The aim of the work presented in this thesis is to establish a design strategy for broadband Voltage-Controlled-Oscillators (V COs). The design methodology is based on a one pôrt negative resistance/conductance technique and an accurate characterisation of the V CO circuit. The circuit optimisation for maximising the available tuning bandwidth concentrates on the coupling between the active device and the tuning device. Small-signal and large-signal characterisation of the active device (TED) is carried out and presented as a two terminal admit tance over the desired frequency range. A suitable topology of the V CO is then chosen, taking into account the fabrication constraints due to the packages of the devices and the microwave medium. An equivalent circuit model for the V CO is then established and experimental investigations of the tuning circuit, the output circuit and the active device are carried out over the desired frequency range. Finally, a Computer-Aided-Design program is developed for the circuit optimisation process and performance determination at small and large-signals. Since the use of a two terminal active device is in principle straightforward in oscillator design, the Transferred-Electron-Device (TED) is used as the active device in this work. The nonlinear characterisation of the TED is accompli shed by the use of a physical model based on a one-dimensional drift-diffusion description of the intrinsic device. The v CO is built on microstrip using packaged devices for both the varactor and the TED. Apart from the nonlinear admit tance of the intrinsic TED, the rest of the equivalent circuit model of the V CO is described in the frequency domain. However , the evaluation of the V CO circuit performance required by the optimisation process requires fast synthesis of the microstrip circuitry and hence approximate empirical expressions are used. Both distributed and lumped elements of the equivalent circuit are treated as multi-ports. The synthesis of the V CO has demonstrated the dependence of the tuning bandwidth of the chosen configuration of the V CO on the parameters and dimensions of the coup1ing elements between the varactor diode and the TED. An efficient overall Computer-Aided-Design (CAD) program has been developed and the comparison between the experimental and computed results has validated the design strategy. Minor differences are thought to be due to uncertainties in the values of the equivalent circuit elements of the device packages.