For an x-ray tube at constant voltage I know that the maximum energy Bremsstrahlung x-ray that can be produced is directly related to the voltage on the tube. But what about the minimum energy? Is there a limit, or can all energies below the maximum be produced? I realize that adding a filter could change the value for the minimum energy x-ray observed, but I mean without any filtration, could radio waves for instance be produced? Thanks for any help!