PURPOSE: To obtain a double frequency signal of high accuracy, by digitizing a reference signal to obtain a level signal of each phase point from maximum and minimum levels of the sample value of the digitized signal and comparing the output of the level signal with a reference digital signal to deliver a coincident pulse.

CONSTITUTION: A frequency motor output signal FGA supplied from an input terminal 1 undergoes A/D conversion 2, and this sampling frequency is sent to a detecting circuit 3 having maximum value Nmax and minimum value Nmin. Then signals A and B are detected as shown in the equations from the output of the circuit 3. For signals A and B, levels y1, y2 and y3 are calculated 5 by a detecting circuit 5 at phase points of π/4, 3π/4; 0, π; 5π/4, 7π/4 of the signal FGA respectively and then compared with the digital value of the signal FGA through a comparator 6. When the coincidence is obtained from the comparison, a coincident pulse is delivered. In the same way, a zero cross level y2' is calculated for a signal FGB of a motor of another frequency which is given from an input point 7. Then a pulse coincident with the signal FGB is delivered. This coincident pulse is obtained from a comparator 6 as a signal 8FG.