Patent Publication Number: US-2010127796-A1

Title: Electronic filter and an electronic circuit for use in a switching application

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of electronic filters and electronic circuits for use in switching applications, and which have particular—but by no means exclusive—application to the field of asymmetric digital subscriber line (ADSL) filters for use with customer premises equipment. 
     BACKGROUND OF THE INVENTION 
     An electronic filter is used to select and reject signals based on their frequency. An electronic filter typically comprises a combination of discrete components (such as a capacitor, inductor and/or resistor) that determine the frequency response characteristics of the filter. More recently, however, electronic filters based on digital signal processors (DSP) technology have been developed. The frequency response characteristic of DSP based filters is determined by software that is executed by the DSP. 
     Filters are commonly classified in to one of several categories based on their frequency response characteristics. The categories include, for example, low-pass, high-pass, band-pass and band-stop. As the category names suggest, a low-pass filter allows (selects) low frequency signals to pass while blocking (rejecting) high frequency signals. In contrast to a low-pass filter, a high-pass filter blocks low frequency signals and allows high frequency signals to pass. A band-pass filter allows signals within a certain frequency range to pass while blocking signals outside the frequency range. In contrast to a band-pass filter, a band-stop filter blocks signals within the frequency range while allowing signals outside of the frequency range to pass. 
     Electronic filters have a wide range of applications and, as an example of a recent application, have been used to filter asymmetric digital subscriber line services provided over copper based subscriber lines that form part of the plain old telephone system (POTS). More specifically, the filters used to filter asymmetric digital subscriber line services have been designed to be connected to customer premises equipment (CPE) such as a telephone. Furthermore, an asymmetric digital subscriber line filter is essentially a low-pass filter that allows DC and voice signals (which are in the frequency range of DC to around 4 KHz) to pass through to the customer premises equipment while blocking asymmetric digital subscriber line signals (which are in the frequency range of around 25 KHz to 1104 KHz for ADSL, and around 25 KHz to 2208 KHz for ADSL 2+) so that they do not pass through to the customer premises equipment. 
     While today&#39;s asymmetric digital subscriber line filters generally perform well as low-pass filters they do have some drawbacks. A notable drawback is that many asymmetric digital subscriber line filters are not suitable for being installed in a distributed arrangement at a customer premises. In a distributed arrangement a separate filter is plugged into each telephone socket located on the customer premises, which essentially results in multiple filters being connected, in parallel with each other, to a single subscriber line. Unfortunately, when existing asymmetric digital subscriber line filters are connected in parallel with each other the filtering performance of each individual filter can be degraded. More specifically, the degradation in the filtering performance can be attributed to the parallel loading effect caused by the impedance of the low-pass filtering circuits (which are in parallel with each other). 
     Many of today&#39;s electronic filters comprise semiconductor based switching circuits (that are typically based on transistors) to selectively switch electronic components in or out of the filtering circuitry. The semiconductor devices may, for example, be employed in an asymmetric digital subscriber line filter to switch electronic components in or out in response to the presents or absence of a subscriber line loop current. On examining existing semiconductor based switching circuits used in many of today&#39;s electronic filters, the inventor surprising discovered that the existing semiconductor based switching circuits can introduce significant noise in to the pass-band of the filter. Upon further investigation the inventor discovered that the noise is often the result of the non-linear characteristics of the pn-structure that exists between the collector and base of a transistor used in the semiconductor based switching circuits in the absence of a subscriber line loop current (such as filter connecting to CPE in on-hook state and in parallel to other filters on the same line). 
     In the case of asymmetric digital subscriber line filters, the inventor discovered that traditional semiconductor based switching circuits generated little unwanted noise when used in conjunction with early asymmetric digital subscriber line technology (signals). However, the inventor also discovered that when used with later asymmetric digital subscriber line technology (signals) such as ADSL2+ the traditional semiconductor based switching circuits generated considerable unwanted noise in the pass-band of the filter in the absence of a subscriber line loop current (such as filter connecting to CPE in on-hook state and in parallel to other filters on the same line), which the inventor discovered was the result of the higher signal power used in ADSL2+. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided an electronic filter comprising: 
     a filter section; 
     an impedance correction section that is electrically coupled to the filter section; and 
     a circuit altering section that is arranged to alter at least one property of the impedance correction section to effect a shift in a resonant frequency of the impedance correction section from within a pass band of the filter section to within a stop band of the filter section. 
     An advantage of a filter according to an embodiment of the first aspect of the present invention is that by shifting the resonant frequency of the impedance correction section it is possible to improve the stop band attenuation of the filter when, for example, a telephone (or any other customer premises equipment) connected to the filter is in an on-hook state. The effect of improving the stop band attenuation is that the filter can still provide reasonable amounts of attenuation against signals outside of the voice band, which is particularly useful for facilitating the exchange of on-hook communication signals such as coded DTMF signals, metering pulse signals or caller ID signals. 
     Preferably, the circuit altering section is arranged to alter at least one property of the filter section to effect an increase in a stop band impedance of the filter section. 
     An advantage of increasing the stop-band impendence is that it minimizes the parallel loading effect on other filters that may be connected in parallel to the filter. 
     Preferably, the property of the filter section comprises a capacitance, and wherein the circuit altering section is arranged to effect the increase in the stop band impendence by reducing the capacitance to a value that is greater than zero Farad. 
     An advantage of reducing the capacitance to greater than zero Farad is that instead of completely removing the capacitance (reduced to zero) it ensures the filter section retains some stop-band attenuation when, for example, the telephone connected to the filter is in an on-hook status. 
     Preferably, the circuit altering section is arranged to alter the property of the impedance correction section and to alter the property of the filter section based on a subscriber line loop current. 
     An advantage of altering the property based on the subscriber line loop current is that it provides an easy method for switching based on whether a telephone is in an on-hook state or an off-hook state. 
     Preferably, the property of the impedance correction section comprises a damping resistance and a capacitance, and wherein the circuit altering section is arranged to effect the shift in the resonant frequency by decreasing the capacitance, and wherein the circuit altering section is further arranged to increase the damping resistance. 
     Preferably, the filter section comprises a plurality of cascaded low pass filters arranged such that the cut-off frequency associated with the pass band and the stop band is around 12 KHz. 
     Preferably, the impedance correction section comprises a parallel resonant circuit that is electrically coupled to a first of the cascaded low pass filters and a second of the cascaded low pass filters. 
     Preferably, the circuit altering section comprises at least one semiconductor device arranged to alter the property of the impedance correction section and to alter the property of the filter section. 
     According to a second aspect of the present invention there is provided an electronic filter comprising: 
     a filter section; and 
     a circuit altering section that is arranged to alter at least one property of the filter section to effect an increase in a stop band impedance of the filter section. 
     Preferably, the filter further comprises an impedance correction section, and wherein the circuit altering section is arranged to alter at least one property of the impedance correction section to effect a shift in a resonant frequency of the impedance correction section from within a pass band of the filter section to within a stop band of the filter section. 
     Preferably, the property of the filter section comprises a capacitance, and wherein the circuit altering section is arranged to effect the increase in the stop band impendence by reducing the capacitance to a value that is greater than zero farad. 
     Preferably, the circuit altering section is arranged to alter the property of the impedance correction section and to alter the property of the filter section based on a subscriber line loop current. 
     Preferably, the property of the impedance correction section comprises a damping resistance and a capacitance, and wherein the circuit altering section is arranged to effect the shift in the resonant frequency by decreasing the capacitance, and wherein the circuit altering section is arranged to increase the damping resistance. 
     Preferably, the filter section comprises a plurality of cascaded low pass filters arranged such that the cut-off frequency associated with the pass band and the stop band is around 12 KHz. 
     Preferably, the impedance correction section comprises a parallel resonant circuit that is electrically coupled to a first of the cascaded low pass filters and a second of the cascaded low pass filters. 
     Preferably, the circuit altering section comprises at least one semiconductor device arranged to alter the property of the impedance correction section and to alter the property of the filter section. 
     According to a third aspect of the present invention there is provided an electronic circuit for use in a switching application, the electronic circuit comprising: 
     a transistor switch arrangement comprising a base semiconductor region of a first type; and 
     a semiconductor device having a pn-structure comprising a semiconductor region of the first type that is electrically coupled to the base semiconductor region. 
     An advantage of an electronic circuit embodying the third aspect of the present invention is that it has the potential to ameliorate pass-band noise that may be generated by existing semiconductor based switching circuits. 
     Preferably, the semiconductor device comprises a transistor actuator arrangement arranged to switch the transistor switch arrangement between an on-state and an off-state. 
     Preferably, the transistor switch arrangement and the transistor actuator arrangement define a complementary transistor pair in which: a collector of the transistor actuator arrangement is electrically coupled to a base of the transistor switch arrangement; and a base of the transistor actuator arrangement is electrically coupled to an emitter of the transistor switch arrangement, which is also electrically coupled to an emitter of the transistor actuator arrangement. 
    
    
     
       A BRIEF DESCRIPTION OF THE FIGURES 
       Notwithstanding any other embodiments that may fall within the scope of the present invention, an embodiment of the present invention will now be described, by way of example only, with reference to the accompanying figures, in which: 
         FIG. 1(   a ) is a schematic representation of an embodiment of a filter according to the present invention; 
         FIGS. 1(   b ) to  1 ( e ) are schematic representations of alternative embodiments of a section of the filter depicted in  FIG. 1(   a ); and 
         FIGS. 2(   a ) to  2 ( e ) are schematic representations of transistor based switching circuits. 
     
    
    
     AN EMBODIMENT OF THE INVENTION 
       FIG. 1(   a ) depicts a schematic diagram of an electronic filter  100 , in accordance with an embodiment of the present invention, which is particularly suited to use as an asymmetric digital subscriber line filter for customer premises equipment (for instance, a home telephone). As such, persons skilled in the art will readily appreciate that the filter  100  is intended to be electrically coupled to a subscriber line of a plain old telephone system and electrically coupled to the customer premises equipment. 
     Persons skilled in the art will also appreciate that the filter  100  basically acts as a low-pass filter that allows DC and signals in a voice band (which means approximately DC to around 4 KHz) on the subscriber line to pass through to the customer premises equipment, while blocking (attenuating) signals in an asymmetric digital subscriber line band (which extends approximately between 25 KHz to over 2208 KHz) from passing through to the customer premises equipment. 
     As can be seen in  FIG. 1(   a ) the filter  100  comprises an inverted-L low-pass filter  102  and a T-type low-pass filter  104 , which together form a filter section. In addition to the filters  102  and  104 , the filter  100  comprises a pair of parallel resonant circuits  106  and  108 , which together form an impedance correction section, that are electrically interposed between the inverted-L filter  102  and the T-type filter  104 . The filter  100  also comprises several transistor based circuits  110  to  116 , which together form a circuit altering section. One of the circuits  110  is electrically connected to the inverted-L low pass filter  102 , while two of the circuits  112  and  114  are electrically connected to the parallel resonant circuits  106  and  108 . Furthermore, one of the circuits  116  is electrically connected to the T-type low-pass filter  104 . 
     The inverted-L low-pass filter  102  comprises two inductors  118  and  120 , each of which has an inductance of about 4 mH. One of the inductors  118  is intended to be electrically coupled in series to the “tip” line of the subscriber line, while the other inductor  120  is intended to be electrically coupled in series to the “ring” line of the subscriber line. It is envisaged that the tip and ring lines maybe reversed. The low-pass filter  102  includes the two inductors  118  and  120  to maintain the balanced aspect of the subscriber line. In addition to the inductors  118  and  120 , the inverted-L low-pass filter  102  comprises two capacitors  122  and  124  that are electrically connected in series with each other. The capacitors  122  and  124  are also electrically connected across the inductors  118  and  120  to form an inverted-L low-pass filter arrangement. One of the capacitors  122  has a capacitance of 22 nF, while the other capacitor  124  has a capacitance of 6.8 nF. The capacitors  122  and  124  are also known as “shunt capacitors”. 
     As persons skilled in the art will readily appreciate, the values of the inductors  118  and  120  and the values of the capacitor  122  result in the low-pass filter  102  having a pass-band in the frequency range of approximately DC to 12 KHz and a stop-band in the frequency range of about 25 KHz to over 20 MHz. The pass-band of the low-pass filter  102  is such that DC and voice (audio) information on the subscriber line will pass through the low-pass filter  102 , while the stop-band band will prevent (attenuate) asymmetric digital subscriber line signals on the subscriber line from passing through the low-pass filter  102 . 
     As outlined in previous paragraphs of this specification, the filter  100  also comprises a pair of parallel resonant circuits  106  and  108 . One of the resonant circuits  106  is connected in series with one of the inductors  118  of the inverted-L low-pass filter  102 , while the other resonant circuit  108  is connected in series with the other inductor  120  of the inverted-L low-pass filter  102 . The filter  100  includes the two resonant circuits  106  and  108  to maintain the balanced characteristic of the subscriber line. Each resonant circuit  106  and  108  includes an inductor  126  and  128 , a damper resistor  130  and  132  and a capacitor  134  and  136 , all of which are electrically connected in parallel with each other. Each of the inductors  126  and  128  has a value of about 4.7 mH (two windings of a 1:1 transformer), while the resistors  130  and  132  each have a value of 5.1 kOhm. Each capacitor  134  and  136  has a value of 1.5 nF. Each resonant circuit  106  and  108  also comprises another resistor  138  and  140  and another capacitor  142  and  144 . The additional resistor  138  and  140  and capacitor  142  and  144  are connected in parallel with each other. Furthermore, the additional resistor  138  and  140  and capacitor  142  and  144  are electrically connected to the inductor  126  and  128 , the resistor  130  and  132 , and the capacitor  134  and  136 . The additional resistor  138  and  140  and capacitor  142  and  144  is also electrically connected to the transistor based circuit  112  and  114 . 
     As persons skilled in the art will readily appreciate, the values of the inductors  126  and  128  and the capacitors  134 ,  136 ,  142  and  144  determine the resonant frequency of the resonant circuits  106  and  108 , while the damping resistors  130 ,  132 ,  138  and  140  determine the damping factors (Q) of the resonant circuits  106  and  108 . The primary purpose of the resonant circuits  106  and  108  is to ensure the impedance of the filter  100  is approximately equal to the impedance of the subscriber line, which in turn sets the return loss of the filter  100  as high as possible. As outlined in more detail in subsequent paragraphs of this specification, the resonant circuits  106  and  108  are also used (selectively) to increase the attenuation of signals (ADSL) in the stop-band of approximately 30 KHz to 1104 KHz. 
     As described in previous paragraphs of this specification, the filter  100  also comprises a T-type filter  104 . The T-type filter  104  comprises four parallel resonant circuits  146  to  152  and two capacitors  154  and  156 . Two of the resonant circuits  146  and  148  are electrically connected in series with each other, and are also electrically connected in series with one of the resonant circuits  106  that forms the impedance correction section. The other two resonant circuits  150  and  152  of the T-type filter  104  are also electrically connected in series with each other, and are also electrically connected in series with the other resonant circuit  108  that forms the impedance correction section. Having the resonant circuits  146  to  152  organised in symmetric pairs that are electrically connected to the resonant circuits  106  and  108  preserves the balanced characteristic of the subscriber line. The two capacitors  154  and  156  are electrically connected in series with each other. Furthermore, the two capacitors are electrically disposed between a first pair of the resonant circuits  146  and  150  and a second pair of the resonant circuits  148  and  152 , to thereby form a T-type low pass filtering section. 
     Each parallel resonant circuit  146  to  152  comprises an inductor  158  to  164 , a capacitor  166  to  172 , and a damping resistor  174  to  180 , all of which are electrically connected in parallel with each other. The inductor  158  and  162  in two of the resonant circuits  146  and  150  has an inductance of about 2700 μH, while the capacitor  166  and  170  of the resonant circuits  146  and  150  has a capacitance of about 10 nF. Furthermore, the resistor  174  and  178  in each of the two resonant circuits  146  and  150  has a resistance of about 5.1 kOhm. The inductor  160  and  164  in the other two resonant circuits  148  and  152  has an inductance of bout 1100 μH, while the capacitor  168  and  172  in the other two resonant circuits  148  and  152  has a capacitance of about 15 nF. The resistors  176  and  180  in the other two resonant circuits  148  and  152  has a resistance of about 680 Ohms. 
     Persons skilled in the art will readily appreciate that the values of the inductors  158  to  164 , the capacitors  166  to  172 , and the resistors  174  to  180  results in the T-type filter  104  having a cut-off frequency at about 13 KHz, and further provides sharp and deep attenuation at around 30 KHz. The pass-band of the T-type filter  104  is such that voice (audio) information including DC in the pass band (that is, DC to around 13 KHz) will pass through the filter  104 , while the stop-band band will prevent (severely attenuate) asymmetric digital subscriber line signals on the subscriber line from passing through the filter  104 . 
     As outlined in a previous paragraph of this specification, the filter  100  comprises several transistor based circuits  110  to  116 . As described in more detail in subsequent text of this specification, the main function provided by two of the transistor based circuits  112  and  114  is to bring about a change in at least one properties of the impedance correction circuits  106  and  108  in order to effect a change in the circuits&#39; resonant frequencies. The main function provided by the other two transistor circuits  110  and  116  is to bring about a change in at least one property of the inverted-L filter  102  and the T-type filter  104  in order to effect an increase in a stop-band impedance of the filters  102  and  104 . 
     The transistor circuits  112  and  114  used to bring about a change in the property of the impedance correction circuits  106  and  108  each comprises an NPN transistor  182  and  184  and a PNP transistor  186  and  188 . The other transistor circuits  110  and  116  each comprise two PNP transistors  190  to  196 . Furthermore, the transistor based circuits  110  and  116  each comprises two diodes  198  to  204  arranged to form a full bridge (or in an alternative embodiment two NPN transistors with diodes  198  to  204  reversed) as well three resistors  206  to  216 . The NPN transistors  182  and  184  are in the form of 3904 series transistors, while the PNP transistors  186  to  196  are in the form of 3906 series transistors. The diodes  198  to  204  are of the 1N4148 type. Two of the resistors  206 ,  210 ,  212  and  216  in each transistor circuit  110  and  116  have a resistance of 68 Ohms, while the other resistor has a resistance of 100 kOhms. 
     With regard to the transistor circuits  110  and  116  that are used to bring about a change in a property of the inverted-L filter  102  and the T-type filter  104 , the two transistors  190  to  196  in each circuit  110  and  116  have their collectors electrically coupled together and also electrically coupled between the two capacitors (shunt capacitors)  122 ,  124 ,  154  and  156  of the filters  102  and  104  such that the collectors are electrically disposed between the two capacitors  122 ,  124 ,  154  and  156 . Furthermore, the collectors of the transistors  190  to  196  are electrically coupled to the common anode of the diodes  198  and  200 . When there is no loop current through the filter  100  (that is, the customer premises equipment is in an on-hook state), the transistor circuits  110  and  116  are in a turned-off state, the common anodes of the diodes  198  and  200  and the collector of transistors  190  and  196  exhibit a high impedance, the shunt capacitor  122  and  156  is isolated from being electrically connected to the invented-L low pass filter  102  and the T-type filter  104 , except bleeding through small capacitance  124  and  154  in series with  122  and  156 , and forming weak inverted-L filter  102 , and weak T-type filter  104 , which is the result of the effective shunt capacitor being reduced. However, any information exchange between Line port and POTS port of the filter  100  during this ON HOOK state of the customer premises equipment is maintained via the by pass resistors  226  and  228  and  230  and  232  which are electrically across the transistor-diode bridge circuits  110  and  116 . When there is loop current through (CPE Off Hook) filter  100 , the transistor-diode bridge  110  and  116  is turn on, the common anode and collector joining point of the transistor-diode bridge  110  and  116  exhibit low impedance, and the shunt capacitor  122  is connected to inductor  120  of inverted-L filter  102 , and the impedance correction resonant circuit  108 , and becomes a stronger inverted-L filter  102 , shunt capacitor  156  is connected to two adjacent resonant circuits  146  and  148  and becomes a stronger T-type filter  104 . 
     The emitters of the transistors  190  and  192  in one of the transistor circuits  110  are electrically disposed (connected) between an inductor  120  of the inverted-L low-pass filter  102  and the parallel resonant circuit  108  to which the inductor  120  is electrically connected in series with. The emitters of the transistors  194  and  196  in the other transistor circuit  116  are electrically disposed (connected) between two of the series connected resonant circuits  146  and  148  in the T-type filter  104 . The base of each transistor  190  to  196  is electrically connected, via one of the resistors  206 ,  210 ,  212  and  216  to the base of the other transistor  190  to  196  in the associated transistor based circuit  110  and  116 . Each diode  198  to  204  in the transistor based circuits  110  and  116  is electrically connected across the emitter and collect of a unique transistor  190  to  196  in the transistor circuits  110  and  116 . 
     With regard to the transistor circuits  112  and  114  used to bring about a change in the property of the impedance correction circuits  106  and  108 , the two transistors  182  to  188  in each of these circuits  112  and  114  have there bases electrically connected together. Furthermore, the bases of the transistors  182  and  186  in one of the circuits  112  are electrically disposed (connected), via a 1 kOhm resistor  218 , between two of the series connected resonant circuits  146  and  148  in the T-type filter  104 . In addition to being electrically connected together, the bases of the transistors  184  and  186  in the other transistor circuit  114  are electrically disposed (connected), via a 1 kOhm resistor  220 , between an inductor  120  of the inverted-L filter  102  and the corresponding series connected parallel resonant circuit  108 . 
     The emitters of the transistors  182  and  186  in one of the transistor circuits  112  are electrically connected together and are also electrically connected to one of the inductors  118  in the inverted-L filter  102 . The collectors of the same transistors  182  and  186  are also electrically connected together and are electrically connected to the additional resistor  140  and the capacitor  142  in one of the resonant circuits  106 , which forms the impedance correction section. The emitters of the transistors  184  and  188  in the other transistor circuit  114  are electrically connected together and also electrically connected to one of the resonant circuits  150  in the T-type filter  104 . The collectors of the same transistors  184  and  188  are electrically connected together and also electrically connected to the additional resistor  138  and the additional capacitor  144  in one of the parallel resonant circuits  108 , which forms the impedance correction section. 
     The primary function provided by two of the transistor based circuits  112  and  114  is to selectively switch the additional resistors  138  and  140  and capacitors  142  and  144  in or out of in the parallel resonant circuits  106  and  108 , to thereby alter the properties (being the resistance and capacitance) of the circuits  106  and  108  in order to shift the resonant frequencies of the circuits  106  and  108  between the pass-band and stop-band of the inverted-L filter  102  and the T-type filter  104 . The primary function provided by the other two transistor circuits  110  and  116  is to selectively ‘switch’ capacitors  124  and  154  in or out of the inverted-L filter  102  and T-type filter  104 . The purpose of switching the capacitors  124  and  154  in or out of the filters  102  and  104  is to bring about a change in a property (capacitance) of the filters  102  and  104  to either increase or decrease the stop-band impedance of the filters  102  and  104 . 
     In relation to the operation of the transistor circuits  110  and  116 , each comprises two diodes  198  to  204 , and two transistors  190  to  196  and their base resistors  206  to  216 , which form a bridge such that when there is loop current through filter  100  (CPE Off Hook), a transistor  190  to  196  and a diode  198  to  204  in series in the bridge are forward biased, and the bridge center joint (collectors and anodes joining together) is conducted (switched on) through the forward biased transistor and diode  198  to  204 . When there is no loop current through filter  100  (CPE On Hook), none of the transistors  190  to  196  nor diodes  198  to  204  is forward biased, the bridge center joint (where collectors and anodes are joining together) is high impedance (switch off), however information exchange through filter  100  during the CPE ON HOOK state is maintained via the by-pass resistors  226  to  232 , which are electrically across the circuits  110  and  116 . 
     In relation to the transistor circuit  110  and  116 , the transistors  190  to  196  the PNP transistors are in the form of 3906 series transistors; however, NPN type of transistor in the form of 3904 series are also possible (in an alternative embodiment of the present invention) to the same effect when the polarity of the diodes  198  to  204  is reversed. The NPN transistors  182  and  184  are in the form of 3904 series transistors, while the PNP transistors  186  to  196  are in form of 3906 series transistors. The diodes  198  to  204  are of the IN4148 type. 
     With regard to the transistor circuits  110  and  116  that are used to bring about a change in a property of the inverted-L filter  102  and the T-type filter  104 , the transistor circuit  110  and  116  have their collectors of transistor  190  to  196  and anodes of diodes  198  to  204  electrically coupled together (bridge center joint), and also electrically coupled between the two capacitors (shunt capacitors)  122 ,  124 ,  154 , and  156  of the filters  102  and  104 . The emitters of the transistors  190  and  192 , and the cathodes of diodes  198  and  200  in one of the transistor circuits  110  are electrically disposed (connected) between an inductor  120  of the invert-L low pass filter  102  and the parallel resonant circuit  108  to which the inductor  120  is electrically connected in series. The emitters of the transistors  194  and  196 , and the cathodes of diodes  202  and  204  in the other transistor circuit  116 , are electrically disposed (connected) between two of the series connected resonant circuit  146  and  148  in the T-type filter  104 . 
     The primary function provided by two of the transistor based circuits  110  and  116  is to selectively alter the shunt capacitance of the inverted-L low pass filter  102 , and alter the shunt capacitance of the T-type low pass filter  104 . When there is no loop current through (CPE ON Hook) filter  100 , the transistor-diode bridge  110  and  116  is turned off, the bridge center (where collectors of transistor  190  to  196  and anodes of diodes  198  to  204  are electrically coupled together) of circuit  110  and  116  exhibit high impedance, the shunt capacitor  122  and  156  is isolated from connecting to the inverted-L low pass filter  102  and T-type filter  104 , except bleeding through a small capacitor  124  and  154  in series with  122  and  156  to form a weak inverted-L filter  102  and a weak T-type filter  104 , which is the result of the effective shunt capacitor being reduced. However, any information exchange between Line port and POTS port during the ON HOOK state of CPE is maintained via the by-pass resistors  226  to  230  and  232 , which are across the transistor-diode bridge  110  and  116 . When there is loop current through (CPE Off Hook) filter  100 , the transistor-diode bridge  110  and  116  is turned on, the bridge center (where collectors of transistor  190  to  196  and anodes of diodes  198  to  204  are electrically coupled together) of circuit  110  and  116  exhibit low impedance, and the shunt capacitor  122  is disposed (connected) between an inductor  129  of the inverted-L filter  102 , and the impedance correction resonant circuit  108  to become a stronger inverted-L filter  102 . Shunt capacitor  156  is disposed (connected) between two resonant circuits  146  and  148  and become a stronger T-type filter  104 . 
     With regard to the transistor circuit  112  and  114  used to bring about a change in the property of the impedance correction resonant circuit  106  and  108 . The two transistors  182  to  188  in each of these circuits  112  and  114  have their emitters electrically connected to one end of the inductors  126  and  128  of the resonant circuit  106  and  108 , and their collectors electrically connected to the parallel resonant capacitors  142  and  144 , and damper resistors  138  and  140  which are in turn connected to the other end of the inductors  126  and  128 . The bases of the transistor circuits  112  and  114  are connected via a base resistor  218  and  220  to the far end of transistor circuit  110  and  116  as a source of detecting on off hook status of the CPE connected to the filter  100 . The transistor circuit  112  and  114  comprise a complementary pair of transistor NPN and PNP in form of 3906 and 3904 series transistor. However, only one of the transistors acts as a transistor based switch depending on the DC polarity on the tip and ring terminal of the subscriber line. 
     The primary function provided by two of the transistor based circuits  112  and  114  is to selectively ‘switch’ the resonant capacitors  142  and  144  and the damper resistors  138  and  140  in or out of the parallel resonant circuit  106  and  108 , to thereby alter the properties (being the resonant capacitor and damping resistor) of the resonant circuits  106  and  108  in order to shift the resonant frequencies of the circuit  106  and  108  depending on the CPE connected to the filter  100  being “off hook” or “on hook”. When the CPE connected to filter  100  is “Off hook”, the transistor based circuit  112  and  114  is conducting (switched ON), resulting in the total resonant capacitor being the sum of capacitors  134  and  140 , and sum of capacitors  136  and  144 , the resonant frequency of circuit  112  and  114  is within the voice band, and the damping resistance being parallel resistors  130  and  140  in circuit  112 , and the damping resistor being parallel resistors  132  and  138  in circuit  114 . Circuit  112  and  114  serve to compensate for the impedance of filter  100  to the line (when the CPE connected to filter  100  is OFF Hook). When the CPE connected to filter  100  is “On hook”, the transistor based circuit  112  and  114  is off (non conducted), the capacitors  142  and  144 , and resistor  138  and  140  are removed from circuit  112  and  114 , with much less capacitance with only capacitor  134  and  136  in circuit  106  and  108  the resonant frequency of circuit  106  and  108  is shifted to a much higher frequency in the stop band at around 30 kHz. The circuits  106  and  108  serve as additional parallel resonant tank circuits to the T-type low pass filter  104 , which provide filter  100  with more attenuation to ADSL signals even when CPE is in an ON-HOOK state without scarifying the parallel loading effect to any other filters in parallel. 
     The subscriber line loop current will flow through the filter  100  when the filter  100  is electrically connected to a subscriber line and customer premises equipment (such as a home telephone). Furthermore, the subscriber line loop current will flow when the customer premises equipment is in an off-hook state. When the customer premises equipment is in an on-hook state the subscriber line loop current will not flow through the filter  100 . 
     As persons skilled in the art will readily appreciate, the present invention is not restricted to the particular arrangement of electronic components depicted in  FIG. 1(   a ). For instance, it is envisaged that instead of using the resonant circuits  106  and  108  depicted in  FIG. 1(   a ), the filter  100  could employ the alternative resonant circuits  106  and  108  depicted in  FIGS. 1(   b ) to  1 ( e ). 
     As indicated in the “Background of the Invention” section of this specification, the inventor has surprisingly discovered that semiconductor based switching circuits, such as the transistor based circuits  110  to  116  depicted in  FIG. 1(   a ), can be susceptible to generating unwanted noise in the pass-band of the filter sections  102  and  104  in the absence of a subscriber line loop current (such as filter connecting to CPE in on-hook state and in parallel to other filters on the same line). While the transistor based circuits  110  to  116  tend not to generate unwanted noise when used in conjunction with early asymmetric digital subscriber line technology (signals), the circuits  110  to  116  may have a tendency to generate unwanted noise in the filters&#39;  102  and  104  pass-band when used with later asymmetric digital subscriber line technology (signals) such as ADSL2+. To ameliorate the unwanted noise when used with later asymmetric digital subscriber line technology (signals), the transistor based circuits  110  to  116  can be replaced with switching arrangements based on the circuits  200  depicted in  FIGS. 2(   a ) and  2 ( b ). 
     With reference to  FIGS. 2(   a ) and  2 ( b ), each circuit  200  comprises a transistor switch arrangement  202  comprising a transistor  204  that has a collector  206 , a base  208  and an emitter  210 . The transistor  204  depicted in  FIG. 2(   a ) is a PNP transistor and consequently the collector  206  and the emitter  210  are each associated with a separate P-type semiconductor region. On the other hand, the base  208  is associated with an N-type semiconductor region. In contrast to  FIG. 2(   a ), the transistor  204  depicted in  FIG. 2(   b ) is an NPN transistor and as such the collector  206  and the emitter  210  are each associated with a separate N-type semiconductor region. The base  208 , however, is associated with a P-type semiconductor region. 
     In addition to the transistor switch arrangement  202 , each circuit  200  comprises a semiconductor device  212  that has a pn-structure that comprises a semiconductor region that is of a type that is the same as the region associated with the base  208  of the transistor  204 . As a result, the pn-structure has an N-type region in the case of the transistor  204  depicted in  FIG. 2(   a ) and a P-type region in the case of the transistor  204  depicted in  FIG. 2(   b ). More specifically, the semiconductor device  212  is in the form of an NPN transistor in the circuit  200  of  FIG. 2(   a ), or a PNP transistor in the circuit  200  of  FIG. 2(   b ). Consequently, the transistor switch arrangement  202  and the semiconductor device  212  forms a complementary transistor pair; that is, two different types of transistors consisting of an NPN transistor and a PNP transistor. 
     The base  208  of the transistor switch arrangement  202  electrically coupled to the collector  214  of the semiconductor device  212 , while the base  216  of the semiconductor device  212  is electrically coupled to the emitter  210  of the transistor switch arrangement  202  via a resistor  218 . The emitter  220  the semiconductor device  212  is electrically coupled to the emitter  210  of the transistor switch arrangement  202  via another resistor  222 . 
     The semiconductor device  212  is arranged to switch the transistor switch arrangement  202  between an on-state and an off-state by setting the electrical potential of the base  208  of the transistor  204  relative to the emitter  210  of the transistor  204 . In the on-state, current or signals can flow through the transistor  204  via its collector  206  and emitter  210 . In the off-state, current or signals cannot flow through the transistor  204  via its collector  206  and emitter  210 . 
     An advantage of the circuits  200  depicted in  FIGS. 2(   a ) and  2 ( b ) is that the non-linear characteristics of the pn-structure between the collector  206  and the base  208  of the transistor  204  are ameliorated by the pn-structure between the collector  214  and the base  216  of the semiconductor device  212 . By ameliorating the non-linear characteristics of the pn-structure, the switching circuits  200  are less likely to generate unwanted noise in the pass-band of the filters  102  and  104  when used in conjunction with later asymmetric digital subscriber line technology (signals) such as ADSL2+. 
     It will be readily appreciated by persons skilled in the art that various different embodiments of the basic circuits  200  shown in  FIGS. 2(   a ) and  2 ( b ) can be used. In this regard,  FIGS. 2(   c ) to  2 ( e ) depict alternative embodiments. Persons skilled in the art will also appreciate that the various circuits shown in  FIGS. 2(   a ) to  2 ( e ) are not limited to being implemented using transistors. It is envisaged that other suitable semiconductor devices could be used including, for example, field effect transistors or an appropriate arrangement of discrete diodes. It will be further appreciated by those skilled in the art that the circuits depicted in  FIGS. 2(   a ) to  2 ( e ) are not restricted to being used with the filter  100  and have application to other fields where semiconductor based switching circuits are required. 
     While the present invention has been described with reference to the aforementioned embodiment, it will be understood by those skilled in the art that alterations, changes and improvements may be made and equivalents may be substituted for the elements thereof and steps thereof without departing from the scope of the present invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the present invention without departing from the central scope thereof. Such alterations, changes, modifications and improvements, though not expressly described above, are nevertheless intended and implied to be within the scope and sprit of the invention. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the embodiment for carrying out this invention, but that the invention will include all embodiments falling within the scope of the independent claims.