Direct Complex Envelope Sampling of Bandpass Signals With M-Channel Time-Interleaved ADCs

Classical receiver architectures demodulate a bandpass signal to baseband before sampling the in-phase and quadrature components. With the advent of faster analog-to-digital converters (ADCs) and wide bandwidth sample and hold (S/H) circuits, it has become practicable to sample a bandpass signal directly without any demodulation operation and then process it with robust DSP technology. Direct sampling methods do present their own challenges. When a single channel is used to digitize the signal, not all frequencies above the Nyquist rate are allowed. As first shown by Kohlenberg, this restriction can he removed with a two-channel time-interleaved ADC (TIADC) where two ADCs separated by a timing offset independently sample the signal. In this paper, we propose a general and flexible technique for sampling the complex envelope of a bandpass signal using a nonuniformly interleaved M-channel TIADC. The signal is sampled directly by the subADCs which are separated by timing skews. Reconstruction of the envelope entails inverting a matrix of filters resulting in some timing skews being forbidden. The proposed sampling scheme requires the implementation of M digital FIR filters and can be used to digitize bandpass signals with any carrier frequency in software defined radio applications. Reconstruction analysis is provided for the case of two, three, and four channels. Multi-tone and MSK signals are used in simulations to validate the proposed method and assess its performance.