Selectable impedance line interface circuit

An interface circuit for a.c. coupling a transmission line, of any one of n predetermined characteristic impedances, to a port (13) of a telecommunications facility, said port being of a predetermined and fixed impedance, the interface circuit comprising:

• a series resistance network including at least 2n+3 resistor elements (20-28), each being of a predetermined ohmic value, the resistance network being connected to ground by a one of said resistor elements (20);
• first and second amplifiers (14, 16) each having an input and an output;
• a switching device including 2n switching elements, n of the switching elements (32-34) being connected between n of the resistor elements and the input of the first amplifier and n of the switching elements (36-38) being connected between another n of the resistor elements and the input of the second amplifier, whereby signals are coupled to said port;
• a transformer (9) having a first winding (10) for connection to the transmission line, a second winding (11) being connected between the output of the first amplifier and a junction between said one (20) of the resistor elements and at least one other of the resistor elements, and a third winding (12) being connected between ground and a point in the resistance network other than said junction;
• means for coupling signals between at least one of the said n switching elements and the port of the telecommunication facility;
• and
• a control means (2) being connected to the switching device for controlling each of said switching elements to be of one of a high resistance state and a lower resistance state.

An interface circuit for a.c. coupling a four wire transmission line of any one of n predetermined characteristic impedances with corresponding input and output ports of a telecommunication facility, the interface circuit comprising:

• first, second third amplifiers (46, 14, 16), each having an output and an inverting input;
• a first resistance network (51a, 51b, 51c, 52a, 52b, 52c, 54) including 2n+1 resistors, and a second resistance network (20-28) including 2n+3 resistors being connected in series between ground and output ports;
• a first transformer (39) including a first winding (40) for connection to a pair of the transmission line wires, a second winding (41) being connected between the output of the first amplifier and ground via a first resistor in the first resistance network, and a third winding (42) being connected between a junction of n resistors (51a, 51b, 51c) in the first resistor network and ground;
• a first switching device (6) including n transfer switches (57-59), each including a BREAK portion connected to ground, a MAKE portion connected to the inverting input of the first amplifier, and a MAKE BREAK junction between said portions, each MAKE BREAK junction being connected to corresponding ones of the n resistors;
• n resistors (52a, 52b, 52c) in the first resistor network being connected at a junction of the first resistor and the second winding;
• a second transformer (9) including a first winding (10) for connection to another pair of the transmission line wires, a second winding (11) being connected between the output of the second amplifier and ground via a first resistor in the second resistance network, and a third winding (12) being connected to a junction in the second resistance network and round, said junction being n+1 resistors removed from the first resistor;
• a second switching device including first and second groups of n MAKE portions, the first group of n MAKE portions being connected between the inverting input of the second amplifier and respective n junctions of the n+1 resistors, and the second group of n MAKE portions being connected between the inverting input of the third amplifier and respective n junctions of a remaining n+1 resistors of the second resistance network;
• and
• register means being connected to each of the switching devices for controlling states of the respective MAKE and BREAK portions such that any one of n operating interface circuit impedances is selectable.