  High resolution studies of circular polarization allow us see where it arises
in a jet, study its local fractional level and spectrum, and compare these
results to local measures of linear polarization and Faraday rotation. Here we
not only review past results from Very Long Baseline Array (VLBA) circular
polarization studies, but we also present preliminary new results on two
quasars. In the core of PKS 0607-157, we find strong circular polarization at 8
GHz and much weaker levels at 15 GHz. Combined with the linear polarization
data, we favor a simple model where the circular is produced by Faraday
conversion driven by a small amount of Faraday rotation. In the core of 3C345,
we find strong circular polarization at 15 GHz in a component with distinct
linear polarization. This core component is optically thick at 8 GHz, where we
detect no circular polarization. With opposite trends in frequency for PKS
0607-157 and 3C345, it seems clear that local conditions in a jet can have a
strong effect on circular polarization and need to be taken into account when
studying inhomogeneous objects with multi-frequency observations.
