A Sub-mW Integrating Mixer SAR Spectrum Sensor for Portable Cognitive Radio Applications

A low power mixed-signal integrating mixer successive approximation register (SAR) architecture is proposed for direct spectrum estimation in portable transceivers targeting IEEE 802.22 wireless regional area network cognitive radio applications. The integrating mixer SAR implements the short-time Fourier transform in the analog domain and digitizes its amplitude at the end of the integration period. The architecture consists of an array of folded double balanced mixers connected to a common subset of binary-weighted capacitive loads. Current domain windowing is applied in the first stage followed by mixing, integration and analog-to-digital conversion (ADC) in the second stage. Windowing sets the detection bandwidth and provides flexible low pass filtering while a load capacitor array enables integration with programmable time constant and acts as the sampling capacitors of a SAR ADC. A prototype chip is fabricated in IBM’s $0.13\mu {\text{m}}$ CMOS process. The measured results indicate an average dynamic range of 27.9–25.7 dB over a frequency range of 0.05–1.25 GHz, while consuming 0.88 mW from 1.1/1.2 V supplies.