The dynamical equations for the superconducting phase differences of a conventional d.c. SQUID in the presence of small structural asymmetry and coupling energy inhomogeneity can be studied by means of a reduced two-junction interferometer model. In this context, the resulting dynamics can be described by a single first-order nonlinear differential equation, in which adjustable parameters are present. As a consequence, a ratchet-like potential can be obtained for a convenient choice of parameters. We show, in a zero thermal noise case, that asymmetric d.c. SQUIDs can operate as voltage rectifiers under an external deterministic a.c. signal. Average voltage quantization in the I–V characteristics of the device can be observed as an effect of phase-locking mechanisms. Analytic expressions for the constant voltage steps appearing in the I–V characteristics can be found as a function of the bias current and of the a.c. amplitude of the forcing term.