Telecommunication industry claims for a one-chip radiofrequency receiver. It is called Software Defined Radio (SDR). It is a re-configurable radio architecture accepting all the cellular or non-cellular standards working in a 0-5 GHz frequency range. A fully digital circuit could be the salvation. But, the analog to digital conversion and the digital operations face issues like power supply and processing speed. To overcome this technological bottleneck, a pre-processing circuit is interfaced between the antenna and a Digital Signal Processor (DSP) to pre-condition the RF signal. This paper presents the design of an analog discrete-time device located between the antenna and a DSP in standard radio architecture. It uses the principle of the Discrete Fourier Transform (DFT) to reduce the frequency of the DSP-input-signal treatment to fulfil the SDR purpose. An application to RF digital modulation is exhibited.