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Although isolated P-wells (IPW) can provide a high degree of immunity from substrate noise in mixed-signal circuits, their superior characteristics makes it extremely difficult to measure their isolation properties using the traditional approach of s-parameter measurements on 2-port rf test structures configured in a ground-signal-ground (GSG) pattern. It has been proven that measurement results obtained from such test structures are dominated by parasitic coupling between metal interconnect layers and the substrate, while the amount of noise that is actually coupled through the IPW region into the Si substrate is substantially smaller. In this report a new technique for separating the intrinsic isolation characteristics of IPW regions from parasitic coupling effects is presented. The commonly used 2-port test structure is treated internally as 3-port system which can be fully represented by a nine element y-parameters matrix. These nine elements can be obtained using two additional structures which are simple variations of the traditionally used test structure. This new approach has been verified by comparison with 3D TCAD simulations which were calibrated to measured Si data.