Analysis of noise up-conversion in microwave field-effect transistor oscillators

The conversion process of the low frequency noise into phase noise in field-effect transistors (FET) oscillators is investigated. First, an evaluation of the baseband noise contribution to the oscillator phase noise is provided from the analysis of the baseband noise and the frequency noise spectra. A distinction is made within the different components of the low frequency noise contributions to close-in carrier phase noise. Next, the frequency noise of the oscillator circuit is analyzed in terms of the FET's low frequency noise multiplied by the oscillator's pushing factor. Though this product usually provides a good evaluation of the phase noise, experimental results presented here show the inaccuracy of this method at particular gate bias voltages where the pushing factor decreases to zero. To account for these observations, a new nonlinear FET model involving at least two noise sources distributed along the channel is proposed.