| 摘要 |
1348882 Line transmission STANDARD TELEPHONES & CABLES Ltd 19 April 1971 [10 April 1970] 17210/70 Heading H4R A frequency-division-multiplex line-transmission system includes terminal equipments connected to the line, at least one terminal equipment controlling the level of the signal it transmits to line in accordance with the level A it receives from the line. The Figure shows one subscriber equipment. A plurality of such equipments each employing different carrier frequencies are each coupled through line transformer and lightning protection devices 4 to a common transmission line which is terminated by an exchange not shown. Signals transmitted from the exchange to the subscriber equipment shown pass through a filter 2 are amplified at 5 demodulated at 7 and after further amplification at 8 applied to the substation receiver. The output of amplifier 5 is rectified and compared with a reference signal in a circuit A.G.C. and the difference signal used to control the gain of the amplifier 5 and a similar amplifier 11 in the transmit path. Amplifiers 5 and 11 each comprise (Fig. 4) two fixed gain stages (represented by transistors) separated by a controlled attenuator 24 or 26. The attenuator includes a variable resistive source 41, a variable resistive shunt arm 42 and a controlled shunt arm including a variable resistor 43 and a thermistor 44 the resistance of which is controlled by the difference signal applied from the comparator to the heater wire 46, 47. Before installation the variable resistors are adjusted in accordance with the frequencies allocated to that terminal equipment setting both the initial insertion loss and the shape of the control/loss characteristic. The terminal can then be added to any point on the transmission line and will automatically adjust to transmit to the exchange at the correct level. At the exchange end of the line all signals are at equal level. |
