Digital time-division multiplexed N-path system

The use of a time-division multiplexed digital filter as the time-invariant part of an
N
-path filter is proposed and examined. The use of a digital filter, and the resultant requirements for only one input modulator, alleviates the problem of closely matching the transmission characteristics of the paths. As a consequence of the improving matching, the time-varying modulation products in the output signal should be suppressed 10-15 dB more than in conventional implementations. The center frequency, bandwidth, and passband shape of the filter are all electronically adjustable. It is possible to time division multiplex a processor using serial arithmetic for a bandpass filter with a bandwidth of several kilohertz using high-speed bipolar logic circuits. The speed requirements for the memory are such that MOS circuitry, with its attendant size advantage, is suitable. This approach to realizing an inductorless, electronically adjustable bandpass filter should become economical in the near future because of the decreasing cost and the increasingly complex functions available in both bipolar and MOS circuitry.