FEEDBACK TECHNIQUE IMPROVES OSCILLATOR PHASE NOISE

GENERATION of microwave and millimeter-wave frequencies can be achieve d by using lower-frequency voltage-controlled oscillators (VCOs) in co njunction with multipliers (for example, comb generators) or by employ ing hybrid gallium-arsenide (GaAs) field-effect- transistor (FET)-base d oscillators with external resonators. Although the best phase-noise performance is often obtained with yttrium-iron-garnet (YIG) oscillato rs, the physical size and cost of these components do not always provi de a practical solution. While there are several noise sources in an o scillator, significant noise reduction is possible with the applicatio n of appropriate techniques. A novel feedback circuit uses internally generated noise to provide cancellation of close-in oscillator noise u p to a 1-MHz bandwidth. This results in more than 15-dB phase-noise im provement at microwave frequencies for the same topology, In fact, the feedback system can be incorporated in the oscillator's biasing circu it, This method can be used over a wide frequency range, and works wel l from the very-high-frequency (VHF)/ultra-high-frequency (UHF) bands up to the millimeter-wave range.