  On the basis of Boltzmann's equation, and including anisotropic scattering in
the collision operator, we investigate the effect of one-dimensional
superlattices on two-dimensional electron systems. In addition to superlattices
defined by static electric and magnetic fields, we consider mobility
superlattices describing a spatially modulated density of scattering centers.
We prove that magnetic and electric superlattices in $x$-direction affect only
the resistivity component $\rho_{xx}$ if the mobility is homogeneous, whereas a
mobility lattice in $x$-direction in the absence of electric and magnetic
modulations affects only $\rho_{yy}$. Solving Boltzmann's equation numerically,
we calculate the positive magnetoresistance in weak magnetic fields and the
Weiss oscillations in stronger fields within a unified approach.
