  Convective dynamo simulations are performed in local Cartesian geometry. We
report the first successful simulation of a large-scale oscillatory dynamo in
rigidly rotating convection without stably stratified layers. A key requirement
for exciting the large-scale dynamo is a sufficiently long integration time
comparable to the ohmic diffusion time. By comparing two models with and
without stably stratified layers, their effect on the large-scale dynamo is
also studied. The spatiotemporal evolution of the large-scale magnetic field is
similar in both models. However, it is intriguing that the magnetic cycle is
much shorter in the model without the stable layer than with the stable layer.
This suggests that the stable layer impedes the cyclic variations of the
large-scale magnetic field.
