The measured amplitude M(q,t) in a pulsed field gradient spin echo experiment probes the pore structure using two length scales, the gradient length q(-1), and the diffusion length [D(t)t](1/2). M(q,t) can be well represented by a convolution of the structure factor of the connected pore space with an appropriate Gaussian propagator. This ansatz provides a model-independent way of obtaining the structure factor. In this paper, we consider the forward problem of understanding the amplitude M(q,t) using the simplest possible models for the structure factors which involve only one length scale. The most prominent among the simple models is the Debye model, which provides us with an understanding of the various length scales in the problem. We study several other structure factors, all definable by a single parameter-collections of tubes, collections of sheets, and collections of impenetrable spheres. We discuss periodic geometries for which the ansatz becomes exact in the long time limit. Implications for a ball and stick model for the pore geometry are discussed.