GB428293A - Arrangements for use in electric signalling systems - Google Patents

Abstract

428,293. Modulating and demodulating circuits. STANDARD TELEPHONES & CABLES, Ltd., 63, Aldwych, London.-(Assignees of Cowan, F. A., and Caruthers, R. S.) Nov.- 10, 1933, Nos. 31383 and 31384. Convention dates, Nov. 25, 1932 and Dec. 1, 1932. [Class 40 (v)] A modulating or demodulating arrangement comprises a bridge of rectifiers or other non- linear resistances to which a carrier wave is applied together with the modulating signals or modulated wave as the case may be. Two arrangements comprise third-order modulation. In the arrangement shown in Fig. 1, a source of carrier frequency 17 is applied through transformer 21 across one diagonal of the bridge 11 .. 14, the other diagonal of which is across the line connecting the modulating source 24 and filter FV to the output filter FSB which passes a single side band. The bridge is balanced for the carrier source so that the carrier is suppressed in the output, but modulating signals unbalance the bridge in accordance with their amplitude and sign. The biassing voltage 20 is such that the direction of currentflow through the rectifiers or resistances 11 .. 14 is always the same. When rectifiers are used the bridge may be regarded as intermittently short-circuiting the line at carrier frequency. The carrier source may be directly connected across the bridge, transformer 21 being omitted. In the modification shown in Fig. 4, the bridge is arranged in series in the line, resistance 22 forming a self-biassing device. In the further modification shown in Fig. 5, the carrier source 17 and modulating signals from 24 are connected to the same terminals 18, 19 through suitable filters FC, FV. The carrier passes through rectifiers 11, 14 and 12, 13 in alternate half cycles so that the resistances of these pairs are alternately reduced, with the result that the direction of signal-current flow in the output is reversed at carrier frequency. Third-order modulation is provided for in the arrangements shown in Figs. 7 and 8. The line in Fig. 7, between modulating source 24 and output FSB is shunted by two rectifier bridges associated with the carrier source 17 and biassing resistances 22, 52 which normally make the rectifiers nonconducting. The bridges are alternately brought into the condition in which they short-circuit the line during successive half-cycles of the carrier, the carrier and bias voltage being such that the carrier overcomes the bias for a quarter - cycle at the carrier voltage peaks. This results in short-circuiting the line at double the carrier frequency. In the modified system shown in Fig. 8, one pair of rectifiers 12, 13 is used in common for both bridges (11 .. 14 and 12, 13, 43, 44) which are fed with carrier frequency over a centre-tappedsecondary transformer 61, a common bias resistance 22 being also provided. This arrangement operates like that shown in Fig. 7. All the systems are described as applicable to demodulating-arrangements. The arms of the bridges may, in all cases, be provided with additional resistances to effect balance.