The possibility of using a three-level-A-scheme for the excitation of the clock hyperfine transition in ~(87)Rb is analyzed. The physics involved opens up two distinct avenues useful for the implementation of an atomic frequency standard, avenues based on two distinct phenomena: the electro-magnetically induced transparency (EIT or Dark Line) and the coherent microwave emission (Maser without population inversion). In spite of the close connection between the two phenomena, which are generated by the coherence created in the ground state by means of two coherent laser radiation fields (Coherent Population Trapping), the characteristics of an atomic frequency standard based on either of these phenomena may be quite different. The paper discusses the possibility of implementing on the basis of these two phenomena, new ~(87)Rb atomic frequency standards having an excellent frequency stability, and makes explicit their differences.
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