Abstract:
This invention relates generally to electronic filter circuits on POTS (Plain Old Telephone System) lines and more particularly to an electronic filter circuit which contains a magnetically saturable inductor switch. The switch enables multiple POTS device micro-filter combinations to be simultaneously active without the drawbacks of the prior art.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to electronic filter circuits on Plain Old Telephone System (POTS) lines and more particularly to an electronic filter circuit which incorporates a saturable magnetic core. The filter circuit enables multiple POTS devices with their associated filters to be placed on a POTS line without causing undesirable signal attenuation.  
           [0003]    2. General Background and State of the Art  
           [0004]    The use of a Digital Subscriber Line (DSL) Internet access service has gained widespread popularity as a technology in which advanced modems are used to increase data transmission speeds over regular telephone lines, sometimes referred to as a Plain Old Telephone System (POTS) lines. DSL, as used in this document, is understood to include, but is not limited to, various modes of DSL known as HDSL, ADSL, VDSL. In any establishment using a POTS line, such as for example residential homes and office complexes, communications devices such as telephones, facsimile machines, DSL modems and other devices are typically connected in parallel across the common POTS line. Deployment of DSL modems on a POTS line requires the installation of filters on all of the POTS communication devices on the line. The filter blocks certain frequencies ensuring that voice transmission over the telephone lines is not disturbed during data transmission by a DSL modem. However, each filter connected to a POTS device constitutes an electrical load on the POTS line. This electrical load causes attenuation of the electrical signal, resulting in increased signal reception errors and degraded DSL performance.  
           [0005]    Specifically, the filter typically contains an electrical capacitor that bypasses a portion of the electrical signal around the receiving circuitry. This capacitor may cause attenuation of the electrical signal(s) going through the POTS line even when the device to which the filter capacitor is connected is not in use. If multiple telephones, facsimile machines, DSL modems, and/or other POTS communication devices are connected to the same DSL line, each with its own filter, undesirable attenuation of the incoming DSL signal may occur due to the shunting effect of the capacitors in each of the filters. However, if the POTS device connected to the circuit is not in use, there is no need for the filtering function caused by the filter capacitor for these POTS devices.  
           [0006]    Existing filters may use active and/or solid state components to switch off the filter&#39;s capacitor when the POTS device to which it is attached is not in use. However, these filters are relatively expensive, and may require power even when the POTS device or phone is on-hook which may cause interference with the telephone company switching circuits.  
           [0007]    Therefore, there is a need for a passive electronic filter circuit in which the filter capacitor may be activated when the associated POTS device is in use and deactivated when the associated POTS device is not in use.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides a passive electronic filter circuit that enables a large number of POTS devices to be placed on one POTS line without the filter circuits causing attenuation of a DSL signal. In one exemplary embodiment, the present invention employs a POTS filter circuit which is a low-pass filter having a magnetically saturable core serving as a switch. The magnetically saturable core is wrapped with a coil which is connected to the DC voltage impressed upon the line by the telephone company such that a voltage is always present when the POTS device is off-hook. When the POTS device attached through the filter to the POTS line is not in use (on-hook), there is no direct current power to the coil and thus the core is not magnetically saturated and the filter simulates an open circuit. When a POTS device attached through the filter to the POTS line is in use (off-hook), the DC power provided on the incoming line energizes the coil, magnetically saturating the core and thereby interconnecting a filtering capacitor imposing a DSL signal filter on the POTS line.  
           [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 description 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 adapted to mate with a POTS communication device; and  
         [0012]    [0012]FIG. 2 shows an alternative circuit diagram of a filter circuit of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]    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.  
         [0014]    [0014]FIG. 1 shows an exemplary circuit diagram of a filter circuit  10  of the present invention which is adapted to mate with a POTS communication device  12  and a POTS line  14 . The filter circuit  10  contains a saturable core transformer  20  and a filter transformer  22  both of which are coupled to a filter capacitor  24 .  
         [0015]    The first component connected to the POTS line  14 , i.e. the incoming telephone line, is the saturable core transformer  20 . The saturable core transformer  20  includes windings or coils  30  and  32  each having many turns of a conductive element such as a wire  34  and  36 , respectively, wrapped around a core  38  of high magnetic permeability saturable ferromagnetic material so as to present a very high inductive reactance (impedance) at all frequencies in both the telephony and DSL bands. Windings or coils  30  and  32  are magnetically coupled to one another and to core  38 . Said coupling is such that an electrical current I 1  flowing into coil  30  from POTS line  14  and an electrical current I 2  flowing into coil  32  from conductor  44  generate magnetic fluxes in core  38  that augment or aid one another.  
         [0016]    When the telephone or POTS communication device  12  is not in use (on-hook), no direct current (DC) flows through the wires  34  and  36  of the coils  30  and  32 , and the filter circuit  10  has a very high impedance, effectively switching OFF or virtually open-circuiting the filter circuit  10  and POTS communication device  12  from the POTS line  14 . The inductance and resultant inductive reactance of the coils or windings  30  and  32  are large, so that the very small alternating current (AC) that flows, limited by the large inductive reactance, is insufficient to cause magnetic saturation of the core  38 .  
         [0017]    When the POTS communication device  12  is in use (off-hook), a direct current (DC) flows through the wires  34  and  36  of the coils or windings  30  and  32 , causing magnetic saturation of the core  38 . As a consequence, the inductance drops from the on-hook large value to a residual value (not zero) that is, or is approximately, equal to the preferred design value for the inductance required for a low-pass filter having the desired characteristics of cutoff frequency and rate of gain roll-off with frequency. Thus, saturation of the core  38  results in effectively switching ON or virtually closed-circuiting the filter circuit  10  to the telephone line  14 . Even a small direct current, on the order of one milli-ampere in the preferred embodiment, is sufficient to magnetically saturate the transformer core  38 . When the core  38  saturates, the transformer  20  retains a relatively small residual inductance that is fairly constant over a range of direct currents.  
         [0018]    In other words, the saturable core transformer  20  can be effectively electrically connected via saturation or disconnected via unsaturation of the ferromagnetic core  38  by the windings or coils  30  and  32 , the saturation and unsaturation occurring as a consequence of a supervisory signal consisting of an electrical direct current of sufficient magnitude through the windings or coils  30 ,  32 .  
         [0019]    The geometry of the core  38 , as well as the core material, the lamination stacking technique and number of windings or turns of the coil(s) are chosen to provide a residual saturated inductance that is appropriate for the design value that the input inductance should possess for a low-pass filter with the desired gain-frequency characteristics such as cut-off frequency for a DSL signal. As a consequence, at saturation the incoming telephone line is connected to a low-pass filter and its input impedance is whatever input impedance a low-pass filter of that design should possess at any given frequency.  
         [0020]    In the embodiment of the invention illustrated in FIG. 1, the filter circuit  10  is a double L-section (LCLC) passive 4 th  order Chebyshev low-pass filter having saturable core transformer  20  and filter transformer  22  wired as two cascaded coupled transformers, a filter capacitor  24  placed across the conductors  42 ,  44  connecting the transformers  20  and  22 , and a shunt capacitor  40  placed across the output lines  46 ,  48  of the filter transformer  22 . The saturable core transformer  20  includes two coils wound bifilarly, each having  260  turns of #35 AWG SPN enamel coated copper wire (magnet wire) on a custom bobbin, made by cutting off a side of two bobbins (coil formers), P/N BE-16H, DWG. No. P-1626, manufactured by the Pin Hsiang Group (Taiwan). The cut bobbins are then glued together, for example with Loctite 444 Instant Adhesive, so as accommodate twice the core cross-sectional area as one bobbin alone. The bobbin was then wound with one layer of 0.001 inch thick yellow mylar tape to provide additional mechanical support. The resulting DC current resistance, RDC is 21.86 ohms (measured), for the two series connected coils.  
         [0021]    The input coupled saturable core transformer  20  has a core  38  constructed out of a stack of EE laminations, Magnetic Metals lamination type 16 ELM, material type HyMu80, performance designation SUPERPERM80. Forty seven laminations, each 0.008 inch thick are fully interleaved with pairs of laminations alternatively inserted from one side of a bobbin and then from the other side of a bobbin, except that the last lamination is inserted alone.  
         [0022]    The capacitor  24  in this embodiment may be, for example, a ceramic, X7R formulation 0.1 microfarad, 100V capacitor. The shunt capacitor  40  may be a mylar, 0.056 μfd, 50V capacitor.  
         [0023]    In the filter circuit  10  illustrated in FIG. 1, the filter transformer  22  is a cascaded ferrite-core transformer. The filter transformer  22  in this embodiment includes two coils  50 ,  52  wound bifilarly, each having 229 turns of #33 AWG SPN wire  54 ,  56  respectively, on a EP17 bobbin or core  58 . For example, a Ferroxcube (Philips) type CSH-EP17-1S-8P component may be used for core  58 . The direct current resistance, RDC, for both coils  50 ,  52  measured in series is approximately 9 ohms.  
         [0024]    The core  58  of the filter transformer  22  may alternatively be a Ferroxcube (Philips) EP17-3E27 (A L =160 nHy/turnsqd.) with the center-post of one of the core halves ground down to create a sufficient air gap to provide and open circuit inductance (L S ) of 33.4 mHy at 1.0 kHz, 100 mVRMS.  
         [0025]    The foregoing configuration of the filter circuit  10  provides a passive electrical filter network composed of transformers and capacitors which, upon a change in a switching mechanism in the POTS communication device  12 , will cause the electrical input impedance looking into the filter circuit  10  from the POTS line  14  to be changed from its normal off-hook operational input impedance to a very high on-hook input impedance. A very high input impedance in the filter circuit  10  results in very small loading on the POTS line  14  circuit that drives the POTS line  14  and results in the filter circuit  10  appearing as a virtual open-circuit on the POTS line  14 . In this way a switched impedance-blocking function is implemented.  
         [0026]    The ultimate purpose of switching the passive filter network to a high input-impedance state is to lower the incidence of reception errors on the associated DSL line connected across the filter circuit  10  input terminals and to improve reception on the POTS communication device  12  connected across the filter circuit&#39;s output terminals. The POTS line  14  from the telephone company central office has a certain impedance which is usually  600  ohms in the United States. This impedance and the combined parallel input impedances of the filter circuit  10  connected across the incoming POTS line  14  act as a voltage divider that lowers the useable signal level. Smaller signal levels result in more reception errors; larger signal levels result in fewer reception errors.  
         [0027]    The present invention exploits magnetic saturation of the core  38  of the saturable core transformer  20  as the switching agent. Additional components are not required, except that in the first embodiment of the invention a larger number of transformer core laminations are required than what would be used for a similar transformer without the impedance switching (impedance-blocking) capability.  
         [0028]    Accordingly, one purpose of this invention is to provide a passive inductor-capacitor filter, inductor-resistor filter, or inductor-capacitor-resistor filter, the capacitor(s) being effectively electrically connected to the telephone line via saturation of the ferromagnetic core  38  of the series connected transformer  20 , or at least one of the transformers, if a plurality of saturable transformers is employed. The saturation and unsaturation occur as a consequence of a supervisory signal consisting of an electrical direct current of sufficient magnitude through the windings or coils  30 ,  32  of transformer  20 . In one embodiment of this invention, the transformer  20  is a coupled inductor transformer.  
         [0029]    In the present invention, the saturable transformer  20  has many turns of wire to provide the large inductance and impedance required. Moreover, the diameter of the coil wire is chosen to be as large as possible so that DC electrical resistance of the windings or coils  30  and  32  are as small as possible. In the detailed embodiment of the invention illustrated in FIG. 1 and discussed above, the low-pass filter circuit  10  is a 4 th  order Chebyshev filter. While the invention has been described in reference to a specific embodiment, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims. For example, a 3 rd  order Butterworth low-pass filter, or 5 th  order Bessel low-pass filter are considered to be equivalent circuits which those skilled in the art would understand to fall within the scope of the invention.  
         [0030]    The filter circuit  10  detailed above provides an approximation to a passive 4 th  order Chebyshev low-pass filter having 0.1 dB passband ripple, with saturable core transformer  20  implementing an impedance-blocking function in response to an electrical direct current IDC flowing through the saturable transformer windings or wires  34 ,  36 . This direct current magnetically saturates the core  38  of the saturable core transformer and effectively switches ON the low-pass filter. At less than 1 mADC in the preferred embodiment the transformer&#39;s magnetic core  38  will be saturated. The switching innovation is designed to reduce circuit loading on a plain-old-telephone system (POTS) line in DSL applications when more than one such circuit is connected in parallel with the others. The filter circuit  10  may have a response of −0.1 dB@4.1 kHz to −3.01 dB@5.0 kHz and a high frequency rolloff of −60 dB/decade. When the two coils are connected in the intended series-aiding configuration, L P =24.4 mHy (OCL)@1.0 kHz, 100 mVRMS.  
         [0031]    [0031]FIG. 2 depicts an alternative embodiment of the present invention of filter circuit  60  of the present invention. In the filter circuit  60 , the input comes from a POTS line  14  to a saturable inductor  64 . The output of the saturable inductor is connected to the input of a filtering inductor  66  as well as to one side of a filter capacitor  68 . The output of the filtering inductor  66  is connected to a shunt capacitor  70  as well as in series to the POTS communication device  12  represented as a  600  ohms resistant load. The opposite terminal of the output of the POTS communication device  12  (represented as a  600  ohm resistant load) is connected first to the opposite side of shunt capacitor  70  and subsequently to the opposite side of filtering capacitor  68  and then output to the POTS line  14 . In this single ended configuration, the saturable core transformer  20  of FIG. 1 is replaced with the saturable inductor  64  having an inductance of 22 milliHenries. The filtering inductor  66  has an inductance of 33.4 milliHenries. The filtering capacitor  68  has a capacitance of 100 nanofarads and the shunt capacitor  70  has a capacitance of 56 nanofarads. However, there may be some performance degradation in the use of this configuration as opposed to the balanced configuration of FIG. 1.  
         [0032]    Notwithstanding that FIGS. 1 and 2 show coupled transformers or 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 non-saturable transformers as the second filtering transformer whose windings create magnetic fields that oppose one another; that is, the scope of this invention includes “common-mode” transformers. Such transformers can be placed in cascade with any other transformer(s), provided that a saturable inductor or transformer is the transformer connected to the telephone line.  
         [0033]    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.