  We investigate circular polarization in pulsar radio emission through
simultaneous observations of PSR B1133+16 at two frequencies. In particular, we
investigate the association of the handedness of circular polarization with the
orthogonal polarization mode phenomenon at two different frequencies. We find
the association to be significant across the pulse for PSR B1133+16, making a
strong case for orthogonal polarization modes determining the observed circular
polarization. The association however is not perfect and decreases with
frequency. Based on these results and assuming emission occurs in superposed
orthogonal polarization modes, we present a technique of mode decomposition
based on single pulses. Average profiles of the polarization of each mode can
then be computed by adding the individual mode-separated single pulses. We show
that decomposing single pulses produces different average profiles for the
orthogonal polarization modes from decomposing average profiles. Finally, we
show sample single pulses and discuss the implications of the frequency
dependence of the correlation of the circular polarization with the orthogonal
polarization mode phenomenon.
