Patent GB-923374-A (GB)
Title: Improvements in or relating to electronic trigonometrical function generators
IPC: G06G7/22
Filed: 1959-04-09
Abstract: 923,374. Function generators. GENERAL PRECISION Inc. April 9, 1959 [April 9, 1958], No. 12101/59. Class 37. A sine or cosine function generator comprises an electronic switching circuit (11 in Fig. 3) receiving as an input a reference sine wave potential (from 301 in Fig. 3), an averaging circuit such as a filter (13 in Fig. 3) receiving the output of the switching circuit, and timemodulating circuitry (307 in Fig. 3) receiving a variable D.C. input (Vx), the independent variable, which is arranged to control the switching circuit (11) so that the reference sine wave potential is delivered to the switching circuit output for portions of the wave cycle determined by the instantaneous magnitude and sign of Vx whereby the averaged potential delivered by the averaging circuit represents cos Vx. The invention is based on the principle that: and if a sine reference potential is interrupted for a particular time during each cycle, the time being determined by the value of Vx, and the remaining, or uninterrupted, portion of the sine wave is averaged or integrated, the output voltage will vary in accordance with cos Vx (since a is then equal to Vx). In the embodiment of Fig. 3, Vx applied at terminal 320 is fed to the secondary winding 303 (through amplifier U-300) of a transformer which is supplied with the reference sine wave A.C. The secondary output is therefore a sine wave whose positive and negative excursions are time- or widthmodulated in accordance with Vx, and is applied to a transistor squaring amplifier 307 which provides correspondingly width-modulated output pulses in push-pull on conductors 308, 310, since the time duration of the pulses depends on the potential of terminal 306 with respect to earth. These pulses are applied to a transistor switch circuit 312 in the feedback circuit of amplifier U.300 consisting of two emitter-connected transistors having D.C. voltages V R  applied to their collectors, so that the output pulses of 312 are of constant amplitude and have areas proportional to the widths of the input pulses but independent of the amplitude thereof. The output pulses from circuit 312 are averaged or demodulated by a filter 313 and applied as a feedback voltage to amplifier U.300. It is proved in the Specification that provision of the feedback voltage ensures that, provided U.300 is a high-gain amplifier, the time modulation is directly proportional to V x  and independent of the amplitude distortion and frequency of the reference sine wave. It is also inversely proportional to V R  so that the circuit so far described may be used as a dividing circuit and the apparatus as a whole will give an output corresponding to cos Vx/V R  (i.e. the cosine of the ratio of two variable input voltages if V R  is also varied). The required cosine function output is obtained by applying the push-pull output pulses of the squaring amplifier 307 to the bases of a switching circuit 11 consisting of two emitter connected transistors T 1 , T 2 , the collectors of which are supplied with the reference sine wave (from the transformer input) and connected to earth respectively. Transistors T 1 , T 2  will cut off alternately in accordance with the pulses applied to their bases, the potential at 316 corresponding to the reference sine wave if T 1  is conducting and to zero if T 2  is conducting, so that it will correspond to that portion of the reference sine wave during which T 1  conducts which in turn depends on the width of the pulses on conductors 308, 310 and therefore on Vx. The pulses on conductors 308, 310 are shown at 204, 205 in Fig. 2B when Vx is zero so that they are of the same width (t = t<SP>1</SP>) and t will be equal to one halfcycle of the reference sine wave 203 (Fig. 2A). Transistors T 1 , T 2  will therefore switch over at the end of the positive part of said cycle and the output at 316 (202 in Fig. 2A) will be a maximum for Vx = 0, so providing the correct function for a cosine generator. If Vx is positive (Fig. 2c) the pulse input to transistors T 1 , T 2  is as shown at 207, 208 and the potential at 316 as shown at 209 which is provided as input to averaging filter 13, the filter than giving an output corresponding to cos Vx. The various potentials when Vx is negative are shown on the right-hand side of Fig. 2C. The range of operation of this embodiment of the generator is from- 90 degrees to + 90 degrees and the operation of the apparatus to provide a sine function generator or for operation in other quadrants may be achieved by the provision of D.C. potential to amplifier U.300 through resistor R399 and by the connection of several function generators in parallel, as is described in the Specification. An alternative arrangement is described (Fig. 4, not shown) in which two channels are used each similar to the one of Fig. 3, the transistor switch of each channel being operated in push-pull by two reference sine waves and the outputs from the two filters being added in a summing amplifier. This circuit is stated to result in better accuracy and larger output signals.