Patent GB-1385266-A (GB)
Title: Line repeater
IPC: H04L25/24
Filed: 1972-06-08
Abstract: 1385266 Digital transmission; bipolar repeaters; pulse regenerators INTERNATIONAL STANDARD ELECTRIC CORP 8 June 1972 [16 June 1971] 26762/72 Headings H4R H3T and H4P A two way repeater designed to receive distorted bipolar P.C.M. signals and retransmit regenerated undistorted bipolar P.C.M. signals has for each direction of transmission an automatic equalizer arranged to yield a constant peak amplitude bipolar P.C.M. signal at both a non-inverting and an inverting output of the equalizer, a clock extraction circuit for producing two clock waveforms associated with the bit rate of the bipolar signal, a decision circuit for eliminating noise pulses of amplitude less than half the constant peak amplitude of the bipolar signal and a pulse regenerator circuit for producing undistorted bipolar signals in accordance with the distorted signal received. A block diagram of such a repeater is shown in Fig. 1, the corresponding transistor circuitry in Figs. 3A and 3B (not shown) and the clock waveforms in Fig. 2 (not shown). A bipolar P.C.M. signal incoming from the line DE is connected via a protection circuit 3, comprising a diode bridge having a Zener diode connected across one of the diagonals, and a powder isolating transformer to the non-inverting input of a differential input, differential output operational preamplifier 5, which supplies a non-inverting and an inverting output at the terminals +, - respectively. An equalization circuit comprising peak rectifier 6, amplitude comparator 7, a reference voltage, and line compensating network 8 connected between the two outputs and the non-inverting input of preamplifier 5 ensure that the peak amplitude of the bipolar P.C.M. signal is kept constant at both the inverting and non-inverting outputs and at a value equal to a reference voltage which is connected to the amplitude comparator 7, thereby compensating for line losses. The line compensating network essentially comprises a pair of variable resistance diodes and the amplitude control is achieved by altering the bias current through the diodes. A capacitor and resistor ensure that isolated noise peaks do not affect the operation. A negative feedback loop is connected between the non- inverting outputs and the inverting input of the preamplifier 5. The inverted and non-inverted equalized signals are then fed to a clock extraction circuit 9 and to decision circuits 15, 16, which decision circuits produce a binary 1 output when the signal level at the corresponding preamplifier output exceeds a threshold voltage which is half the reference voltage used in the equalization stage. By this method noise pulses of up to half the signal amplitude are eliminated. The outputs of the decision circuits in association with the clock waveforms are then used to produce square wave bipolar P.C.M. signals at the output AB. The clock extraction circuit 9 includes a tank circuit 11 which produces a sinusoidal signal having positivenegative zero crossings lagging each zero crossing of the distorted but equalized incoming bipolar signals by 90 degrees. From this sinusoidal signal is produced a first clock waveform TM2 which is a square wave having positive to negative transitions corresponding to each peak, positive or negative, of the incoming bipolar signal and a second clock waveform TM1 consisting of positive 50 ns. wide pulses coinciding in time with the negative going transitions of waveform TM2, the waveform TM1 being produced by differentiating and inverting the negative going transitions of waveform TM2. By means of bi-stables 19, 20, transistors Q23, Q24 and transformer T2 a square pulse is produced at the output corresponding in time and sign to the positive and negative excursions of the distorted incoming bipolar P.C.M. signal. Identifying faulty repeaters.-Each repeater incorporates facilities that enable interrogation from a central office. A pulse train which isolates the bipolar pattern is transmitted from the central office at a low frequency rate. Each repeater has its own characteristic low pass filters coupled to terminals F, and H of sides 1 and 2 which pick up the low frequency signal and return it to the central office over a fault pair. Power supply.-The power supply for the two-way repeater is derived from two 5À6 volt Zener diodes CR21 and CR22 connected as shown and being fed over the transmission line. Connections T1-T4 allow the current to be fed on to the next repeater or to be looped back to the other direction of transmission.