Inductance variation module and radio frequency filter comprising such a module

Abstract

The invention relates to an inductance variation module adapted for varying an inductance in an electrical circuit based on a setpoint current, said module (1) comprising a ferrite plate (10) having a central opening (11), an electromagnet (20) comprising two electromagnet parts (21, 22), each electromagnet part (21, 22) having a main body (210, 220) and an electromagnetic coil intended to be wound around said main body (210, 220), said main body (210, 220) having a U-shape defining a central space (212, 222). The electromagnet parts (21, 22) are arranged opposite each other on either side of the ferrite plate (10) at the central opening (11), such that the central spaces (212, 222) of said electromagnet parts (21, 22) extend the central opening (11) substantially perpendicularly to said central opening (11). Figure for the abbreviation: Fig. 1

Description

Title of the invention: Inductance variation module and radio frequency filter comprising such a module technical field

[0001] The present invention relates to an inductance variation module which can be applied to any radiocommunication product requiring the implementation of frequency-tunable filters, in particular when these filters are positioned at the output of power stages. Previous technique

[0002] In most radio communication equipment, particularly tactical and aeronautical, there is a need to implement tunable filters to improve out-of-band noise or the level of harmonics, particularly in the VHF (for "Very High Frequency") range.

[0003] Historically, the best filtering performance on broadband filters is obtained either by the implementation of varactor diodes or by switching capacitances using PIN diode switches as disclosed in document FR3059496.

[0004] The use of varicaps allows for continuous control of a filter within a frequency band, for example, a frequency band between 30 and 43 MHz. However, varicaps cannot withstand RF levels exceeding a few mW. Furthermore, varicaps used in the VHF and UHF bands are becoming obsolete.

[0005] The implementation of PIN diode switched capacitance filters allows for higher RF power (up to 200 W), but its implementation requires complex and bulky control circuits, implementing high currents and voltages.

[0006] The solutions currently used are bulky, energy-intensive and expensive to produce, particularly for switched-capacitor power filters.

[0007] It is known to vary the inductance by acting on the equivalent permeability of an assembly of ferrite materials. This approach has been the subject of various publications, but all the proposed solutions implemented an auxiliary winding through which a direct current was applied. The drawback is that while it is indeed possible to vary the permeability of the ferrite, and therefore the inductance, sufficiently, this also causes an increase in magnetic losses, which makes this approach unusable for implementing a high-performance radio frequency filter, in which losses are a key factor. Another drawback is related to the high coefficient The coupling of the two windings, that of the control circuit and that of the radio frequency circuit, is complex. To decouple the two circuits, it is necessary to create two magnetic circuits and connect them in such a way as to cancel the mutual inductance, which significantly complicates the inductance variation modulus.

[0008] There is therefore a need to propose such a module for varying an inductance, enabling the realization of a high-performance power radio frequency filter with low losses. Description of the invention

[0009] The present invention aims to remedy at least partially this need.

[0010] To this end, a first object of the invention relates to an inductance variation module designed to vary an inductance in an electrical circuit based on a setpoint current, said module comprising: - a ferrite plate with a central opening; - an electromagnet comprising two electromagnet parts or magnetic bodies, each electromagnet part having a main body and an electromagnetic coil intended to be wound around said main body, said electromagnetic coil being capable of being supplied by the set current, said main body having a U-shaped or similar form defining a central space. The electromagnet parts are arranged on either side of the ferrite plate opposite each other at the central opening, such that the central spaces of said electromagnet parts extend the central opening substantially perpendicularly to said central opening.

[0011] The invention thus proposes to exploit the magnetic property of an assembly consisting of a magnetic circuit with an air gap in which a low-loss ferrite core is placed. Unlike known prior art devices, the main inductance is varied not by switching secondary inductances (for example, with PIN diodes) but by manipulating the permeability of the ferrite. A continuous magnetic field is then applied perpendicular to the radio frequency magnetic field present in the inductance. This avoids any negative interaction. The configuration of such a variation module leads to a significant variation in the inductance value without an increase in magnetic losses, or even a decrease in magnetic losses, unlike more conventional configurations in which the continuous and radio frequency magnetic fields are collinear.Furthermore, the invention allows for the introduction of an innovative approach in the realization of broadband UHF electronic antennas.

[0012] In a particular embodiment, the central spaces of the electromagnet parts extend the central opening by forming an angle with said central opening between 80 and 100 degrees.

[0013] In a particular embodiment, the central spaces of the electromagnet parts extend the central opening by forming an angle with said central opening of between 85 and 100 degrees.

[0014] In a particular embodiment, the central spaces of the electromagnet parts extend the central opening perpendicularly to said central opening.

[0015] In a particular embodiment, the electromagnet is made of soft iron.

[0016] For the control circuit, it is therefore proposed to select a material ma Soft magnetic materials with strong saturation magnetization, such as soft iron, iron-nickel alloys, amorphous magnetic alloys, or nanocrystalline materials, can be used. This allows for a reduction in control power.

[0017] In a particular embodiment, the ferrite plate is composed of a NiZnCuCo spinel ferrite.

[0018] The permeability of such a ferrite is between 50 and 200 and it exhibits low magnetic losses in the HF to VHF band.

[0019] In a particular embodiment, the ferrite plate has a thickness between 0.1 and 2 mm.

[0020] In a particular embodiment, the ferrite plate has a thickness less than or equal to 1 mm.

[0021] In a particular embodiment, the module includes means for maintaining contact between the electromagnet and the ferrite plate.

[0022] Another object relates to a radio frequency filter comprising at least one variation module of an inductance according to the invention.

[0023] In a particular embodiment, the radio frequency filter comprises: - a first module for varying an inductance, - a second module for varying an inductance, - a capacitor, said capacitor being connected to the first variation module of an inductance and to the second variation module of an inductance.

[0024] The present invention will be better understood upon reading the detailed description of embodiments taken by way of non-limiting examples and illustrated by the accompanying drawings, in which:

[0025] [Fig.1] [Fig.1] schematically illustrates a variation module of an inductance according to the invention;

[0026] [Fig.2] [Fig.2] schematically illustrates the variation modulus of [Fig.1], said module being connected to receive a setpoint current;

[0027] [Fig.3] [Fig.3] is a graphical representation of a variation of an inductance, of a quality factor measured as a function of a setpoint current;

[0028] [Fig.4] [Fig.4] is a graphical representation of a variation of the inductance in function of a setpoint current;

[0029] [Fig. 5] [Fig. 5] illustrates a radio frequency filter comprising at least one module of variation of an inductance according to figures 1 and 2;

[0030] [Fig.6] [Fig.6] is a graphical representation of a variation of the radio filter frequency of the [Fig.4] over a wide frequency band.

[0031] The invention is not limited to the embodiments and variants shown and other embodiments and variants will be obvious to a person skilled in the art.

[0032] Figures 1 and 2 schematically illustrate a variation module for an inductor according to the invention. This module comprises: - a ferrite plate 10; - an electromagnet 20; - means of maintenance 30.

[0033] The ferrite plate 10 extends mainly along an X direction and a Y direction and has a thickness along a vertical direction Z perpendicular to the horizontal X and Y directions. This thickness is between 0.1 and 2 mm. Preferably, the thickness of the plate 10 is less than 1 mm. Preferably, the thickness of the plate 10 is 0.5 mm.

[0034] The ferrite plate 10 is composed of a NiZnCuCo ferrite.

[0035] This ferrite plate 10 comprises a central opening 11 passing through said plate, so that said ferrite plate 10 has a torus shape. This central opening 11 has an overall rectangular shape.

[0036] The electromagnet 20 frames the ferrite plate 10. It comprises two electromagnet parts 21, 22. Each electromagnet part has a main body 210, 220 and an electromagnetic coil 211, 221 wound around the main body 210, 220. Each electromagnetic coil 211, 221 is intended to be supplied with a set current. Each main body 210, 220 has a U-shape extending primarily along the Y direction and along a vertical direction Z'. The various U-shapes define central spaces 212, 222 extending along the vertical direction Z'. The electromagnet parts 21, 22 are arranged on either side of the ferrite plate 10 opposite each other at the central opening 11, so that the central spaces 212, 222 of said electromagnet parts 21, 22 extend the central opening 11 substantially perpendicularly to said central opening 11.By substantially perpendicular, we mean that the vertical axis Z' associated with the electromagnet 20 makes an angle 0 with the horizontal axis X of the ferrite plate 10, between 80° and 100°.

[0037] Preferably, the vertical axis Z' associated with the electromagnet 20 makes an angle 0 with the horizontal axis X of the ferrite plate 10, between 85° and 95°.

[0038] Preferably, the vertical axis Z' associated with the electromagnet 20 makes an angle 0 with the horizontal axis X of the ferrite plate 10, exactly equal to 90°. In this case, the vertical axis Z' and the vertical axis Z coincide.

[0039] The electromagnet 20 is here made of soft iron.

[0040] The retaining means 30 allow the electromagnet 20 to be held against the ferrite plate 10. These retaining means 30 are in the form of a tab which will act in compression on each main body 210, 220 to hold the parts of the electromagnet 21, 22 against the ferrite plate 10.

[0041] Hereafter, we will call a DC circuit a soft iron control circuit consisting of the two U-shaped electromagnet parts 21, 22 and an HF circuit formed by the ferrite plate 10

[0042] After insertion of the ferrite plate 10 between the two electromagnet parts 21, 22, the objective is to saturate the parts of the NiZnCuCo ferrite using the setpoint current applied in the DC circuit.

[0043] The DC magnetic field produced by the soft iron circuit is perpendicular to the HF magnetic field created in the HF inductor. Thus, the permeability p of the NiZnCuCo ferrite in contact with the DC circuit will enter a rotational regime, which allows for a faster variation of the ferrite's permeability p with the applied magnetic field without increasing magnetic losses.

[0044] As illustrated in [Fig. 2], the biasing circuit was made by winding 50 turns of enameled copper wire around each of the two pre-machined U-shaped electromagnet parts 21, 22. The toroidal plate-shaped HF core is inserted between these two U-shaped cores.

[0045] A first series of measurements illustrated in [Fig.3] was carried out by winding about twenty turns on each U-shaped core.

[0046] The inductance and quality factor were measured as a function of the DC current injected into the windings.

[0047] We thus observe a variation in inductance greater than a factor of 3 for a setpoint current which varies between 0 and 1.6 A. We also note that the quality factor increases especially at high frequency which indicates a decrease in magnetic losses.

[0048] Knowing the number of turns, it is possible to calculate, using Ampère's law, the magnetic field applied to the HF ferrite as a function of the polarizing current: HDC = Ni / Lm. For Lm = 46.1 mm and N = 100, we have, for 1 A applied, HDC = 2170 A / m, or 27.1 Oe. We thus obtain BDC = 2850 Gauss, or 285 mT. A more precise measurement with a B(H) meter allows us to take into account non-linear effects.

[0049] Figure 4 illustrates a variation of the HF inductance as a function of the applied DC current; a variation of a factor of 3 is observed for a current between 0 and 600 mA, which corresponds to a control power of 220 mW. A factor of 5 is obtained for a current between 0 and 1200 mA and a power of 865 mW.

[0050] Thus, the use of a "soft" iron makes it possible to obtain an inductance variation of a factor of 3 for an applied DC current of 0.6 A without an increase in losses, unlike a so-called conventional configuration where the applied DC magnetic field is parallel to the HF magnetic field.

[0051] This physical principle of current-controlled inductance variation is implemented for the realization of a prototype filter in the 30-50 MHz band. Such a filter F is illustrated in [Fig. 5]. This filter F comprises: - a first AI variation module of an inductor, - a second IB variation module of an inductor, - a capacitor 2, said capacitor 2 being connected to the first variation module IA of an inductor and to the second variation module IB of an inductor. Such a filter F is implemented based on a Chebyshev structure whose main characteristics are: - constant Q filter, the bandwidth is proportional to the center frequency; - operation with two identical inductances in a second-order filter; - maximum target losses of less than 3 dB.

[0052] The setpoint current implemented to verify the operation of the filter was positioned between 0 and 2A.

[0053] Figure 6 illustrates the results obtained for filter F. The losses of filter F are slightly higher than expected and the covered band is smaller, but the principle of varying the permeability of the assembly of the two materials by a current control is demonstrated as well as its effect for controlling the center frequency of the filter.

[0054] The invention makes it possible to produce compact tunable power filters over a wide frequency band without the quality of the filter, in particular the losses, being affected.

[0055] This invention can be implemented on low power filters, a few tens of milliwatts, as well as on higher power filters, from a few watts to 50 watts, or even 100 watts.

[0056] The solution provided implements a physical phenomenon linking two materials with different magnetic properties and associated in an original way allowing a perpendicularity of the magnetic fields.

[0057] The invention thus provides the following advantages: - it allows the implementation of a multi-octave frequency tuning device; - it allows for maintaining optimal RF performance (losses, VSWR, selectivity, etc.); - it helps to reduce manufacturing costs; - it makes it possible to greatly reduce the volume and manufacturing cost of antenna tuning boxes for HF radios.

Claims

Claims

1. Module for varying an inductance adapted to vary an inductance in an electrical circuit from a setpoint current, said module (1; IA, IB) comprising: - a ferrite plate (10) comprising a central opening (11);- an electromagnet (20) comprising two electromagnet parts (21, 22), each electromagnet part (21, 22) having a main body (210, 220) and an electromagnetic coil (211, 221) intended to be wound around said main body (210, 220), said electromagnetic coil (211, 221) being capable of being powered by the setpoint current, said main body (210, 220) having a U-shape defining a central space (212, 222), characterized in that the electromagnet parts (21, 22) are arranged on either side of the ferrite plate (10) facing each other at the central opening (11), so that the central spaces (212, 222) of said electromagnet parts (21, 22) extend the central opening (11) substantially perpendicular to said central opening (11).;

2. An inductance variation module according to claim 1, wherein the central spaces (212, 222) of the electromagnet parts (21, 22) extend the central opening (11) forming an angle 0 with said central opening (11) of between 80 and 100 degrees.

3. An inductance variation module according to any one of claims 1 or 2, wherein the central spaces (212, 222) of the electromagnet parts (21, 22) extend the central opening (11) forming an angle 0 with said central opening (11) of between 85 and 100 degrees.

4. An inductance variation module according to any one of claims 1 to 3, wherein the central spaces (212, 222) of the electromagnet parts (21, 22) extend the central opening (11) perpendicular to said central opening (11).

5. An inductance variation module according to any one of claims 1 to 4, wherein the electromagnet (20) is made of soft iron.

6. An inductance variation module according to any one of claims 1 to 5, wherein the ferrite plate (10) is composed of a NiZnCuCo alloy.

7. An inductance variation module according to any one of claims 1 to 6, wherein the ferrite plate (10) has a thickness

8.

9.

10.

11. between 0.1 and 2 mm. An inductance variation module according to any one of claims 1 to 7, wherein the ferrite plate (10) has a thickness less than or equal to 1 mm. Module for varying an inductance according to any one of claims 1 to 8, said module (1; IA, IB) comprising means (30) for holding the electromagnet (20) in contact with the ferrite plate (10). Radiofrequency filter comprising at least one variation module (IA, IB) of an inductance according to any one of claims 1 to 9. Radiofrequency filter according to claim 10, said filter comprising: - a first variation module (IA) of an inductance, - a second variation module (IB) of an inductance, - a capacitor (2), said capacitor (2) being connected to the first variation module (IA) of an inductance and to the second variation module (IB) of an inductance.

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