Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01P—WAVEGUIDES; RESONATORS, LINES, OR OTHER DEVICES OF THE WAVEGUIDE TYPE
  • H01P1/00—Auxiliary devices
  • H01P1/20—Frequency-selective devices, e.g. filters
  • H01P1/215—Frequency-selective devices, e.g. filters using ferromagnetic material
  • H01P1/217—Frequency-selective devices, e.g. filters using ferromagnetic material the ferromagnetic material acting as a tuning element in resonators

Definitions

• the invention relates to a microwave resonant circuit as well as a frequency tunable microwave filter using the resonant circuit.
• the invention applies to any transmission / reception device implementing frequency tuning from a magnetic or mechanical control in the microwave domain such as, for example, multi-band mobile telephones.
• Frequency tunable filters are a particularly important family of microwave functions. There are different ways of making frequency tunable filters according to the known art.
• the frequency tuning can, for example, be obtained using electronic components of the diode type (varactor diode or PIN diode). Filters with electronic components then exhibit significant insertion losses and high noise levels due to the use of electronic components.
• Frequency tunable filters can also be made with ferroelectric materials. These filters have the advantage of having relatively low noise levels but require control voltages which can be high and are characterized by significant insertion losses.
• Tunable filters using magnetic material are also known.
• Filters using ferrimagnetic materials are the most common. They have the disadvantage of requiring a large static magnetic control field, which implies the use of coils traversed by a current of high intensity. Their operation, based on the evolution of the gyromagnetic permeability under the effect of an external field, requires to overcome a field called "demagnetizing field" to create a given magnetic field inside the magnetic component.
• the control field must be equal to the internal field, increased by the demagnetizing field.
• the demagnetizing field can be calculated according to the shape of the sample. Consider for example a flattened parallelepiped of ferrite whose height to side ratio is 1/10. The demagnetizing field can then reach values of the order of 7% of the saturation magnetization. For a ferrite, this represents a control field of the order of 24 A / m to be added to the useful field. Such values are penalizing.
• Ferromagnetic materials are also used to make microwave filters. Unlike ferrites, the conductive nature of ferromagnetic materials imposes additional constraints to prevent losses by conductivity preventing the propagation of waves.
• In-line microstrip filters have been produced which include one or more ferromagnetic layers (cf. "Tuneajle microstrîp device controlled by a weak magnetic field using ferromagnetic laminât ions" ALAdenot, O. Acher, T Taffary, P. Quéffélec, G. Tanné, JOURNAL OF APPLIED PHYSICS, 1 May 2000).
• the layer or layers of ferromagnetic material are inserted between the input port and the output port of a microstrip line.
• the filters thus produced are band stop filters, the band width of which is solely a function of the width of the gyromagnetic absorption line of the ferromagnetic material.
• the filtering is then due to the selective losses presented by the ferromagnetic material.
• the width of the absorption line is of the order of a few hundred MHz and can hardly be changed.
• the invention does not have the drawbacks and limitations of the various known filters mentioned above.
• the invention relates to a microwave resonant circuit comprising at least one resonant microstrip line element, the resonant microstrip line element comprising a conductive strip and a mass.
• the microwave resonant circuit comprises at least one composite element consisting of alternating ferromagnetic layers and insulating layers placed between the conductive tape and the ground plane.
• the invention also relates to a frequency tunable microwave filter comprising at least one microwave resonant circuit.
• the microwave resonant circuit is a resonant circuit according to the invention and the microwave filter comprises means for applying a magnetic field to the composite element.
• a composite element consisting of alternating ferromagnetic layers and insulating layers will also be referenced by the acronym LIFT for "Lamellar Ferromagnetic Insulator on the Trench".
• LIFT Lamellar Ferromagnetic Insulator on the Trench.
• the resonant microstrip line element can be, for example, an open circuit or short circuit stub - circuit of respective length ⁇ g / 4 or ⁇ g / 2 or a line element of length substantially equal to ⁇ g / 2, ⁇ g being the length of the wave which propagates in the line element.
• the term “stub” should be understood to mean an open circuit or short circuit line element placed in derivation of a main propagation line.
• the ferromagnetic and insulating layers are stacked parallel to the conductive tape and the ground plane.
• the layers ferromagnets have a thickness between 0.05 ⁇ m and 2 ⁇ m and the insulating layers have a thickness between 2 ⁇ m and 50 ⁇ m.
• the volume fraction in ferromagnetics is preferably between 0.2% and 20%.
• by the ferromagnetic volume fraction f is between 0.5 and 300.
• the saturation magnetization of the ferromagnetic layers is preferably greater than 400kA / m.
• a LIFT structure comprises, for example, a stack of ferromagnetic layers deposited on a flexible substrate of ylar or kapton.
• the stacked layers are glued together to achieve, for example, a stacking thickness of between
• the use of a LIFT composite advantageously makes it possible to control the tuning in frequency with relatively weak magnetic fields.
• the magnetic field is between 80A / m and 25kA / m. This also makes it possible to produce in large series easier and much less expensive than the use of ferrimagnetic material.
• the device for controlling the resonant frequency and the gyromagnetic permeability of LIFT composites can consist of a source of static magnetic field acting on the LIFT in a direction parallel to the ferromagnetic layers.
• the magnetic field source can be, for example, a system of coils traversed by a current or a permanent magnet.
• Frequency control can also be achieved by a stress applied to the LIFT, parallel to the plane of the ferromagnetic layers.
• the ferromagnetic layers which constitute the LIFT must have a non-negligible magnetostriction coefficient, for example of the order of 3 to 35 ⁇ 10 ⁇ 6 in absolute value.
• the applied stress then makes it possible to modify the intensity and the direction of the internal field of the ferromagnetic layers.
• the stress exerted can be, for example, between 10 and 800Mpa.
• FIG. 2 shows, as an example, the transmission coefficient of a structure consisting of a microstrip line and a LIFT composite as a function of frequency, for different line widths;
• FIG. 3A and 3B show a first embodiment of the microwave resonant circuit according to the invention
• - Figure 4 shows the transmission coefficient of a frequency tunable microwave filter comprising a resonant circuit as shown in Figures 3A and 3B
• FIG. 5 represents a microwave resonant circuit of the impedance jump resonator type according to the invention
• Figure 6 shows the responses in reflection and transmission of a frequency-tunable microwave filter comprising a resonant circuit as shown in Figure 5, there
• FIG. 7 shows a resonant microwave circuit with capacitive coupling according to the invention.
• FIG. 1 represents the measured relative permeability of a layer of ferromagnetic film.
• the layer of ferromagnetic film has a thickness of 0.43 ⁇ m.
• FIG. 1 represents the real part ⁇ ′ and the imaginary part ⁇ ′′ of the relative permeability ⁇ as a function of the frequency.
• the natural resonance frequency of the ferromagnetic material is characterized by the change to 1 of the real part ⁇ 'and by a maximum value for the imaginary part ⁇ ".
• the resonance frequency is around 1.6 GHz.
• the width of the peak of the imaginary permeability ⁇ " is typically a few hundred MHz (for example 700 MHz in the case studied).
• the relative permeability is essentially real. There is therefore little or no loss.
• the ferromagnetic material is used according to the invention.
• FIG. 2 represents, by way of example, the transmission coefficient of a structure made up of a microstrip line and of a LIFT composite as a function of the frequency, for different line widths.
• a microstrip line consists of a conductive tape and a ground plane, the conductive tape and the ground plane being separated by a dielectric medium.
• the ferromagnetic composite is placed between the conductive tape and the ground plane of the microstrip line.
• the width of the ribbon is 4.2mm.
• ferromagnetic laminate composites in microwave causes losses due to the appearance of induced currents in the ferromagnetic layers. These induced currents result from the presence of components of the microwave electric field in the plane of the ferromagnetic layers. To limit these losses, it clearly appears in FIG. 2 that the strip must have a width greater than or equal to that of the LIFT ferromagnetic composite.
• FIGS. 3A and 3B represent a first exemplary embodiment of a microwave resonant circuit according to the invention.
• Figure 3A is a top view of the resonant circuit and
• Figure 3B is a view along section AA 'in Figure 3A.
• This first example of a resonant circuit shows the feasibility of a first order filter of the type variable frequency notch according to the invention. Frequency agility is then ensured by variations in the magnetic properties of the LIFT composite under the action of an external static field Ho or an external stress.
• a ribbon 1 of width R is mounted in derivation of a ribbon 2 of width typically corresponding to the input and output impedances of the device.
• a LIFT composite 3 is placed between the ribbon 1 and the ground plane 4.
• the ribbon 1 of width W mounted in parallel with the ribbon 2 constitutes a line element resonant.
• the resonant frequency of the tape cutter function is controlled by the length L and the width R of the tape 1 and by the intrinsic parameters (permittivity and permeability) of the medium that separates the tape 1 from the ground plane 4.
• FIG. 4 thus illustrates the transmission coefficient in decibels (S 2 ⁇ dB) of a microwave filter using a resonant circuit such as shown in Figures 3A and 3B for different values of the applied magnetic field Ho (Ho varies from 0 A / m to 20 kA / m).
• the advantage of the filtering device according to the invention is to be able to control, within a certain limit, the bandwidth of the filter.
• the bandwidth of the filter advantageously depends on the electrical characteristics of the "stub", for example its length and its width. Filtering devices of the known art which use a ferromagnetic material do not have this advantage since they only use the gyromagnetic losses to fix the bandwidth. According to the invention, it is thus possible, for example, to reduce the bandwidth by doubling the length of the stub and by replacing the open circuit with a short circuit (the bandwidth at -3 dB is then divided by a factor of at least 2).
• the LIFT 3 composite consists of a set of layers which constitutes, for example, a rectangular parallelepiped.
• the deposition is carried out, for example, by magnetron sputtering, under vacuum, of the ferromagnetic material on a kapton film unwound continuously in front of the magnetron.
• the residual magnetic field of the magnetron present at the level of the substrate orients the magnetization of the material in a privileged direction of its plane. This direction is called "easy magnetization axis".
• the permeability relative to a microwave field applied in the direction of easy magnetization is close to unity, while it has high levels in the direction of the plane of the orthogonal curve to the direction of easy magnetization.
• the control magnetic field Ho can be applied using conventional means for applying a field, such as one or more coils, with or without magnetic poles or a permanent magnet.
• the Ho field is applied to a low volume (of the order of magnitude of the volume of the LIFT), which advantageously results in low consumption of the control circuit.
• the intensity of the static magnetic field can then be, for example, less than or equal to 20 kA / m.
• a variant filter according to the invention consists in tuning the filter no longer using a magnetic control but using a mechanical constraint.
• the LIFT component is produced not from a layer of CoNbZr, whose coefficient of magnetostriction is low, but with a more strongly magnetostrictive material, such as an FeCoSiB alloy, with the exception of the compositions of which the ratio between the rate of iron and the rate of Cobalt is between 2 and 10%, for which it is known that the coefficient of magnetostriction is quite low.
• An alloy of type F 66 ⁇ 8 Si ⁇ B ⁇ 4 has for example a magnetostriction coefficient of the order of 30.10 "6
• the CoNbZr of the previous example has a magnetostriction coefficient of the order of 10 " 6 .
• This material also has the advantage of having a magnetization at high saturation, of 1430 kA / m. It is known that a mechanical stress is equivalent to an external magnetic field which is added to or subtracted from the anisotropy field of the layer (depending on the sign and the direction of application of the stress). In the previous example, a stress of 1 MPa in compression in the plane of the layer is equivalent to an external field of the order of 56 A / m applied in the plane of the layer, perpendicular to the stress. The equivalent external field is proportional to the constraint. The equivalent of an external control magnetic field of 8 kA / m is therefore obtained, by exerting a stress of the order of 140 MPa in ferromagnetics.
• the average stress to be exerted on the LIFT is lower than these values, of the order of 8 MPa for a LIFT composed of a ferromagnetic layer of thickness 0.4 ⁇ m over 12 ⁇ m mylar substrate.
• the forces at play, taking into account the small size of the LIFTs, are therefore advantageously very low and make the piezoelectric control effective.
• a ferromagnetic thickness of 0.43 ⁇ was preferably chosen because, for the material considered, significantly increasing the thickness would lead to the appearance of additional losses in below the resonance frequency (losses linked to the skin effect) and significantly reducing this thickness would significantly reduce the ferromagnetic charge rate of the LIFT and therefore the permeability levels. It should however be noted that it is possible to maintain or increase the level of permeability of the LIFT even with smaller thicknesses of ferromagnetic, provided that the thickness of insulation of the LIFT is reduced (the thickness of the insulation is given by the sum of the adhesive thickness and the thickness of the dielectric substrate on which the ferromagnetic layer is deposited). It is thus possible to use dielectric layers of mylar with a thickness of 3.5 ⁇ m, or even 1.6 ⁇ m, for depositing the ferromagnetic material.
• the ferromagnetic deposition on flexible film is structured in the form of a stack using an epoxy adhesive, the thickness of the adhesive not exceeding 5 ⁇ m.
• the thickness of the multilayer composite is chosen to be slightly less than the thickness of the substrate of the microstrip line, namely 0.625 mm in the example presented. Then, parallelepipedic pieces of LIFT materials are machined to the desired dimensions, so as to place the ferromagnetic lamellae parallel to the ground plane of the microstrip line.
• FIG. 5 represents a resonator circuit with jump of impedance according to the invention.
• a microwave filter which uses a jump impedance resonator will also be called SIR filter (SIR for "Stepped Impedance Resonator").
• SIR filters have the disadvantage of having parasitic feedback at harmonic frequencies. It is shown (cf. "Improvement of global performances of band-pass filters using non-conventional stepped impedance resonators", S.Denis; C.Person; S.Toutain;
• the SIR filters according to the invention advantageously make it possible to eliminate the existence of part of the parasitic lifts.
• the suppression of parasitic lifts is then obtained by making them coincide with the gyromagnetic resonance of the LIFT material. It is then possible to make a variable frequency filter while controlling the first parasitic ascent.
• FIG. 5 The topology of an SIR filter according to the invention is shown in FIG. 5.
• a ribbon 5 of length L is between a first set of coupled lines 6 and a second set of coupled lines 7.
• the LIFT element 8 is placed under the ribbon 5.
• the assembly formed by the coupled lines 6 and 7 and the ribbon 5 forms the resonator of total length substantially equal to ⁇ g / 2. In practice, depending on the impedance ratio, the length of the resonator will be slightly greater or less than ⁇ g / 2.
• the LIFT element is centered between the two sets of coupled lines so as not to modify the bandwidth of the filter which is essentially fixed by the coupling level of the coupled lines.
• a static magnetic field only the central frequency of the filter is modified by variation of the electrical length of the line ⁇ g / 2.
• the input and output couplings are not disturbed by the magnetic field and the bandwidth of the filter remains almost insensitive to the static field applied.
• the responses measured for different values of the static magnetic field are presented in Figure 6.
• FIG. 6 represents, as a function of the frequency, the reflection coefficients Sn (dB) and transmission S 2 ⁇ (dB), in decibels, of a microwave filter which uses a resonant circuit as shown in FIG. 5 for different values of the applied magnetic field Ho (Ho varies from 0 A / m to 20 kA / m). A variation of ⁇ 24% is obtained around fo ⁇ l.O ⁇ GHz.
• the width of the filtered band is significantly smaller than the width of the gyromagnetic loss peak, which illustrates well the advantage and the versatility of the filters according to the invention, compared to existing tunable magnetic filters.
• FIG. 7 represents a third example of a resonant circuit according to the invention.
• the circuit shown in FIG. 7 is a circuit with capacitive coupling and with a ⁇ g / 2 resonator.
• a line element 10 of length ⁇ g / 2 is between two lines 9 and 11.
• the capacitive coupling is carried out by a first space el which separates the line 9 and the line element 10 and a second space e2 which separates the line 9 and the line element 11.
• a LIFT composite 12 is placed, centrally, under the line element 10.

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• 2001
  • 2001-08-02 FR FR0110395A patent/FR2828337B1/fr not_active Expired - Lifetime
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