A tuneable, Bandwidth-adjustable solid-state filter

Abstract

A tuneable, adjustable bandwidth filter is realized using the N-path system, in a form implemented by solid-state circuits. The N-path system is a time-varying system that can perform a low-pass to bandpass filter transformation. It is used to shift the passband of an active RC low-pass filter to a new center about a frequency \\Gw 0 = 2π/ T, where T is the period of a locally generated modulating signal. A multilevel approximation to the sinusoid is used as the modulating signal. A two-pole Butterworth bandpass filter was built to provide experimental verification of the theory. The center frequency of this basic N-path system is adjustable from 200 to 500 kHz and the bandwidth is adjustable from 4 to 12 kHz. An image signal rejection of 37 dB was measured. This value compares favorably with the 40 dB figure predicted analytically.