  A nondegenerate four-level N-type scheme was experimentally implemented to
observe electromagnetically induced transparency (EIT) at the $^{87}$Rb D$_{2}$
line. Radiations of two independent external-cavity semiconductor lasers were
used in the experiment, the current of one of them being modulated at a
frequency equal to the hyperfine-splitting frequency of the excited 5P$_{3/2}$
level. In this case, apart from the main EIT dip corresponding to the
two-photon Raman resonance in a three-level $\Lambda$-scheme, additional dips
detuned from the main dip by a frequency equal to the frequency of the HF
generator were observed in the absorption spectrum. These dips were due to an
increase in the medium transparency at frequencies corresponding to the
three-photon Raman resonances in four-level N-type schemes. The resonance
shapes are analyzed as functions of generator frequency and magnetic field.
