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In the present paper we have first recalled the basic phase-noise equations for a fairly general PLL system, and suggested their normalisation with respect to the square of the output frequency. In this way it is possible to proceed with a graphic solution for the noise spectrum, even in cases of complicated frequency synthesisers, using one and the same figure throughout, since the normalisation brings all sources to their frequency multiplication or division invariant noise power spectral densities. Further, we discuss the influence of the thermal noise, generated in low-pass PLL filters, and emphasise the importance of third-order type-2 loops for low-noise design. Theoretical findings are illustrated by two examples.