  In this paper, we derive a framework to understand the effect of
imperfections on the phasematching spectrum of a wide class of nonlinear
systems. We show that this framework is applicable to many physical systems,
such as waveguides or fibres. Furthermore, this treatment reveals that the
product of the system length and the magnitude of the imperfections completely
determines the phasematching properties of these systems, thus offering a
general rule for system design. Additionally, our framework provides a simple
method to compare the performance of a wide range of nonlinear systems.
