Atomic clocks using coherent population trapping (CPT) allow to improve performances of vapour cell conventional microwave clocks, or else to miniaturize them thanks to of a full optical interrogation. In this paper, we describe a prototype developed at LNE-SYRTE. It combines two original techniques: an excitation scheme with two linearly and orthogonally polarized beams, and a Ramsey interrogation. This enables the observation of narrow resonances with a good signal-to-noise ratio. A few major effects influencing the frequency stability are addressed; they are the effect of the buffer gas, of the magnetic field, of the laser power, and finally the effect of the local oscillator noise or Dick effect. The measured frequency stability is 7×10-13 at 1 s and 2×10-14 at 2 000 s.