Subsampling digital IF receiver implementations

This paper introduces the concept of direct digital down-conversion via subsampling and digital quadrature tuning. In theory, a bandpass filtered intermediate frequency (IF) signal can be sampled by an analog-to-digital converter (ADC) with a sampling rate consistent with the pass band width and center frequency of the signal of interest. During the sampling process, the bandpass signal is constructively aliased to baseband. The practical limits of such an implementation are that the bandwidth and timing accuracy of the ADC's track and hold circuitry must be consistent with the IF location of the signal of interest. The subsampling ADC can be immediately followed by a quadrature digital tuner to provide fine tuning and produce a baseband quadrature signal ready for baseband processing. The combination of the subsampling ADC and digital quadrature tuner forms a complete digital IF processing chain.