We have used the RATAN-600 radio telescope to study the spectral characteristics of a uniform sample of 504 radio sources from the NVSS catalog (Cat. VIII/65) near the North Celestial Pole at 6 frequencies from 1.1 to 21.7GHz, with the aim of selecting sources possessing inverted spectra near 22GHz, to be included in the program of the RadioAstron future space VLBI mission. We found 17 radio sources with the desired spectral characteristics. Data from spectral studies over a wide wavelength range testify that the spectral behavior of our sample differs from that for a complete sample of sources with the same initial parameters but selected at 20GHz. We find a 6% deficit of inverted-spectrum sources, which can be explained as an effect of the spectral characteristics of "sub-threshold" sources that were not included in the initial sample at 1.4GHz.