[en] Two ways of increasing synchrotron frequency spread in a bunched proton beam circulating in magnetic field flat-top are compared. Instability thresholds for the lowest multipole modes are practically unchanged, the accelerating voltage amplitude being decreased, and for higher-order modes may be decreased due to multipole mode coupling. On the contrary, after Landau resonator is switched, particle bunch parameters, excluding synchrotron frequency spread, are slightly changed and lowest multipole mode thresholds in practically interesting cases may be increased approximately by an order. Landau resonator may increase instability thresholds in case of higher-order modes, the value of synchrotron frequency natural spread in bunches being sufficiently low. Basic characteristics of Landau resonator for the IHEP accelerators are calculated as an example