  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 $\lesssim 10$
ps and a quality factor of $\gtrsim 2\times 10^5$ across the entire X-band,
that ranges from 6.79 GHz to 11.48 GHz. Tuning is achieved by varying the
injection current $J$.
