  We show how small-signal analysis, a standard method in electrical
engineering, may be applied to thermoelectric device performance measurement by
extending a dc model to the dynamical regime. We thus provide a physical ground
to \textit{ad-hoc} models used to interpret impedance spectroscopy of
thermoelectric elements from an electrical circuit equivalent for
thermoelectric systems in the frequency domain. We particularly stress the
importance of the finite thermal impedance of the thermal contacts between the
thermoelectric system and the thermal reservoirs in the derivation of such
models. The expression for the characteristic angular frequency of the
thermoelectric system we obtain is a generalization of the expressions derived
in previous studies. In particular, it allows to envisage impedance
spectroscopy measurements beyond the restrictive case of adiabatic boundary
conditions often difficult to achieve experimentally, and hence \emph{in-situ}
characterization of thermoelectric generators.
