A 400-800 MHz spectrum sensing system is designed and implemented using 0.18-mu m CMOS technology for cognitive radios in the UHF TV band. Based on envelope detection, the system employs a down converter, a digitally controlled oscillator, a low-pass filer (LPF), a rectifier, a comparator, and a digital control unit to realize spectrum sensing with 10-bit frequency representation. With variable-gain function of the LPF, signal strength can be measured and presented with a 5-bit signal strength index.