In this paper, theoretical and experimental results for wideband photonic downconversion systems operating from microwave frequencies through millimeter-wave frequencies are presented. The system consists of a low phase-noise optical heterodyne local oscillator (LO) generator derived froma two-frequency diode-pumped Nd:YAG laser, a millimeter-wave Mach-Zehnder modulator, and a high-speed photodiode. The sum and difference frequency products between the optical LO and the input RF signal are generated upon photodetection. An analysis of photonic heterodyne downconversion is presented, and preliminary experimental downconversion results at Ka-band are presented that are in good agreement with the theoretical prediction of 6 dB conversion loss. Due to the high degree of correlation between the phase fluctuations of the laser modes, the phase noise is much lower than that of previous heterodyne sources, which were typically too noisy for many applications. The free- running optical LO has measured phase noise better than L(1 kHz) equals -90 dBc/Hz at X-band, limited by the measuring system. Finally, novel microwave and millimeter-wave system architectures with enhanced performance and flexibility are discussed, and compared to conventional downlink systems employing electronic mixers.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.