1. Field of the Invention
The present invention relates generally to components used in electronics applications, and particularly to an improved inductive devices used in, inter alia, filter and splitter apparatus for a digital subscriber line (DSL) or similar telecommunications system.
2. Description of Related Technology
Today, Digital Subscriber Line (DSL) installations are often what is known as “self-install” or specifically where the subscriber installs a micro-filter or in-line phone filters on each telephone to isolate the phones (including faxes, answering machines, etc.) from the line and the DSL signal path. FIG. 1 illustrates a typical installation of such in-line filters.
The self-installable micro-filter is a challenging design, largely because it must have sufficient stop band in the DSL band to protect and preserve DSL performance, but at the same time should also have negligible effect on the voice band performance.
FIG. 1a illustrates a typical prior art in-line filter configuration used in DSL applications. Such prior art filter designs, however, often do not satisfy some of the telecom customer's requirements for both return loss and DSL stop band. One significant problem is that the total capacitance required for the DSL stop band requirements also produce excessive side tone in the upper band of the telephones, a highly undesirable result. Furthermore, the return loss problem becomes worse as more micro-filters are added for each of the subscriber's phones.
In certain countries, filter circuit requirements can be stringent. One major challenge, for example, is providing the 30 KHz stop band while providing the very high voice band return loss.
Prior art inductive devices are often not well adapted for use in the foregoing applications, based in large part on their inductance characteristic. As used herein, the term “inductance characteristic” refers generally to the inductance profile, or variation in inductance as a function of dc current through the inductor. FIGS. 2a and 2b illustrate the inductance characteristics associated with typical prior art inductors having either fixed inductance or variable inductance, respectively. Note that in the typical “fixed” inductor, the inductance characteristic 102 is essentially flat or constant as a function of current, until comparatively high currents are reached. In comparison, the inductance profile of the variable inductor varies as a function of current, either in a substantially linear fashion 106, or in a somewhat “soft stepped” fashion 108, as shown in FIG. 2b. FIG. 2b is generally representative of the types of prior art device manufactured by, inter alia, Coilcraft Corporation of Cary, Ill., USA, such as the DT1608 Series SMT power inductors.
FIG. 3 illustrates the construction of the aforementioned Coilcraft device. As shown in FIG. 3, the device 300 comprises a two-piece core 302 having a base 304 with an off-centered post 306. The upper core piece 308 has an aperture 310 which is oversized with respect to the diameter of the post 306. This arrangement creates what amounts to a continuously variable gap between the outer surface of the post 306 and the interior surface of the aperture 310, ranging from a minimum gap at the closest point of approach of the two surfaces, to a maximum at the diametric opposite of the point of closest approach. This continuously variable gap has at least two disabilities, including: (i) a continuously variable or “soft stepped” inductance characteristic, which is undesirable or less than optimal in certain applications, and (ii) high cost of manufacturing, since two core pieces with precise relative tolerances must be provided (including precise alignment of the upper core piece 308 with the base 304. Furthermore, there is additional cost associated with manufacturing the “off-center” post 306, irrespective of its tolerances with the other core piece 308. Such off-center arrangement is also not generally conducive to use of well known alignment aids, such as the split-pin arrangement described subsequently herein.
Certain applications, including for example some DSL filter circuits where higher stop band loss is needed (such as for Caller ID functions), require inductive devices with an inductance characteristic different than those of FIG. 2a or 2b. In the case of the aforementioned Caller ID function, higher stop band loss is needed in the on-hook state to protect the Caller ID device from current overload via the DSL signals. Consider the exemplary filter circuit described in co-pending PCT Application No. PCT/US01/45568 entitled “High Performance Micro-Filter and Splitter Apparatus” filed Nov. 14, 2001 and assigned to the Assignee hereof, which is incorporated herein by reference in its entirety. In this circuit, removal of most of the capacitance during the on-hook state reduces filter stop loss, thereby necessitating an additional or alternate mechanism for increasing the stop loss as previously described.
Similarly, for the exemplary filter circuit described in, inter alia, U.S. Pat. No. 6,212,259 entitled “Impedance Blocking Filter Circuit” and issued Apr. 3, 2001, also assigned to the Assignee hereof, an improved inductive device is needed whereby sufficient inductance is present to allow the circuit to pass the on-hook stop band loss for a plurality of filters, while still allowing a larger off-hook capacitance.
In addition to desirable inductive performance characteristics, low cost of manufacturing for inductive devices is also a highly desirable attribute. Inductive device markets (as well as DSL filter circuit markets) are characteristically quite price competitive; hence, even small improvements in cost efficiency or reductions in pricing of these components can have significant impact on the viability of a manufacturer's product(s). Prior art approaches of controlling device inductance are generally complex (e.g., the “swinging inductance” approach of the aforementioned Coilcraft devices) and dictate comparatively high costs of manufacturing, due to increased labor and/or parts associated with generating the desired inductance characteristic.
Board and interior space consumption is also an issue with many electronic devices (including DSL filter circuits); hence, in addition to the desired performance characteristics and low cost, minimal physical size and footprint is also very desirable. A device which performs well electrically and is inexpensive to manufacture, yet takes up appreciable board or interior space, is often not commercially viable.
ETSI Technical Standard 952, Part 1, Sub-part 5 (ETSI TS 952-1-5) entitled “Access network xDSL transmission filters; Part 1: ADSL filters for European deployment; Sub-part 5: Specification of ADSL/POTS distributed filters” specifies requirements and test methods for DSL distributed filters and distributed filters installed at the Local Exchange side of the local loop and at the user side near the network termination point (NTP). The Standard specifies requirements and test methods for distributed ADSL/POTS distributed filters valid at the user end of the local loop. Per the Standard, on-hook voiceband electrical requirements comprise two conditions: (i) a DC feeding voltage of 50 V, and using the impedance model ZON (10 kΩ), or (ii) a DC loop current in the range of 0.4 mA to 2.5 mA flowing through the distributed filter; and using an impedance model of 600 Ω to terminate the LINE and POTS port of the distributed filter at voice frequencies. The Standard's on-hook ADSL band electrical requirements may be met with a DC feeding voltage of 50 V, and using the impedance model ZON (10 kΩ). Off-hook electrical requirements may be met with a DC current of 13 mA to 80 mA. These requirements are comparatively stringent, especially for simple low-cost inductive devices.
Based on the foregoing, an improved inductive device having both low cost of manufacturing and desirable inductance characteristics is needed for use in, inter alia, digital subscriber line (DSL) signals. Such improved apparatus would ideally (i) have the desired inductance characteristics in the on hook and off-hook states, so as to support for example functions such as Caller ID which require higher on-hook stop band loss (ii) be highly cost-effective to manufacture, (iii) be reliable, and (iv) be physically compact in both volume and footprint.