The noise characterisation of differential circuits is considered. A new method for the determination of the differential mode noise power at the output of these circuits is presented. It is demonstrated that this differential noise power depends not only on the output noise powers but also on the correlation of the output noise waves. An analytical expression of this correlation in terms of the S-parameters and the output noise powers is determined. Based on the presented theory, a fast and simple technique is developed for the noise figure measurement of differential circuits.
