  We evaluate the use of Coherent Population Trapping (CPT) excited with
parallely polarized laser fields in vapour cell atomic clocks. We study the
resonance shape, the discriminator slope and signal-to-noise ratio as a
function of relevant parameters such as the laser frequency detuning and the
applied magnetic field. We show that a stability of $6 \cdot 10^{-12}$
$\tau^{-1/2}$ may be reached in a compact system using a modulated VCSEL.
