  A method of dynamic continuous-wave spectroscopy of coherent population
trapping (CPT) resonances using phase modulation of the jump type is developed.
The time evolution of the spectroscopic signal is investigated. A method for
the formation of an error signal for frequency stabilization is proposed. We
show that our approach has a reduced sensitivity to the lineshape asymmetry of
the CPT resonance. The experimental results are in good qualitative agreement
with theoretical predictions based on a mathematical model of a three-level
$\Lambda$ system in a bichromatic field. This method can be used in atomic
frequency standards (including chip-scale atomic clocks).
