We investigate the oscillation characteristics of active modulated periodic slabs such as those used for distributed feedback (DFB) lasers. In particular, we examine the effect of modulation wave number on the threshold gain and oscillation frequency of these structures by employing the vector coupled-mode approximation. Significant changes in threshold gain are found for large modulation indices and moderate couplings as compared to the unmodulated case. Small changes in the oscillation frequency are also noted. Methods for lowering threshold gain are suggested.