EP0337825B1 - Microstrip fashion microwave rejection filter - Google Patents

Description

The present invention relates to a microwave band cut filter in micro-band technology.

Tunable microwave notch filters are used in particular in instantaneous very broadband microwave receivers generally having numerous signals to be processed simultaneously, typically radar signal receivers.

Because of the very large instantaneous band, the signals are very often disturbed by the presence of strong parasitic signals which saturate the reception chains. The function of a notch filter is thus to attenuate the disturbing signals in order to be able to analyze and identify the signals of lower amplitudes.

These disturbing signals generally having neither a frequency known in advance nor a stable frequency, it is necessary to provide a notch filter which is tunable.

Hitherto, a YIG filter (Yttrium-Iron Garnet: yttrium-iron garnet) has been suitably polarized for this purpose to tune it to the frequency to be eliminated.

This technique, despite its interesting performance (high rejection rate, low attenuated bandwidth), has the drawback of significantly increasing the dimensions and mass of the circuit and presenting a transition time ("rallying time"). ) relatively long, on the order of 10 to 20 ms, and to require a complex control circuit.

To at least partially remedy the aforementioned drawbacks, tunable microwave notch filters have also been proposed based on the so-called coupled line structure, that is to say comprising a transmission line in the form of a microstrip associated with at least one filter cell comprising a microstrip segment arranged parallel to and at a distance from the transmission line.

Such a filter structure is described more precisely in the article by M. Mehdizadeh et al. "High speed varactor tuned notch filter" 1985 IEEE - MTT-S International Microwave Symposium Digest, June 4-6, 1985, St. Louis, Missouri, pages 531-534. In this article, a coupled line filter is described where each line segment is connected to the ground potential at both ends, a varactor being inserted in series at one end to form a resonator. By adjusting the capacity of the varactor, resonance is obtained when this capacity is adapted to the inductive reactance of the line segment. However, the tuning range obtained is limited and its center frequency is not easily adjustable.

The object of the invention is to remedy these drawbacks.

• faibles pertes d'insertion hors bande réjectée,
• fonctionnement en large bande, compatible avec les performances des chaînes de réception actuelles (typiquement, largeur de bande de 2 à 18 GHz),
• plage d'accord en fréquence importante,
• réjection élevée à la fréquence d'accord,
• système d'accord sans consommation électrique,
• temps de ralliement très faible,
• dimensions très réduites, permettant une intégration aisée en micro- électronique.

More precisely, as will be seen below, the invention makes it possible to combine the following advantages:

• low rejection out of band rejections,
• broadband operation, compatible with the performance of current reception channels (typically, bandwidth from 2 to 18 GHz),
• large frequency tuning range,
• high rejection at the tuning frequency,
• tuning system without power consumption,
• very short rally time,
• very small dimensions, allowing easy integration into microelectronics.

Characteristically of the present invention, the connection to the ground potential is carried out for each cell with the interposition of a tunable LC resonant circuit and the other end of the line segment is in open circuit.

Very advantageously, the capacitive element of the tunable resonant circuit LC comprises a varactor, the anode of which is brought to an adjustable continuous potential, so that the control of this continuous potential allows the variation of the central rejection frequency of the notch filter.

More specifically, the subject of the invention is a microwave band cut filter as defined in the claims.

• la figure 1 montre la structure de base d'un filtre coupe-bande à lignes coupées de l'art antérieur, dont la fréquence d'accord, fixée par construction, n'est pas ajustable,
• la figure 2 est un diagramme atténuation/fréquence correspondant au filtre de la figure 1,
• la figure 3 est homologue de la figure 1, pour le filtre accordable selon la présente invention,
• la figure 4 est homologue de la figure 2, pour le filtre accordable selon la présente invention, les courbes d'atténuation ayant été représentées pour les deux fréquences extrêmes d'accord de ce filtre,
• la figure 5 représente, en perspective, une réalisation du filtre de la présente invention montrant la manière dont on peut l'intégrer avec les techniques de la micro-électronique,
• la figure 6 est la courbe de réponse en fonction de la fréquence, mesurée pour un exemple pratique de filtre réalisé selon les enseignements de la présente invention,
• la figure 7 est une vue agrandie de la bande de fréquences centrale de la figure 6, montrant les courbes de réponse obtenues pour diverses valeurs d'accord s'étendant à l'intérieur de la plage de réglage du filtre.

Other characteristics and advantages of the present invention will appear on reading the detailed description below, made with reference to the appended figures in which:

• FIG. 1 shows the basic structure of a band-cut filter with cut lines of the prior art, the tuning frequency of which, fixed by construction, is not adjustable,
• FIG. 2 is an attenuation / frequency diagram corresponding to the filter of FIG. 1,
• FIG. 3 is homologous to FIG. 1, for the tunable filter according to the present invention,
• FIG. 4 is homologous with FIG. 2, for the tunable filter according to the present invention, the attenuation curves having been represented for the two extreme tuning frequencies of this filter,
• FIG. 5 represents, in perspective, an embodiment of the filter of the present invention showing the way in which it can be integrated with the techniques of microelectronics,
• FIG. 6 is the response curve as a function of frequency, measured for a practical example of a filter produced according to the teachings of the present invention,
• Figure 7 is an enlarged view of the center frequency band of Figure 6, showing the response curves obtained for various tuning values extending within the filter adjustment range.

FIG. 1 shows the structure, in itself known, of a notch filter of the type known as "with coupled lines" produced in microstrip technology: such a filter comprises a transmission line 1 in the form of a microstrip connecting a generator 2 of microwave signals to a load impedance 3, and there is provided at least one filtering cell (five, in the example illustrated) formed by a microstrip segment 4 arranged parallel to the transmission line , and having an electrical length corresponding substantially to a quarter of the wavelength at the central rejection frequency which it is desired to give to the filter. Each of the segments 4 has one of its ends in open circuit and the other directly connected to the ground potential.
The attenuation provided by such a filter is illustrated in FIG. 2, the central frequency F0 being determined by the length of each segment 4 and the bandwidth rejected depending on the number of cells and the coupled line impedance of each d 'between them.

According to the invention, as shown in FIG. 3, the direct connection to earth of one of the ends of each segment is replaced by a resonant LC circuit formed by an inductor 5 in series with an adjustable capacity 6, this circuit constituting therefore a charge for the coupled line.
Very advantageously, the adjustable capacity 6 consists of a varactor, the cathode of which is connected to ground and the anode of which is connected on the one hand to one of the terminals of the inductor 5 and on the other hand to a negative continuous potential source -V (the respective potentials -V1, -V2, ... applied to the varactors of the different cells are in fact not entirely identical, the theoretical calculation showing that, even in the presence of components having a dispersion null, it is necessary to calibrate the potentials -V1, -V2, ... beforehand to different values (only a few percent, however) to tune all the cells on the same central frequency. The capacity of the varactor being a function of the potential applied to its terminals, the tuning frequency of the resonant circuit LC will vary with the control voltage of the varactor, the operation of the filter will be modified and its tuning in frequency will depend essentially on ement of the DC voltage applied to the varactor (of course, for a filter with several cells, all the potentials -V1, -V2, etc. are varied simultaneously and in the same way in order to maintain the correct tuning of the filter).
The attenuation provided by such a filter is illustrated in FIG. 4, where it can be seen that the attenuation curve is similar to that of FIG. 2, but that its central frequency can move between a value F0min and a value F0max as a function of the potential applied to the cathode of the varactor, the minimum frequency being obtained for the maximum capacity of the varactor, itself corresponding to the lowest control potential.
Furthermore, if we remember that the tuning frequency of an LC circuit whose capacitive element is a varactor varies from substantially logarithmically with the applied voltage, it is conceivable that the control circuits of the tuning frequency of the filter will become particularly simple, in particular compared to the tuning circuits currently used for filters tunable to YIG.
Furthermore, although a five-cell filter has been shown of which all the LC circuits are similar, this number of five cells is in no way limiting, and essentially depends on the selectivity that is desired for the filter (by increasing the number of cells, we restrict the width of the ejected strip), the space available on the substrate to integrate the cells, etc.

An exemplary layout of the components has been described in FIG. 5, which shows how it is possible without difficulty to produce the filter of the present invention with the integration techniques known in microelectronics.
The filter is for example produced on a dielectric substrate 7 of alumina (relative permittivity of 9.8) of small thickness, the lower face 8 of which is metallized so as to constitute both the ground plane and the mechanical support of the circuit .
Transmission line 1 is a conventional transmission line, with an impedance close to 50 Ω, comprising a microstrip extending between an entry point 9 and an exit point 10.
On either side of this line 1, five microstrip segments 4 have been distributed forming a coupled line; the tunable filter was therefore produced with five cells but, as we have just indicated, this number largely depends on the final electrical characteristics that one wishes to obtain. Opposite these segments 4 are provided throttles 11, 11 and 12, 12 allowing, in known manner, to adjust the impedances (in even and in odd mode) of each of the coupled lines.

One end of each segment 4 is in open circuit, while the other end is connected by a connecting wire 5 to the cathode of a varactor 6, this connecting wire forming the inductance 5 of the diagram of the figure 3.
The varactor 6 is preferably a component produced in the form of a micropave carried over to the surface; the cathode of the varactor is connected to ground by means of a metallized via 16 connecting the circuit area on which the micropavé is welded to the underlying ground plane 8.
The continuous potential -V is applied to the anode of the varactor by means of a low-pass filter comprising a decoupling capacitor of high capacity 13 and a connecting wire 14 of considerable length constituting an impedance of high value, passing above a trench 15 delimiting the microwave circuits proper and the dielectric alumina substrate; the voltage control circuit of the varactors is thus made completely neutral in the microwave domain.

In addition, provision can be made to make the filter switchable by replacing the series resonant circuit with a parallel resonant circuit and by varying the polarity of the voltage applied to the varactors.
In this case, it is advantageous to put several notch filters of the same type in cascade, each being tunable over a different frequency range. By selective switching of one or other of the filters, it is thus possible to cover a much wider frequency range than that of a single filter, typically extending over several octaves.

Example of realization

• impédance caractéristique 50
• bande passante (hors fréquence réjectée) : 2 à 18 GHz,
• plage d'accord possible : 7 à 10 GHz,
• largeur de bande atténuée à - 25 dB : 300 MHz.

A five-cell filter was produced with the layout of FIG. 5, intended to operate under the following operating conditions:

• characteristic impedance 50
• bandwidth (excluding rejected frequency): 2 to 18 GHz,
• possible tuning range: 7 to 10 GHz,
• bandwidth attenuated at - 25 dB: 300 MHz.

A filter meeting these conditions is obtained by taking the following characteristics:
- five cells,
- ZOE impedances in even mode and ZOO in odd mode of the coupled lines (in ohms) of each cell: cell number ZOE impedance (even mode) ZOO impedance (odd mode) 1 70.14 38.8 2 68.8 33.9 3 71.2 31.5 4 73.4 35.5 5 63.8 36.1
- capacity of the varactor at -4V: C = 0.45 pF,
- maximum capacity variation ratio C0 / C25 = 3.7,
- inductance of the connection wire connecting the varactor to the coupled line: L = 0.73 nH.

The performances of the filter thus produced are given in FIGS. 6 and 7, which both represent the response of the filter (FIG. 6 for the entire width W of the operating band; FIG. 7 in the range of variation of the filter). It is noted that the frequency of the filter can vary, substantially logarithmically as a function of the voltage applied to the varactor, between approximately 6.5 and 9.8 GHz, with an attenuated bandwidth w from 240 MHz to -25 dB and a maximum rejection of around -40 dB, values substantially constant whatever the tuning frequency.

Claims (5)

1. UHF notch filter in microstrip technology of the type with coupled lines, including a transmission line in microstrip (1) form associated with at least one filtering cell comprising a microstrip segment (4) arranged parallel to the transmission line and at a distance from the latter, this microstrip segment being linked, at a first one of its ends, to the earth potential with the interposition of a variable capacitive element, characterised in that the electrical length of the microstrip segment corresponds substantially to a quarter wavelength of the central rejection frequency of the filtering cell, in that the other end of the microstrip segment is open-circuited, and in that an inductive element (5) produced in the form of a wire for linking the microstrip segment (4) to the varactor (6) is inserted in series with the variable capacitive element (6) between the earth and the first end of the segment in order to form a tunable resonant LC circuit.
2. Filter according to Claim 1, characterised in that the capacitive element of the tunable resonant LC circuit comprises a varactor (6) being arranged on the same dielectric substrate (7) as this microstrip segment, whose anode is taken to an adjustable direct current potential (-V), so that the control of this direct current potential permits the central rejection frequency of the notch filter to be varied.
3. Filter according to Claim 2, characterised in that the direct current potential is applied to the anode of the varactor with the interposition of a low-pass filter (13, 14).
4. Filter according to one of the preceding claims, characterised in that it further comprises switching means in order selectively to open-circuit the said first end of each microstrip segment (4) instead of linking it to the earth potential.
5. UHF notch filter in microstrip technology, characterised in that it comprises a plurality of filters according to Claim 4 mounted in cascade, the switching means for each filter being controlled selectively in such a way as to switch only the elementary filter(s) whose range of variation of the rejection frequency contains the frequency or frequencies to be eliminated.

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