A differentiator for A-C computers

This paper describes an electronic circuit which differentiates, with respect to time, information represented by the amplitude modulation of an a-c carrier wave. A circuit of this type is useful in a-c fire-control computing systems where rate information must be obtained from positional input data. In any fire-control computer, the choice of the particular type of differentiator to be used depends upon the form of the input data. If these data are in the form of d-c signals, accurate electronic differentiating circuits are available. If they are in the form of a-c signals a positional servo coupled to an a-c tachometer is usually used1 (especially for rectangular co-ordinate fire-control systems). The accuracy of servo differentiators, however, is inadequate for many applications. The electronic circuit to be described offers a means of obtaining, for an a-c fire-control computing system, more accurate differentiation than has so far been obtained using servo differentiators. Basically, the circuit described is a modification of a well-known d-c differentiator circuit which utilizes a Miller integrator in a feedback path, achieving differentiation by the implicit function technique.2 A magnetically keyed demodulator3,4 has been incorporated into the design so that the d-c circuit can be used in an a-c computing system. This demodulator circuit has excellent linearity characteristics and a wide operating range, making the electronic differentiator design feasible for many applications.