A digital-intensive receiver front-end using VCO-based ADC with an embedded 2nd-Order anti-aliasing Sinc filter in 90nm CMOS

Abstract

One of the recent trends in multimode multiband (MMMB) receivers is to remove the analog filter or variable-gain amplifier (VGA) in the receiver chain and employ a wide-dynamic-range ADC directly after the mixer or include the mixer in the ADC. While such architecture provides ease of programmability once the signals are digitized, it puts a large burden on the ADC and anti-alias filter. Hence, ADCs typically use high-performance analog circuits for wide dynamic range, even though it is difficult to implement these circuits using low-voltage nanoscale CMOS processes. A promising ADC architecture for an MMMB receiver is the VCO-based ADC, since it offers 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> -order noise-shaping from its open-loop digital-intensive nature, thus allowing high sampling rate and high SNR. Furthermore, the VCO-based ADC provides an inherent anti-aliasing 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> -order Sinc filter due to the innate integrating ability of the VCO. Unfortunately, for multiband receivers that do not have an RF pre-filter, a Sinc filter alone does not provide enough out-of-band rejection and hence higher-order anti-aliasing filters are required. In order to solve this problem, we propose a 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> -order anti-aliasing Sinc filter (Sinc <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> filter) that provides double the rejection ratio of a Sinc filter. Furthermore, the proposed technique is highly digital and can be embedded in a VCO, resulting in little overhead.