In this study, three voltage controlled oscillator (VCO) circuits are realised using Monolithic Microwave Integrated Circuit (MMIC) technology. Two of the VCOs are in the capacitive feedback topology, whereas the last one is designed by using the inductive feedback topology. GaAs MESFETs are used as both active devices and varactor diodes. Designed for a 50il system, the circuits operate in 8.88-10.40GHz, 8.7T10.23GHz and 8.96-12.14GHz ranges. Their output powers are well above the 9.5dBm for most of the oscillation band. All three VCOs have harmonic suppressions better than 30dBc. Both small signal and large signal analysis are carried out. The layouts are designed by GEC Marconi’s F20 process rules and the circuits are produced in this foundry