  We show that a characteristic two photon lineshape asymmetry arises in
coherent population trapping (CPT) and three photon (N) resonances because both
resonances are simultaneously induced by modulation sidebands in the
interrogating laser light. The N resonance is a three-photon resonance in which
a two-photon Raman excitation is combined with a resonant optical pumping
field. This joint CPT and N resonance can be the dominant source of lineshape
distortion, with direct relevance for the operation of miniaturized atomic
frequency standards. We present the results of both an experimental study and
theoretical treatment of the asymmetry of the joint CPT and N resonance under
conditions typical to the operation of an N resonance clock.
