Differential RCS characterizes the aptitude of a tag to modulate the backscattered power. This parameter is classically estimated based on the variation of the IQ channels in the time domain. This paper introduces a generalization of the RCS backscattered by a tag and a new definition of the delta RCS in the frequency domain. The analytical model is based on schochastic processes and allows to estimate the delta RCS from the power spectral density of the modulated signal backscattered by the tag. The associated measurement is compared to the classical time domain based methods. Results shows a good agreement with a difference between the different approaches less than 0.5 dB. Finally, the proposed method relies on simple synchronization between the emitter and receiver to estimate the delta RCS while maintaining the accuracy.