A double Nyquist digital product detector for quadrature sampling

A technique for digitally obtaining the in-phase (I) and quadrature (Q) components of an IF signal is presented. Initially, the input bandpass signal is mixed to a carrier frequency that is one-fourth of the sampling rate of a single A/D converter. The digitized bandpass signal is converted into its I and Q components at one-half the A/D sample rate by a digital product detector (DPD) composed of a commutator, two sign alternators, and two FIR fractional-phase interpolator filters. This simple structure can yield image performance that is limited by A/D quantization using relatively low interpolator filter orders and IF bandwidths as large as one-half the sampling rate of the A/D converter. The DPD performs Nyquist limit demodulation of the sampled bandpass signal and, therefore, requires a minimal sampling rate. The theory of operation, an analytic proof, design methodology, and simulated performance results are presented. Simulated results show that -86 dB images can be obtained with 8-tap FIR interpolators and a 12 bit A/D converter. A VLSI implementation is also presented