Electroclinic effect is theoretically well known in SmA but is not in Sm $C^*$. In the generalized Landau free energy in SmA phase and in Sm $C^*$ phase, we added helical deformation energy. As a result, we could confirm that helical twist and flexoelectric contribution influence on electroclinic tilt angle. We studied soft mode dielectric behaviour as a function of temperature, frequency and bias electric field. In Sm $C^*$ phase we performed nonlinear curve fitting to obtain linear electroclinic coefficient and spontaneous tilt angle more exactly. From the data of two separate experiments we could obtain soft mode viscosity, structural coefficient and piezoelectric coefficient and find similar behaviour between soft mode inverse dielectric strength and induced tilt angle in electrooptic experiment. Furthermore we examined soft mode relaxation and electroclinic response and found similar behaviours of nearly the same decay time.
