Patent GB-790578-A (GB)
Title: Improvements relating to electrical computing apparatus
IPC: G06G7/161
Filed: 1955-12-23
Abstract: 790,578. Electric analogue calculating systems. GENERAL ELECTRIC CO. Dec. 23, 1955 [Dec. 29, 1954], No. 36879/55. Class 37. In an electronic multiplier, analogue voltages representing two input quantities are respectively added and subtracted and the results superimposed on a common cyclically recurring alternating voltage to derive modulated signals which are then squared and averaged in rectifier circuits to produce resultant signals, which are oppositely combined to derive a difference signal from which unwanted signals proportional to either input signal are cancelled or removed to yield a signal proportional to the square of the inputs. Fig. 1 shows a multiplying circuit wherein direct or low-frequency alternating analogue voltages X, Y are summed over equal input resistances 11, 12 and amplifier 10 with equal fedback resistance 13, while analogue voltages X, - Y are similarly summed over resistances 15, 16 and amplifier 14; the resultant voltages - (X + Y) and - (X - Y) being respectively added in transformers 20, 21 to a periodically recurring, e.g. triangular carrier waveform from generator 18 having peak amplitude C greater in amplitude than (X+Y) and greater in frequency than the maximum frequency thereof. The resultant voltages (- X - Y+C) and (- X+Y+C) are half-wave rectified by vacuum tube or copper oxide or selenium diodes 22, 23 and 26, 27 connected to remove the negative portion of the combination of the triangular waveform and the input signals, to voltages shown mathematically to be analogous to (- X - Y+C)<SP>2</SP> and (- X+ Y +C)<SP>2</SP> which are averaged by resistance capacitance circuits and additively combined at the input of feedback amplifier 34 after inversion of the second voltage by feedback amplifier 31. The resultant output voltage is shown mathematically to represent (4XY - 4YC) and the latter term is eliminated by the introduction of a signal proportional thereto over resistance 38, so that the output of feedback amplifier 34 is analogous to the product (XY). In a modification (Fig. 3), the input voltages analogous to X, Y are additively applied to feedback amplifier 40 over resistances 41, 42 equal to the feedback resistance 46, together with the triangular carrier waveform of amplitude C derived from generator 43 over capacitance 44 and resistance 45 equal to 46 as before. The Y voltage is applied over resistance 48 to the input of amplifier 47 (whose feedback resistance 51 is twice the value of input resistance 48) and is combined with the output of amplifier 40 over resistance 50 whose value is equal to 51. The output of amplifier 40 is then analogous to - (X-)-Y-)-C) and that of amplifier 47 to - (2Y - X - Y - C) or (X - Y+C), and these outputs are respectively rectified by oppositely poled pairs of rectifiers 52, 53 and 56, 57; averaged by resistance capacitance circuits 54, 55 and 58, 60 and additively combined at the input of feedback amplifier 61 over resistances 62, 63 with a signal derived from the Y input over variable resistances 66 and resistances 67, 68 connected to the outputs of the respective averaging circuits. It is shown that the resultant output is analogous to - (XY + CY) and the value of Y is adjusted to eliminate the latter term so that the output of amplifier 61 is analogous to (XY). Capacitance 65 across the feedback path eliminates any remaining carrier components and for approximate accuracy the carrier waveform may be sinusoidal.