EP0337825A1 - Microstrip fashion microwave rejection filter - Google Patents

Abstract

This microband technology microwave band-rejection filter is of the type with coupled lines, that is to say containing a transmission line in the form of a microstrip (1) associated with at least one filtering cell comprising a microstrip segment (4) arranged parallel to the transmission line and remote from it, this microstrip segment having one of its ends open-circuited and the other connected to the earth potential. Connection to the earth potential is effected for each cell with interposition of a tunable LC resonant circuit (5, 6). Advantageously, the capacitive element of the tunable LC resonant circuit comprises a varactor (6) whose anode is brought to an adjustable DC potential (-V), so that control of this DC potential allows variation in the central rejection frequency of the band-rejection filter, and the inductive element of the tunable LC resonant circuit is produced in the form of a wire (5) connecting the microstrip segment (4) to the varactor (6), the latter being arranged on the same dielectric substrate (7) as this microstrip segment. <IMAGE>

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 many 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.

Until now, a YIG filter (Yttrium-Iron Garnet: yttrium-iron garnet) has been used for this purpose, polarized appropriately to tune it to the frequency to be eliminated.
This technique, despite its interesting performance (high rejection rate, low attenuated bandwidth), has the disadvantage 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.

One of the aims of the present invention is to provide a tunable microwave notch filter which has electrical properties comparable to YIG filters, but which eliminates the aforementioned drawbacks.

More precisely, as will be seen below, the invention makes it possible to combine the following advantages:
- low rejection out-of-band insertion losses,
- 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.

To this end, the present invention uses the basic structure known as coupled lines, that is to say comprising a transmission line in the form of a microstrip associated with at least one filtering cell comprising a microstrip segment disposed parallel to the line of transmission and at a distance therefrom, this microstrip segment having one of its ends in open circuit and the other connected to the ground potential.

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.

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

According to other preferred characteristics of the present invention:
- the inductive element of the tunable LC resonant circuit is produced in the form of a wire connecting the microstrip segment to the varactor, that being placed on the same dielectric substrate as this microstrip segment;
- the continuous potential is applied to the anode of the varactor with the interposition of a low-pass filter;
- the ratio of possible extreme rejection frequencies is at least equal to 1.5;
the filter further comprises switching means for selectively placing said other end of each microstrip segment in open circuit instead of connecting it to the ground potential;

In the latter case, it is very advantageous to produce a complex filter comprising a plurality of such filters connected in cascade, the switching means of each filter being selectively controlled so as to switch only the elementary filter (s) ( s) whose range of variation of the rejection frequency contains the frequency (s) to be eliminated.

• - la figure 1 montre la structure de base d'un filtre coupe-bande à lignes couplées 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 coupled 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 with 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 central frequency band of Figure 6, showing the response curves obtained for various tuning values extending within the adjustment range of the filter.

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 microwave signal generator 2 to a load impedance 3, and there is provided at least one filter 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 FO 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 then be modified and its tuning in frequency will essentially depend on the DC voltage ap plicated with 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 keep 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 FOmin and a value FOmax 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

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 to - 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 (7)

1. Microwave band filter in microstrip technology of the coupled line type, comprising a transmission line in the form of a microstrip (1) associated with at least one filtering cell comprising a microstrip segment (4) arranged parallel to the line of transmission and at a distance therefrom, this microstrip segment having one of its ends in open circuit and the other connected to the ground potential, characterized in that, for each cell, the connection to the ground potential is performed with the interposition of a tunable resonant circuit LC (5,6). 2. Filter according to claim 1, characterized in that the capacitive element of the tunable resonant circuit LC comprises a varactor (6) whose anode is brought to an adjustable continuous potential (-V), so that the control of this continuous potential allows variation of the central rejection frequency of the notch filter. 3. Filter according to claim 2, characterized in that the inductive element of the tunable LC resonant circuit is produced in the form of a connecting wire (5) from the microstrip segment (4) to the varactor (6), that being arranged on the same dielectric substrate (7) as this microstrip segment. 4. Filter according to one of claims 2 or 3, characterized in that the continuous potential is applied to the anode of the varactor with the interposition of a low-pass filter (13,14). 5. Filter according to one of the preceding claims, characterized in that the ratio of the possible extreme rejection frequencies is at least equal to 1.5. 6. Filter according to one of the preceding claims, characterized in that it further comprises switching means for selectively placing in open circuit said other end of each microstrip segment (4) instead of connecting it to the potential of the mass. 7. Microwave band microwave cut-out filter, characterized in that it comprises a plurality of filters according to claim 6 connected in cascade, the switching means of each filter being selectively controlled so as to switch only the one (s) elementary filter (s) whose range of variation of the rejection frequency contains the frequency (s) to be eliminated.

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