Lowpass circuit for physical party line applications in subscriber carrier telephone system

Claim 2

A lowpass circuit comprising a first port of the circuit; a second port of the circuit; a lowpass filter comprising first and second filter ports and at least two filter sections that are connected in series; each of said filter sections including a first inductor and first capacitor; said inductor of one filter section being connected in series with one terminal of said first filter port; first means connecting said first capacitors of said two filter section across said first filter port through at least said first inductor of said one filter section; second means connecting the other filter section to said second filter port; said filter exhibiting one series resonance across saId first filter port at a frequency in the filter passband when said second filter port is terminated by a load having an impedance that is substantially greater than the characteristic impedance at said second filter port; a nonpolarized semiconductor device that conducts when a voltage across it exceeds a first prescribed value; third means electrically connecting said semiconductor device in series between the one terminals of said first circuit and first filter ports; fourth means electrically connecting the other terminals of said first circuit and first filter ports; fifth means electrically connecting terminals of said second filter port to respective terminals of said second circuit port; and a second capacitor connected in parallel with said device, said second capacitor having a capacitance for cooperating with the reactive elements of said filter for translating the in-band series resonant frequency of said filter to occur at a frequency outside the filter passband; said semiconductor device being essentially nonconducting when the current through said device is less than a second prescribed value; said device conducting for electrically connecting said filter to said first circuit port when the current through saId device is greater than the second prescribed value.

Claim 3

A lowpass circuit comprising a first port of the circuit; a second port of the circuit; a balanced lowpass filter comprising first and second filter ports and at least two filter sections that are connected in series; each of said filter sections including first and second inductors and a first capacitor; said first inductors being connected in series between the one terminals of said filter ports; said second inductors being connected in series between the other terminals of said filter ports; said first capacitors of the one and other of the two filter sections being connected across said first filter port through said inductors of said one filter section and through said inductors of the one and other filter sections, respectively; said filter exhibiting one series resonance across saId first filter port at a frequency in the filter passband when said second filter port is terminated by a load having an impedance that is substantially greater than the characteristic impedance at said second filter port; first and second nonpolarized semiconductor devices each of which conducts when a voltage across it exceeds a first prescribed value; first means electrically connecting said first and second semiconductor devices in series between associaTed terminals of said first circuit port and first filter port; second means electrically connecting terminals of said second filter port to respective terminals of said second circuit port; and second and third capacitors connected in parallel with said first and second semiconductor devices, respectively, the capacitances of said second and third capacitors having values cooperating with reactive elements of said filter for translating the in-band series resonant frequency of said filter to occur at a frequency outside the filter passband; said semiconductor devices being essentially nonconducting when the current therethrough is less than a second prescribed value; said devices being conducting for electrically connecting said filter to said first circuit port when the current through said devices is greater than the second prescribed value.

Claim 4

A lowpass circuit comprising a first port of the circuit; a second port of the circuit; a balanced lowpass filter comprising first and second filter ports and at least two filter sections that are connected in series; each of said filter sections including first and second inductors and a first capacitor; said first inductors being connected in series between the one terminals of said filter ports; said second inductors being connected in series between the other terminals of said filter ports; said first capacitors of the one and other of the two filter sections being connected across said first filter port through said inductors of said one filter section and through said inductors of the one and other filter sections, respectively; said filter exhibiting one series resonance across saId first filter port at a frequency in the filter passband when said second filter port is terminated by a load having an impedance that is substantially greater than the characteristic impedance at said second filter port; first and second nonpolarized semiconductor devices each of which conducts when a voltage across it exceeds a first prescribed value; first means electrically connecting said first and second semiconductor devices in series between associaTed terminals of said first circuit port and first filter port; second means electrically connecting terminals of said second filter port to respective terminals of said second circuit port; and second and third capacitors connected in parallel with said first and second semiconductor devices, respectively, the capacitances of said second and third capacitors having values cooperating with reactive elements of said filter for translating the in-band series resonant frequency of said filter to occur at a frequency outside the filter passband; said semiconductor devices being essentially nonconducting when the current therethrough is less than a second prescribed value; said devices being conducting for electrically connecting said filter to said first circuit port when the current through said devices is greater than the second prescribed value.

Claim 5

The circuit according to claim 2 including third and fourth nonpolarized semiconductor devices connected in series with associated first and second devices, said second and third capacitors being connected across the series combinations of said first and third devices and said second and fourth devices, respectively.

Claim 6

The circuit according to claim 2 wherein said semiconductor devices are varistors.

Claim 7

In a subscriber carrier telephone system with a physical party line having at least a pair of physical telephones separately connected to a cable pair, each physical telephone having a lowpass circuit connected between the cable pair and the associated physical telephone for passing dc and low-frequency voiceband signals and for blocking carrier frequency signals, each of said lowpass circuits comprising a first port of the circuit for connection to the cable pair; a second port of the circuit for connection to an associated telephone set; a lowpass filter including first and second filter ports and having at least two sections that are connected in series; each filter section comprising a first inductor and a first capacitor, said first inductors being connected in series between associated one terminals of said filter ports, both of said first capacitors being connected in series across said first filter port through the first inductor of one of said filter sections; and including first means connecting the other associated terminals of said first and second filter ports; second means connecting associated terminals of the second circuit and second filter ports; said filter exhibiting a series resonance across said first filter port at a frequency in the filter passband when said second circuit port is terminated in an impedance substantially greater than the filter characteristic impedance at the second filter port by an on-hook telephone; a nonpolarized semoconductor device that conducts when a voltage across it exceeds a first prescribed value; third means electrically connecting said semiconductor device in series between associated one terminals of said first circuit and first filter ports; fourth means connecting the other terminals of said first circuit and first filter ports; and a second capacitor connected in parallel with said device, said second capacitor having a capacitance for cooperating with the reactive elements of said filter for translating the in-band series resonant frequency of said filter to occur at a frequency outside the filter passbnad; said semiconductor device being nonconducting when the second circuit port is terminated by an on-hook telephone and the voltage on the cable pair is less than the first prescribed value; said device conducting for connecting said filter to said first circuit port when the current through said device is greater than a second prescribed value.

Claim 8

In a subscriber carrier telephone system with a physical party line having at least a pair of physical telephones separately connected to a cable pair, each physical telephone having a lowpass circuit connected between the cable pair and the associated physical telephone for passing dc and low-frequency voiceband signals and for blocking carrier frequency signals, each of said lowpass circuits comprising a first port of the circuit for connection to the cable pair; a second port of the circuit for connection to an associated telephone set; a lowpass filter including first and second filter ports and having at least two sections that are connected in series; each filter section comprising a first inductor and a first capacitor, said first inductors being connected in series between associated one terminals of said filter ports, both of said first capacitors being connected in series across said first filter port through the first inductor of one of said filter sections; and including first means connecting the other associated terminals of said first and second filter ports; second means connecting associated terminals of the second circuit and second filter ports; said filter exhibiting a series resonance across said first filter port at a frequency in the filter passband when said second circuit port is terminated in an impedance substantially greater than the filter characteristic impedance at the second filter port by an on-hook telephone; a nonpolarized semoconductor device that conducts when a voltage across it exceeds a first prescribed value; third means electrically connecting said semiconductor device in series between associated one terminals of said first circuit and first filter ports; fourth means connecting the other terminals of said first circuit and first filter ports; and a second capacitor connected in parallel with said device, said second capacitor having a capacitance for cooperating with the reactive elements of said filter for translating the in-band series resonant frequency of said filter to occur at a frequency outside the filter passbnad; said semiconductor device being nonconducting when the second circuit port is terminated by an on-hook telephone and the voltage on the cable pair is less than the first prescribed value; said device conducting for connecting said filter to said first circuit port when the current through said device is greater than a second prescribed value.

Claim 9

In a subscriber carrier telephone system with a physical party line having at least a pair of physical telephones separately connected to a cable pair, each physical telephone having a lowpass circuit connected between the cable pair and the associated Physical telephone for passing dc and low-frequency voiceband signals and for blocking carrier frequency signals, each of said lowpass circuits comprising a first port of the circuit for connection to the cable pair; a second port of the circuit for connection to an associated telephone set; a balanced lowpass filter including first and second filter ports and having at least two sections that are connected in series; each filter section comprising first and second inductors and a first capacitor; said first inductors being connected in series between associated one terminals of said filter ports; said second inductors being connected in series between associated other terminals of said filter ports; said first capacitors being connected across said first filter port through said inductors of one of said filter sections; first means connecting associated terminals of the second circuit port and second filter port; said filter exhibiting a series resonance across said first filter port at a frequency in the filter passband when said second circuit port is terminated in an impedance substantially greater than the filter characteristic impedance at the second filter port by an on-hook telephone; first and second nonpolarized semiconductor devices each of which conducts when a voltage across it exceeds a first prescribed value; second means electrically connecting said first and second semiconductor devices in series between associated terminals of said first circuit port and first filter port; and second and third capacitors connected in parallel with said first and second semiconductor devices, respectively, the capacitances of said second and third capacitors having values cooperating with reactive elements of said filter for translating the in-band series resonant frequency of said filter to occur at a frequency outside the filter passband; said semiconductor devices being essentially nonconducting when the second circuit port is terminated by an on-hook telephone and the voltage on the cable pair is less than the first prescribed value; said devices being conducting for connecting said filter to said first circuit port when the current through said devices is greater than a second prescribed value.

Claim 10

In a subscriber carrier telephone system with a physical party line having at least a pair of physical telephones separately connected to a cable pair, each physical telephone having a lowpass circuit connected between the cable pair and the associated Physical telephone for passing dc and low-frequency voiceband signals and for blocking carrier frequency signals, each of said lowpass circuits comprising a first port of the circuit for connection to the cable pair; a second port of the circuit for connection to an associated telephone set; a balanced lowpass filter including first and second filter ports and having at least two sections that are connected in series; each filter section comprising first and second inductors and a first capacitor; said first inductors being connected in series between associated one terminals of said filter ports; said second inductors being connected in series between associated other terminals of said filter ports; said first capacitors being connected across said first filter port through said inductors of one of said filter sections; first means connecting associated terminals of the second circuit port and second filter port; said filter exhibiting a series resonance across said first filter port at a frequency in the filter passband when said second circuit port is terminated in an impedance substantially greater than the filter characteristic impedance at the second filter port by an on-hook telephone; first and second nonpolarized semiconductor devices each of which conducts when a voltage across it exceeds a first prescribed value; second means electrically connecting said first and second semiconductor devices in series between associated terminals of said first circuit port and first filter port; and second and third capacitors connected in parallel with said first and second semiconductor devices, respectively, the capacitances of said second and third capacitors having values cooperating with reactive elements of said filter for translating the in-band series resonant frequency of said filter to occur at a frequency outside the filter passband; said semiconductor devices being essentially nonconducting when the second circuit port is terminated by an on-hook telephone and the voltage on the cable pair is less than the first prescribed value; said devices being conducting for connecting said filter to said first circuit port when the current through said devices is greater than a second prescribed value.

Claim 11

The circuit according to claim 6 including third and fourth nonpolarized semiconductor devices connected in series with associated first and second devices, said second and third capacitors being connected across the series combinations of said first and third devices and said second and fourth devices, respectively.

Claim 12

The circuit according to claim 6 wherein said semiconductor devices are varistors.

Claim 13

A lowpass circuit comprising a first port of the circuit; a second port of the circuit; a lowpass filter comprising first and second filter ports and at least two filter sections that are connected in series; at least one of said filter sections including an inductor and a first capacitor; said inductor of the one filter section being connected between one terminal of said first filter port and one terminal of said second filter port through the other filter section; first means connecting said first capacitor of said one filter section across said first filter port through at least said inductor of said one filter section; said filter exhibiting one series resonance across said first filter port at a frequency in the filter passband when said second filter port is terminated by a load having an impedance that is substantially greater than the characteristic impedance at said second filter port; a nonpolarized semiconductor device that conducts when a voltage across it exceeds a first prescribed value; second means electrically connecting said semiconductor device in series between the one terminals of said first circuit port and first filter port; third means electrically connecting the other terminals of said first circuit port and first filter port; fourth means electrically connecting Terminals of said second filter port to respective terminals of said second circuit port; and a second capacitor connected in parallel with said device, said second capacitor having a capacitance cooperating with the reactive elements of said filter for translating the in-band series resonant frequency of said filter to occur at a frequency outside the filter passband; said semiconductor device being essentially nonconducting when the current through said device is less than a second prescribed value; said device conducting for electrically connecting said filter to said first circuit port when the current through said device is greater than the second prescribed value.

Claim 14

A lowpass circuit comprising a first port of the circuit; a second port of the circuit; a lowpass filter comprising first and second filter ports and at least two filter sections that are connected in series; at least one of said filter sections including an inductor and a first capacitor; said inductor of the one filter section being connected between one terminal of said first filter port and one terminal of said second filter port through the other filter section; first means connecting said first capacitor of said one filter section across said first filter port through at least said inductor of said one filter section; said filter exhibiting one series resonance across said first filter port at a frequency in the filter passband when said second filter port is terminated by a load having an impedance that is substantially greater than the characteristic impedance at said second filter port; a nonpolarized semiconductor device that conducts when a voltage across it exceeds a first prescribed value; second means electrically connecting said semiconductor device in series between the one terminals of said first circuit port and first filter port; third means electrically connecting the other terminals of said first circuit port and first filter port; fourth means electrically connecting Terminals of said second filter port to respective terminals of said second circuit port; and a second capacitor connected in parallel with said device, said second capacitor having a capacitance cooperating with the reactive elements of said filter for translating the in-band series resonant frequency of said filter to occur at a frequency outside the filter passband; said semiconductor device being essentially nonconducting when the current through said device is less than a second prescribed value; said device conducting for electrically connecting said filter to said first circuit port when the current through said device is greater than the second prescribed value.

Claim 15

In a subscriber carrier telephone system with a physical party line having at least a pair of physical telephones separately connected to a cable pair, each physical telephone having a lowpass circuit connected between the cable pair and the associated physical telephone for passing dc and low-frequency voiceband signals and for blocking carrier frequency signals, each of said lowpass circuits comprising a first port of the circuit for connection to the cable pair; a second port of the circuit for connection to an associated telephone set; a lowpass filter including first and second filter ports and having at least two sections that are connected in series; at least one of said filter sections comprising an inductor and a first capacitor; said inductor being connected in series between associated one terminals of said filter ports through the other filter section; said first capacitor being connected in series across said first filter port through said inductor; first means connecting associated terminals of the second circuit and second filter ports; said filter exhibiting a series resonance across said first filter port at a frequency in the filter passband when said second circuit port is terminated in an impedance substantially greater than the characteristic impedance of the filter at the second filter port by an on-hook telephone; a nonpolarized semiconductor device that conducts when a voltage across it exceeds a first prescribed value; second means electrically connecting said semiconductor device in series between associated one terminals of said first circuit and first filter ports; third means connecting the other terminals of said first circuit and first filter ports; and a second capacitor connected in parallel with said device, said second capacitor having a capacitance cooperating with the reactive elements of said filter for translating the in-band series resonant frequency of said filter to occur at a frequency outside the filter passband; said semiconductor device being nonconducting when the second circuit port is terminated by an on-hook telephone and the voltage on the cable pair is less than the first prescribed value; said device being conducting for connecting said filter to said first circuit port when the current through said device is greater than a second prescribed value.

Claim 16

In a subscriber carrier telephone system with a physical party line having at least a pair of physical telephones separately connected to a cable pair, each physical telephone having a lowpass circuit connected between the cable pair and the associated physical telephone for passing dc and low-frequency voiceband signals and for blocking carrier frequency signals, each of said lowpass circuits comprising a first port of the circuit for connection to the cable pair; a second port of the circuit for connection to an associated telephone set; a lowpass filter including first and second filter ports and having at least two sections that are connected in series; at least one of said filter sections comprising an inductor and a first capacitor; said inductor being connected in series between associated one terminals of said filter ports through the other filter section; said first capacitor being connected in series across said first filter port through said inductor; first means connecting associated terminals of the second circuit and second filter ports; said filter exhibiting a series resonance across said first filter port at a frequency in the filter passband when said second circuit port is terminated in an impedance substantially greater than the characteristic impedance of the filter at the second filter port by an on-hook telephone; a nonpolarized semiconductor device that conducts when a voltage across it exceeds a first prescribed value; second means electrically connecting said semiconductor device in series between associated one terminals of said first circuit and first filter ports; third means connecting the other terminals of said first circuit and first filter ports; and a second capacitor connected in parallel with said device, said second capacitor having a capacitance cooperating with the reactive elements of said filter for translating the in-band series resonant frequency of said filter to occur at a frequency outside the filter passband; said semiconductor device being nonconducting when the second circuit port is terminated by an on-hook telephone and the voltage on the cable pair is less than the first prescribed value; said device being conducting for connecting said filter to said first circuit port when the current through said device is greater than a second prescribed value.