Terahertz (THz) signals can be generated by the periodic motion of ballistic electron bunches inside a quantum well cavity surrounded by two asymmetric resonant tunneling structure implemented in an InAlAs/InGaAs heterostructure. The electron bunches are produced in an adjacent active quantum well by optical excitation with a femtosecond laser pulse, and transferred into the cavity by resonant tunneling. The device contains also a superlattice that eliminates the electrons, which exit the cavity in the direction of the active quantum well. In consequence, the device output consists of a train of ultrashort pulses, of less than 100 fs, which preserve their coherence. Electrical signals up to few tens of THz can be obtained, depending on the resonant filter or antenna placed at the end of the device.