This paper provides a new IF quadrature sampling and processing approach by sampling IF signal directly and produces quadrature complex signal by digital signal processing. Digitized in-phase(I) and quadrature(Q) components are obtained through choosing middle point Bessel interpolation function implemented easily in engineering project. Its phase errors derived from theory and a lot of tests are an order of magnitude less than conventional detectors, and no amplitude errors. This approach provides improved accuracy. A basic hardware scheme and a typical experimental result are also given.