Abstract: A circuit for high speed testing of the periodicity of an alternating signal (60) independently of signal amplitude. A bistable multivibrator (128) changes state in response to each positive slope zero-crossing of the signal and alternately activates each of a pair of integrators (116, 118, 180 and 117, 114, 181) on alternate cycles of the signal. A pair of high speed gated comparators (112, 115) compares the integrator voltages at the end of each pair of cycles. A reference voltage (130) which increases with the number of cycles is provided to maintain a narrow comparator window as the integrator voltages increase.