ELECTRONIC IMPEDANCE-MATCHED LINE REPEATER

Abstract: An electronic line repeater for connecting a two-wire line to a subscriber exchange is impedance-matched to the two-wire side with the aid of a feedback circuit from the two-wire side of the repeater to the incoming four-wire side. The feedback includes a differential amplifier (F1) which senses the speech voltage drop on the two-wire side, and a resistor (R) connected to the line amplifier (F2) of the repeater. Impedance matching can be achieved by definite values of the gain provided by the differential amplifier and the line amplifier.

Description: ELECTRONIC IMPEDANCE-MATCHED LINE REPEATER TECHNiCAL FIELD The present invention relates to an electronic impedance-matched line repeater for connecting a two-wire line to a subscriber exchange. BACKGROUND ART A line repeater has the task of forming a junction between a two-wire line (the two-wire side) and two four-wire sides with the object of subdividing the traffic flow through the line into two one-way paths. Particularly with external line repeaters, i e such repeaters as are arranged between a subscriber exchange and an outgoing line (a line to the main station), it is desired to have as low-ohmic DC impedance as possible, seen from the line side (two-wire side). Furthermore, there is also desired a very well-defined AC impedance, which is in most cases somewhat higher, seen from this side. In known structures this impedance matching has been solved by accurate and relatively complicated dimensioning of hybrid transformers, e g according to the Swedish Patent Application No 8102610-6. Impedance matching has been carried out in such cases with the aid of RC components in the hybrid transformer, the hybrid itself having been implemented in the form of a transformer coupling. DISCLOSURE OF INVENTION The object of the present invention is to provide an electronic line repeater, particularly an external line repeater, with impedance matching in respect of both AC and DC, as seen from the two-wire side. This object is achieved in accordance with the invention, such that a feed-back loop containing amplifier elements is arranged from the two-wire side of the repeater to the incoming four-wire side, whereby desired DC and AC impedance can be obtained by given values for the selected working point and for the gain of the amplifier elements incorporated in the feed-back loop. The invention is characterized by the features set forth in the characterizing portion of claim 1. BRIEF DESCRIPTION OF DRAWING The invention will now be described in detail while referring to the appended drawing, illustrating a circuit diagram of the line repeater in accordance with the invention. MODE FOR CARRYING OUT THE INVENTION In the circuit diagram of the Figure, the input terminal to the left forms the two-wire side of the repeater. A voltage U1 appears across this terminal which comprises a direct component (the supply direct voltage) and an alternating component (the speech signals from and out, respectively, the line). The two-wire loop contains two resistors R1 and R2 having the same resistance, and constituting protective resistances to overvoltages. A balanced line amplifier F2 contains a driving stage D which is an operational amplifier circuit and an LED light-emitting diode for galvanic isolation between the two- and four-wire sides, described in the Swedish Patent Application No 8200740-2. The LED in the driving stage D is optically coupled to a phototransistor T, the collector emitter circuit of which is connected to both resistors R1 and R2. The phototransistor T can also be connected to a Darlington circuit comprising two transistors, as is illustrated in the application mentioned. In accordance with the invention, a feed-back loop is provided, comprising a differential amplifier circuit F1 and a resistor R. The circuit F1 has two input pairs, of which one is connected across the input of the two-wire side and thus senses the potential drop U1, the other pair being connected to the output side of the balanced line amplifier F2 and thus senses the potential drop U2. The differential amplifier circuit F1 provides an output U3 = F1 (U1-U2) and contains three operational amplifiers OP1-OP3. Both amplifiers OP1 and OP2 sense the potential drops U1 and U2, respectively, and each transmits signals to both inputs of the amplifier OP3. This amplifier subtracts and filters the output signals from the amplifiers OP1, OP2 and gives the output signal u3' which does not contain any DC-component from the two-wire side but only the speech signal component, since a difference signal is formed in the amplifier OP3. The output signal from I the amplifier OP3 thus constitutes an amplified speech signal from the two-wire side. This signal is connected via the resistor R to the input of the line amplifier F2. The four-wire side receiver and transmission paths contain buffer steps OP4 and OPS, respectively, suitably looped operational amplifiers of known design. The receiver path buffer step OP4 is connected via the resistor R to the input of the line amplifier. The incoming speech signal is denoted u5. If low-ohmic output is assumed for the buffer step OP5, as with the output of the final amplifier OP3 (which is the general case for operational amplifiers), the input signal u4 to the line amplifier F2 (which thus only contains the speech signal) is obtained by voltage division between the resistances of the resistors R, i e u4 u3/2 + u512 if the resistors R have the same resistance. The following calculations are applicable for the line impedance matching in respect of DC and AC impedance matching. DC Impedance Matching The DC resistance on the two-wire side input (DC impedance) is determined by the resistance of the resistors R1 and R2 and also by the selected working point of the transistor T. Assume that the voltage U2 has a direct component U02 and that the working point of the transistor T is selected so that its collectoremitter current is ICE, then the DC impedance Zdc = R1 + R2 + U02/ICE The differential amplifier F1 is assumed to have very high-ohmic inputs and thus does not influence the value of dc Since R1 + R2 are constant, the working point of the transistor T can be acljusted so that the desired value of Uo2/ICE is obtained. The transistor T must however operate in its linear range. AC Impedance Matching From the circuit diagram it will be seen that the desired input impedance can be obtained if the amplification F1 and F2 in the units F1 and F2, respectively, are selected so that correct negative feed-back gives the desired input impedance. By superposition is obtained input impedance ZAC = EMI3.1 From the block diagram it will be seen that the following equation relationships can be set up: u2 = F2 . U4 U3 = F1 (u1 - u2) EMI4.1 For the no-load state: i = 0, U1 = U2, which gives U3 = 0 and u2 = F2 . U5/2, where u1, u2 denote the signal magnitudes of U1 and U2 respectively. For short-circuiting: EMI4.2 which gives u3 = - F1 u2 and U4 = EMI4.3 EMI4.4 so that EMI4.5 The input impedance ZAC for alternating current will thus be: EMI4.6 For example, if an AC impedance ZAC = 600 ohms is desired and R1 = R2 = 30 ohms, there is obtained according to the above that F1 . F2 = 18. Assume that F2 = 0,2 which is a usual value for the coupling factor of an optocoupler, there is obtained F = 90. This amplification of the block F1 is very moderate, and can be achieved without difficulty and be kept constant by suitably selecting the resistors determining the gain. By introducing integrating or differentiating networks in a manner known per se, in the operational amplifier block F1, the gain F1 can be made complex, i e F1 = F1 (jw) whereby a complex ZAC may be obtained.

Claims: CLAIMS 1 Electronic impedance-matched line repeater included in a two-wire- to four-wire converter for connecting a two-wire line to a telephone exchange including a two-wire side as well as an incoming and an outgoing four-wire side, the two-wire side of the repeater comprising an external line loop containing resistors connected as overvoltage protection and a balanced line amplifier (F2), the output side of which is connected in series with said resistors (R1) and the input side of which forms one (incoming) four-wire path via a first resistor (R), characterized in that a feedback loop is provided for impedance matching to the two-wire line, and constitutes a differential amplifier circuit (F1) with two input pairs, of which one is connected directly to the two-wire input and the other is connected across the output side of the line amplifier, the output of the differential amplifier (F1) being connected via a second resistor (R) to the input of the line amplifier, and furthermore forms the second (outgoing) four-wire path. 2 Electronic line repeater as claimed in claim 1, characterized in that said differential amplifier (F1) has a low-ohmic output which, via the first and the second resistor (R) is connected to a low-ohmic output at one (incoming) four-wire side (OP4). 3 Electronic line repeater as claimed in claim 2, characterized in that said first and second resistors (R) have the same resistance. 4 Electronic line repeater as claimed in claims 1-3, characterized in that the amplification of the differential amplifier (F1) is frequency-dependent, so that complex impedance-matching with the two-wire side is obtained.

Inventors: LENELL, LARS ANDERS
Applicants: TELEFONAKTIEBOLAGET L M ERICSSON
Application year: 1983
Main classification: H04B1
IPC codes: H04M19, H04B1