  We predict peculiar changes in spectral distributions of radiation emitted by
ultra-relativistic positrons and electrons in periodically bent crystals with
variation of the bending amplitude. It is shown that the changes, being
sensitive to the projectile energy, manifest themselves differently for
negatively and positively charged projectiles. We demonstrate that the features
observed are due to the interplay of different radiation mechanisms occurring
in periodically bent crystals. The observations are important for design,
practical realization and channeling experiments with periodically bent
crystals as the key element of the novel light sources. The analysis presented
is based on the grounds of accurate numerical simulations of the channeling
process.
