Abstract:
The current technology involved with overlaying any type of digital subscriber loop (xDSL) service with plain old telephone service (POTS) makes use of two separate transformers, one for POTS and another for xDSL. This invention provides a transformer which combines the POTS transformer and the xDSL transformer into one transformer. This considerably reduces the weight volume and cost of the overlaid POTS and xDSL circuits. In combining the two transformers the magnetic coupling between any one of the windings used for POTS and any one of those used for xDSL must remain weak despite their close proximity. In addition, any two windings of the same type of service, either POTS or xDSL, must remain strongly coupled. This is achieved by choosing a special geometric form for the core and choosing strategic locations for the windings. A portion of the core is dedicated to serve as a shunt for each component of the magnetic field produced by the windings. Strongly coupled windings are wound around a same portion of the core whereas weakly coupled windings are wound around different core portions which are separated by the shunt.

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of plain old telephone services (POTS) and any type of digital subscriber loop (xDSL). More specifically the invention pertains to the transformers used in overlaid POTS and xDSL technology. 
     BACKGROUND OF THE INVENTION 
     Digital subscriber loop (DSL) technology that offers the subscriber a very large bandwidth is engineered to overlay the existing analogue plain old telephone services (POTS). There are several types of DSL systems and the notation for specifying a DSL system of any type is xDSL. The xDSL system requires minimal equipment retrofit. It can be installed very quickly and easily, and is a cost-effective solution for high-bandwidth requirements. xDSL uses the existing copper analogue loop between the central office (CO) and the customer premises equipment (CPE) as its transmission medium, transporting voice in the traditional 4 kHz bandwidth where it has always been, while higher bandwidth digital services are relegated to higher frequency domains. A specific problem faced by overlaid POTS and xDSL technology, as well as other high-performance transmission systems, is the need to keep circuit costs low and packaging density high. Current technology makes use of a transformer for POTS and a second transformer for xDSL services in combination with a low pass filter (LPF) to combine POTS and xDSL services. The transformers have a significant volume and contribute significantly to the cost. Solutions for reducing the cost, volume and number of components in these circuits are therefore sought in the industry. Reducing the number of components also results in a reduction in inventory. In addition, due to the cost of real estate, an increased line density is required and would result in most of the savings per line (per customer) by sharing common equipment costs among a large number of lines. 
     SUMMARY OF THE INVENTION 
     The current technology involved with overlaying any type of digital subscriber loop (xDSL) services with plain old telephone services (POTS) makes use of two separate transformers, one for POTS and another for xDSL. In this invention the POTS transformer and the xDSL transformer are combined into one transformer. This is achieved by choosing a special geometric form for the core and by choosing strategic locations for the windings. A portion of the core is dedicated to serve as a shunt for each component of the magnetic field produced by the inductors. The geometric form of the core also provides a closed circuit of high permeability to restrict, to the core, the magnetic field produced by the current in the inductors. The windings of strongly coupled inductors are wound around the same portion of the core whereas the windings of weakly coupled inductors are wound around different core portions which are separated by the shunt. In overlaid POTS and xDSL applications a weak magnetic coupling between any one of the inductors used for POTS and any one of those used for xDSL services can therefore be achieved, despite their close proximity, by arranging the windings used for POTS and those of xDSL services around different portions of the core. Combining two transformers into one considerably reduces the weight volume and cost of the POTS+xDSL circuits. 
     In accordance with a first broad aspect, a transformer consists of a core formed by two coil portions, one central portion and connecting portions such that the central portion is spaced between the two coil portions and the connecting portions interconnect both ends of the central portion with the corresponding ends of the two coil portions. There is at least one first primary winding and at least one first secondary winding wound around one of the two coil portions of the core. There is also at least one second primary winding and at least one second secondary winding wound around the other coil portion of the core. The central portion is adapted to provide a shunt for components of the magnetic field produced by electric current in the windings. 
     The transformer may have two first primary windings and one first secondary winding. It may also have two second primary windings and one second secondary winding. The turn ratio of either of the two first primary windings to the first secondary winding may be 1:1 and the turn ratio of either of the two second primary windings to the second secondary winding may be 1:1. The transformer may be connected such that the two first primary windings are connected to a capacitor and connected to two conductors of a copper analogue loop, which is used to connect subscriber customer premise equipment (CPE) to a central office (CO). The two conductors are referred to as TIP and RING. The two first primary windings and the capacitor may form a high pass filter (HPF) and the first secondary winding may be connected to any type of digital subscriber loop (xDSL) circuit. The two first primary windings may be connected to one side of a low pass filter (LPF). The two second primary windings may be connected to another side of the LPF and the second secondary windings may be connected to a plain old telephone service (POTS) circuit. 
     The core of the transformer may be iron, laminated iron, powdered iron, ferrite or any other suitable magnetic material. 
     The cross-sectional area of the central portion of the transformer, in relation to the cross-sectional of its coils and connecting portions, may be specified to regulate the extent to which components of the magnetic field produced by electric currents in the windings are shunted through the central portion. In this way the strength of the magnetic coupling between any one of the first windings and any one of the second windings can be tuned by controlling the extent to which the components of the magnetic field are shunted through the central portion. 
     Each portion of the transformer may be a rectangular parallelepiped. The approximate width, height and depth of the central portion may be 12 mm, 12 mm and 6 mm, respectively. The approximate width, height and depth of the coil portions may be 1.5 mm, 12 mm and 6 mm, respectively. Finally, the approximate width, height and depth of the connecting portions may be 3 mm, 1.5 mm and 6 mm, respectively. The magnetic coupling between any one of the first windings and any one of the second windings may be in the range 0.01 to 0.25, whereas the magnetic coupling between any two first windings or any two second windings may be in the range 0.9 to 0.9999. 
     An air gap may be inserted into the core between the first primary and secondary windings of the transformer and its thickness may be approximately 0.1 mm. A second air gap may also into the core between the second primary and secondary windings of the transformer and its thickness may be approximately 0.1 mm. 
     The invention makes use of a combined POTS and xDSL transformer to reduce the cost and volume of the overlaid POTS and xDSL circuits. The combined transformer results in a reduced inventory. In addition, the combined transformer allows an increased line density which results in savings per line (per customer) by sharing common equipment costs among a large number of lines. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described with reference to the attached drawings in which: 
     FIG. 1 is a diagram of a typical electrical circuit which is used in overlaid plain old telephone services (POTS) and any type of digital subscriber loop (xDSL) services; and 
     FIG. 2 is a transformer which combines two transformers of FIG. 1, one for POTS and another for xDSL services, into a single transformer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a diagram of a typical arrangement of components which are used in overlaid plain old telephone services (POTS) plus any type of digital subscriber loop (xDSL) services. The arrangement of FIG. 1 includes a transformer  10 . The transformer  10  consists of two first primary windings  20  and  30 , a first secondary winding  40  and a capacitor  45 . Traditionally, the two conductors of a copper analogue loop, which is used to connect the subscriber customer premise equipment (CPE) to the central office (CO), are referred to as the TIP and RING. Leads A 20  and B 20  of the first primary winding  20  are connected to the TIP and to a terminal A 1  of a low-pass filter (LPF)  55 , respectively. Terminals A 30  and B 30  of the first primary winding  30  are connected to the RING and a terminal A 2  of the LPF  55 , respectively. Terminals A 40  and B 40  of the first secondary winding  40  are connected to a xDSL circuit. The terminals B 20  and B 30  of the first primary windings  20  and  30 , respectively, are connected to the capacitor  45 . 
     Terminals A 3  and A 4  of the LPF  55  are connected to resistors  58  and  59 , respectively. The resistors  58  and  59  are connected to a transformer  85 . The transformer  85  consists of two second primary windings  50  and  60  and a second secondary winding  70 . The resistor  58  is connected to a terminal A 50  of the second primary winding  50  and a terminal B 50  of the second primary winding  50  is connected to ground. The resistor  59  is connected to a terminal A 60  of the second primary winding  60  and a terminal B 60  of the second primary winding  60  is connected to a CO (− 48  V) battery  95 . Terminals A 70  and B 70  of the second secondary winding  70  of the transformer  85  are connected to the voice circuit of the CO. 
     Both POTS and xDSL signals are transmitted and received at the TIP and RING. The combination of the capacitor  45  and the first primary windings  20  and  30 , which operate as inductors, serve as a high pass filter for the high frequency xDSL signal. The first secondary winding  40  is used to couple the xDSL signal between terminals A 20  and A 30  of the first primary windings  20  and  30 , respectively, and terminals A 40  and B 40  of the first secondary winding  40 . 
     The LPF  55  connected to the terminals B 20  and B 30  of the first primary windings  20  and  30 , respectively, serves to transmit the low frequency POTS signal in the traditional 4 kHz channel bandwidth to its terminals A 3  and A 4 . The resistors  58  and  59  are used to provide terminating impedance and a direct current (DC) loop current limit to the copper analogue loop and the LPF  55 . 
     The transformer  85  is used to provide a direct current (DC) to the copper analogue loop in addition to the POTS signal. The DC current is needed to operate a telephone set at the CPE. By connecting the terminal B 50  of the second primary winding  50  to ground and connecting the terminal B 60  of the second primary winding  60  to the CO (− 48  V) battery  95  a potential difference between the TIP and RING is created driving a component of current through the copper analogue loop. The second primary windings  50  and  60  and the second secondary winding  70  couple the POTS signal between the terminals A 50  and A 60  of the second primary windings  50  and  60 , respectively, and the terminals A 70  and B 70  of the second secondary winding  70 . 
     The POTS transformer  85  and the xDSL transformer  10  are in the same physical component. The POTS signal and the xDSL signal are isolated from each other because of a small magnetic coupling coefficient, k, between them. 
     Referring to FIG. 2 which shows a transformer according to the invention, the transformer has a ferromagnetic core  10  which is rectangular in cross-section and is provided with two parallel rectangular holes  5  and  6  extending therethrough. It is convenient in describing the structure and operation of the transformer to refer to portions  11 ,  12 ,  14 ,  16 ,  17 ,  18  and  19  of the core. Portions  11 ,  12 ,  14 ,  16 ,  17 ,  18  and  19  are rectangular parallelepipeds. Portion  11  is a central portion or leg defined between the two holes  5  and  6 . Portions  12  and  14  are two marginal portions of the core  10  each defined by one of the holes  5  and  6  and one outside edge of the core  10 . Portions  12  and  14  may be referred to as coil portions because they are designed to carry windings or coils  20 ,  30 ,  40 ,  50 ,  60  and  70  which correspond to the windings  20 ,  30 ,  40 ,  50 ,  60 ,  70  shown in FIG.  1 . Thus coil portion  12  carries a first two primary windings  20  and  30  and a first secondary winding  40  and a coil portion  14  carries a second two primary windings  50  and  60  and a second secondary winding  70 . The coil portions  12  and  14  may also have optional air gaps  80  and  90 , respectively. The air gaps  80  and  90  prevent magnetic saturation of the core  10  by the DC current. 
     Typical dimensions of the core  10  are 12 mm, 12 mm and 6 mm for the width (L), height (M) and depth, respectively. The holes  5  and  6  each have width (W) 3 mm and height (V)  9  mm. The central portion  11  has a width (X), height (M) and depth of 3 mm, 12 mm and 6 mm, respectively. The width (y 1 ), height (M) and depth of the coil portion  12  is 1.5 mm, 12 mm and 6 mm, respectively. The width (Y 2 ), height (M) and depth of the coil portion  14  is 1.5 mm, 12 mm and 6 mm, respectively. The air gaps  80  and  90  are 0.1 mm wide. Portions  16 ,  17 ,  18  and  19  all have a width (Z 1 ), height (Z 2 ) and depth of 3 mm, 1.5 mm and 6 mm, respectively. The turn ratio of either first primary winding  20  or  30  to the first secondary winding  40  is 1:1. Similarly, the turn ratio of either second primary winding  50  or  60  to the second secondary winding  70  is 1:1. In another embodiment of the invention, portions  12  and  14  may have more than one secondary winding and the turn ratios between primary and secondary windings may be different than 1:1. 
     Each of the first primary windings  20  and  30 , the first secondary winding  40 , the second primary windings  40  and  50  and the second secondary winding  70  produces a component of the total magnetic field inside the core  10 . The set of first primary windings  20  and  30  and first secondary winding  40  produce magnetic field lines  100  and the set of second primary windings  50  and  60  and second secondary winding  70  produce magnetic field lines  110 . 
     The terminal leads Ai and Bi of winding i (where i=20, 30, 40, 50, 60 or 70) in FIG. 2 correspond to the respective terminal leads Ai and Bi in FIG.  1 . 
     The first primary windings  20  and  30  and the first secondary. winding  40  are used for xDSL and the second primary windings  50  and  60  and the second secondary winding  70  are used for POTS services. Together, they are used in overlaid POTS and xDSL applications. The first primary windings  20  and  30 , the first secondary winding  40 , the second primary windings  40  and  50  and the second secondary winding  70  are coupled to each other through the core  10 . In overlaid POTS and xDSL applications a strong magnetic coupling is required between any of the windings used for POTS. Similarly, a strong magnetic coupling is required between any of the windings used for xDSL. On the other hand, a weak magnetic coupling between any one winding used for POTS and any one winding used for xDSL is required. In the arrangement of FIG. 1 the magnetic coupling M 15  coefficient, k, between any of the first primary windings  20  and  30  and first secondary winding  40  is between 0.9 to 0.9999. Similarly, the magnetic coupling coefficient between any of the second primary windings  50  and  60  and second secondary winding  70  is between 0.9 to 0.9999. This strong magnetic coupling is compatible with existing overlaid POTS and xDSL technology which uses two transformers in lieu of the combined transformer of this Figure. On the other hand, the magnetic coupling coefficient between any one of the first primary windings  20  and  30  and first secondary winding  40 , and any one of the second primary windings  50  and  60  and second secondary winding  70  is between 0.01 to 0.25. This weak magnetic coupling is low enough so that the signals (noise) due to the coupling does not affect the performance of the overlaid POTS and xDSL service. 
     The weak magnetic coupling between any one of the first primary windings  20  and  30  and first secondary winding  40  and any one of the second primary windings  50  and  60  and second secondary winding  70  is achieved despite the fact that they are in close proximity. The central portion  11  acts as a shunt for the magnetic field lines  100  produced by electrical currents in the first primary windings  20  and  30  and the first secondary winding  40 . Similarly, the central portion  11  acts as a shunt for the magnetic field lines  110  produced by electrical current in the second primary windings  50  and  60  and the second secondary winding  70 . Since the magnetic field lines  100  and  110  are shunted through the central portion  11 , the magnetic flux through any one of the second primary windings  50  and  60  and second secondary winding  70  due to components of the magnetic field produced by any one of the first primary windings  20  and  30  and first secondary winding  40  is small. As a result the mutual inductance, and consequently the magnetic coupling coefficient, between any one of the first primary windings  20  and  30  and first secondary winding  40  and any one of the second primary windings  50  and  60  and second secondary winding  70  is low. On the other hand, the magnetic flux through a winding in portion  12  or  14  due to the component of magnetic field produced by a winding in the same portion is large and consequently the magnetic coupling coefficient is high. 
     The shunt effect can be understood from the reluctance of the circuit. The reluctance in a magnetic circuit decreases with decreasing length of the circuit, increasing cross-sectional area of the circuit and increasing permeability of the material. In FIG. 2, a component of magnetic field produced by the first primary windings  20  and  30  and the first secondary winding  40  may follow a closed circuit through the central portion  11  or through the portions  19 ,  14  and  18 . The cross-sectional of the central portion  11  is greater than that of the portions  19 ,  14  and  18  and the length of a circuit through the central portion  11  is shorter than the length of a circuit through portions  19 ,  14  and  18 . In addition, the presence of the air gap  90  also increases the reluctance of portion  14 . The reluctance of the central portion is therefore much lower than that of the combined portions  19 ,  14  and  18 , and the air gap  90 . The lower reluctance of the central portion  11  results in the components of the magnetic field produced by the first primary windings  20  and  30  and the first secondary winding  40  to be shunted through the central portion  11 . Similarly, the lower reluctance of the central portion  11  compared to the combined portions  16 ,  12  and  17  results in the components of the magnetic field produced by the second primary windings  50  and  60  and the second secondary winding  70  to be shunted through the central portion  11 . 
     The overall effect of the geometric form of the core  10  is to shield the second primary windings  50  and  60  and the second secondary winding  70  from the magnetic field lines from the first primary windings  20  and  30  and the first secondary winding  40 , and vice-versa, to minimise the magnetic coupling coefficient, k, between windings of opposite sides of the core  10 . This is achieved despite the fact that the two sets of windings are in close proximity to each other. 
     In the preferred embodiment of the invention the cross-sectional area of portion  11  is chosen, in relation to other dimensions of portions of the core  10 , to shunt components of magnetic field through the central portion  11 . The result is a magnetic coupling coefficient between two windings wound around different coil portions of the core  10  in the range 0.01 to 0.25. In another embodiment, the cross-sectional area of portion  11  may be specified to tune the magnetic coupling coefficient from weak coupling to strong coupling. For example, reducing the cross-sectional area of portion  11  results in a decrease in the extent to which the components of the magnetic field are shunted through portion  11  and consequently the magnetic coupling coefficient between two windings wound around different coil portions of the core  10  increases. 
     In the preferred embodiment of the invention, the core  10  is a rectangular parallelepiped. In another embodiment, the core and portions thereof may have different shapes as long as there is at least one shunt portion which can be used to shunt the magnetic field lines of the other portions. The specifications required to produce a shunt effect is a high permeability, short length and large cross-sectional area of the shunt portion. 
     The first primary windings  20  and  30 , the first secondary winding  40 , the second primary windings  40  and  50  and the second secondary winding  70  are oriented such that the magnetic field lines  100  and  110  are in opposite directions throughout the core  11 . The effect is to decrease the total magnetic field throughout the core  11 . In overlaid POTS and xDSL applications, the magnetising inductance of the first primary winding  20  when connected in series with the first primary winding  30  is approximately 2 mH. Since the turn ratio of the first primary windings  20  and  30  to the first secondary winding  40  is 1:1 the magnetising inductance of the first secondary winding  40  is also 2 mH. The magnetising inductance of the second primary winding  50  in series with the second primary winding  60  is approximately 100 mH. Therefore, since the turn ratio of the second primary windings  50  and  60  to the second secondary winding  70  is 1:1, the magnetising inductance of the second secondary winding  70  is also 100 mH. Since the magnetising inductance of the first primary windings  20  and  30  and the first secondary winding  50  is different than the magnetising inductance of the second primary windings  50  and  60  and the second secondary winding  70 , the magnitudes of the DC magnetic fields in portion  11  differ significantly. Therefore, the effect of opposing magnetic fields on the total magnetic field is minimal in overlaid POTS and xDSL applications. As a consequence the effect of opposing magnetic fields on saturation in portion  11  is not very significant. While FIGS. 1 and 2 show one secondary winding for the xDSL circuit and one secondary winding for the POTS circuit it is known to those skilled in the art that there could be more than one secondary winding for xDSL and more than one secondary winding for POTS. In such a case in which portions  12  and  14  have more than one secondary winding, or if the turn ratios between the primary and secondary windings are different than 1:1, the effect of opposing magnetic fields on saturation in portion  11  may be significant. 
     When compared with the two transformers of POTS and xDSL in conventional overlaid POTS and xDSL arrangements, the single transformer in the arrangement of FIG. 2 requires 22% less volume and the cost is reduced to 60%. 
     Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practised otherwise than as specifically described herein.