The difference in the two systems is that in the TWT the speed of propagation in the helix has to be similar to that of the electrons in the beam. This is not the case in the BWO. The waveguide places strict limits on the bandwidth of the signal and sets its propagation speed as a basic function of its construction, but the speed of the signal induced into the electron beam is relative to the speed of the electrons. That means the frequency of the output signal can be changed by changing the speed of the electrons, which is easily accomplished by changing the voltage of the electron gun.