EP0326498B1 - Resonant circuit and filter using it - Google Patents

Description

The present invention relates to a resonant circuit and a filter using this circuit. The technical field of the invention is radioelectricity, electronics, filtering and multiplexing of frequencies, etc.

The invention finds a particular application in the construction of stations for receiving television signals broadcast by satellites.

One of the problems posed in this technique is to produce, in the reception station, a filter operating in the frequency band 950-1750 MHz and making it possible to very easily rearrange the frequency plans, according to the availability of channels and user demand.

Among the many types of existing filters (of the type with coupled LC cells, helical, coaxial, quartz, waveguide, dielectric resonator, ...), there is none that has both a low cost, great ease of adjustment, good stability and an operating range from frequencies as low as a few tens of megahertz to frequencies above 2000 MHz.

The object of the present invention is precisely to remedy this deficiency by proposing a circuit and a filter which have all these advantages.

Ring resonators are known which operate on the principle of establishing a standing wave regime. A conductive tape (or microstrip) is used, the length of which is equal to the wavelength associated with the resonant frequency (or possibly at half the wavelength).

Such ring resonators are described for example in the article entitled "On the Study of Microstrip Ring and Varactor-Tuned Ring Circuits" published in the journal IEEE Transactions on Microwave Theory and Techniques, vol. MIT-35, n ° 12, December 1987, pp. 1288-1294 or even in the patent of the United States of America US-A-4,121,182 for "Electrical Tuning Circuit", or also in the patent of the United States of America US-A-4,641,116 for "Microwave Filter", or in the French patent application FR-A-2 248 621 for "Microwave device provided with a half-wave resonator" and finally in the European patent application EP-A-0 071 508 for "Microwave filter of small dimensions with linear resonators ".

In this prior art, the shape of the ribbon is limited to a few simple shapes capable of leading to a high overvoltage. There are therefore in practice always circular rings or possibly U. In addition, in this prior art, the dimension of the circuit is, in essence, of the order of the wavelength. As soon as the frequency becomes low, the dimensions of the resonator become prohibitive. For example, at 30 MHz a resonator of the prior art constituted by a circular ribbon will have a diameter of 1.60 m.

The object of the present invention is precisely to remedy these drawbacks. To this end, it recommends a resonant circuit still using a conductive tape but under very different operating conditions from those of the prior art. In the invention, the ribbon plays the role of pure inductance in the band of use of the circuit. To do this, its length is taken less than λ / 8, if λ is the wavelength associated with the working frequency of the resonator.

It should be noted that resonant loops comprising an interrupted microstrip loop of length λ / 8 and a capacitor in the opening are already known from the prior art, see for example the document "1986 IEEE-MTT-S INTERNATIONAL MICROWAVE SYMPOSIUM DIGEST , Baltimore, Maryland, June 2-4, 1986, pages 411-414, IEEE, New York, US; M. MAKIMOTO et al .: 'Varactor tuned bandpass filters using microstrip-line ring resonators' ".

The fact that the ribbon is a simple inductive element means that its shape is in no way critical. It is therefore possible to retain any desired shape, in particular shapes that allow the ribbon to be folded up to save space.

Furthermore, limiting the length of the ribbon below λ / 8 has the effect of reducing the size of the circuit. Combined with the folding ability, this arrangement makes it possible to obtain circuits of very small dimensions. So, to use the example of a 30 MHz resonant circuit, the invention makes it possible to build a 15 × 2 cm circuit, compared to the 1.60 m diameter ring of the prior art: the reduction in dimensions is therefore by a factor of 10.

If this reduction is particularly appreciable when the frequency is low, since it makes it possible to avoid filters of prohibitive size, it is not negligible however when the frequency is high, because then the filter then has such small dimensions that integration becomes possible.

The subject of the present invention is a filter comprising several resonant circuits as defined above. These circuits (identical or different) are coupled to each other. The coupling is tight, critical or loose depending on the case. As the shape of the ribbon is not critical, as has been pointed out above, we are free to choose the shape best suited to the chosen coupling.

The invention thus finds a wide field of application. Filters from 30 MHz to 2 or 3 GHz can be produced. Bandwidth ranges from a few fractions of a percent to about 10%.

• la figure 1 montre un circuit résonnant,
• la figure 2 illustre le schéma électrique équivalent du circuit résonnant,
• la figure 3 montre une variante à ruban triangulaire,
• la figure 4 illustre une variante à contour replié,
• la figure 5 montre un filtre selon l'invention dans sa totalité,
• la figure 6 donne les dimensions d'un filtre selon un exemple de réalisation,
• la figure 7 montre une caractéristique d'atténuation d'un filtre passe-bande conforme à l'invention, dans une plage allant de 1 MHz à 2000 MHz,
• la figure 8 montre l'atténuation de ce même filtre autour de la fréquence centrale, dans une bande de largeur 100 MHz,
• la figure 9 montre la caractéristique d'un filtre passe-bande selon l'invention autour de la fréquence centrale dans une bande de 40 MHz,
• la figure 10 montre une autre caractéristique (temps de groupe) d'un filtre passe-bande selon l'invention autour de la fréquence centrale, dans une bande de 100 MHZ.

In any case, the characteristics of the invention will appear better in the light of the description which follows. This description relates to examples given by way of non-limiting explanation and it refers to the attached drawings in which:

• FIG. 1 shows a resonant circuit,
• FIG. 2 illustrates the equivalent electrical diagram of the resonant circuit,
• FIG. 3 shows a variant with triangular ribbon,
• FIG. 4 illustrates a variant with a folded outline,
• FIG. 5 shows a filter according to the invention in its entirety,
• FIG. 6 gives the dimensions of a filter according to an exemplary embodiment,
• Figure 7 shows an attenuation characteristic a bandpass filter according to the invention, in a range from 1 MHz to 2000 MHz,
• FIG. 8 shows the attenuation of this same filter around the central frequency, in a band of width 100 MHz,
• FIG. 9 shows the characteristic of a bandpass filter according to the invention around the central frequency in a band of 40 MHz,
• FIG. 10 shows another characteristic (group time) of a bandpass filter according to the invention around the central frequency, in a band of 100 MHZ.

A resonant circuit according to the invention is shown in Figure 1, in top view (a), in section (b) and in a variant with a metal case (c). This element comprises a planar substrate 10, made of dielectric material (for example epoxy glass, Teflon, etc.). On the lower face of this substrate, in the variant of Figures (a) and (b), there is a conductive layer 12 (in copper for example) forming a ground plane and on the upper face, a microstrip 14 made of conductive material ( copper for example). In the variant of FIG. C, the circuit is arranged in a metal housing 20 and the ground plane is constituted by the metal walls 22 lower and upper. The microstrip draws an "open" outline in the sense that it incompletely surrounds part of the plane. In other words, it has at least one opening. In Figure 1, this outline is rectangular and the opening (single) is referenced 16. Connected through this opening is a capacitor 18 adjustable or adjusted once and for all.

The equivalent electrical diagram is shown in FIG. 2, again considering the element in plan view (a) and in section (b) in the variant where the ground plane is arranged under the substrate. In this figure, we see an inductance L, due to the non-rectilinear microstrip, a tuning capacitor Ca connected between the ends of the ribbon, and parasitic capacitors Cp, which correspond to the volume separating the microstrip and the ground plan.

The operation of this resonator is then as follows.

We denote by l the length of the microstrip and by λ the wavelength in the substrate at the operating frequency. The length l is always less than or equal to λ / 8 in order to be able to be assimilated to an inductor independent of the frequency.

Let fo be the central frequency of the filter to be produced; this frequency is determined by the classical relation:

où L est l'inductance et C la capacité équivalente du résonateur. $$\text{fo =}\frac{\text{1}}{\text{2π}\sqrt{\text{LC}}}$$

where L is the inductance and C the equivalent capacity of the resonator.

We start by determining a practically achievable value of L. For this, the wavelength λo in the substrate corresponding to fo is determined, then the value of λo / 8 and a microstrip length less than this value is chosen. The value of the inductance L can be obtained approximately by the formula:

$$\text{L = l ×}\frac{\text{Z}}{\text{v}}$$

• L est la valeur de l'inductance exprimée en Henry,
• l est la longueur du microruban, en mètre,
• Z est l'impédance de la ligne, en Ohm, et
• v est la vitesse de phase.

In this expression:

• L is the value of the inductance expressed in Henry,
• l is the length of the microstrip, in meters,
• Z is the line impedance, in Ohm, and
• v is the phase speed.

We also have:

expression dans laquelle :

• c est la vitesse de phase dans le vide (soit 3.10⁸ m/s),
• ε reff est la constante diélectrique efficace du substrat, soit

$$\text{v =}\frac{\text{vs}}{\sqrt{\text{ε reff}}}$$

expression in which:

• c is the phase velocity in a vacuum (i.e. 3.10⁸ m / s),
• ε reff is the effective dielectric constant of the substrate, is

où :

• εr est la constante diélectrique du substrat,
• h est l'épaisseur du substrat,
• W est la largeur du microruban.

$$\text{ε reff =}\frac{\text{εr + 1}}{\text{2}}\text{+}\frac{\text{εr - 1}}{\text{2}}\frac{\text{1}}{\sqrt{\text{1 +10}}\frac{\text{h}}{\text{W}}}$$

or :

• εr is the dielectric constant of the substrate,
• h is the thickness of the substrate,
• W is the width of the microstrip.

The parasitic capacitance Cp value can be obtained by a formula of the type:

où H désigne un coefficient, fonction de la géométrie du circuit. $$\text{Cp ≃ 17.7 εr. H (}\frac{\text{2h}}{\text{W}}\text{)}$$

where H indicates a coefficient, function of the geometry of the circuit.

The value of the tuning capacity Ca to be placed between the two ends of the ribbon is deduced from formula (1), knowing that:

$$\text{C = Ca +}\frac{\text{<figure-callout id="2" label="Cp" filenames="imgf0002.png,imgf0004.png" state="{{state}}">Cp</figure-callout>}}{\text{2}}$$

Thus, the parameters of the resonant circuit are defined. The length of the coupled microstrip and the degree of coupling are determined experimentally.

When the frequency fo increases, the value and the dimensions of the elements L and C decrease and one encounters a limit in the practical realization of the filter. However, this limit can be exceeded if the capacity is distributed along the microstrip.

In Figure 3, for example, we see that the outline (which is triangular) drawn by the microstrip has two openings 16, 16 ′ (instead of only one). Each can be fitted with an adjustable capacitor 18, 18 ′. Each capacitor can then have a capacity of 2 Ca.

Of course, more than two capacitors could be used, if necessary.

To further push these limits, we can use a substrate with a low dielectric constant (relations 2-3 and 4 show indeed that if εr decreases εreff decreases, v increases and L decreases for a given length l).

Conversely, when the frequency fo decreases, the value and the dimensions of the elements L and C increase and another limit is reached in the practical realization of the filter. To cross it, it is possible to reduce the dimensions of the cell by folding up the microstrip as illustrated in FIG. 4. But this solution reduces the length of coupling with the next cell.

FIG. 5 shows a complete filter composed of five circuits C1 to C5 on a single substrate 10 with an input microstrip E and an output microstrip S.

It will also be noted that the filter of the invention uses a direct coupling at the input and output which performs the adaptation between the first and the last resonators and the circuits of use.

In the case illustrated in FIG. 5, the contours drawn by the microstrip are rectangular, two adjacent contours having two parallel sides. But one could also use triangular circuits for example mounted head to tail.

In another variant, the circuits can be nested one inside the other.

Figure 6 gives the main dimensions of an example filter designed to have a center frequency fo equal to 1131.620 MHz. Dimensions are in millimeters. The capacitors are adjustable from 0.5 to 5 pF. The substrate is made of 16/10 mm thick epoxy glass.

In this exemplary embodiment, the contours drawn by the microstrips are U-shaped, that is to say rectangles with one side missing. The orientation of these U alternates from one resonator to another, so that the capacitors are placed on both sides of the filter, sometimes at the top, sometimes at the bottom (in the sense of figure 6).

Figures 7, 8 and 9 show the attenuation characteristic of a filter obtained according to the invention, with different frequency scales (on the abscissa). In FIG. 7, the curve goes from 1 to 2000 MHz; in FIG. 8, the width of the measurement band is 100 MHz; in Figure 9, it is 40 MHz.

The curve in FIG. 10 represents the group time characteristic of the filter in nanoseconds by division.

Naturally, if the foregoing description emphasizes rectangular contours, this is only for explanatory purposes. Any other shape is possible: triangular, circular, elliptical, diamond, etc.

Claims (3)

1. A filter with flat resonators, comprised of at least two coupled resonators, each resonator comprising:

   - a conductor (12) forming an earthing plane,

   - opposite this earthing plane, a conductor micro tape (14) disposed on one of the faces of a dielectric substrate, this micro tape defining a contour having a first side interrupted by an opening (16), a second and a third side which are uninterrupted and are adjacent to the first side, and possibly a fourth side facing the first side, the length of the micro tape being less than λ/8, where λ is the wavelength associated with the resonance frequency of the resonator,

   - a capacitor (18) connected across the opening (16) interrupting the first side,

   this filter being characterised in that:

   a) it comprises more than two coupled resonators disposed in cascade (C1, C2, C3, C4, C5),

   b) the coupled resonators are disposed such that the second and third uninterrupted sides of two adjacent resonators are placed side by side, the electrical coupling between two adjacent resonators (C1, C2), (C2, C3), (C3, C4), (C4, C5) being effected by the second and third sides; the first sides of the resonators with their capacitors as well as the possible fourth sides being disposed along the longitudinal sides of the filter alternately on one of the longitudinal sides then the other,

   c) the first resonator (C1) of the filter is connected to a general input by an input micro tape (E) connected directly to the micro tape of the first resonator, this input micro tape (E) producing an adaptation between the first resonator (C1) and load circuits,

   d) the last resonator (C5) of the filter is connected to a general output by an output micro tape (S) connected directly to the micro tape of the last resonator, this output micro tape (S) producing an adaptation between the last resonator (5) and load circuits.
2. A filter according to claim 1, characterised in that each micro tape only comprises three sides and is triangular in shape, the different triangles of the different resonators being mounted head to tail.
3. A filter according to claim 1, characterised in that each micro tape comprises a folded part.

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