FR2628571A1 - MICRO-BAND TECHNOLOGY HYPERFREQUENCY CUTTER FILTER - Google Patents

Abstract

This microwave band-stop filter in microstrip technology is of the coupled line type, that is to say comprising a transmission line in the form of a microstrip 1 associated with at least one filtering cell comprising a segment of microstrip 4 arranged parallel to the transmission line and at a distance from it, this microstrip segment having one of its ends in open circuit and the other connected to ground potential. According to the invention, the connection to the ground potential is made for each cell with the interposition of a tunable LC resonant circuit 5, 6. Very advantageously, the capacitive element of the tunable LC resonant circuit comprises a varactor 6, the anode of which is brought to an adjustable DC potential -V, so that the control of this DC potential allows the variation of the central filter rejection frequency. band cutter, and 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, the one being arranged on the same dielectric substrate 7 as this microstrip segment.

Description

Byperféquenoe band-stop filter in micro-band technology This

  The invention relates to a microwave band-stop filter in

micro-band technology.

  Tunable microwave bandpass filters are particularly used in very wide microwave receivers. instant band usually having many signals to process

  simultaneously, typically the radar signal receivers.

  Due to the very wide instantaneous band, the signals are very often disturbed by the presence of strong parasitic signals which saturate the reception channels. The function of a notch filter is thus to attenuate the disturbing signals in order to analyze and identify the

signals of smaller amplitudes.

  Since these disturbing signals generally have neither a known frequency nor a stable frequency, it is necessary to provide a

  notch filter that is tunable.

  Until now, a YIG (Yttrium-Iron 2) Garnet: yttrium-iron garnet filter, appropriately polarized for

  tune it to the frequency to be eliminated.

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

  One of the aims of the present invention is to propose a cut-off filter.

  tunable microwave band which has electrical properties comparable to YIG filters, but which eliminates the aforementioned drawbacks. More precisely, as will be seen later on, the invention makes it possible to combine the following advantages: - low insertion loss out of band rejection, - operation in broadband, compatible with the performance of current reception channels (typically, width 2 to 18 GHz band), - high frequency tuning range, - high tuning frequency rejection, - tuning system without power consumption, - very low tuning time, - very small size, allowing easy integration in

microelectronics.

  For this purpose, the present invention uses the basic structure called coupled lines, ie having a microstrip transmission line associated with at least one filter cell comprising a microstrip segment disposed parallel to the line of microstrip transmission and at a distance from it, this microstrip segment having one of its

  ends in open circuit and the other connected to the potential of the mass.

  In a characteristic manner of the present invention, the connection to the potential of the mass is carried out for each cell with the interposition of a circuit

resonant LC tunable.

  Very advantageously, the capacitive element of the tunable LC resonant circuit comprises a varactor whose anode is brought to an adjustable continuous potential, so that the control of this continuous potential

  allows the variation of the center frequency of rejection of the filter cutter

  gang. According to other preferred features of the present invention, the inductive element of the tunable LC resonant circuit is formed as a connecting wire of the microraban segment to the varactor, that being arranged 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 1.5; the filter further comprises switching means for selectively open-circuiting said other end of each microstrip segment instead of connecting it to the potential of the mass; In the last case indicated, it is very advantageous to produce a complex filter comprising a plurality of such cascaded filters, 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.

  Other features and advantages of the present invention

  will appear on reading the detailed description below, made in

  reference to the appended figures in which FIG. 1 shows the basic structure of a prior art coupled-line band-stop filter whose tuning frequency, 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 to FIG. According to the present invention, the attenuation curves having been represented for the two tuning extreme frequencies of this filter, FIG. 5 represents, in perspective, an embodiment of the filter of the present invention showing the manner in which the FIG. 6 is the response curve as a function of frequency, measured for a practical example of a filter made according to the teachings of the present invention; Fig. 7 is an enlarged view of the central frequency band of Fig. 6, showing the response curves obtained for various tuning values extending within the range of

filter adjustment.

  2D In FIG. 1, there is shown the structure, itself known, of a band-stop filter of the "coupled-line" type produced in microstrip technology: such a filter comprises a transmission line 1 in the form of a a microstrip connecting a generator 2 of microwave signals to a load impedance 3, and there is provided at least one filter cell (five, in the illustrated example) formed of a microstrip segment 4 arranged parallel to the line of transmission, and having an electrical length substantially corresponding to a quarter of the wavelength at the central rejection frequency that is to be given to the filter. Each of the segments 4 has one of its ends in

  open circuit and the other connected directly to the potential of the mass.

  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 rejection bandwidth depending on the number of cells.

  and the coupled line impedance of each of them.

  According to the invention, as shown in FIG. 3, the direct earth connection of one of the ends of each segment is replaced by a resonant LC circuit formed of an inductance 5 in series with a capacitance

  adjustable 6, this circuit thus constituting a load for the coupled line.

  Very advantageously, the adjustable capacitance 6 consists of a varactor whose cathode is connected to ground and whose anode is connected on the one hand to one of the terminals of the inductor 5 and on the other hand to a negative potential source -V (the potentials -Vi, -V2, ... respectively applied to the varactors of the different cells are not quite identical, the theoretical calculation showing that even in the presence of components having a dispersion zero, it is necessary to calibrate previously -V1, -V2, ... potentials to different values (only a few percent, however) to tune all cells

on the same center frequency).

  Since the capacitance of the varactor is a function of the DC 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 frequency will depend essentially on the DC voltage applied to the varactor (of course, for a multi-cell filter, it is varied simultaneously and in the same way

  all potentials -V1, -V2, ... to maintain the correct agreement 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 one remembers that the tuning frequency of an LC circuit whose capacitive element is a varactor varies substantially logarithmically with the applied voltage, it is conceivable that the control circuits of the frequency of tuning of the filter will become particularly simple, especially compared to tuning circuits

  currently used for YIG tunable filters.

  Moreover, although a five-cell filter has been represented, all the LC circuits of which are similar, this number of five cells is in no way limiting and depends essentially on the selectivity that is desired for the filter. (by increasing the number of cells, the width of the rejected band is restricted), the space available on the substrate to integrate the cells, etc. FIG. 5 is an example of implementation of the components, which shows how easily the filter of the

  present invention with the integration techniques known in micro-

electronic.

  The filter is for example made on a dielectric substrate 7 of alumina (relative permittivity of 9.8) of small thickness, the lower face 8 is metallized so as to constitute both the plane of

  mass and the mechanical support of the circuit.

  The transmission line 1 is a conventional transmission line, with impedance close to 50 n, 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 thus made with five cells but, as just indicated, this number

526285.7 1

  depends largely on the final electrical characteristics that one wishes to obtain. Opposite these segments 4 are provided restrictions 11,11 and 12,12 allowing, in known manner, to adjust the impedances (in even mode and 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 made in the form of a micropave reported on the surface; the cathode of the varactor is connected to ground by means of a metallized via 16 connecting the range of

  circuit on which the micropave is welded to the ground plane 8 underlying.

  The continuous potential -V is applied to the anode of the varactor via a low-pass filter comprising a high capacitance decoupling capacitor 13 and a long connecting wire 14 constituting a high value impedance, passing above a trench 15 delimiting the microwave circuits themselves and the dielectric substrate of alumina; this makes the control circuit voltage of the varactors totally neutral in the field

microwaves.

  In addition, provision can be made to make the filter switchable by replacing the series resonant circuit with a parallel resonant circuit.

  2 and playing on the polarity of the voltage applied to the varactors.

  In this case, it is advantageous to advantageously

  band of the same type in cascade, each being tunable over a different frequency range. By selective switching of one or the other of the filters, it is thus possible to cover a much wider range of frequencies than that of a single filter, typically extending over several octaves. Embodiment 36 A five-cell filter was made with the implementation of FIG. 5, intended to operate under the following operating conditions: characteristic impedance 50 n-bandwidth (excluding frequency referenced): 2 to 18 GHz, - possible tuning range: 7 to 10 GHz,

  - attenuated bandwidth at - 25 dB: 300 MHz.

  A filter meeting these conditions is obtained by taking the following characteristics:

262857 1

  - five cells, - ZOE impedances in even mode and ZOO in odd mode of the coupled lines (in ohms) of each cell n of impedance cell ZOE impedance ZOO (even mode) (odd mode)

1 70.14 38.8

68.8 33.9

3 71.2 31.5

4 73.4 35.5

63.8 36.1

  -capacity of the varactor at -4V: C = 0.45 pF, - maximum capacity variation ratio Co / C25 = 3.7, -inductance of the connecting wire connecting the varactor to the coupled line:

L = 0.73 nH.

  The performance of the filter thus produced is given in FIGS.

  7, which both represent the filter response (Figure 6 for the entire.

  width W of the operating band; figure 7 in the range of

filter variation).

  It can be seen that the filter frequency can vary substantially logarithmically as a function of the voltage applied to the varactor between approximately 6.5 and 9.8 GHz, with attenuated bandwidth w of 240 Mz at 25 dB and rejection. maximum of -40 dB, values

  substantially constant regardless of the tuning frequency.

Claims (7)

  1. A microwave band-stop filter with a coupled-line type microstrip technology, comprising a microstrip transmission line (1) associated with at least one filtering cell comprising a microstrip segment (4) arranged parallel to the line. of transmission and at a distance thereof, this microstrip segment having one of its ends in open circuit and the other connected to the potential of the mass, characterized in that, for each cell, the connection to the potential of the mass is achieved with the interposition of a resonant circuit LC (5,6) tunable. 2. The filter of claim 1, wherein 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 the control of this potential continuously allows the variation of the center frequency of rejection of the notch filter.   3. The filter of claim 2, wherein the inductive element of the tunable LC resonant circuit is formed as a connecting wire (5) of the microstrip segment (4) to the varactor (6), that being arranged on the   same dielectric substrate (7) as this microstrip segment.   4. The filter of claim 2, wherein the DC potential is applied to the anode of the varactor with the interposition of a low-pass filter. (13,14).   5. The filter of one of the preceding claims, wherein the   ratio of possible extreme repair frequencies is at least 1.5.   6. The filter of one of the preceding claims characterized in that   it further comprises switching means for selectively open circuiting said other end of each segment   microstrip (4) instead of connecting it to the potential of the mass.   7. A microwave band-eliminator filter in microstrip technology, characterized in that it comprises a plurality of filters according to claim 6 mounted in cascade, the switching means of each filter being selectively controlled so as to switch only the (s) ) elementary filter (s) whose range of variation of the frequency of   rejection contains the frequency (s) to be eliminated.