EP0326498A1 - Resonant circuit and filter using it - Google Patents

Abstract

The resonant circuit comprises, on a dielectric substrate (10), conductive microstrips (14) each describing an open contour, a capacitor (18) being connected across each opening. The length of the microstrip is less than lambda /8. Application to radio and in particular in the construction of filters for stations for receiving satellite broadcast television signals. <IMAGE>

Description

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 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 also 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. Furthermore, 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 prior art resonator 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.

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 however negligible when the frequency is high, because then the filter then has such small dimensions that integration becomes possible.

The present invention also relates to 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,
• - Figure 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 flat 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 (made of 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 case 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 Figure 2 while still considering the element in top 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.

The length of the microstrip is designated by l 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.

où L est l'inductance et C la capacité équivalente du résonateur.Let fo be the central frequency of the filter to be produced; this frequency is determined by the classical relation:

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 λ o / 8 and a microstrip length less than this value is chosen. The value of the inductance L can be obtained approximately by the formula:

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.

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,

où :
- εr est la constante diélectrique du substrat,
- h est l'épaisseur du substrat,
- W est la largeur du microruban.We also have:

expression in which:
- c is the phase speed in a vacuum (i.e. 3.10⁸ m / s),
- ε reff is the effective dielectric constant of the substrate,

or :
- εr is the dielectric constant of the substrate,
- h is the thickness of the substrate,
- W is the width of the microstrip.

)      (5)
où H désigne un coefficient, fonction de la géométrie du circuit.The parasitic capacitance Cp value can be obtained by a formula of the type:
Cp ≃17.7 εr. H (

) (5)
where H denotes a coefficient, a function of the geometry of the circuit.

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

(6)

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 provided 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 we could also use triangular circuits for example mounted head to tail.

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

FIG. 6 gives the main dimensions of an example of a filter designed to have a central 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 Figure 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 (6)

1. Resonant circuit including:
- a conductor (12) forming a ground plane,
- opposite this ground plane, a conductive microstrip (14) deposited on one of the faces of a dielectric substrate, this microstrip drawing an outline having an opening,
- a capacitor (18) connected through the opening (16) of the contour,
characterized in that the length of the microstrip is less than λ / 8, where λ is the wavelength associated with the resonant frequency. 2. Resonant circuit according to claim 1, characterized in that the conductor forming a ground plane is a conductive layer deposited on the other face of the substrate (10) made of dielectric material. 3. Resonant circuit according to claim 1, characterized in that it is disposed in a conductive housing (20) and that the conductor forming a ground plane is constituted by the housing (20-22). 4. Resonant circuit according to claim 1,
characterized by the fact that the contour has at least part of a shape taken from the group which includes rectangular, triangular, circular, elliptical, lange, U-shaped, etc. shapes 5. Resonant circuit according to claim 1, characterized in that the contour has several openings (16, 16 ′) each provided with a capacitor (18, 18 ′). 6. Filter with planar resonators, characterized in that it comprises several resonant circuits according to any one of claims 1 to 6, coupled together.

Images