Patent GB-866653-A (GB)
Title: Negative impedance repeater for pulse multiplex circuits
IPC: H04J3/20
Filed: 1959-06-12
Abstract: 866,653. Automatic exchange systems. STANDARD TELEPHONES & CABLES Ltd. June 12, 1959 [June 17, 1958], No. 20215/59. Class 40 (4). In a time division multiplex system in which energy interchange between storage devices takes place repeatedly during short intervals over a common highway on a tuned circuit basis the highway is provided with an opencircuit stable repeater comprising at least one negative resistance and producing a gain greater than or equal to the resistive loss in the circuit. As shown, Fig. 1, the storage devices are normally condensers C fed via low-pass filters from subscribers' lines, and inductances L are provided whereby complete interchange of charges between the condensers is effected during the interval that the gates GA, GA<SP>1</SP> are opened. In the embodiments described the inductive portion of the transfer circuit is included in the repeater and the gain of the repeater is such that when a charge is transferred from an input condenser to an output condenser the charge on the latter is larger than it was on the former, whilst the said input condenser is left in a substantially discharged condition so preventing any undesirable reflections. The Specification shows that if the repeater takes the form of a quadripole, the voltages on the condensers take the form of the sum of two sinusoids with exponentially increasing amplitudes. The parameters of the quadripoles may be chosen so that at the end of the interval during which they are connected the rate of change of voltage across the condensers is zero. This permits some tolerance to slight changes in duration of the pulses opening the gates GA, GA<SP>1</SP>. This condition also corresponds to zero energy being stored in the inductances of the repeater. If # 0 , # 1  are the angular frequencies of the two sinusoids then it is proved that one of the quantities # 0 t 1 , # 1 t 1  is an odd multiple of # and the other is an even multiple of #, and that the rates of exponential rise must be the same for the two sinusoids. It is preferred to choose either 2# 0 t 1  = # 1 t 1  = 2# corresponding to amplification without phase reversal, or 2# 1 t 1  = # 0 t 1  = 2# corresponding to amplification with phase reversal. Fig. 5 illustrates a simple form of quadripole capable of exhibiting the required characteristics. Negative resistances are inserted in the series arms so that when combined with the resistances in the gates and coils the resultant is - 2R. The inductances L are positive, but m may be positive or negative so that the shunt arm may be a negative inductance with a positive resistance or vice versa. m must be smaller than ¢. Amplification with or without phase reversal occurs according as m is negative or positive. If m is negative the three coils shown at TL may be replaced by two loosely coupled series aiding coils as shown in Fig. 6. If m is positive the coils must be in series opposition. These networks may be transformed from star to delta configurations, which result in the networks shown in Figs. 7, 8 for m positive and m negative respectively. In Fig. 7 positive resistances R1 have been retained in the series arms, prior to the transformation. This allows a degree of freedom in the shunt arm negative resistances, which are thereby chosen to minimize the effect of small variations therein. Similar considerations have effected the choice of parameters in Fig. 8. Specifications 753,645, 823,190, 824,221 and 824,222 are referred to.