  We report multi-frequency circular polarization measurements for the four
extragalactic radio sources 0056-00, 0716+71, 3C138 and 3C161 taken at the
Effelsberg 100-m radiotelescope. The data reduction is based on a new
calibration procedure that allows the contemporary measurement of the four
Stokes parameters at different frequencies with single-dish radiotelescopes. We
are in the process of framing the observed full Stokes spectra within a
theoretical model that explains that the level of measured circular
polarization as Faraday conversion.
