Intrinsic noise in ferrite circulators and its impact on microwave oscillator phase noise

This paper deals with intrinsic noise in ferrite circulators and its impact on microwave oscillator phase noise. An approximate formula for spectral density of ferrite circulator intrinsic phase noise with respect to the isolation port is derived and discussed. The derivation is based on a view of a ferrite circulator like a kind of an interferometer with intrinsically noisy arms. It is found that the relative noise with respect to the isolation port might be much more than the noise added to a signal passing between adjacent circulator ports in forward direction. Some examples demonstrate the use of the derived formula in oscillator design. As shown, namely intrinsic circulator noise limits in many cases output phase noise of the advanced ultra- low phase noise microwave sources. The only radical way to overcome that drawback is to replace a ferrite circulator with a suitable hybrid junction.