GB713224A - Variable attenuation correcting electrical impedance network - Google Patents

Abstract

713,224. Multiplex systems; automatic volume control. TELEFONAKTIEBOLAGET L. M. ERICSSON. May 16, 1952 [May 21, 1951], No. 12519/52. Class 40 (4) [Also in Group XL (b)] In an automatic volume control arrangement for a multi-channel carrier system, of the type in which two pilot currents control flat gain and slope, respectively, the slope is maintained at a mean value in the event of failure of the second pilot current. As shown, the pilot currents are extracted from the output of "go" amplifier 15 by filters 16, 18 and are applied by way of receivers 17, 19 to control variable attenuators 14, 13 respectively. Attenuator 13 controls flat gain in the normal way, e.g. pilot current heats a thermistor in an impedance network. The second pilot receiver 17, however, derives from the second pilot two control currents, of which the sum is constant, but which change in opposite senses when the pilot level changes. These two currents each control one of two thermistors in the attenuator 14 to vary the slope. In the absence of the second pilot current, these two control currents cease and the attenuator 14 produces a mean value of slope. The two thermistors form part of an impedance network (see Group XL (b)) which is connected either in series with, or in the negative feedback path of, an additional amplifier, the combination of the impedance network and the additional amplifier forming the attenuator 14.