Radio Frequency Integrated Circuits

Modern Receiver Architectures:  mixer-first, N-path, and “no-gain” architectures
Bram Nauta, University of Twente, Enschede, the Netherlands

In a classical receiver, the first thing designers do is add amplification immediately after the chip’s RF input. This is usually done with a low-noise amplifier (LNA) and is often followed by selectivity and a mixer. This way, the mixer may be noisy, and its design becomes easier. In the current overcrowded radio spectrum, however, adding the first amplifier may result in significant distortion products at the LNA output. These cannot be removed anymore. Therefore, mixer-first architectures have emerged as a candidate for the first circuit after the chip’s RF input. These architectures do not use a low-noise amplifier but instead use a low-loss passive mixer. These passive mixers exhibit very good linearity and offer the option of narrow-band RF filtering at the mixer input. This makes the mixer-first receiver a good candidate for applications where interference is a challenge. The RF filtering is achieved by exploiting the mixer in a so-called N-path filter, which is a filtering technique from forgotten times. New ideas such as higher-order filtering and passive voltage gain via capacitor stacking will also be presented in this lecture. An outlook of fully passive receivers, even without active linear amplification, is also given as a possible future direction.