Using an optical phase-locked loop (OPLL), millimeter-wave signals were generated at frequencies up to 70.5 GHz by heterodyning the outputs of two extended-cavity semiconductor lasers. One of the lasers had an intracavity electro-optic crystal to control the lasing frequency. The FM-response characteristics of the laser were examined and improved by giving a mechanical constraint to the crystal. For the generation of millimeter-wave signals, a portable spectrum analyzer was employed as a replacement for a local oscillator and a mixer. This implementation permitted a wide-range tuning operation from 5 GHz to 50 GHz without any alterations to the setup. The continuous tuning operation of over 150 MHz could also be achieved at every frequency. With the aid of an external harmonic mixer, the millimeter-wave frequency was extended up to 70.5 GHz.