Laser diodes with optical feedback can exhibit periodic intensity oscillations at or near the relaxation-oscillation frequency. We demonstrate optoelectronic oscillators based on external-cavity semiconductor lasers in a periodic dynamical regime tunable over the entire $X$ -band. Moreover, unlike standard optoelectronic oscillators, we need not employ the time-dependent optical intensity incident on a photodiode to generate the microwave signal, but rather have the option of generating the electrical microwave signal directly as a voltage V(t) at the laser-diode injection terminals under constant current operation, no photodiode need be involved, thus circumventing optical-to-electrical conversion. We achieve a timing jitter of 10 ps and a quality factor of 2× 105 across the entire X -band, that ranges from 6.79 to 11.48 GHz. Tuning is achieved by varying the injection current J. © 1965-2012 IEEE.