Oscillator resistor noise optimisation paradigm

Abstract

A theoretical explanation is given for the significant reduction in the phase noise of varactor tuned voltage controlled oscillators achievable by the use of by multiple parallel varactor diodes. The noise temperature of the total resonator resistance lowered by the number of parallel diodes used. Minimal electromagnetic coupling gives maximum de-correlation and minimal total noise for multiple equal separated noise sources. Thus we show that the noise temperature of any varactor, inductor or resonator may be made significantly below ambient by suitable design. Resonator resistance defines the unloaded Q factor irrespective of its noise temperature. To exploit this fully for a low-noise oscillator its amplifier must have a noise temperature lower than that of the resonator. The Underhill Q-Multiplier oscillator model shows how this may be done using minimal `noiseless' coupling to the resonator and predicts the achievable performance. The optimum coupling occurs when the resonator and amplifier noise contributions approximately equal. `Optimised Noiseless Coupling' is thus a new paradigm and way forward in low noise oscillator design.