  Multi-frequency VLBI observations allow studies of the continuum spectrum in
the different parts of the parsec scale jets of AGN, providing information on
the physical properties of the plasma and magnetic fields in them. Since VLBI
networks cannot be scaled, the range of spatial frequencies observed differs
significantly between the different observing frequencies, which makes it
difficult to obtain a broadband spectrum of the individual emission features in
the jet. In this paper we discuss a model-fitting based spectral extraction
method, which can significantly relieve this problem. The method uses a priori
knowledge of the source structure, measured at high frequencies, to allow at
lower frequencies the derivation of the sizes and flux densities of even those
emission features that have mutual separations significantly less than the
Rayleigh limit at the given frequency. We have successfully used this method in
the analysis of 5-86 GHz VLBA data of 3C273. The spectra and sizes of several
individual jet features were measured, thus allowing derivation of the magnetic
flux density and the energy density of the relativistic electrons in the
different parts of the jet. We discuss the results, which include e.g. a
detection of a strong gradient in the magnetic field across the jet of 3C273.
