"RF channel filtering: a revival of N-Path filters in Nanometer CMOS?"
- Bram Nauta, Professor & Chair IC Design, University of Twente, The Netherlands
Back in the 1960’s quite some research was carried out on so called “N-path filters”. These filters exploited down conversion, filtering and – again - up conversion. This way band-pass filters could be made from simple low-pass filters. The techniques lost attention because the matching of capacitors turned out to be a problem and soon “real” oversampled Switched Capacitor filters took over attention, which have become classical. However in modern nanometer CMOS the capacitor matching is almost perfect and the speed of up the down and up-converters is in the GHz range, which makes it worth to have a look at these techniques and to revisit and improve the architectures. Moreover with today's speed of digital circuits, the required multiphase clocks signals can be easily generated for RF frequencies. The results are filters, which show more or less analog behavior while just using switches, digital logic gates and capacitors. After intuitively discussing the basics of N-path filters, several examples of recently published N-path filters will be shown. These filters are tunable with a clock signal and show characteristics, which were until recently only thinkable with SAW, or BAW technologies.