Narrow-bandpass filtering with modulation

To overcome the problems of carrier leak etc. experienced with N-path filters incorporating active lowpass filters, it has been suggested that N-path filters incorporating bandpass filters (which must be well matched) be used to produce a pair of passbands away from the carrier frequency, with another filter at the required final frequency to discriminate between the wanted and the image passbands. An alternative and practical solution is described here in which neither the well matched low-frequency filters nor the additional high-frequency filter are required; instead a single low-frequency bandpass filter and two pairs of elementary phase-shift networks are used. The circuit is well suited to realisation as a thin-film or hybrid integrated circuit, since all the frequency-selective circuits operate at the low frequency.