An analog frequency-measuring circuit accurate to 0.1 percent

During the course of work on an electromechanical analog computer it was necessary to convert frequencies in the range of 2 to 20 kc into shaft rotations. The shaft rotation had to be proportional to frequency with a departure from linearity of less than ±0.1% of full scale. The circuit was required to operate over the temperature range of ¿55 to +55 C (degrees centrigrade).