A distinct operation frequency cascaded N-path filter with improved harmonic rejection ability

A cascaded differential 20-path and 3-path filter which realizes a more than 20 dB stopband rejection and harmonic rejection ability is proposed. Compared to a differential N-path filters with a passive low-pass filter connected between the signal source and the filter itself, the proposed cascaded N-path filter provides better flexibility in tuning and narrower full width half maximum (FWHM) value for higher frequency selectivity. The first stage 20-path filter which acts as a tunable low frequency band-pass filter operating at one third of switching frequency of second stage N-path filter, attenuating the undesirable high frequency signals. The second stage 3-path filter then filters off the remaining standards providing sufficient stopband rejection for the whole system. The different operating frequencies for both stages provides a harmonic rejection ability as the high frequency components causing the harmonic folding of second stage frequency are first removed before passing through the second stage N-path filter for a desirable standard.