Every high frequency circuits need inductors for proper work. This is more and more true w i t h new radio-frequency MOS transistors systems. But with t h e increase in frequency, lower inductors are required and MOS transistors offer less gain, Radio-frequency circuits i n MOS technology are also obedient to the layout of low inductance high quality factor inductors.It has been shown that t h e y can be better realized by microstrips. However, classical MOS digital technologies have no back ground plane under the substrate. This article analyzes the influence of parasitic back ground planes i n the case of an often used coplanar solution : coplanar waveguide.