A novel RF quadrature under-sampling technique, which we have developed, makes it possible to extract baseband inphase (I) and quadrature (Q) data from an RF signal directly. The technique is applicable to advanced modulations, such as demodulating an OFDM without losing any vector information. Its only drawback is that it requires a precise sampling frequency and timing to be synchronized with the input RF carrier frequency. Therefore, we have also developed a new frequency comparator, symbol timing synchronization circuit, and a phase tracking circuit. To prove the validity of the new under-sampling technique, an all digital OFDM receiver fabricated using FPGA devices has been demonstrated experimentally.