Here we present an analysis of microwave emission spectra from flare-productive active regions (FPAR). For that we used regular observations in a wide frequency range with high spectral resolution and polarization sensitivity made with the RATAN-600 radio telescope. The multi-frequency observations make possible to detect small changes of magnetic field structures at different heights of the solar atmosphere. Observations with RATAN-600 and other large radio heliographs revealed an existence of narrow-band irregularities in the circular polarization spectra of FPARs radio emission. The new receiver complex, recently installed at RATAN-600, significantly improves its technical capabilities for the FPARs further study. Now it provides 1% (or 100 MHz) resolution in the frequency range from 6 GHz to 18 GHz in simultaneous registration. We present some observational results obtained with this new equipment.