Patent GB-1319349-A (GB)
Title: Automatic electronic test equipment and method
IPC: G01R31/319
Filed: 1970-11-23
Abstract: 1319349 D/A Converters MARTIN MARIETTA CORP 23 Nov 1970 [18 Aug 1970] 55556/70 Heading G4H [Also in Division G1] A system for testing electronic equipment includes a plurality of D/A converters, all of which insert a reference A.C. or D.C. potential into selected rungs of a ladder network. In the first D/A converter, Fig. 6, a - 5v D.C. reference potential 146 is applied by a transistor switch logic unit 152 (Fig. 7, not shown) to selected rungs of a ladder network 154 (not described in detail) under the control of ten input bits 130, each bit being represented by either + 15v or -15v. The output of the ladder is effectively amplified and stored on a capacitor 164 by an arrangement in which the capacitor 164 is allowed to charge towards a + 120v supply via a switch 158 until a preset fraction of the potential across the capacitor 164, extracted by a potentiometer 168, is equal to the output from the ladder. The potential across the capacitor 164 is available at an output terminal 66 in true or inverse form under the control of a sign bit (either 0v or +5v) 178. In the second D/A converter (Fig. 8, not shown) the output from the ladder network is applied to an operational amplifier (188) having two feed back loops, selectable under the control of a range bit, so that the output can be scaled by one or by ten. This converter also includes a further amplifier whereby an output D.C. current in the range - 1ma to + lma may be obtained. In the third and fourth converters, Fig. 9, either an A.C. or D.C. reference potential can be applied to selected rungs of the ladder network 206 so that the output, from an amplifier 210 can be either A.C. or D.C. The amplifier 210 includes four feedback paths, two being associated with a contact 214 for D.C. and two with a contact 218 for A.C. One of these paths is selected by the combined action of a mode switch 252 (i.e. A.C. or D.C.) and a range switch 250. However, when the A.C. mode is selected an additional A.C. range switch 254 is also operable, giving in all a total of four ranges from the two secondaries of an output transformer, and a single range direct from the amplifier 210. In the fifth converter (Fig. 10, not shown) the outputs of two ladders (264, 258) energized respectively by the two phases of two-phase A.C., are combined to produce a signal controllable in both amplitude and phase. In the final D/A converter, Fig. 11, referred to as a digital-to-synchro converter, three in-phase A.C. outputs are produced having amplitudes in the ratio sin # : (# + 120) : sin (# + 240), where # is the number represented by the input digital signal. Only an outline of the way in which this circuit operates is given. Two input bits, on a terminal 178 are applied to a logic unit 344 to close one pair of four pairs of contacts 336, 284; 302, 286; 304, 288; and 306, 290, which correspond respectively to angles 0, 11À25, 22À5 and 33À75 degrees. The resulting signals from amplifiers 314 and 300 are crosscoupled to switch logics 280 and 278 respectively, for selective routing, under control of seven input bits on a terminal 130, to appropriate rungs of ladders 318 and 136, and the output signals are amplified 320, 330, and inverted 326, 340 to provide signals +Sin #, - Sin #, + Cos # and - Cos # to an octant selector 282. A further three bits of the input signal, also on terminal 130, control the octant selector (Fig. 12, not shown) so that the appropriate pair of the four inputs are selected, and this pair is applied to a Scott transformer arrangement to produce the required output signals. The octant selector circuit also includes amplifiers having two feed back paths so that two voltage output ranges, selected by an input bit on terminal 178, are available.