HK1159871A - Filter circuit having a fe-based core - Google Patents

Description

Filter circuit with iron-based core

The present application is a divisional application of an invention patent application having an application date of 2003, 2/3, application number of 03807917.8, entitled "filter circuit having core based on iron".

Technical Field

The present invention relates to the field of digital and analog information transmission; and more particularly to filter circuits having iron-based amorphous metal cores for bandpass filtering in telecommunications applications such as DSL communication circuits.

Background

Telephone communication lines are currently used to generate a wide variety of signals for commercial and domestic users of products or services. Such products or services of course include conventional voice telephony and communications as well as an ever-increasing number of ancillary services. Supplementary services are generally intended to transmit information over existing telephone lines. Such information currently includes computer data, i.e., data transmitted over a two-way internet connection, as well as an ever-increasing number of other output services. Other supplementary services that are currently being developed or envisioned include unidirectional and bidirectional continuous broadcast information such as pure audio (radio broadcast), video streams, mixed audio/video streams, etc.

The aforementioned communication modes are made possible by existing telephone lines, which provide a network capable of carrying a wide range of such information. Although the typical range of available bandwidth is from about 0 to 3000kHz, the actual bandwidth requirements of conventional voice-only telecommunications approach up to about 6 kHz. Therefore, there is a possibility of communicating communication information using such existing networks at higher frequencies.

Techniques for providing information over a range of frequencies, i.e., a limited "bandwidth," enable different signals to be transmitted simultaneously along a single circuit. This obviates the need for multiple circuits in which signals transmitted along the communication line can be kept within separate frequency bands, i.e. separate bandwidths in the available spectrum of the circuits. Traditionally, this is achieved by using a "band pass filter", which typically includes a coil and a capacitor. The operating characteristics of the coil and the capacitor are selected such that only a limited subset of the total available frequency spectrum passes through the communication line. A band pass filter is typically provided between the communication source and the communication line, thereby limiting the output of the communication device to only fall within the frequency range set for the particular band pass filter. A plurality of band pass filters each having outputs of non-overlapping frequency ranges within the available frequency spectrum may also be used, whereby a plurality of communication devices may share a common communication line.

In the past, choke coils have been used in telecommunication circuits. Although advantageous, the use is not without drawbacks. Such disadvantages include controlling the incompatibility of the desired choke coil performance necessary for the relevant use in, for example, band pass filter circuits. These disadvantages have created a need for materials with "softer" and more controllable magnetic properties. In some cases, cobalt containing an amorphous metal alloy is used to form the choke coil core. While iron-rich amorphous metal cores are more common and less expensive, they have not been used in choke cores because of their inductive properties which are considered unsuitable for use in bandpass filters.

Disclosure of Invention

The present invention provides an inductor having a core consisting essentially of an Fe-based amorphous metal alloy.

In one aspect, the permeability of the core is substantially constant over a frequency range of about 1 to 1000 kHz. In particular, the permeability of the core is substantially constant over a field strength range of about-15 to +15 oersted (Oe).

In another aspect, the invention provides a filter circuit comprising a choke coil including a core having a substantially constant permeability over a frequency range from about 1 to 1000 kHz.

In another aspect, the present invention provides a method for segmented frequency communications that employs a filter circuit having a core permeability that is substantially constant at field strengths of about-15 to +15 Oe.

Advantageous structural features are incorporated into the elements of the present invention. Filter circuits comprising a core made of an Fe-based amorphous metal alloy provide as good or better performance as filter circuits employing a Co-based core. Furthermore, cores composed of Fe-based amorphous alloys are much cheaper than Co-based cores. As such, the Fe-based amorphous metal alloy core provides a low cost solution for communication applications requiring filter circuitry. Filter circuits employing Fe-based cores are particularly useful in communication applications requiring tunable bandpass filters to select frequency bands of digital and analog signals over a communication channel such as DSL.

Drawings

The present invention will be more fully understood and further advantages will become apparent, when reference is made to the following detailed description of the preferred embodiments of the invention and the accompanying drawings, in which:

fig. 1 is a schematic diagram illustrating a communication system according to the present invention;

FIG. 2A is a circuit diagram illustrating a filter circuit of the present invention;

FIG. 2B is a graph depicting the frequency response of a filter circuit according to the present invention;

FIG. 3 is a graph illustrating the magnetization curves of a core according to the present invention and a prior art core;

FIG. 4A is a graph depicting the permeability of the core of the present invention as a function of applied frequency;

FIG. 4B is a graph depicting the permeability of the core of the present invention as a function of applied field strength;

FIG. 5 is a graph depicting the center resonant frequency f shown in FIG. 2B as a function of DC bias field for a core according to the present invention and a prior art core_cA plot of the deviation of (a);

FIGS. 6A-D are circuit diagrams of examples of bandpass filter circuits incorporating cores constructed in accordance with the invention;

FIG. 7 is a graph depicting the relationship between core permeability and annealing temperature, with different curves depicting materials having different crystallization temperatures;

FIG. 8 is a graph depicting the relationship between core permeability and annealing temperature for different annealing times;

FIG. 9 is a diagram depicting a loading configuration of a core for annealing to achieve temperature uniformity within a few degrees;

FIG. 10 is a graph depicting core loss as a function of DC bias field and frequency;

FIG. 11 is a graph depicting core permeability as a function of DC magnetizing force;

FIG. 12 is a Scanning Electron Microscopy (SEM) picture illustrating a cross-section of a ribbon after annealing; and

FIG. 13 is a graph depicting core permeability as a function of volume percent crystallinity.

Detailed Description

Referring to fig. 1 of the drawings, there is illustrated a communication system 20 in accordance with the present invention. A communication source 22, such as a telephone central office, communicates high bandwidth analog and digital signals to a subscriber 24, such as a company or home, using DSL technology over a communication channel 26. A filter circuit 28, such as a band pass filter circuit, may be provided at the central office 22 and the subscriber 24. The operating characteristics of the filter circuit 28 may be adjusted to limit the frequency spectrum passing through the communication line 26. In another embodiment, the filter circuit 28 may be a low pass filter whose operating characteristics may be adjusted to define the frequency range allowed to pass.

Fig. 2A is a circuit diagram illustrating the filter circuit 28 of fig. 1 in accordance with the present invention. In one embodiment, the filter circuit of fig. 2A is a band pass filter circuit comprising an electrical choke L and a capacitor C combined into a parallel circuit. The filter circuit of FIG. 2A accepts inputAnd generates an output signal that is dependent on the frequency of the input signal and the operating characteristics of the filter circuit. Referring to FIG. 2B, the filter circuit of FIG. 2A has a center resonant frequency f_c＝1/([2π(LC)^1/2]Where L is the inductance of the inductor L and C is the capacitance of the capacitor. The filter circuit having a frequency indicative of the resonance frequency f relative to the centre_cBandwidth BW of the frequency range passed. Selection of a particular center resonant frequency f by adjusting the values of inductor L and/or capacitor C_cAnd BW. For example, in a DSL embodiment, one or more filter circuits are employed, each having its own center resonant frequency f_cSuch that each filter circuit passes signals in a particular sub-band within a particular kHz bandwidth used in the DSL communication system.

The inductor L is an energy storage element comprising a ferromagnetic core wound with current carrying wires. For toroidal inductors, the stored energy is W-1/2 [ (B)²A_cl_m)/(2μ₀μ_r)]Wherein B is the magnetic flux density, A_cIs the effective magnetic area of the core, /)_mIs the average magnetic path length, μ₀Is the permeability of free space, mu_rIs the relative magnetic permeability in the material.

By introducing a small gap in the annular ring, the magnetic flux in the air gap remains the same as in the ferromagnetic core material. However, since the permeability of air (μ ≈ 1) is significantly lower than that in typical ferromagnetic materials (μ ≈ several thousand), the magnetic field strength (H) in the gap becomes much higher than that in the rest of the core (H ═ B/μ). The energy stored per unit volume in the magnetic field is W-1/2 (BH), indicating that it is mainly concentrated in the air gap. In other words, the energy storage capacity of the core is increased by introducing a gap.

The gaps may be discrete or distributed. The distributed gap can be introduced by using ferromagnetic powder fixed together with a non-magnetic binder or by partially crystallizing amorphous alloy. In the second case, the ferromagnetic crystalline phases are separated and surrounded by a non-magnetic matrix. This partial crystallization mechanism is utilized in conjunction with the choke of the present invention.

FIG. 3 is a graph depicting the magnetization curves of an Fe-based amorphous alloy core according to the present invention and a prior art Co-rich amorphous alloy-based core. The graph shows that the permeability defined by μ ═ B/H for the core of the present invention is substantially linear. The magnetic field strength H varies over a range from about-40 Oe to +40Oe, resulting in a linear variation of the corresponding magnetic flux density B over a range of about +13 to-13 kG. The linear permeability property makes Fe-based cores suitable for bandpass filter circuits in DSL communication systems. On the other hand, the permeability of the prior art core is only linear up to an induction level of about 7kG, which is significantly lower than the 13kG level achieved in the Fe-based core of the present invention. A greater degree of usable inductance of the core according to the invention is desirable because the core can operate at greater current levels in the telecommunications line.

Figure 4A is a graph depicting the core permeability of the bandpass filter of the present invention as a function of applied field frequency. An Alternating Current (AC) signal is applied to a bandpass filter having a core consisting essentially of an Fe-based amorphous metal alloy, wherein the permeability is approximately 700. The frequency was varied over the range of 1-10000kHz while measuring the permeability. The graph shows that the permeability is constant up to a range of about 1000 kHz. The permeability gradually decreases linearly from 700 to 20 as the frequency varies from 1000kHz to 20000 kHz.

Figure 4B is a graph depicting the core permeability of the bandpass filter of the present invention as a function of applied field strength. A band pass filter having an Fe-based core with a permeability of about 700 was placed in a magnetic field H varying over a range of 0 to 35Oe while measuring the permeability of the core. The graph shows that the permeability does not change significantly in the magnetic field H range of about 0 to 15 Oe. As the magnetic field H changes beyond 17Oe, the permeability gradually decreases in a linear manner from 700 to 300. Ferromagnetic cores may be used in filter circuits that are part of a communications circuit such as DSL. The ferromagnetic core exhibits a linear permeability as a function of frequency and magnetic field strength over a range representative of communication applications such as DSL.

FIG. 5 compares the center resonant frequency f defined in FIG. 2B for a core according to the present invention and a prior art core_cOf (3) is detected. These cores, having an OD of about 13mm, an ID of about 8mm and a height of about 7mm, each together with 150 turns of copper wire, form an inductance L of about 8 mH. The inductor is connected in parallel with a1 muF capacitor, resulting in a central resonance frequency F of about 1800Hz_c. The prior art core saturates at DC bias fields above 10Oe, beyond 10Oe, the core becomes inoperable, while the core according to the present invention works well beyond 10 Oe. A lower center resonance frequency shift is required for stable filter operation.

Figure 6 shows an example of an Fe-based core according to the invention for use in a bandpass filter circuit in an inductor L. All these circuits have a voltage of 1/[2 π (LC)^1/2]Given the same center resonant frequency.

FIG. 7 is a graph depicting the relationship between permeability of a core and annealing temperature, with different curves depicting different crystallization temperatures T_xThe material of (1). The permeability was measured at a frequency of 10kHz, 8 turns jig and 100mV AC excitation using a commercially available inductive bridge. The annealing time was kept constant at 6 hours. All cores were annealed in an inert gas atmosphere. The different curves represent Fe-based alloys with small changes in chemical composition and therefore in their crystallization temperature. The crystallization temperature was measured by Differential Scanning Calorimetry (DSC). For a constant annealing time, the decrease in permeability is observed with increasing annealing temperature. For a given annealing temperature, the permeability scales with the crystallization temperature, i.e. for the alloy with the highest crystallization temperature, the permeability is highest.

FIG. 8 depicts the permeability of an annealed Fe-based core having the same chemical composition as a function of annealing temperature. Different curves represent different annealing times. The curves show that for temperatures above 450 ℃, the effect of the annealing temperature far exceeds the effect of the annealing time.

Based on the information in fig. 7 and 8, an appropriate combination of annealing temperature and time is selected for Fe-based boron and silicon containing amorphous metal alloys. Provided that the crystallization temperature (T) of the alloy_x) And/or the chemical composition is known, such selection can be made. For example, for having T_x507 deg.C Fe₈₀B₁₁Si₉In order to obtain a permeability in the range of 100 to 400, an annealing temperature in the range of 420 to 425 ℃ for 6 hours is appropriate. To obtain a permeability of about 700, T is provided_xFe-based amorphous alloy 527 ℃ was heat treated at 430 ℃ for 6 hours as depicted in fig. 7.

An improvement in the linearity of permeability is achieved by applying a magnetic field perpendicular to the direction of magnetic excitation of the core during the heat treatment. For example, the Fe-based core of the present invention used in FIGS. 3, 4A, 4B and 5 is subjected to a heat treatment under a vertical magnetic field of about 200 Oe. The field annealing is performed in conjunction with the discussion of fig. 7 and 8.

Referring to fig. 9, it is shown that reproducibility and uniformity for a given permeability value are obtained when maintaining a temperature change of less than one or two degrees. A specialized charging configuration was developed for the annealing process to establish temperature uniformity and reproducibility in the furnace. For the cassette inert gas furnace, the wire mesh a1 plates 72 were stacked as per fig. 9, the arrangement being designed in the center of the furnace. The a1 plate 72 is a substrate that holds the core 71 during annealing.

Typical magnetic characterization data for the inductor core, such as core loss and DC bias, are shown in fig. 10 and 11. The core loss data is plotted as a function of the DC bias field, with different curves representing different measurement frequencies. The data shown are for a core with an OD of 25 mm. An important parameter for core performance is the percentage of initial permeability that is maintained when the core is driven by a DC bias field. FIG. 11 depicts a typical DC bias curve for a core with an OD of 35 mm.

Scanning Electron Microscopy (SEM) and x-ray diffraction (XRD) were performed to determine the distribution and the percentage of crystallization of the annealed core. FIG. 12 depicts a typical cross-sectional SEM showing bulk alloy and surface crystallization with a thickness of about 20 μm. The volume percent of crystallization was determined from the SEM and XRD data and is plotted as a function of permeability in fig. 13. For permeability in the range of 100 to 400, a large amount of crystallization in the range of 5% to 30% is required.

Having thus described the invention in rather full detail, it is to be understood that such detail need not be strictly adhered to, but that additional changes and modifications may suggest themselves to one skilled in the art. For example, filter circuits having Fe-based amorphous metal cores may be used for communications other than DSL. These and other embodiments are intended to be within the scope of the present invention as defined by the appended claims.

Claims (27)

1. A band pass filter comprising an inductor having a core consisting essentially of Fe-based amorphous metal alloy strips having a crystallization temperature in the range from 507 ℃ to 527 ℃, wherein:

the metal alloy strip is annealed to produce a linear BH loop therein having a squareness ratio approaching zero over a field strength range of about-15 to +15 oersted, an

The core has a substantially constant permeability over a frequency range of 1kHz to 1000 kHz.

2. A bandpass filter as recited by claim 1, wherein said substantially constant permeability ranges from 100 to 400.

3. The bandpass filter of claim 1, wherein the substantially constant permeability exists over a field strength range of approximately-1.2 to +1.2 kA/m.

4. An inductor comprising a core consisting essentially of an Fe-based amorphous metal alloy strip having a crystallization temperature in the range from 507 ℃ to 527 ℃, wherein:

the metal alloy strip is annealed to produce a linear BH loop therein having a squareness ratio approaching zero over a field strength range of about-15 to +15 oersted, an

The core has a substantially constant permeability over a frequency range of 1kHz to 1000 kHz.

5. The inductor of claim 4 wherein the substantially constant permeability ranges from 100 to 400.

6. The inductor of claim 4 wherein the substantially constant permeability is present over a field strength range of-1.2 to +1.2 kA/m.

7. An improvement in a method for defining frequency communications, wherein an inductor is employed having a core consisting essentially of an Fe-based amorphous metal alloy strip having a crystallization temperature in the range from 507 ℃ to 527 ℃, wherein:

the metal alloy strip is annealed to produce a linear BH loop therein having a squareness ratio approaching zero over a field strength range of about-15 to +15 oersted, an

The core has a substantially constant permeability over a frequency range of about 1kHz to 1000 kHz.

8. The method of claim 7, wherein the substantially constant permeability ranges from 100 to 400.

9. The method of claim 7, wherein the core permeability is substantially constant over a field strength range of the magnetic field of-1.2 to +1.2 kA/m.

10. A bandpass filter comprising an inductor having a core consisting essentially of Fe-based amorphous metal alloy ribbon, wherein:

the metal alloy strip is annealed to produce a linear BH loop therein having a squareness ratio approaching zero over a field strength range of about-15 to +15 oersted, an

The core has a substantially constant permeability over a frequency range of 1kHz to 1000 kHz.

11. A bandpass filter as recited by claim 10, wherein said substantially constant permeability ranges from 100 to 400.

12. The bandpass filter of claim 10, wherein the substantially constant permeability exists over a field strength range of approximately-1.2 to +1.2 kA/m.

13. An inductor comprising a core consisting essentially of Fe-based amorphous metal alloy ribbon, wherein:

the metal alloy strip is annealed to produce a linear BH loop therein having a squareness ratio approaching zero over a field strength range of about-15 to +15 oersted, an

The core has a substantially constant permeability over a frequency range of 1kHz to 1000 kHz.

14. The inductor of claim 13 wherein the substantially constant permeability ranges from 100 to 400.

15. The inductor of claim 13 wherein the substantially constant permeability is present over a field strength range of-1.2 to +1.2 kA/m.

16. An improvement in a method for defining frequency communications, wherein an inductor is employed having a core consisting essentially of Fe-based amorphous metal alloy ribbon, wherein:

the metal alloy strip is annealed to produce a linear BH loop therein having a squareness ratio approaching zero over a field strength range of about-15 to +15 oersted, an

The core has a substantially constant permeability over a frequency range of about 1kHz to 1000 kHz.

17. The method of claim 16, wherein the substantially constant permeability ranges from 100 to 400.

18. The method of claim 16, wherein the core permeability is substantially constant over a field strength range of the magnetic field of-1.2 to +1.2 kA/m.

19. A bandpass filter comprising an inductor having a core consisting essentially of Fe-based amorphous metal alloy ribbon, wherein:

the metal alloy strip is annealed to produce a linear BH loop therein having a squareness ratio approaching zero over a field strength range of about-15 to +15 oersted, an

The core has a substantially constant permeability over a frequency range of 1kHz to 1000kHz, an

The core is produced under one of the following conditions:

annealing at a temperature in the range of 415 to 450 ℃ for 6 hours;

annealing at a temperature in the range of 440 to 450 ℃ for 4 hours; or

Annealing was carried out at 450 ℃ for 2 hours.

20. A bandpass filter as recited by claim 19, wherein said substantially constant permeability ranges from 100 to 400.

21. The bandpass filter of claim 19, wherein the substantially constant permeability exists over a field strength range of approximately-1.2 to +1.2 kA/m.

22. An inductor comprising a core consisting essentially of Fe-based amorphous metal alloy ribbon, wherein:

the metal alloy strip is annealed to produce a linear BH loop therein having a squareness ratio approaching zero over a field strength range of about-15 to +15 Oersted,

the core has a substantially constant permeability over a frequency range of 1kHz to 1000kHz, an

The core is produced under one of the following conditions:

annealing at a temperature in the range of 415 to 450 ℃ for 6 hours;

annealing at a temperature in the range of 440 to 450 ℃ for 4 hours; or

Annealing was carried out at 450 ℃ for 2 hours.

23. The inductor of claim 22 wherein the substantially constant permeability ranges from 100 to 400.

24. The inductor of claim 22 wherein the substantially constant permeability is present over a field strength range of-1.2 to +1.2 kA/m.

25. An improvement in a method for defining frequency communications, wherein an inductor is employed having a core consisting essentially of Fe-based amorphous metal alloy ribbon, wherein:

the metal alloy strip is annealed to produce a linear BH loop therein having a squareness ratio approaching zero over a field strength range of about-15 to +15 Oersted,

the core has a substantially constant permeability over a frequency range of about 1kHz to 1000kHz, an

The core is produced under one of the following conditions:

annealing at a temperature in the range of 415 to 450 ℃ for 6 hours;

annealing at a temperature in the range of 440 to 450 ℃ for 4 hours; or

Annealing was carried out at 450 ℃ for 2 hours.

26. A method as recited by claim 25, wherein said substantially constant permeability ranges from 100 to 400.

27. A method as recited in claim 25, wherein said core permeability is substantially constant over a field strength range of a magnetic field of-1.2 to +1.2 kA/m.