Theory and applications of all-digital N-path filters

The all-digital
N
-path filter structure for the processing of discrete-time signals is introduced. The structure is made of
N
parallel paths having identical time-invariant digital filters. The filters are connected to a common input signal path through input modulators, and a common output signal path through output modulators. The modulators produce identical periodic sequences but are staggered in phase from filter to filter. A detailed time-domain and frequency-domain analysis of the
N
-path filter is provided. It is shown that for certain bandlimited modulating sequences, or by appropriately bandlimiting the input and output signals, the overall time-varying structure can be made to appear as a timeinvariant network. This approach leads to filters with adjustable characteristics, and it is shown that by time-sharing the time-invariant digital filter path
N
times, it is possible to design filters with complex characteristics quite economically. A number of useful modulating functions are described, and some possible applications are discussed.