A general overview of radio receivers and transmitters is presented. Introducing the principle of frequency translation, classical heterodyne and homodyne RX and TX architectures for analog telecommunications are discussed. Digital telecommunications are addressed and, with them, modern polyphased transceiver architectures and the insertion of an ADC within the receiver chain. First the ADC is considered at the baseband level, then at higher frequency stages, and up to the antenna yielding to the ultimate Software Defined Radio (SDR) principle. At each location of the ADC within the receiver chain, the converter characteristics are reviewed, leading to the choice of a possible architecture. Nonidealities are discussed in order to highlight problems that researchers will have to address in the future to fulfill the SDR concept.