An X-band microstrip VCO with 900MHz tuning range

A low phase-noise, microwave voltage controlled oscillator (VCO) is built to operate between 7.8GHz and 8.7GHz. The circuit is built on Roger RO6002 with a thickness of 20 mils, and a dielectric constant of 2.94. The loss tangent of the substrate is 0.0009. The active element used is a dual emitter Infineon (former Siemens) BFP420 low noise bipolar transistor. It is in a SOT-343 package and used in a common collector configuration. The oscillator circuit contains both coarse and fine tune controls. An active bias circuit is used to power the oscillator. The output is loosely coupled, resulting in better phase noise. Two varactors used in the coarse tune circuit are connected back-to-back to improve the noise performance. As the coarse tune voltage is changed from −1.0Vdc to −19.0Vdc, the frequency of oscillation changes from 7.8GHz to 8.7GHz. For a fixed coarse tune voltage, when the fine tune voltage is changed from 6.5V dc to 13.5V dc, the oscillation frequency varies by about 5MHz per volt. The circuit was designed using the small signal scattering parameters (s-parameters) of the transistor provided by the manufacturer. The circuit prototype, including the impedance matching sections, was simulated using Serenade, Ansoft Ensemble, and other simulators.