Patent GB-779571-A (GB)
Title: Analogue digital decoding apparatus
IPC: H03M1/00
Filed: 1955-09-28
Abstract: 779,571. Digital-to-analogue converters. SPERRY RAND CORPORATION. Sept. 28, 1955 [Oct. 4, 1954], No. 27632/55. Class 40 (1). [Also in Group XL (c)] An apparatus for converting to an analogue voltage a serial binary number in the form of pulses for the " 1 " digits and no-pulses for the "0" digits with the least significant digit occurring first, comprises a capacitor 10 which is connected by switch 12 to a potential source E in response to the " 1 " digits and is connected by switch 14 to earth in response to "0" digits of a binary number fed to the decoder. A second capacitor 16 of the same capacity as capacitor 10 is momentarily connected across capacitor 10 by a switch 18 after each operation of switch 12 or 14, the charge on capacitor 10 being halved at each step and the final potential across capacitor 10 or 16 being the analogue voltage of the input digital code, The apparatus may be used to convert negative as well as positive numbers to an analogue voltage, the negative binary numbers being expressed as the complement of the corresponding positive number and the most significant digit being utilized to identify the sign, all positive numbers having as the most significant digit " 0 " and negative numbers having " 1." The circuit is arranged so that the last digit received (the most significant) actuates the switch 12 if it is a " 0 " and the switch 14 if it is a "1." In the arrangement shown in Fig. 6, the switches 12, 14, 18, for the capacitors 10, 16, are of the diode bridge type. Two series pairs of diodes are connected across an input transformer 50 having two secondary windings with a potential source 52 connected between them to normally maintain the diodes non- conducting. When an input pulse is applied to the transformer 50 of such potential as to overcome the potential of source 52, the diodes conduct and clamp the output point 54 to the potential of the input point 56 which is a potential E for diode switch 12, and earth potential for switches 14 and 18. To trigger the switches 12 and 14 in response to a pulse or no-pulse, timing pulses Sa, which are derived from clock pulses a, Fig. 5, are applied through a gate circuit 78, which is held open by waveform c applied through capacitor 80, to the cathodes of a pair of triodes 86, 88. The control grid of valve 88 is held at a fixed negative bias and that of triode 86 is connected through a clamping circuit to the binary number pulse input. The grid of triode 86 is normally at earth potential, but when a negative pulse is received, representing digit " 1," valve 86 becomes non-conducting and valve 88 conducts, the reverse taking place when no-pulse is received. By this means each timing pulse Sa gated by valve 78 is coupled to a monostable multivibrator 58 or 60 to operate the corresponding switch 12 or 14. The switch 18 is similarly operated through a gate 100 and isolating triode 102 by pulses Sc which are delayed behind pulses Sa. To provide for positive and negative numbers the action of switches 12 and 14 is reversed on the last digit by a pulse Sb applied through a gate 104 to the cathodes of a triode pair 106, 108, which are connected to the multivibrators 58, 60, in opposite sense to the triodes 86, 88. The analogue voltage is derived across the capacitor 16 through a cathode follower 110.