc. The microstrip line can therefore have various shapes in L, T, etc.

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

All known or future techniques allowing so many microstrips to be applied to the invention: use of a dielectric substrate, triplate technology, suspension in a housing, existence of a ground plane under a substrate or use metal walls of a housing to kill mass, etc. The use of a high permittivity dielectric makes it possible to reduce the dimensions. But, if these become too weak, the use of air as a dielectric makes it possible to 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 schematically shows a filter according to the invention with a single cell;

- Figure 3 illustrates an embodiment with a fully rectilinear microstrip;

- Figure 4 illustrates an embodiment with microstrip with inclined branches;

FIG. 5 illustrates an embodiment with a microstrip having a coupling branch of variable width;

- Figure 6 illustrates an embodiment with microstrip with second branch of variable width;

- Figure 7 shows a mask for the realization of 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 attenuation of the filter as a function of frequency, in a range from 1 to 200 MHz;

- Figure 11 shows the attenuation towards high frequencies up to 2000 MHz;

- Figure 12 shows the standing wave rate in a frequency range 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. On the upper side, there are two resonators RI, R2 each consisting of a microstrip MR1, MR2, and a capacitor Cl, C2. Each microstrip comprises a first straight part (or branch) L1 (respectively L2) and a second part (or branch) The I (L'2) which, in the illustrated variant, forms, with the part L1 (L2), a T. The end el (e2) of the branch L1 (L2) is connected to the ground plane 22 by a stud and a conductive passage 24/1, (24/2). The end e'I (e'2) of the branch I (L'2) is connected to one of the armatures of a capacitor Cl (C2), the other armature of the capacitor being connected to the ground plane 22 by a pad and a conductive passage 26/1 (26/2).

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

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

All of these means form a C cell.

Figures 3, 4, 5 and 6 illustrate some embodiments of the different branches of the microstrip. In FIG. 3, first of all, the branches Ll

(L2) and I (L'2) are in line with each other and the microstrip is straight. There is no longer, strictly speaking, a second part, the first being able to be considered as forming the entire ribbon.

In FIG. 4, the branches I (L'2) not used for coupling are inclined by a certain angle (θ) on the branches L1 (L2) used for coupling. The branches I, L'2 thus form, between them, an angle double (2Θ). We can take for example θ≈45 °, in which case the branches I, L'2 are at right angles.

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

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

In FIG. 6, it is the branch I which sees its width increase.

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

FIG. 7 firstly shows the mask used to constitute the printed circuit on the upper face of the substrate. This mask is shown on a scale of 3, which makes it possible to appreciate the reduced dimensions of the filter of the invention. This mask comprises two parts which are symmetrical with respect to a point O. Each part comprises an entry access strip ME and exit MS, and two juxtaposed T strips forming a set Ml, 2 (M3,4) which will correspond to both cells.

Figure 8 shows the electrical diagram corresponding to Figure 7, once the capacitors C1, C2, C3, C4 have been reported. We recognize the coupled resonators RI and R2 for the first cell Cl-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 any coupling, as in the prior art, but a simple serialization.

Furthermore, the two cells are arranged in such a way that they are as far apart as possible from one another to avoid any coupling between them. Thus, the second cell C3-4 is not available in the extension of the first Cl-2, but placed symmetrically with respect to the element 30.

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

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

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

Figure 10 gives the same attenuation but over a wider frequency range, from 1 MHz to 200 MHz. Figure 11 shows the attenuation towards high frequencies, up to 2000 MHz. We see a parasitic peak appear but with a considerable attenuation of the order of 70 dB. This peak therefore does not matter in practice. Finally, Figure 12 shows the standing wave rate (TOS) as a function of frequency. In bandwidth, this rate drops to around -22 dB.

Claims

1. 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, hereinafter designated by first and second resonator, each resonator of a cell comprising: - a conductive microstrip line (MR1, MR2) playing, at the working frequency, the role of an inductor, this line comprising a first rectilinear part (Ll, L2) re¬ having at least a part of the microrub, this first part (L1, L2) having one end (el, e2) connected to an electrical ground (22), the first two parts (L1, L2) specific to the first and to the second resonators (RI , R2) of the same cell (C) being juxtaposed and ensuring the coupling between the first and the second resonators (RI, R2), the line still comprising a second part (I, L'2) if the first does not is not the whole line), this second p part with one end (e'I, e'2),

a tuning capacitor (Cl, C2) having a frame connected to the end (e'I, e'2) of the second part and another frame connected to the electrical ground (22),

- an access (E, S) at a point located between the end (e'I, e'2) of the second part and the tuning capacitor (Cl, C2).

2. Bandpass filter according to claim 1, characterized in that it comprises several elementary filter cells connected directly to each other consecutively, the proper access to the second resonator of a cell being connected to the specific access to the first resonator of the cell that follows.

3. Bandpass filter according to claim 1, characterized in that the second part (I,

L'2) of each microstrip makes a certain angle (θ) with the first part (L1, L2), the two second parts (I, L'2) of the microstrips of the two resonators making a double angle between them (2θ ).

4. Bandpass filter according to claim 3, characterized in that the angle (θ) made by the second part (I, L'2) relative to the first (Ll, L2) is equal to 90 °, each microstrip having an L shape.

5. Bandpass filter according to claim 3, characterized in that the angle (θ) made by the second part (I, L'2) relative to the first (Ll, L2) is zero, the second part (I, L'2) being an extension of the first (L1, L2), each microstrip having a rectilinear shape.

6. Bandpass filter according to claim 1, characterized in that the microstrip has a T shape.

7. Bandpass filter according to claim 1, characterized in that the first part (L1, L2) of the microstrip has a variable width.

8. Bandpass filter according to claim 1, characterized in that the second part (I, L'2) of the microstrip has a variable width.

9. Bandpass filter according to any one of claims 1 to 7, characterized in that the ends (el, e2) of the first parts (L1, L2) of the two microstrips specific to two coupled resonators (RI, R2) are connected to a single electric mass (24/1, 24/2).

10. Bandpass filter according to claim 1, characterized in that the successive cells (Cl-2, C3-4) are mounted in staggered rows.

11. Band-pass filter according to any one of claims 1 to 10, characterized in that it operates in a frequency band centered around 100 MHz.