Linear stable plant with unknown coefficients in the presence of an unknown-but-bounded disturbance is considered. Finite-frequency technique for identification of the plant makes use of a test signal with minimal quantity of harmonics (this value is equal to a plant state-space dimension).In order to provide the prescribed boundaries of the plant output a procedure of test signal amplitude is proposed. It is shown, if frequencies of the test signal are chosen outside a natural frequencies band (where log magnitude of plant has corner frequencies) then identification results may very strong depend on errors of a frequency characteristics determination.In order to find a estimate of the natural frequencies band boundaries a procedure of test signal amplitude is given.