Power consumption is of utmost importance in portable devices as they are operated from a limited energy supply. Wireless radio is one of the most power-hungry blocks in these systems and needs to be activated opportunistically. Radio frequency (RF) spectrum sensing can be performed with a dedicated low power radio to control the main wireless radio. A low power receiver architecture is proposed in this work to monitor the 2.4 GHz Industrial, Scientific, and Medical (ISM) band for communication standards such as Wireless Local Area Network (WLAN), Zonal Intercommunication Global-standard (ZigBee), and Bluetooth-Low-Energy (BLE). The whole ISM band is down-converted to zero-intermediate-frequency (ZIF) and a tunable base-band complex band-pass filter (CBPF) is used to scan the spectrum. A widely tunable first order ultralow-power transconductor-capacitor (Gm-C) CBPF architecture was designed and fabricated in GF 22FDX technology, which occupies 0.0049mm(2) area. A frequency shift of +/- 60MHz was achieved with 5 - 40MHz bandwidth range. Power consumption ranges from 6.7 mu W to 99.2 mu W with the best and the worst figure-of-merit of 0.034 fJ/pole and 0.082 fJ/pole that is the energy consumption per pole normalized by the spurious free dynamic range.