Abstract:
The present invention provides a passive electronic filter circuit for telephony equipment used in conjunction with xDSL that reduces the number of discrete components over existing filters. In one exemplary embodiment the present invention employs a POTS filter having a coupled-inductor array made up of two cascaded pairs of coupled inductors that share one common ferrite core. The use of a common ferrite core reduces the number of coupled inductor packages required to perform the same filtering.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to electronic filter circuits for plain old telephone system (POTS) lines and more specifically to an integrated coupled inductor for use in a POTS filter.  
           [0003]    2. General Background and State of the Art  
           [0004]    The growing demand for broadband data reception has lead to the growing popularity of digital subscriber lines (DSL). DSL provides for high speed data transference and reception over regular twisted-pair copper telephone lines, sometimes referred as plain old telephone system (POTS) lines. DSL provides for the transmission of both voice and high speed data transmission over the POTS line. For example, in one implementation of asymmetrical DSL or ADSL, the signals on the telephone line are split into three distinct bands: the voice band operating from 0-4 kHz, an upstream data band operating between 25 and 160 kHz and a downstream data band operating above 240 kHz. The downstream data band has greater bandwidth than the upstream data band because the typical user receives more information than he/she sends. Various implementations of DSL exist including: ADSL; very high bit rate DSL, or VDSL; symmetrical DSL or SDSL, and high bit rate DSL or HDSL. DSL or xDSL, as used in this document refers to these and any other types of DSL. Where DSL is deployed, communication devices such as telephones, fax machines, DSL modems, and other devices are all connected in parallel across an existing POTS line.  
           [0005]    The deployment of DSL modems in residences and businesses (the customer&#39;s premise) typically requires the installation of a filter on all of the devices (known as POTS devices) sharing the same POTS line as the DSL modem. This is because intrusion in the form of noise may occur in one channel (such as the voice channel) due to signal transmissions in another channel signal (such as the upstream data channel). For example, xDSL signals and POTS signal can interact with magnetically non-linear components in the POTS device to cause audible noise, such as a hum, in a voice telephone conversation. Also, when a POTS device goes from an on hook status to an off hook status the impedance of the POTS device changes, which can result in transient noise in the xDSL channel.  
           [0006]    The POTS filter may be implemented as part of a POTS splitter which both splits and filters the incoming POTS line and/or as a filter connected directly to the POTS device. The POTS filter functions to separate the low frequency telephony signals from the higher frequency data signals by filtering out the xDSL data signals.  
           [0007]    Currently, passive POTS filters are manufactured using discrete capacitors and a plurality of inductors or coupled inductors. Because current filters require a large number of discrete components they require more space and are more expensive. What is needed is a way to reduce the components of a POTS filter, resulting in smaller filters and reduced costs.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides a passive electronic filter circuit for telephony equipment used in conjunction with xDSL that reduces the number of discrete components over existing filters. In one exemplary embodiment the present invention employs a POTS filter having a coupled-inductor array made up of two cascaded pairs of coupled inductors that share one common ferrite core. The use of a common ferrite core reduces the number of coupled inductor packages required to perform the same filtering.  
           [0009]    Many modifications, variations and combinations of the methods and systems of filtering are possible in light of the embodiments described herein. The description above and many other features and attendant advantages of the present invention will become apparent from a consideration of the following detailed descriptions when considered in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    A detailed description with regard to the embodiments in accordance with the present invention will be made with reference to the accompanying drawings; wherein:  
         [0011]    [0011]FIG. 1 shows an exemplary circuit diagram of a filter circuit of the present invention which is adopted to mate with a POTS communication device;  
         [0012]    [0012]FIG. 2 shows a diagram of a common ferrite core;  
         [0013]    [0013]FIG. 3 is an overhead view of two coil formers installed on an EE10 core; and  
         [0014]    [0014]FIG. 4 is a side view of two coil formers installed on an EE10 core.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    The following description should not be taken in a limiting sense but is made for the purpose of illustrating the general principles of the invention. The section titles and overall organization of the present detailed description are for purposes of convenience only and are not intended to limit the present invention. As used in this document, a coupled inductor is a device that uses electromagnetic induction to transfer electrical energy from one circuit to another, usually with a change in voltage or current.  
         [0016]    [0016]FIG. 1 shows an exemplary circuit design of a filter  100  of the present invention, which is adapted to mate a POTS communication device  102  with a POTS line  104 . The unfiltered xDSL signal  105  is typically sent to a DSL modem. Filter  100  includes a coupled inductor array  120  comprising a first coupled inductor  122  and a second coupled inductor  124 . First coupled inductor  122  comprises a first inductor  126  and a second inductor  128 . Second coupled inductor  124  comprises a third inductor  130  and a fourth inductor  132 . First coupled inductor  122  and second coupled inductor  124  share a single common core  106 . A first shunt capacitor  134  is provided between lines  112  and  114  and a second shunt capacitor  136  is provided between output lines  116  and  118  of second coupled inductor  124 .  
         [0017]    In a preferred embodiment, common core  106  is a MnZn ferrite core of EE10 geometry as illustrated in FIG. 2. An exemplary ferrite core  106  is Part No. FCI-9.70/12.4/2.85 manufactured by Nippon Ceramic Co., Ltd. Common core  106  includes a right leg  210 , a left leg  214  and a center leg  206 . The cross-sectional area of center leg  206  is chosen to be relatively large as compared to the other cross-sectional areas of the magnetic paths in common core  106 . This gives center leg  206  a low reluctance path for magnetic flux passing through it and functions as a low reluctance return path for magnetic fluxes created by the first coupled inductor  122  and second coupled inductor  124 . This results in less magnetic flux transferred between first coupled inductor  122  and second coupled inductor  124 . This decouples third and fourth inductors  130  and  132  from first and second inductors  126  and  128 . The inductor pairs  126  and  128  in first coupled inductor  122  and the inductor pairs  130  and  132  of second coupled inductor  124  posses high magnetic coupling. In one exemplary embodiment, the magnetic coupling between the first inductor  126  and second inductor  128  of first coupled inductor  122  and between third inductor  130  and fourth inductor  132  of second coupled inductor  124  has a magnetic coupling coefficient of 0.95 (close to the theoretical value of one). Third inductor  130  and fourth inductor  132  are decoupled magnetically from first inductor  126  and second inductor  128 . In one exemplary embodiment, the magnetic coupling coefficient is 0.003, which is close to the ideal value of zero needed for total decoupling. Thus, first coupled inductor  122  is almost totally isolated from second coupled inductor  124 . While two coupled inductors,  122  and  124 , are shown, more than two coupled inductors can be used without departing from the scope of the present invention.  
         [0018]    Core  106  is divided into a first half  202  and a second half  204 . The face of the left leg  214  and the face of the right leg  210  of one of the halves (such as the first half  202 ) are grounded down, milled down or shaved away to create a first air gap  216  and a second air gap  218 . In an exemplary embodiment, the gap thickness is 0.18 mm+/−0.05 mm. Varying the gap thickness varies the open circuit inductance of the inductors  126 ,  128 ,  130  and  132 . In an exemplary embodiment, the thickness of the first air gap  216  and the second air gap  218  are chosen such that the open circuit induction of inductors  126 ,  128 ,  130  and  132  are sufficient to give the filter  100  the desired filter response. In an exemplary embodiment the open circuit inductance for each inductor  126 ,  128 ,  130  and  132  is 4.5 mHy at 1.0 kHz and 100 mVrms.  
         [0019]    In one exemplary embodiment, a thin layer of epoxy resin, such as Nagase ChemTex XNR3501SL, is optionally applied between the matting face of center leg  206  of the first half  202  and the second half  204  of common core  106 . Pressure is then provided to control the resin layer thickness. Once hardened the thin layer of epoxy creates a very small center leg adjustment gap  220 . Center leg adjustment gap  220  is non-ferromagnetic and helps to reduce the magnetic reluctance of the center leg  206  and decrease the magnetic coupling coefficient. Slight adjustments to the center leg adjustment gap  220  can adjust the magnetic coupling coefficient of the center leg  206 . Instead of using an adjustable gap, the magnetic coupling coefficient can be adjusted by other means known to those in the art including varying the cross-sectional area of center leg  206 .  
         [0020]    Shunt capacitors  134  and  136  are, in an exemplary embodiment, film capacitors. In one exemplary embodiment, shunt capacitor  134  is a 33 nFd capacitor and shunt capacitor  136  is a 47 nFd capacitor.  
         [0021]    In the embodiment of the invention as illustrated in FIG. 1, the filter  100  is a double L-section (LCLC) passive 4 th  Order Chebyshev low pass filter. The desired filter response can be chosen by providing appropriate core path lengths, core path cross sectional areas, adjustable gap thickness and air gap thickness. Of course, other filters can be utilized such as a 3 rd  Order Butterworth low pass filter and a 5 th  Order Bessel low pass filter, wherein two or more coupled inductors in those filters share a common core. In an exemplary embodiment, filter  100  has an insertion loss of −1.5 dB between 2.2 kHZ and 3.5 kHz, a passband ripple of 1.5 dB and a high frequency roll-off of −55 dB to −65 dB over 30 kHz to 1.1 MHz.  
         [0022]    By careful selection of component values and parameters, the responses of the filter of the present invention will be almost the same as a filter with a conventional design using separate cores. Thus, the filter of the present invention will filter out the xDSL signal such that it does not reach the POTS device. Also, the relatively high impedance looking into the filter from the line side, swamps out the impedance changes occurring on the other side of the POTS filter in the POTS device.  
         [0023]    In an exemplary embodiment, the coupled inductor array  120  is installed on a base for mounting on a printed circuit board. As seen in FIGS. 3-4, a first coil former  302  and second coil former are installed around core  106  to form a base. The first coil former  302  and the second coil former  304  are a combination of a mounting base and winding bobbin. An exemplary coil former is PIN Base-SLF 1312-F8P, manufactured by Sumida Corp. of San Diego, Calif. In one exemplary embodiment, first coupled inductor  122  includes two coils, each coil wound bifilarly and each coil having 195 turns of #34.5 AWG HPN enamel coated wire (magnet wire) in 14 layers on first coil former  302 . In FIGS. 3-4 the wire coils are not pictured in order to better see the first coil former  302  and the second coil former  304 . In the exemplary embodiment, second coupled inductor  124  also includes two coils, each coil wound bifilarly and each coil having 195 turns of #34.5 AWG HPN enamel coated wire (magnet wire) in 14 layers on second coil former  302 .  
         [0024]    First coil former  302  and second coil former  304  are mechanically secured to each other and around core  106  to form a package that can be mounted on a printed circuit board. As seen in FIG. 4, there is a plurality of electrical terminals  402  for use in mounting the package on a printed circuit board and connecting to external components such as RJ-11 connectors for coupling the POTS line and POTS devices as well as the shunt capacitors in order to form a complete POTS filter unit.  
         [0025]    Notwithstanding that FIG. 1 shows coupled inductors whose windings aid one another rather than oppose one another in the establishment of the magnetic fields within their respective cores, the scope of this invention includes the incorporation of coupled-inductors, such as second coupled inductor  124 , whose windings create magnetic fields that oppose one another; that is, the scope of this invention includes “common-mode” coupled inductors. Such transformers can be placed in cascade with any other transformer(s), provided that the first coupled inductor  122  is the coupled inductor connected to the telephone line.  
         [0026]    Although specific components with particular operating parameters are described in the preferred embodiment, a variety of different components with varying operating parameters may be used which do not depart from the scope of the present invention. The preferred embodiment described above are for exemplary purposes only. While the filter circuit can be configured as a separate electrical element, it should be appreciated that the circuit can readily be incorporated into the design of a telephone or other device connected to the POTS line. The invention applies to all types of combinations and/or rearrangements of the methods and systems described. It is to be understood that the invention is not limited to these specific embodiments. With respect to the claims, it is the applicant&#39;s intention that the claims not be interpreted in accordance with the sixth paragraph of 35 U.S.C. § 112 unless the term “means” is used followed by a functional statement.