Patent GB-482382-A (GB)
Title: Gain regulating arrangements for electrical signalling systems
IPC: H04B3/10
Filed: 1936-10-02
Abstract: 482,382. Repeating systems. STANDARD TELEPHONES & CABLES, Ltd. (Western Electric Co., Inc.) Oct. 2, 1936, No. 26784. [Class 40 (iv)] [Also in Group XL] In a multiplex carrier or other transmission system comprising a long repeatered line subject to variable attenuation due to temperature and other changes, compensation of some changes is effected at repeaters at frequent intervals and of other changes at more widely spaced repeaters. In one system, control is effected by two pilot frequencies at the edges of the transmission band, the upper pilot which is more sensitive to temperature variations being used at frequent intervals (attended and unattended repeaters) for compensating the effect of such variations over the whole frequency range whereas the lower pilot is used at less frequent intervals (attended repeaters) for compensating changes in the flat gain of the repeaters due for example to variation in valve gain. In another system, compensation of the second kind is controlled by short inter-repeater pilot wires and of the first kind by longer pilot wires between the more widely spaced repeaters. Pilot frequency control. One of the more widely spaced repeaters at which control by both frequencies is effected is shown in Fig. 4 ; the other repeaters have the high freqeuency pilot control omitted. The signals together with the pilot frequencies are applied over equalizer EQ to an amplifier comprising three pentodes 10-12 in cascade, a stabilizing feedback connection 13 being taken from the anode circuit of 12 to the cathode of 11 and the feed-back being equalized over the frequecny range by a network 15<1>. The coupling between valves 10 and 11 comprises a variable shunt network 23 of inductance, capacity and resistance and includes silver sulphide resistance 25 mounted in a chamber with heating coil 26. The Specification gives an example of the values of the network components. The high frequency pilot in the output of the amplifier passes through filter 41 over a control amplifier 50 to valve 51 operating as a grid rectifier and tending to apply positive bias to grid 60 of valve 55 having a static negative bias applied from battery 59 and the associated resistance network. The grid of 55 is supplied with A.C. from transformer 57 so that amplified A.C. flows in the output circuit of an amplitude depending on the amplitude of the pilot frequency and increases the temperature of heating coil 26 which reduces the resistance of the silver sulphide element 25 in the shunt circuit. Part of the output of 55 is fed back over transformer 66 to rectifier 56 which aids the bias on 55 due to the pilot frequency to give better regulation of the control. The anode of grid rectifier 51 is fed with A.C. so that in the absence of pilot frequency due to failure of the line or other equipment, the grid of 51 has only a small negative bias from battery 59, and anode current operates relay 76 which brings in an alarm. The Specification discusses in much detail the effect of the amount of feed back (controlled at 81), the time constant of the circuit of rectifier 56, and the rate of heating or cooling of the sulphide element 25, on the regulation. The output level of the repeater may be arranged to increase, remain constant, or decrease with increase of input. The low frequency pilot wave at main repeaters is separated by filter 40 and after amplification and rectification at 42, 43 biasses a rectifier 44 fed with A.C. to provide varying potentials across resistance 47, for biassing valve 10 of the repeater. The control amplifying valve 42 is also biassed by resistance 47 so as to increase the effect of a variation of input to valve 42. Testing. The high frequency pilot control circuit may be tested by opening the input and output circuits 41. 29 and manually varying the bias of valve 55, the output of which is read by a meter transformer-coupled to the output circuit. Pilot wire control, Figs. 9 to 11. In this system, relatively short pilot wires PL1-PL3 between repeaters are used to control the flat gain of the repeaters and a longer pilot wire PL4 between attended repeaters compensates for variable attenuation changes over the frequency range. Pilot wire PL4 also exercises a control of the pilot wire circuit PL3 at the attended repeater RR. Fig. 10 shows the repeater and dual control arrangements used at RR, but the portion of the control which is similar to that used at unattended repeaters R8, R9 will first be considered and for this purpose switch 127 is assumed thrown to connect the short pilot wire PL3 directly in one arm of bridge 123- 126. The variable network 118 is also not required at unattended repeaters. The repeater 108 has a negative feed back path over equalizer 114, the gain of the repeater being increased by increasing the loss in the feed back path by means of a condenser potentiometer arrangement C1-C3 shown in Fig. 11. The gain of the amplifier is adjusted manually to the required normal by varying C2, C3 keeping C2 + C3 constant by using, for example, a differential condenser for C2, C3. The automatic control operates on condenser C1., and the law of change due to rotation of C1 may be linear or otherwise depending on the setting of a trimming condenser 216. When the resistance of the pilot wire changes, the bridge is unbalanced and a galvanometer at 129 causes shaft 131 to be driven whereby the bridge is re-balanced by variable resistance 128 and a master controller 133 operating on the resultant field principle sets controller 135 of amplifier 108 and controllers corresponding amplifiers of other lines at the same repeating station. The controller 135 drives condenser C1 through reduction gearing 137. The dual control at attended repeaters involving the whole of the equipment in Fig. 10 with switch 127 in the position shown will now be considered. Change of resistance of long pilot wire PL4 unbalances an automatically re-balancing bridge 151 in a similar manner to that described for the short pilot wire and the controller 159 adjusts variable resistances 164, 165 to alter the characteristics of networks such as 118 at the associated repeaters. As variation of network 118 involves a variation of overall loss, it is arranged that controller 159 also varies a resistance 163 in series with the short pilot wire whereupon bridge 123 .. 126 re-balances and drives condenser C1 to off-set the overall change produced by network 118.