Depending on the composition of [001] oriented short period SimGen superlattices (m monolayers Si/n monolayers Ge) there are six different space groups. In case of m+n odd the microscopic periodicity is doubled to 2(m+n) monolayers due to the tetrahedral bonding. We prepared several superlattices namely Si2Ge3, Si3Ge3 and Si4Ge6 with low temperature molecular beam epitaxy. Using selected area diffraction in the transmission electron microscope we found the microscopic periodicity of 10 monolayers for Si2Ge3 and Si4Ge6. For Si3Ge3 the vertical periodicity is six monolayers.