A low power integrating mixer successive approximation register (SAR) prototype chip for spectrum sensing is fabricated for portable transceivers targeting IEEE 802.22 cognitive radio applications. The integrating mixer SAR combines mixing, current-domain windowing, and integration to implement the short-time Fourier transform. Integration with programmable time constant is incorporated within the mixer by utilizing binary-weighted capacitive loads, which double as the sampling capacitors of a SAR ADC. The design operates over a frequency range of 0.05–1.25GHz, consumes 0.88mW from 1.1/1.2V supplies and obtains an average dynamic range (DR) of 25.7–27.9dB.