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We have developed an undersampling technique that can extract individual in- and quadrature-phase baseband signals from a quadrature-modulated RF carrier. The proposed technique enables vector information to be maintained by satisfying certain phase and frequency relationships between the RF carrier and the sampling clock despite the use of an undersampling technique. Moreover, it can extend the bandwidth of the RF front end because it requires neither 90-degree hybrids nor phase shifters but can utilize the down-conversion effect caused by undersampling. It is also suitable for the fabrication of LSIs in digital radio receivers because the analog parts for separating the in- and quadrature-phase components are eliminated. To determine the effectiveness of the proposed technique, we fabricated direct conversion digital radio receivers for QPSK and 16-QAM with FPGA devices. The measured bit-error-rate of the receivers showed that the performance was almost as strong as the theoretical performance of conventional quadtarure modulations. This demonstrates that the direct conversion digital receiver using RF direct quadrature undersampling is as practical as conventional analog receivers.