Optically frequency modulated 6 GHz MMIC oscillator

Optical illumination is used to modulate a 6 GHz GaAs MMIC VCO. Tuning can be achieved by illuminating either the MESFET or the Varactor. The MESFET tuning is caused by photocurrents varying the self-bias condition. Varactor optical tuning results from both a change in the junction capacitance and by external voltages generated by a photocurrent. Although both devices achieved similar D.C. optical tuning levels, tuning via the Varactor was found to be frequency limited. Preference is therefore given to illumination of the MESFET as the optical interface component. The resulting FM modulation bandwidth achieved is sufficient for the transmission of video signals with bandwidths up to 10 MHz.