A realtime spectrum trigger system on a realtime oscilloscope considers both the magnitude and phase of an input signal having a periodic component so that successive acquisitions of the input signal are time aligned. A user inputs a frequency, threshold and phase for triggering on the periodic component. Input signal samples are filtered by quadrature filters according to the frequency input to produce quadrature signal components. The square of the magnitude of the input signal is computed from the quadrature signal components, as well as the phase ratio and sign, for comparison with calculated values. When enabled by the magnitude of the input signal, a phase crossing determinator compares the phase ratio and sign with calculated values to determine the phase crossing to generate a trigger, resulting in successive acquisitions of the input signal being in time alignment.