  It is shown that the frequency distribution of the degree of circular
polarisation for a homogeneous source is sensitive to the properties of the
synchrotron emitting plasma. Most of the circular polarisation comes from the
region around the turn-over frequency, where the synchrotron radiation becomes
optically thick. However, nearly circular characteristic waves result in
circular polarisation dominated by frequencies above the turn-over frequency,
while for nearly linear characteristic waves it is dominated by frequencies
below. Observations argue in favour of nearly circular characteristic waves.
This implies a low energy cut-off in the electron distribution substantially
below that corresponding to the turn-over frequency and, simultaneously,
provides an upper limit to the fraction of electron-positron pairs.
