Design of Harmonic Oscillators for Millimeter Wave Signal Generation in Optical Systems

A new approach is presented for the design of harmonic microwave or millimeter wave (MAW) VCOs. The active element of the oscillator circuit is driven into saturation and below pinch-off at the positive and negative peaks of the output signal, respectively. This strongly nonlinear operation is achieved by applying strong feedback. The operation point and thus the output harmonic content can be adjusted by varying the transfer function of the feedback circuit and the gate bias voltage of the active device. The stability of the operating point is also investigated The new method is demonstrated on a MESFET oscillator which is designed to produce high third harmonic output. The circuit is utilized as a harmonic VCO in a photonic system to generate the quasi MMW carrier by using an optically transmitted subharmonic reference. Due to the low cost of the optical parts the system concept is very attractive for MMW picocell applications.