A new RF quadrature undersampling technique for an ideal software-defined radio system

We propose a quadrature undersampling technique that makes it possible to extract baseband inphase (I) and quadrature (Q) data from an input RF signal directly. These I and Q data are derived from three sampling points which lay in one cycle of the RF signal. A traveling-wave type of sample and hold (S/H) circuit with phase delay per-section of θ can sample an input signal at the three points with a phase interval of θ at one time, where θ can take not only π/2, but also an arbitrary phase angle between 0 and π in the RF signal. The sampling frequency is set so that the carrier frequency `f<sub>c</sub>´ and sampling frequency `f_s´ satisfy the relationship f_s = f_c/n, wherein undersampling factor n is a positive integer. In a feasibility study, a prototype receiver fabricated using FPGAs showed good demodulation characteristics from a measured constellation diagram, where we employed 16-QAM as the modulation, f_c of 302.4 MHz, f_s of 75.6 MHz, and an undersampling factor n of 4.