A method and circuit for measuring a time interval between transitions of periodic signals at nodes of a circuit-under-test (CUT), the signals having a periodic clock frequency, the method includes periodically latching a digital value of a first periodic signal at edges of an undersampling clock, simultaneously periodically latching a digital value of a second periodic signal at edges of the undersampling clock, combining the latched digital values of the first and second periodic signals to produce a combined output whose duty cycle is proportional to the time interval between a median edge of latched digital values of the first periodic signal and a median edge of latched digital values of the second periodic signal; and counting the number of undersampling clock cycles in which the combined output is a predetermined logic value within a predetermined time interval whereat the number is proportional to a time interval between a transition of the first periodic signal and a transition of the second periodic signal.