  We present investigations of radio-frequency resonances observed in a
rubidium vapor. With measurements of a systematic shift of radio-frequency
resonances, induced by an off-resonance component of an oscillating field (the
Bloch-Siegert shift), we demonstrate limitations of the rotating-wave
approximation. Experimental results are supported with numerical calculations,
reproducing all features of the observed signals. Based on the studies, we
develop experimental scheme free from the problem. By liberating the
measurements from a systematic error (the resonance shift), the new approach
will increase accuracy of spectroscopic measurements and magnetic-field
detection.
